Animal tissues - Hoërskool Patriot - ONS GLO...Animal tissues We concluded the previous lesson with plant tissues. Just as plant tissues, animal tissue is a collection of cells and

Copy right © R. Botha (2016) May nor be copied or distributed

Animal tissues

TYPES OF ANIMAL TISSUES

10.2.1


Animal tissues

We concluded the previous lesson with plant tissues.

Just as plant tissues, animal tissue is a collection of cells and their

intercellular material that are more or less similar in differenciation

and functions as a unit.

A definition of tissue is:

A tissue is a group of cells with a specific structure and function.


Animal tissues:

Four basic types of tissues are found in animals, viz.:

epithelial tissue,

connective tissue,

nervous tissue, and

muscle tissue


Animal tissues: Epithelium

Greek for epi = on top; theli = tissue.

Epithelial tissues are continuous cell layers that line body cavities

and cover the surfaces of tissues and organs.

It also occurs in glands where it is responsible for the secretion of

hormones and enzymes.

Epithelial tissue consists of closely packed cells with little intercellular

material and almost no intercellular spaces.

Epithelia occur in one or more layers of cells and are divided with regard to the number of layers into simple epithelium (one cell layer)

and compound epithelium (more than one cell layer).



Functions of epithelium include the following:

Excretion

Selective absorption

Protection

Trans cellular transport

Sensors of extracellular conditions (intercellular communication)



Epithelial cells are closely packed by tight junctions and

desmosomes (type of intercellular sutures).

Epithelium lies on top of connective tissue, but is separated from it

by the basement membrane (or basal lamina).

Epithelium is innervated (contains nerves), but is not vasculated

(does not contain blood vessels).

Epithelium receives nutrients from the connective tissue by diffusion

through the basement membrane.



Epidermis:

The epithelium that forms the outer layer of the skin.

Endothelium:

The epithelium that lines the blood vessels (arteries, veins and inside of the heart).

Capillary vessels consist of endothelium only.

Basement membrane

Supplies the framework on which the epithelium lies.

It is a thin layer of loosely associated cells and fibers found between the epithelium and the connective tissue.

Remark:

Cancers of the endothelium and the mesothelium (middle layer of epithelium layers) are known as sarcomata (singular: sarcoma), while cancers of any other epithelium are known as carcinomata (singular: carcinoma).



Epithelium is classified with regard to the following:

Shape of cells:

Squamous epithelium

Cubic epithelium

Cylinder (columnar) epithelium

Intermediate epithelium

Stratification (more than one cell layer)

Specialisation

Keratinized (hard, dead epithelium for protection, e.g. outer layer of skin.)

Ciliated epithelium (epithelium that possesses cilia, e.g. mucous membranes

or air passages).



Epithelium and excretion

Invaginations of the epithelium form glands.

These invaginations may be simple or compound and may be imbedded in the underlying connective tissue.

Both exocrine and endocrine glands are formed by epithelium.

Exocrine glands secrete their contents by means of tubes.

The product of endocrine glands is secreted directly on the surface of the glandular epithelium.

Sensory function of epithelium

The epithelial cells may contain receptors that can track specific chemical compounds e.g. in the nose, be heat sensitive, e.g. in the skin and able to observe mechanical vibrations, e.g. in the ear.



Squamous epithelium

nucleus

basement membrane

Columnar epithelium

basement

membrane

nucleus

cilia

Cubic epithelium

basement

membrane

nucleus

cytoplasm


Animal tissues: Epithelium –

Squamous epithelium

Locality

Forms the walls of the lung alveoli.

Lines the blood vessels.

Lines the mouth, oesophagus and vagina.

Structure

Consists of a single layer of thin, flat cells arranged next to one onother to often form a mosaic pattern.

Function:

Diffusion of gases effectively takes place through thin squamous epithelium.

Endothelium that lines blood vessels decreases friction of blood flow.

Covers and protects underlying tissues.



Squamous epithelium

cytoplasm

nucleus

basement

membrane


Animal tissues: Epithelium – Cubic

epithelium

Locality

Found in glands, like the thyroid gland, salivary glands and the renal

tubules.

Also found in the skin in sweat glands and sebaceous glands.

Structure

Cells have a cubical structure – breadth, width and height are more or

less the same.

Function

Cubic epithelium secretes substances, like saliva by the salivary glands

and sebum or oil by the sebaceous glands.

It excretes substances, like urine and the waste products in sweat.


Animal tissues: Epithelium – Cubic

epithelium

cytoplasm

nucleus

basement

membrane



Columnar (cylindrical) epithelium

Locality

Found in the stomach and small intestine.

Villi consist of a single layer of columnar epithelium.

Structure

The cells are rodlike and the nucleus occurs near the bottom of the cell.

Goblet cells (unicellular glands) occur among the rodlike cells.

Function

The columnar epithelium in the villi is responsible for the absorption of

digested food, water, vitamins and minerals.

Goblet cells in the villi secrete mucus.



Columnar epithelium

Goblet cell

Cytoplasm

nucleus

basement

membrane


Animal tissues: Epithelium – Ciliated

columnar epithelium Locality

Occurs in moist environments of the body and lines the nasal passages, trachea, bronchi, Fallopian tubes and sperm ducts.

Structure

Consists of rodlike columnar epithelium with cilia on the free end of each cell.

Among the ciliated epithelium cells, goblet cells are found.

Function

The combination of mucus and the wavelike movements of the cilia enable the movement of substances inside the tubules.

Expulsion of dust particles and foreign substances from the air passages to protect the lungs.

The movement of egg cells and sperm cells is caused by the wavelike movements of the cilia in the Fallopian tubes and sperm ducts.


Animal tissues: Epithelium – Ciliated

columnar epithelium

basal granule

with cilia

cytoplasm

nucleus

basement

membrane

mucus

Goblet cell


Animal tissues: Epithelium – Glandular

epithelium Locality

Unicellular glands (goblet cells) are found in the air passages and in the small intestine.

Multicellular glands, like the salivary glands, the thyroid gland and sebaceous glands in the skin are formed by invaginations of the epithelium.

They consist of cubic or columnar epithelium.

Unicellular glands consist of single, isolated glandular cells, e.g. goblet cells, spread among other non glandular cells.

Multicellular glands originate by invagination of the epithelium.

Function

Goblet cells produce mucus that serves as lubricant in the digestive canal to enhance the movement of food and to keep the air passages moist and slimy.

Multicellular glands secrete enzymes for the digestion of food as well as hormones, sweat, wax and saliva.



Glandular epitheliumSimple gland Compound gland

Tube

Secretory

part


Animal tissues: Connective tissue

Connective tissue consists of a matrix of living cells and non-living ground material.

The ground material consists of an organic part (usually proteins) and an inorganic part (water and minerals).

The most important cells of connective tissue are fibroblasts, that inter alia synthesise fibers. Almost all types of connective tissues contain fibers.

Fibroblasts are mobile cells, can undergo mitosis and can synthesise all types of connective tissue.

Macrophages, lymphocytes and sometimes leucocytes are found in some types of connective tissue.

Some types of connective tissue contain specialised cells that are not found in other connective tissues.

The matrix of connective tissue with a high concentration of cells and fibers is relatively less dense.



The organic part of connective tissue, the protein fibers are

collagen, elastic or reticular fibers.

Collagen gives sturdiness and strength and prevents that

connective tissue separates from surrounding tissues.

Elastic fibers consist of the protein elastin, that can stretch one or

one and a half times their length and return to their original length

and shape.

Elastic fibers make the connective tissue pliable.

Reticular fibers consist of a network of thin collagen fibers that give

support to the tissue and organs to which it is attached.



Type of connective

tissue

Cells Fibers Locality

Loose/areolar connective tissue

Fibroblasts, macrophages, few lymphocytes and neutrophils

Few collagen or elastic and reticular fibers

Around blood vessels; anchors epithelium

Dense fibrous connective tissue

Fibroblasts, macrophages

Mainly collagen Tendons and ligaments; sometimes skin

Cartilage Chondrocytes, chondroblasts

Hyaline: few collagen fibersFibrous cartilage: large

amount of collagen

Bones of foetus; human outer ear; intervertebral discs;

Skeleton of sharks

Bone Osteoblasts, osteocytes, osteoclasts

Few collagen and elastic fibers

Skeletons of Vertebrata

Adipose tissue Adipocytes Few fibers Adipose tissue around organs, like the eye.

Blood Red blood cells, white blood cells

No fibers Blood


Animal tissues: Connective tissue –

Areolar connective tissue Locality

Surrounds blood vessels and organs.

Structure

Areolar connective tissue contains parts of all other types of connective tissue and has the consistency of a ball of cotton pulled apart.

Collagen fibers are wide and elastic fibers are thin.

The cavities among the cells and fibers are filled with matrix.

Function

Joins organs and supports cell and tissue structures.

Is elastic and allows movement.

Assists the immune system in the defence against diseases and the healing of wounds.

Anchor epithelium.



Areolar connective tissueElastic fiber Fibroblast Collagen fiber


Animal tissues: Connective tissue – Dense

fibrous connective tissue

Locality

Occurs in tendons and ligaments.

Forms part of the skin.

Structure

Contains large amounts of collagen fibers and relatively few cells and little matrix.

May have a regular appearance with fibers that are arranged in parallel, or an irregular appearance. Irregular, fibrous connective tissue is found in localities where there is tension in the tissue, e.g. the dermis of the skin.

Functions

Tendons attach muscles to one another and attach muscles to bones.

Ligaments attach bones to one another.



Dense, fibrous connective tissueFibroblast nucleus Fibroblasts Collagen fibers


Animal tissues: Connective tissue - Cartilage Locality

Skeleton of sharks.

Whole skeleton of foetus.

Hyaline cartilage covers ends of long bones and in joints, between ribs and breast bone, in larynx, bronchi and trachea.

Elastic cartilage in outer ear and part of the larynx.

Fibrous cartilage in the discs between vertebrae and in knee joint.

Structure

Cartilage is connective tissue with much matrix and a variable amount of fibers.

Chondrocytes produce the matrix and fibers. They are found in fluid-filled cavities in the cartilage, the lacunae.

Hyaline cartilage: Few collagen and elastic fibers; milky or clear matrix; lacunae are randomly spread.

Elastic cartilage: Large amount of elastic fibers that makes it flexible.

Fibrous cartilage: Large amount of collagen that makes it very strong.

Functions

Hyaline cartilage on ends of long bones reduces friction and serves as a cushion for the articulation of these bones (joints) and keeps air passages open.

Elastic cartilage gives shape and pliability.

Fibrous cartilage serves as a shock absorber between vertebrae and deepens the articular cavities in some joints to prevent injuries.


Animal tissues: Cartilage

Hyaline

cartilage

Elastic cartilage

Fibrous cartilage

Elastin fibers

Collagen fibers

lacunae


Animal tissues: Connective tissue -

Bone

Bone tissue contains two different types of matrix material, viz.

organic and inorganic matrix. The organic matrix is similar to that of other connective tissue, including collagen and elastic fibers that

give strength and pliability.

The inorganic matrix consists of mineral salts, mainly calcium salts

that make bone hard.

When there is insufficient inorganic matrix in bone tissue bones stay

soft and may bend.



Bone

Bone tissue contains three types of cells, viz. osteoblasts, osteocytes

and osteoclasts.

Osteoblasts produce new bone tissue for growth and repair.

Osteoblasts deposit bone material in the matrix and the matrix

surrounds it and keeps it alive, but in a reduced metabolic state.

Osteocytes are living cells with a high metabolic rate. They are

found in lacunae in bone tissue.

Osteoclasts break bone down for repair and release stored calcium

Osteoclasts usually occur on the surface of bone tissue.


Animal tissue: Connective tissue -

Bone Bone tissue can be divided into two types, viz. compact bone and spongy

bone.

Compact bone is found in the shafts (diaphysis) of long bones and the surface of flat bones, while spongy bone is found at the ends (epiphyses) of long bones.

Compact bone consists of sub units, osteons.

In the centre of each osteon is a canal, called the Haversian canal, that contains a blood vessel, nerve and a lymph duct.

There are concentric layers, called lamellae, around the Haversian canal. In the lamellae small cavities, known as lacunae, are found. Osteocytes occur in the lacunae.

Osteocytes are connected with one another as well as with the Haversian canal and periosteum by tiny canals, viz. canaliculi. Periosteum is connective tissue surrounding bone.

Osteons form Haversian systems, that together form bone tissue.


Animal tissue: Connective tissue -

Bone Spongy bone consists of small platelets, known as trabeculae. They

support and strengthen spongy bone.

Spongy bone is filled with red bone marrow and contains red blood cells in different stages of development.

With age these platelets may break and make bone less resistant to injuries.

Functions of bone tissue:

Bone tissue forms the internal skeletons of vertebrates and gives structure and sturdiness to these animals as well as places for attachment of muscles.

It forms a system of joints and levers to enable movement.

Bones protect delicate organs, e.g. the skull that protects the brain and bones of the ribcage that protect the heart and lungs.

Red bone marrow in spongy bone produces red blood cells.


Animal tissues: Connective tissue - Bone

Compact

bone Spongy bone

Lymph duct

Nerve

Blood vessel

Trabecula

Sponge

bone

Compact bone



Bone

Lacuna (contains osteocyte)

Lamellae

(concentric circles)

Haversian canal

Canaliculi

Osteocytes

Osteoclasts

Osteoblasts

Canaliculi

Lamellae



Adipose tissue

Adipose tissue or fat tissue is regarded as connective tissue, although it does not contain any fibroblasts or a real matrix and only a few fibers.

Adipose tissue consists of adipocytes (fat cells) that collect and store fat in the form of triglycerides for energy metabolism when needed.

Adipose tissue serves for isolation to maintain the body temperature of endothermic animals.

Adipose tissue also protects certain organs against injuries, like the eyes and kidneys.

Under the light microscope adipose tissue looks empty, because the techniques of preparation for microscopy extract the cells from the fat.



Adipose tissue

Cell membrane

nucleus



Blood

Blood is regarded as connective tissue because it has a matrix.

The living cell types in blood are red blood cells (erythrocytes) and

white blood cells (leucocytes).

The fluid part of blood forms the matrix and is known as the plasma.

Erythrocytes are the most abundant in blood. The average amount

of erythrocytes in primates is 4,7 to 5,5 million cells per microliter of

blood.

The erythrocytes of mammals lose their nuclei and mitochondria when they are released from bone marrow into the blood stream.

The main function of erythrocytes is to transport oxygen to tissues.


Animal tissues: Connective tissue - Blood

Leucocytes are the most important white blood cells found in peripheral blood (blood in the blood vessels).

Primates on average have 4 800 to 10 800 leucocytes per μm (microliter) of blood.

Other types of white blood cells that make up part of the immune response of the body are lymphocytes, neutrophils, monocytes, macrophages, eosinophils and basophils.

Lymphocytes protects the body against foreign antigens and other foreign substances. The B lymphocytes produce antibodies that protect the body against antigens.

Neutrophils are phagocytic cells that early in immune control, take in and destroy microorganisms, like bacteria, that enter the body.

Monocytes give rise to phagocytic macrophages that remove dead and damaged cells in the body.

Eosinophils and basophils assist with inflammatory reactions in the body.



Blood

The granular substance among the other blood cells are blood

platelets (thrombocytes) and are cytoplasmic fragments of cells in the bone marrow.

Blood platelets facilitate the clotting of blood.

Blood has different functions of which the most important are to

transport oxygen and nutrients to cells and to remove waste

products.


Animal tissues: Connective tissue - Blood

Neutrophil

Macrophage

Monocyte

Erythrocyte

Lymphocyte

Basophil

Blood platelets

Eosinophil


Animal tissues: Muscle tissue

Muscles consist of muscle fibers, connective tissue, nervous tissue

and vascular tissue.

With regard to striation, number and locality of nuclei, voluntary

muscles and involuntary control and the locality in the body, three

types of muscles are distinguished, viz.:

Smooth muscles

Skeletal / voluntary muscles

Cardiac muscle


Animal tissue: Muscle tissue

Type of

muscle

Striation Nuclei Control Locality

Smooth

muscles

No One nucleus;

central

Involuntary Internal

organs

Skeletal

muscles

Yes Multinuclear;

peripheral

Voluntary Skeleton

Cardiac

muscle

Yes One nucleus;

central

Involuntary Heart


Animal tissues: Muscle tissue –

Skeletal/voluntary/striated muscles

Locality

The muscles that join the tendons to parts of the skeleton.

Structure

Skeletal or striated muscles consist of numerous muscle fibers joined in bundles by a sheath of connective tissue, known as a perimecium.

Each bundle of fibers, surrounded by a perimesium, is known as a fasciculus.

Numerous fassiculi are surrounded by an epimecium.

The entire muscle thus consists of fassiculi surrounded by the epimecium.

Each muscle fiber (± 2,5 mm long) is surrounded by a tough sheath, the sarcolemma. The ground tissue of the muscle fiber (cytoplasm) inside the sarcolemma is called the sarcoplasm.

A number of oval nuclei occur in the sarcoplasm.




Each muscle fiber consists of a number of myofibrils that form the contractile units of the muscle.

The muscle fibers have alternating dark and light bands (striata), that give voluntary muscles their striated appearance.

Nerve fibers are attached to the sarcolemma. The nerve fibers conduct messages from the brain to muscles to initiate the contraction or relaxation of the muscle fibers, which bring about movement. Blood capillaries are found among muscle fibers to assist in the exchange of nutrients, gases and waste products.

Functions:

Skeletal muscles function in pairs to enable coordinated, voluntary movements.

Contaction and relaxation of these muscles enable movement of the body.




Perimysium

Blood vessel

muscle fiber

Fasciculus

Endomysium

Epimysium

Tendon

Bone



Skeletal/voluntary/striated muscle

Segment of a muscle fiber

Sarcolemma

Myofibril

Dark band Light band Peripheral nuclei



Smooth muscles

Locality

Occurs in the walls of organs like the digestive canal, blood vessels, the bladder, uterus and in the eye.

Structure

Consists of spindle shaped cells, each with a central nucleus.

Contains sarcoplasm surrounded by a sarcolemma.

Does not have a striated appearance.

Functions

Is under control of the autonomic nervous system and controls involuntary muscles.

Causes rhythmic contractions and relaxation in the digestive canal, called peristalsis. It pushes the food on along the canal.



Smooth musclesautonomic nerve

nucleus

muscle fibers



Cardiac muscle

Locality

Cardiac muscle is only found in the walls of the heart.

Structure

Consists of branched fibers with nuclei.

Among the muscle fibers connective tissue with blood vessels and nerves occurs.

The branched fibers are joined by small muscle bridges (intercalated discs) to form a continuous muscle mass (sincytium).

Cardiac muscle is also striated but not as conspicuous as skeletal muscles.

Cardiac muscle continuously contracts and relaxes and is involuntary.

Cardiac muscle is responsible for the pump action of the heart.


Animal tissues: Muscle fibers – Cardiac

muscle

Sarcolemma Intercalated disc Connective tissue

Cross bands Nucleus


Animal tissues: Muscle tissue

Functions of muscle (summary)

Muscle fibers contain contractile filaments (myofibrils) that can exert

power and cause movement.

That can lead to the movement of limbs, the entire organism or organs

in the organism.

The contraction of the cardiac and smooth muscles is involuntary and is

necessary for the survival functions of the body, e.g. the beating of the

heart and for peristaltic movements of the intestine to transport food

through the digestive system.

The contraction of the voluntary muscles is delicately controlled. That

enables accurate movements, e.g. the movement of the eye and even

that of the bigger limbs.


Animal tissues: Nervous tissue

Nervous tissue is made up of cells that are specialised to receive

electrical impulses from specific areas of the body and to conduct them to other parts of the body.

Nervous tissue consists of:

Neurons

Macroglia

Microglia


Animal tissue: Nervous tissue

Neurons

Specialised cells to conduct nerve impulses.

Due to their function neurons possess certain characteristics:

They possess cell bodies that contain the nucleus.

Dendrites and an axon are found on the cell body. The dendrites receive impulses and conduct them over the surface of the cell body to the axon.

The nerve impulses are conducted along the length of the axon to the axon terminal.

The axon terminals conduct the impulses to the dendrites of the consecutive neuron by means of synapses.

Synapses are small structures where the dendrites and the axon terminals make contact.

The cells (the axon is the elongated part) sometimes are very long, because the impulses must often be conducted over a long distance.

When the nerve impulses are conducted for short distances the cells are not very long.


Animal tissue: Nervous tissue

A typical neuron



Macroglia

These are cells that support the neurons in different ways.

The shape of these cells is completely different from that of neurons. The shape may be very complex with numerous dendrites (outgrowths on the surface of the cell).

One of the functions of the astrocytes (a sub class of the macroglia) is to keep the neurons in position. They also supply the neurons with nutrients and remove waste products.

Oligodendrocytes (another sub class of the macroglia) form the myelin sheath that surround the neurons of the central nervous system.

The Schwann cells (also a sub class of macroglia) form the myelin sheath that surround the neurons of the peripheral nervous system. The myelin sheath also covers the axons and gives electrical isolation to the neuron.

The macroglia help with the development of new neurons and are involved in the repair of damaged neurons.



Microglia

They are smaller than the macroglia, as their name indicates.

Their structure does not resemble that of a neuron.

These cells originate from cells of the bone marrow that give rise to

blood cells.

The cells move by amoeboid movements (like amoeba, by changing its

shape) through the brain.

Their function is to protect the brain against infections and to remove

dead neurons.



(Macroglia and microglia)

Copy right © R. Botha (2016) May not be copied or distributed

Application of indigenous

knowledge systems and

biotechnology

10.2.2


Traditional medicine

According to the World Health Organization (WHO) tradtional medicine is the sum total of knowledge, skills and practices used in physical iand spiritual health. It is based on the theories, beliefs and experience that are indigenous to different cultures.

The use of traditional medicine often makes use of plants, but animal based therapy as well as spiritual practices are also included in traditional medicine and medication.

Up to 80% of people in Africa and Asia (70% in South Africa) relies on traditional medicine for their health needs.

There are basically two groups of traditional health practitioners, viz. herbalists and witch-doctors.

Herbalists mainly use plants as medicine, but also fungi, bee products, minerals, shells and other animal products. There is more and more reseach into the use of plants as medicine. In 1997 The South African Traditional Medicine Research Group is established at the University of the Western Cape and it is funded by the South African Medical Board. They do research in traditional medicine and also present courses in traditional medicine.

Fortune-tellers / witch doctors make use of rituals to heal people of a variety of diseases. They are not accepted by ortodox medical practices and practioners.


Traditional medicine

In 2010 a 48-bed hospital that treats patients with a combination of

traditional and Western medicine opened in Kwa-Mhlanga.

A well known South African plant with medicinal characteristics is

the African potato (Hypoxis hemerocallidea).

African potato is indigenous to South Africa and has been used for

centuries as a medicinal plant. According to a study that was

carried out, it was the most popular product on the “muti” market in

the Eastern Cape.

Infusions are made of the fleshy underground bulb of the African

potato. It is used for a big variety of conditions, inter alia as a

strengthening tonic and to treat tuberculosis, cancer (mainly prostate cancer), urinary tract infections, rheumatism, anxiety and depression.


Traditional medicineAfrican potato (Hypoxis hemerocallidea)


Biotechnology

Biotechnology is the use of living matter to produce and change products and processes. (“Bio” means life and technology means the production and processing of products and processes.)

Biotechnology has been used for centuries, e.g. in the baking of bread and brewing of beer, where yeasts (a type of fungus) are used in the rising and fermentation process. Lactic acid bacteria are used in the production of yoghurt and cheese.

Modern biotechnology involves new and sometimes controversial products and processes. It includes the following:

Genetically modified food (GM), contains genetic material of other organisms to improve some of their characteristics, e.g. genetically modified food crops that contain genes to make them more resistant to certain diseases, pests and plagues.

The products of biofuels (fuels from organic substances like biogas, biodiesel and alcohol petrol).

The use of stem cells and cloning.


Stem cell research

Stem cells are undifferentiated cells that occur in several parts of the human body.

Stem cells have the potential to develop into different types of cells.

When stem cells divide, each new cell can stay a stem cell or it can develop into a specialised type of cell, with a particular function, like a muscle of nerve cell.

Stem cells have the potential to supply patients with an unlimited stock of a specific cell type needed in transplants, due to diseases like heart failure, leucaemia or diabetes.

Stem cell research started in 2001. It focuses on three types of stem cells:

stem cells of human embryos:

stem cells from bone marrow;

stem cells of blood from the umbilical cord.


Stem cell research

Embryonic stem cells (ES cells)

Embryonic stem cells are taken from embryos formed when an egg cell

is fertilised by a sperm cell inside a test tube. This process is called in

vitro fertilisation.

There are divergent opinions about the harvest (gathering and storage)

of stem cells. People of some religions argue that an embryo is already

a human being and that it is murder to prevent its development.

Bone marrow stem cells

Stem cells that occur in bone marrow are further developed than

embryonic stem cells and are also easier available. The harvest of these

cells is less controversial , because the organism is not destroyed by the

harvest.

These kinds of stem cells are used to form new blood cells as well as

skeletal and cardiac muscle cells.


Stem cell research

Umbilical cord blood stem cells

The blood stem cells of the umbilical cord is obtained during birth,

frozen and is later made available.

It is advised that cord blood stem cells are obtained and stored when:

there is a genetic disease in the family that can be healed by stem cell

transplant;

older childen of the family contract an aquired disease (a condition that a

child is not born with but later developed) that can be healed by stem cell

therapy, like leucaemia.


Cloning

A clone is a new organism or offspring that originates by asexual

reproduction.

It is an identical copy of the parent because it is genetically

identical to the parent.

The use of cuttings of plants is an example of cloning.

Cloning has been use for a long time in agriculture and horticulture

to increase plants with suitable characteristics. It is also used in the

breeding of animals.

The disadvantage of the cloning of organisms is that there is no

variation in the offspring.

Any change in the environment, like a new disease, can lead to the death of all the offspring if the parent organisms did not have

resistance against or a tolerance to this change in the environment.


Cloning in animals

The technique mostly used in the cloning of animals is the transfer of

the nucleus.

In 1997 scientists of Scotland succeeded, after 200 attempts, to

produce the first cloned mammal, the sheep “Dolly”.

In 2002 , when she was six years old, Dolly developed arthritis. Young

sheep seldomly develop this condition.

Dolly died on14 Februarie 2003, after a long period of disease.


Ethical and technical questions

about cloning

Ethics is a set of moral principles applied by people to determine

what is right or wrong.

Although the technique of nucleus transfer sounds easy, the rate of

failure is very high.

Research shows that only 2% of cloned embryos survive to living

offspring.


Vaccinations

Vaccines are weakened pathogens (disease causing) or dead

pathogenic organisms, or molecules obtained from pathogenic

organisms, injected into animals.

The animals experience the substance as a foreign substance to the

body and starts an immune response against it.

When the animal encounters the living, virulent pathogenic

organism, it is resistant to the infection.

Vaccines prevent infections and is the most cost effective method

to control infections.


Organs

The structure of a leaf

10.2.3


In previous lessons we discussed different plant and animal tissues.

Can you still remember the plant tissues we discussed?

Amongst others we discussed vascular tissues, the xylem and the

phloem. We also discussed other different types of tissues, like

parenchyma, collenchyma, sclerenchyma and epidermis tissue.

We shall discuss organs next.

What is an organ?

Try to formulate a definition of an organ.

Introduction


Organs

An organ (in Biology) is a collection of tissues combined in

a particular way into a structure to perform a specific

function.

Both plants and animals possess organs. Examples of

organs in mammals, e.g. humans, are the heart, lungs,

kidneys, liver, stomach, pancreas, etc.

Examples of plant organs are leaves, stems and roots.

The study of the external structure of plant organs is

known as morphology (or organography).


Organs - Leaves

A leaf is a flattened organ with limited growth, borne at a

node of a stem and develops from a leaf primordium just

below the apical meristem of the growth point of the

stem.

A typical leaf is green and consists of a lamina (leaf

blade), a petiole (leaf stalk) and a leaf base where it is

attached to the stem.

The leaf base cannot always be clearly distinguished. In

grasses the leaf base forms a sheath that surrounds the

stem.


Leaves: external morphology

Leaf-like outgrowths, known as stipules, occur at the leaf base of

some plants, e.g. in Pelargonium sp.

The leaf base may be thickened and cushion-like, as in Jacaranda, and

is known as a pulvinus.

In certain types of plants the pulvinus facilitates sleep movements of

the leaves, as in certain Acacia species where the leaves fold up

during the night. The leaflets of the “touch-me-not” plant (Mimosa

pudica) do not only fold up at night, but also rapidly fold inward and

droop when touched, heated, blown on or shaken. The movements

are caused by the pulvinus at the leaf base.

These movements are caused by a change in the turgor pressure of

cells in the pulvinus. (You will learn more about turgor pressure

later.)



lamina

petiole

axillary bud

node

internode

leaf sheath encircles

the stem



The petiole attaches the lamina to the stem. It occurs in petiolated leaves

only. A sessile leaf does not have a petiole.

The petiole mainly consists of vascular tissue that runs from the stem to the

leaf blade.

sessile semi-sessile petiolated



The venation of a leaf is the arrangement of the vascular tissue in the

leaf.

The mid rib (main vein or primary vein) is the most superficial vein.

Lateral veins (secondary veins) branch from the mid rib and they

usually branch even further, forming teriary veins.

With regard to the arrangement of the major veins leaves are grouped

into leaves with a parallel venation (mostly monocotyledons) and

pinnate or palmate venation, as in dicotyledons.

The smallest veins in mono and dicotyledons usually form reticulate

(net-like) patterns.



Venation

parallel pinnate/reticulate palmate



Division of the lamina

The lamina may be undivided, as in simple leaves, or it may be

subdivided as in compound leaves.

A leaf is regarded as simple when the lamina is not divided into

smaller leaflets.

When the lamina of a single leaf is subdivided into smaller units up to

the mid rib or other prominent veins, the leaf is regarded as a

compound leaf; the smaller units are called pinnae (leaflets),

The vein bearing the pinnae, is called the rachis.

If the pinnae are not divided any further and are borne on either side

of the rachis, the leaf is pinnately compound.


Leaves: External morphology


If all the pinnae are borne on the same part of the

petiole, the leaf is known as a palmately compound leaf.

If the pinnae of pinnately and palmately compound leaves

are subdivided, bipinnately compound and bipalmately

compound leaves arise and the leaflets are called

pinnulae.

The stalk of a pinna is called a petiolule.

The main vein that carries the pinnulae, is called a

rachilla.


Leaves: External morphology


Simple leaves

Pinnately compound leaves Palmately compound leaves

pinna

rachis

petiole

Leaves: External morphologyDivision of the lamina (double pinnately compound leaf)

pinna

pinnula

rachis

rachilla

petiolule

petiole


Leaves: Function

We discussed leaves and in particular their external structure (morphology).

What are the functions of leaves?

The main functions are photosynthesis and transpiration.

You studied photosynthesis in Grade 8. Can you still remember what it is?

Photosynthesis is the process whereby sugar (mainly glucose) is produced in

the leaves (chloroplasts) from water and carbon dioxide. Sunlight is needed

for this process and oxygen is released.

CO2 + H2O + light energy → glucose + O2

Transpiration is the release of water in the form of water vapour through the

stomata of the leaves. In following lessons you will learn more about

transpiration and the role of transpiration in plants.


Leaves: Anatomy

We shall discuss the internal structure (anatomy) of the leaves next.

Keep the functions of leaves in mind while we discuss the anatomy of

leaves. We shall give more attention to that later.

When we make a cross section of a leaf, we notice that the lamina of

the leaf consists of the upper (adaxial) and lower (abaxial) epidermis

with the mesophyll between them and vascular bundles embedded in

the mesophyll.

The upper and lower epidermis are covered by a thick, waxy cuticle

that covers the entire lamina, except the parts above the stomata.


Leaves: Anatomy

epidermis

cuticle

lateral vein

spongy parenchyma

palisade parenchyma

mesophyl

(chloren-

chyma)xylem

vascular cambiumphloemvascular bundle sheath

collenchyma

epidermis

stoma


Leaves: Anatomy

Epidermis consists of living cells, usually without chloroplasts.

Characteristics of epidermis cells:

A single layer of brick-shaped cells without intercellular spaces.

Irregular shape in surface view.

Stomata mostly occur on the abaxial (lower) side. The function of stomata is to

enable the release of water vapour and the intake of carbon dioxide (CO2).

Epidermal cells may be modified as living or non-living hairs (trichomes).

Mesophyll: (mesos = “middle”; phyllo = “leaf”); also known as the cortex or

chlorenchyma (because it contains many chloroplasts) is the part of the leaf

between the upper and lower epidermis, excluding the vascular bundles

(veins). Mesophyll consists of palisade parenchyma and spongy parenchyma.


Leaves: Anatomy

Palisade parenchyma: Consists of elongated parenchyma cells that are

arranged perpendicular to the adaxial (upper) epidermis. Can be more than

one cell layer and contains numerous chloroplasts. There are small

intercellular spaces, except above stomata, where there are big sub-stomatal

cavities (air spaces).

Spongy parenchyma: Loosely packed, irregular cells with big intercellular

spaces. It stretches from the palisade parenchyma to the abaxial (lower)

epidermis. This tissue absorbs CO2 from the atmosphere and transports it to

the palisade parenchyma, as well as organic compounds from the palisade to

the phloem and water and inorganic compounds from the xylem to the

palisade. Sub-stomatal cavities enhance the exchange of gases and water

vapour between the tissue and the atmosphere.


Leaves: Anatomy

Bundle sheath: Forms the border between the vascular bundle and the remainder of the mesophyll. Strengthening tissue (collenchyma or sclerenchyma) occurs above and below the vascular bundles.

Main vein or midrib: Represents the primary vascular tissue of the leaf. The xylem lies on the upper side of the circular vascular bundle, with the phloem below. A cambium is present between the xylem and the phloem.

Stomata: A stoma consists of two bean-shaped guard cells with a pore and an air chamber (stomatal cavity).

Guard cells: Contain chloroplasts and starch granules. Irregular thickenings in the walls regulate the size of the pore and thus regulate gaseous exchange.

Pore: Connects the outer atmosphere with the sub-stomatal cavity and with intercellular air channels.


Leaves: Anatomy

Cuticle

Adaxial epidermis

Palisade

parenchyma

Intercellular

cavities

Vascular bundle

sheath

XylemSpongy

parenchyma

Vascular cambium

Phloem

Collenchyma

Abaxial epidermis

Guard cells


Leaf: Structural suitability for its function

Functions of the leaf are photosynthesis, transpiration and the transport of

organic nutrients, produced in photosynthesis.

The internal structure is such that the raw material for photosynthesis (water

and carbon dioxide) are readily available.

Water is transported from the roots to the leaves by the vascular tissue

(xylem). There is a network of small veins that transport water effectively to

the photosynthesising cells.

Leaves are covered by a waxy cuticle that limits water loss from the leaf.

Mesophyll cells (palisade and spongy parenchyma) are parenchyma cells with

large vacuoles that hold much water for photosynthesis.

Carbon dioxide (CO2) enters the leaf through the stomata, that are mostly

found on the lower side of the leaf (cooler side of the leaf).

Stomata can open and close to regulate the flow of gases of photosynthesis.


Leaf: Structural suitability for its function

Spongy and palisade parenchyma contain chloroplasts that are the centres for photosynthesis.

Spongy parenchyma cells are loosely arranged with numerous intercellular spaces:

CO2 that enters the leaf by the stomata can easily diffuse among the cells where it is needed for photosynthesis.

Oxygen (O2) that is a by-product of photosynthesis can easily move through the air spaces to the stomata.

Palisade parenchyma cells are densely packed to allow glucose (product of photosynthesis) to be transported among the cells to the phloem with ease. From there it is transported to the remainder of the plant.

Stomata open during the day to release water vapour from the xylem, through the loosely packed spongy parenchyma cells to the atmosphere. In this way a “pulling force” and a continuous column of water develop in the vascular tissues to the roots to enable the absorption of water. We shall learn more about that in following lessons.


Let us see what you remember

Internal structure of the leaf:

cuticle

adaxial epidermis

palisade parenchyma

spongy parenchymamesophyll

xylem

phloem

bundle sheath

vascular bundle

collenchyma

stoma

abaxial epidermis

Copyright © EduTop (2014) May not be copied or distributed

Anatomy of dicotyledonous

plants

Root and Stem

10.2.4


Roots

• Nearly all flowering plants have roots.

• The roots of plants usually occur subterranean, but some plants produce roots above the surface of the soil.

• The main root develops from the radicle of the embryo.

• All three types of roots, viz. main root, lateral roots and adventitious roots have the same anatomical structure.

• A calyptra (root cap) protects the root tip and enhances movement through the soil.

• There are no cuticle and stomata in the root epidermis.


Root: external characteristics

• Roots do not have any nodes and internodes.

• Roots do not have any leaves or auxiliary buds.

• Root hairs are found above the root tip.

• Root hairs are modified epidermal cells for the absorption of water and dissolved nutrients.

• A calyptra is always present and can be observed by the naked eye in some plants, e.g. aerial roots of certain orchids.


Roots: functions

• Roots anchor the plant in the soil.

• Roots absorb water and minerals from the soil to be transported to the other

parts of the plant.

• Roots store reserved organic food. All roots store organic food, but some

roots are specially modified for this function.


Root: anatomy

• Epidermis (root epidermis is also known as rhizodermis):

• Single layer of cells without chloroplasts.

• No cuticle or stomata occur in the root epidermis.

• May possess root hairs.

• The function of the epidermis is the absorption of water and dissolved nutrients.

• Exodermis:

• The exodermis is the outer cortex layer that takes over the function of the epidermis in older roots.

• The exodermis consists of cork cells and transfusion cells.


Root: anatomy

• Central cortex:

• The central cortex consists of several layers of living parenchyma cells with intercellular

air spaces.

• These cells are usually colourless, but the aerial roots of

of epiphytes (plants that completely grow above

ground) contain chloroplasts.

An epiphytic orchid with green

aerial roots with chloroplasts for

photosynthesis.


Root: anatomy

• Endodermis:

• It is the inner cortex layer and consists of cells with Casparian strips.

• The cell walls of these cells also form horseshoe-shaped thickenings when they become

older and consist of thickened cells with passage cells (transfusion cells) among them.

• In monocotyledons the thickened endodermis takes over the function of the cortex and

epidermis.


Root: anatomy

• Casparian strips:

• The radial and tangential walls of the endodermis cells are impregnated with strips of watertight lignin and suberin that form the Casparian strips.

• The thickenings occur in the region of elongation of the root a little above the tip of the root and up to the mature root hair region.

• The Casparian strips prevent water from moving through the cell walls into the vascular tissues, but only through the cell membranes and cytoplasm of the endodermis cells to control the absorption of water. (The meganism will be explained later)


Root: anatomy

• Pericycle:

• The pericycle is found on the inside of the endodermis and consists of thin-walled cells without intercellular spaces, usually one cell layer only.

• In older monocotyledons it may consist of sclerenchyma cells.

• The pericycle has latent meristematic characteristics and gives rise to lateral roots and a part of the vascular cambium that originates in older roots and causes secondary thickening of roots.

• In dicotyledonous plants the cork cambium (phellogen) differentiates from the pericycle.


Root: anatomy

• Central cylinder (stele)

• Consists of groups of xylem alternated by groups of phloem. They are separated by parenchymatic sheaths (conjunctive tissue).

• The xylem and phloem groups are arranged radially and not in vascular bundles.

• In dicotyledonous plants most roots do not possess a pith (medulla) and the xylem is found in the centre of the root. Dicotyledonous roots possess 2 – 8 vascular groups, but mostly 3-6.

• In woody plants the pith is almost always absent. If the pith is present, it is sclerenchymatic in some cases.

• In monocotyledonous plants numerous vascular groups occur and a well developed pith is present.


Root: anatomy

Cross section of a dicotyledonous root


Stems

• The part of the flowering plant above the soil mostly consists of an upright

growing stem, that is branched to a certain extent to form a framework of

lateral branches that carry leaves, flowers and fruit.

• These lateral branches and/or leaves are only borne on regularly spaced parts

of the stem, called nodes.

• The part of the stem between two consecutive nodes is called an internode.


Stems

• Internally the stem framework represents a route of transportation for water,

dissolved minerals and certain hormones from the roots to the leaves,

flowers, fruit and seeds.

• Organic nutrients produced by photosynthesis, as well as certain hormones

produced in the leaves and tips of the stems, are also carried by the stem

framework to the other parts of the plant.


Stem: anatomy

Dicotyledonous stem without secondary growth

• In a cross section of a young herbaceous stem (without secondary growth) the following types of tissues can be distinguished from outside to inside:

• Epidermis

• Cortex

• Pericycle

• Vascular rays

• Pith rays (medullary rays)

• Pith (medulla)


Stem: anatomy


• Epidermis:

• Single layer of living cells without chloroplasts, but possess leucoplasts.

• The cell walls consist of cellulose and outer walls are thickened and covered by a cuticle.

• Stomata and epidermal hairs (trichomes) may occur.

• Cortex:

• Varies from 5-20 cell layers just below the epidermis (hypodermal), consisting of collenchyma that gradually changes into parenchyma with intercellular air spaces.

• The inner most layer is known as the endodermis, but in contrast to root endodermis, does not contain Casparian strips.


Stem: anatomy


• Pericycle:

• The pericycle consists of a few layers of schlerenchyma cells (phloem fiber cap) above the phloem, alternated by parenchyma of the pith ray inside the endodermis.

• The pericycle strengthens the stem and protects the vascular bundles.

• Pith:

• Consists of parenchyma ground tissue that stores food (end products of metabolism).

• May die and dry up to result in a hollow stem.

• Pith rays (primary vascular rays):

• Lies between vascular bundles and stretches from the pith to the pericycle. Consists of parenchyma cells and, like the pith, serves as storage tissue.

• Each pith ray (medullary ray) consists of a few rows of cells and gives rise to the interfassicular cambium.


Stem: anatomy


• Vascular bundles (arranged in a ring):

• Phloem and xylem lie on the same radius, with the phloem to the outside and the xylem

to the inside.

• Vascular bundles of dicotyledonous plants possess a cambium between the phloem and

xylem (fascicular cambium) in contrast to monocotyledons without a cambium.

• Primary vascular bundles are formed by the fascicular cambium and secondary vascular

bundles by the inter-fascicular cambium. (More about this under secondary thickening)


Stem: anatomyDicotyledonous stem without secondary thickening

pith ray


Stem: anatomy

Dicotyledonous stem – secondary growth

• Secondary growth in thickness or secondary thickening starts as soon as permanent tissue groups are differentiated.

• Parenchyma cells of the pith rays between the fascicular cambium become meristematic and produce secondary vascular tissues (xylem to the inside and phloem to the outside) as well as parenchyma cell rows between the secondary vascular elements.

• Secondary vascular bundles are thus formed between the primary vascular bundles.

• The newly formed parenchyma cell rows between the secondary vascular bundles form secondary pith rays.


Stem: anatomy

Dicotyledonous stem – secondary thickening

Diagram of primary and secondary vascular bundle after the onset of secondary thickening

secondary phloem

secondary parenchyma

interfascicular cambium

fascicular cambium

secondary parenchyma

secondary xileem

pith ray

cortex

pericycle

primary phloem

secondary tissue

fascicular cambium

secondary tissue

primary xileem

pith


Stem: anatomy

Secondary growth – annual rings

• During favourable conditions (rainy season) trees grow fast. It is reflected anatomically by large woody elements (vessels elements) and relatively fewer fibers. The wood formed during these favourable conditions is called springwood (or early wood).

• As conditions become less favourable (dry season) the activity of the cambium decreases and the wood elements that are formed are smaller with thicker walls and more fibres. The wood formed during these unfavourable conditions is called late wood.

• The growth phases depend on the seasons. The springwood and late wood of two consecutive seasons together form one annual ring (or growth ring).

• The age of a tree can be determined by counting the number of annual rings.

• Differences in seasons, like cold and warm or wet and dry periods are also reflected in the wood and thus annual rings reflect the changes in climate during the growth period of the tree.


Stem: anatomy

Secondary growth - periderm

• Secondary growth causes tension in the epidermis and cortex and eventually they tear and burst. The periderm then takes over the function of the epidermis.

• Periderm arises hypodermally when mature cortex cells (even phloem cells) become meristematic and produce a secondary cork cambium (phellogen)

• The phellogen forms cork (phellem) to the outside and phelloderm (secondary cortex) to the inside.

• The phellem, phellogen and phelloderm together form the periderm.

• The periderm, together with parts of the secondary phloem form the bark of the tree.


Stem: anatomy

Secondary growth – annual rings and periderm


Stem: anatomy

Secondary growth – annual rings and periderm

phellem

phellogen

phelloderm

secondary phloem

fasciculur cambium

primary vascular ray

secondary vascular ray

late wood

secondary xileem

spring wood

annual ring

primary xileem

pith


SKELETONS: DIFFERENT TYPES AND FUNCTIONS

10.2.6


DIFFERENT WAYS OF MOVEMENT

Animal bodies need support for movement.

Animals need to move to obtain food, flee from predators, seek shelter and find mates for mating.

Animals have skeletons that support their bodies and enable them to move.


TYPES OF SKELETONS: HYDROSTATIC SKELETON

Hydrostatic skeleton:

In a hydrostatic skeleton support is supplied by fluid that is under pressure in the body cavity.

Hydrostatic skeletons occur in organisms with soft bodies, like earth worms and jellyfish. These organisms secrete a watery fluid (coelomic fluid) that is trapped in the body cavity (coelom).

The fluid-filled coelom forms a resilient structure against which the muscles can contract to enable movement.

The pressure of the coelomic fluid against the muscles makes the body of the animal firm.

Water has important qualities that makes it suitable to serve as a skeleton: It cannot be compressed and as such forms a soft-walled structure.

It distributes changes in pressure evenly.



dorsal blood vessellongitudinal muscles

circular muscle

epidermis

dorsal mesentery

digestive canal

peritoneum

ventral mesentery

coelom

ventral blood vessel



Examples of animals with hydrostatic skeletons:

Sea anemones, jellyfish and earthworms


TYPES OF SKELETONS: HYDROSTATIC SKELETONS

Advantages of a hydrostatic skeleton:

The watery environment in which many animals with a hydrostatic skeleton lives, serves to keep the surfaces moist and prevents desiccation.

The water supplies extra support.

Disadvantages of a hydrostatic skeleton:

Terrestrial animals with a hydrostatic skeleton, like earthworms, move slowly and cannot escape fast from predators, like birds or unfavourable conditions.

Animals with a hydrostatic skeleton is not well protected against mechanical injuries.

Animals with a hydrostatic skeleton are very sensitive to changes in temperature.


TYPES OF SKELETONS: EXOSKELETON

Exoskeletons:

An exoskeleton is a hard skeleton on the outside of the body of an animal.

Arthropods are the most abundant types of animals with an exoskeleton.

Of all arthropods insects are the most mobile with the greatest variety of habitats.

The exoskeletons of insects consist of chitin, a substance that hardens the exoskeleton.



chaeta opening of gland

exo-cuticle

endo-cuticle

rift for attachment of

muscles

epidermis

dorsal blood vessel and heart

muscle

digestive canal

jointed appendages with muscles

ventral cord

Joint, folds of appendages



Examples of animals with exoskeletons:

Crabs, lobsters and insects



Advantages of an exoskeleton:

It supports the body.

It serves as points for attachment of muscles.

It has mobile seams that enhance movement.

It is fairly resistant to changes in temperature.

It protects the animal against injuries, toxins and desiccation.

It assists in camouflage.

Disadvantages of an exoskeleton:

The exoskeleton cannot grow bigger. To become bigger the animal has to moult. During this process the animal is very vulnerable to predators.

It is relatively immobile.


TYPES OF SKELETONS: ENDOSKELETON

Endoskeletons:

An endoskeleton is an internal framework of bone and cartilage.

All vertebrates (animals with a back bone) possess an endoskeleton.

The skeletons of all vertebrates consist of the same basic parts, viz.: a scull,

a backbone consisting of vertebrae,

a ribcage,

limbs,

a pectoral girdle (shoulder girdle)

a pelvic girdle.



Advantages / functions of an endoskeleton:

The hard, bony skeleton protects the internal organs against injuries.

It supports the body.

It supplies points of attachment for muscles and enable movement.

Bone and cartilage, that together make up the endoskeleton, are living tissues that allow growth, because they grow and enlarge together with the organism.

Disadvantages of an endoskeleton:

Bones may crack or break when too much pressure is exerted on them.

Broken bones can immobilise an animal.



Endoskeletons in vertebrates:

Different types of movement include walk, run, fly, crawl and swim.

All of these types of movement require that muscles collaborate against resistance. The bones of the skeleton supply the resistance.

In vertebrates the endoskeleton provides the necessary mechanical support to the body and also serve as framework for the attachment of muscles.

There are several joints in an endoskeleton and together with the muscles attached to them, it functions as a leverage system.

Different animals have different habitats. Birds should be able to fly to obtain food; fish should be able to swim to survive in water.

The skeletons of vertebrates are specially adapted to enable them to survive in their environment.



Endoskeletons in vertebrates - continues:

Fish are adapted to movement in water. Although the body mass of the fish is supported by water, it must surmount resistance for locomotion.

The skeletons and bodies of fish are flattened and streamline.

Fish move forward by making wavy movements with their bodies that propel them forward.

Many bone fish possess a swim bladder that assists in keeping them afloat. Without a swim bladder they would sink when they stop swimming.

The skeletons of birds are adapted to fly and to generate enough power to take off and stay in the air and keep flying against gravity and air resistance.

Birds have a delicate skeleton and hollow bones that lower the body mass and limit the amount of energy needed during flight.




Uptake and translocation

of water and mineral

elements in plants and

transpiration

10.2.5


Water relationships in plants

Plants need water for normal metabolic processes throughout the plant

body.

Plants need water for various reasons:

for mechanical support;

as the main solvent for chemical compounds;

as reagent in several chemical reactions like photosynthesis and the hydrolysis

reactions of macro elements;

as the most important transport medium in cells (cyclosis) and by the plant in the

xylem and phloem;

to assist in the stabilization of the temperature of the plant.


Water relationships in plants

Not only water, but minerals as well are needed in different parts of the plant, like young growing shoots or young photosynthesising leaves.

Water and mineral salts are absorbed by the root hairs of the roots mainly and must be transported from there to all other parts of the plant where it is needed.

Different processes are involved in the maintenance of a constant water balance and the distribution of essential substances through the plant.

Those include:

diffusion,

osmosis,

water potential, osmotic potential and pressure potential,

turgor pressure and plasmolysis,

transpiration and imbibition.


Water relationships in plants:

Diffusion

Diffusion is the nett movement of molecules (or ions) of a substance from a

region with a high concentration to a region with a lower concentration of

the same substance, until the concentration gradient is removed.

Diffusion takes place in all substances (solids, liquids and gases).



Diffusion

The rate of diffusion depends on various factors, like:

the concentration of the substance as well as the difference in concentration

(concentration gradient) between the two regions;

the diffusion coefficient that varies from substance to substance (influenced by

cohesion powers);

temperature (the higher the temperature the faster the rate of diffusion);

the density of the diffusing molecules (solid, liquid or gas);

the medium in which diffusion takes place;

the means of diffusion (through a membrane or no membrane) and the type of membrane (completely permeable or selectively permeable).



Diffusion

The direction of diffusion is determined by the difference in concentration of diffusing substance between the two regions.

Particles move along the concentration gradient from the higher to the lower concentration.

In living cells dissolved substances can diffuse into a cell or from one cell to another to result in the even distribution of the substance.

In this way hydrogen, carbon, oxygen, mineral salts and other substances and organic molecules can enter of leave cells by diffusion.

In living systems it sometimes happen that molecules and ions are transported across cell membranes against the concentration gradient.

For the provision of essential molecules, three types of transport occur in cells, viz. simple diffusion, facilitated diffusion and active transport.

In plants water is usually absorbed by simple diffusion.



Osmosis

Osmosis is a special case of diffusion where the solution (usually with water

as solvent in living systems) moves through a selectively permeable

membrane, from a higher to a lower concentration of the solvent.

Cell membranes (plasma membrane, tonoplast surrounding the vacuole,

etc.) are examples of selectively permeable membranes that allow the

passage of water molecules as well as certain dissolved substances, but

prevent the passage of other molecules, like larger sugars, protein

molecules and certain ions.

Osmosis thus plays an important role to control of the distribution of water in

living organisms.



Water potential and osmotic potential

The concentration of water is determined by the amount of dissolved

particles in it.

Pure water (without any dissolved particles) has the highest concentration

of water molecules.

The more dissolved particles in a watery solution, the lower the water

concentration and the higher the concentration of dissolved particles.




When two solution, separated by a selectively permeable membrane,

contain the same amount of dissolved particles per unit volume on either

side of the membrane, the solution is isotonic.

When two solutions have different amounts of dissolved particles per unit

volume the solution with more dissolved particles per unit volume (lower

water concentration), is hypertonic and the solution with less dissolved

particles is hypotonic.

Osmosis (movement of water) will take place from the hypotonic solution to

the hypertonic solution through the selectively permeable membrane, until the two solutions are isotonic.




Water potential of a solution is the ability of the solution to release water molecules to move through a selectively permeable membrane.

Pure water has the highest water potential, viz. nil (0).

Water molecules are always inclined to move from a region with a higher water potential to a region with a lower water potential.

The osmotic potential of a solution is the ability to absorb water molecules through a selectively permeable membrane.

The higher the concentration of dissolved particles and the higher the temperature of the solution, the higher the osmotic potential and the lower the water potential (more negative) of the solution.

Water molecules diffuse from any solution with a lower osmotic potential (higher water potential) through a selectively permeable membrane until the two solutions have the same osmotic potential (and water potential).




It is important to note that although all dissolved substances contribute to

the osmotic potential of a substance, only those molecules that the

membrane are permeable to will be involved by transport across the

membrane.



Absorption of water by plant cells

The cellulose cell wall of plants usually is permeable to water, dissolved

molecules and ions.

The plasma membrane and tonoplast are permeable to water, but

selectively permeable to most dissolved substances and ions.

When a plant cell absorbs water osmotically from the surrounding solution

(endosmosis), the water collects in the cell sap of the central vacuole and the vacuole constitutes the biggest volume of the cell.

When water leaves the plant cell to the surrounding solution (exosmosis), it

is mainly from the vacuole and it shrinks.



Absorption of water by plant cells - Turgor

When a plant cell is in contact with water, the cell sap with dissolved

substances in the vacuole will have a lower water potential and higher

osmotic potential than the surrounding water.

Due to the higher water potential of the surrounding water, water

molecules will be inclined to diffuse into the plant vacuole.

The volume of the vacuole will increase due to the uptake of water.

Eventually the cell membrane pushes against the cell wall and the cell wall

becomes rigid.

This condition of a plant cell where the cell wall becomes rigid; stretched

due to the increase of the volume of the vacuole, with the cell membrane

pushing against the cell wall (during the absorption of water), is known as

turgidity.




During turgidity the cellulose cell wall is firm and relatively inelastic, with a limited ability to stretch.

The resistance of the cell wall agaist the pressure of the enlarging vacuole, causes a counter pressure that prevents any further absorption of water.

This pressure is called cell wall pressure of the cell or pressure potential of the cell wall.

Due to this potential a plant cell placed in water will not burst.

When a plant cell absorbs water it will fill up to its capacity till it becomes turgid.

In this condition the protoplasm exerts pressure on the cell wall.

This pressure on the cell wall is called turgor pressure (hydrostatic pressure).

In a cell that is totally turgid the cell wall pressure is equal to the turgor pressure, but in opposite directions.

In this condition the difference between water potential and osmotic potential (that inclines to force water into the cell) is balanced and osmosis stops.

When a plant cell loses water, the turgor pressure decreases and more water moves into the cell by osmosis till turgidity is restored.




Turgor is an essential mechanism in plant cells for mechanical support.

One of the first signs that is easy to observe when plants lack water is the

lack of turgidity of the cells in the leaves that gives leaves a wilted

appearance.

Other examples of the influence of the change in turgidity is the opening

and closing of the guard cells of stomata, and the folding up of leaves of

the “touch me not” when touched.


Water relationships in plants:Absorption of water by plant cells - Plasmolysis

When a living plant cell is placed in a solution with the same osmotic

potential than cell sap (isotonic solution) the appearance of the cell will

stay unchanged.

If the surrounding solution is hypertonic to cell sap (higher osmotic and

lower water potential), e.g. in concentrated salt or sugar solution, the cell

will undergo several changes.

Due to the higher water potential of the cell sap, water will move from the

vacuole to the surrounding salt/sugar solution with a lower water potential.

This exosmosis results in a loss of turgidity, the vacuole shrinks and the cell

membrane pulls away from the cell wall.

A cell is this condition is plasmolysed.


Water relationships in plants:Absorption of water by plant cells - Plasmolysis

Plasmolysis is the phenomenon in plant cells where the protoplasm shrinks and pulls away from the cell wall due to the osmotic removal of water from the central vacuole.

The cell wall loses its rigidity and becomes flaccid.

Although the selectively permeable cell membrane only allows the passage of water, the cell wall allows he passage of water as well as salt/sugar molecules. Therefore the space between the cell membrane and the cell wall becomes filled up by the surrounding salt/sugar solution.

In this condition there is no turgor pressure to keep the cell wall rigid.

When the plasmolysed cell is placed in pure water the cell becomes deplasmolysed by endosmosis and the turgidity is restored.

If permanent plasmolysis takes place, the cells become flaccid and the plant wilts. Permanent wilting leads to the death of the cells.


Water relationships in plants:Absorption of water by plant cells – Turgor and plasmolysis

Light microscope photo of turgid epidermal cells Light microscope photo of plasmolysed epidermal cells


Water relationships in plants:Absorption of water by plant cells – Osmosis in living cells

In the same way a plant cell can absorb water from (or lose water to) a

surrounding solution, a cell in close contact with another cell can absorb

water from (or lose water to) that cell.

The water potential of adjacent plant cells may be different and that

causes the movement of water from one cell to another. The one factor

that always determines the movement of water from one cell to another, is

the water potential (or osmotic potential).

Cell A Cell B

High water potential

Low osmotic potential

H2O

Low water potential

High osmotic potential


Water relationships in plants:Routes of water and dissolved minerals through plants

In most cells the cytosol is continuous from cell to cell, separated by plasmodesmata (openings in the cell wall and cell membrane).

This continuum of the cytoplasm is called the symplast.

The continuum of cell walls and intercellular (and extracellular) spaces is known as the apoplast.

Water and minerals move among cells in three ways:

Transmembrane route: from one cell, through the cell wall and cell membrane into another cell.

Symplastic route: via die continuum of the cytosol.

Apoplastic route: via the cell walls and intercellular spaces.

Long distance translocation of water and dissolved substances in plants takes place by the vascular tissues (xylem and phloem).


Water relationships in plants:Routes of water and dissolved minerals through plants



Uptake of water by roots

To maintain their water balance plants must take up big quantities of water.

In terrestrial plants the uptake depends on a large contact surface between the roots and soil water.

Some plants have a much branched adventitious root system that spreads out near the soil surface. Other plants possess a tap-root system that penetrates deeper into the soil. The environment and the type of plant greatly influence the development of the root system of a plant.

Although plants may possess an extensive root system, the absorption of water is mainly restricted to the root hairs.

Millions of root hairs may develop in the root hair region of a single root system.

The root hairs considerably enlarge the surface for water absorption.

In the root hair region water diffuses from the soil, through the epidermal cells into the plant.




Soil water usually has a very low concentration of dissolved substances, while the cell sap of the vacuoles of the epidermal cells, cortex cells, endodermis and pericycle cells contain a relatively high concentration of dissolved substances.

The osmotic potential of soil water thus is low (with a high water potential) and that of cell sap is high (with a low water potential). That results in the movement of water from the soil into the root hair cells.

Water (and dissolved minerals) can enter the plant by two different routes:

Symplastic route: Water and dissolved substances only pass through the cell membranes of the root hairs and then through the symplast of each cell until they reach the xylem.

Apoplastic route: Water and dissolved substances pass through the cell walls of the root hairs and flow through the apoplast until they reach the endodermis. Here the flow of water is blocked by the Casparian strips and water follows the symplastic route until it reaches the xylem.




The Casparian strips lie in the radial and tangential walls of the endodermis

cells. They surround the cells like a ring surrounds a finger.

The Casparian strips contain suberin that prevents the flow of water and

forces water to pass through the endodermis cells to the xylem.

In this manner the amount of water and dissolved minerals that move to

the xylem are controlled.

Only when the water concentration in the endodermis cells becomes lower

than in the cortex cells will water move through the endodermis cells to the

pericycle and xylem.

The following slide illustrates the uptake of water by the roots.




Translocation of water in plants

The water and dissolved minerals taken up by the roots is called plant sap.

Sap moves upwards through the plant body by means of the xylem.

The xylem of the roots, stems and leaves forms a continuous transport system for the upward movement of plant sap.

A pressure of one atmosphere supports a column of water 10,4 m high at sea level. A pressure of ±10 atmosphere is thus needed to support a column of 100 m high (height of some trees). The column should not only be supported but should move upwards at a fast rate (up to 1 m per minute) and all of that against the resistance of the xylem. A much higher pressure is thus required to force the sap upwards.

The question thus is: how does the sap continuously move from the roots by the xylem to all the parts of the plant that may be up to 100 m tall? The tallest tree in the world is a Sequoia sempervirens of ± 116 m tall.

The upward movement of water in plants is caused by root pressure, capillarity and the suction force of transpiration pull.



Translocation of water in the plant – Root pressure

When the trunk of certain plant species are sawn off (or stems are pruned), sap still slowly flows out on the surface for a long time. We call it “bleeding”.

When conditions for water uptake by the roots is optimal, but the humidity is so high that little water is lost by transpiration, water that is under pressure in the plant may be forced out at the ends of the veins on the leaves. This water forms droplets on the edges of the leaves. This type of water excretion in liquid form is called guttation (photo right). The little glands for the excretion of water at the tips and edges of the leaves are called hydathodes.

Root pressure is responsible for both “bleeding” when a plant is sawn off and for guttation.

Root pressure is the force that arises in the xylem due to the osmotic uptake of water by the roots.



Translocation of water in plants – Root pressure Root pressure does not generally arise in all plants.

Root pressure only arises in some plants when conditions are such that transpiration takes place slowly and the soil is very wet.

Even then the movement of water in the xylem is very slow.

Root pressure is thus not an important mechanism for the upward movement of water in plants.

Root pressure arises due to active uptake of mineral ions by the epidermal cells.

The ions move from cell to cell in the cortex, through the endodermis to the central cylinder (vascular cylinder).

The cells of the central cylinder are metabolically less active than the cortex cells, therefore ions move from these cells to the wood vessels elements and tracheids.

In this way the xylem obtains a lower water potential than soil water. That causes water to move osmotically into the central cylinder.

Energy is used to actively “pump” ions to the central cylinder and big amounts of water follows passively. That results in root pressure arising in the xylem.

If such tracheids and wood vessel elements were cut through, xylem sap would slowly flow out of the trunk or stem. In herbaceous plants it would lead to guttation.



Translocation of water in plants - Capillarity

The rise of water in a thin tube is known as capillarity.

In plants capillarity refers to the attraction of the walls of the thin wood

vessel elements and tracheids to water molecules (adhesion).

Capillarity renders additional support to the water column that moves from the roots to the leaves.


Water relationships in plants:Translocation of water in plants - Transpiration pull

Water that is taken up by the roots is needed by the entire plant, but mainly

by the leaves, where photosynthesis takes place.

A small amount of water is used for growth and metabolic activities, but

most of it evaporates into the atmosphere.

The loss of water in the form of water vapour by the aerial parts of the plant

(leaves and stems) is known as transpiration.

Transpiration is the inevitable side effect of gaseous exchange (e.g. cellular

respiration) in plants that causes the continuous loss of water, that should

be replaced again.


Water relationships in plants:Translocation of water – Transpiration pull

The xylem sap usually contains a low concentration of dissolved substances and thus has a high water potential.

The atmosphere (low water potential) causes drying out and evaporation (transpiration) of water from the plant, mainly the leaves, as water difuses from the leaves into the atmosphere.

As water molecules evaporate from the moist cell walls of the mesophyll cells in the leaves, they are replaced by water molecules that diffuse from the vacuoles of the cells to the cell walls.

The water potential of the cell sap of the mesophyll cells thus drops and water molecules from the xylem sap move into the mesophyll cells.

This osmotic chain in the leaf cells causes the loss of water from the veins of the leaves by transpiration and water is drawn from the xylem of the stems.

Due to this pulling force, the water in the xylem system is continuously in a state of tension (negative pressure), that is continuous from the leaves, through the stems and trunks to the roots. This pulling force is called transpiration pull.


Water relationships in plants:Translocation of water in plants – Transpiration pull

Water molecules, drawn from the xylem sap of the roots, cause a decrease

in the water potential.

Water molecules diffuse from the cortex and epidermal cells of the roots to

the xylem. This again lowers the water potential of the cortex and

epidermal cells.

In this way soil water with a very high water potential diffuses into the root

cells.


Water relationships in plants:Translocation of water in plants – Cohesion theory of water transport

What causes a continuous water column inside the plant?

The Irish Botanist, Henry Dixon, was first to formulate the cohesion theory of

water transport.

According to this theory the hydrogen bonds (cohesion forces) are strong

enough to prevent the separation of water molecules when they are pulled

up in the plant.

Cohesion forces between water molecules assist to pull water as a

continuous column through the plant, from the roots to the leaves and

stems, where transpiration takes place. Capillary action causes the rising of

water in the xylem tubes and transpiration supplies the pulling force in the

transport of water through the plant.


Water relationships in plants:Translocation of water in plants – Cohesion theory ofwater transport

When enough water is available the plant will take up sufficient water to

maintain a constant water balance.

When the soil dries out the water potential of the soil will be lower than that

of the root cells and osmosis will stop.

When the transpiration rate exceeds the rate of water uptake, the cells

undergo plasmolysis and the plant wilts.

Depending on the rate of recovery of the soil water, wilting may be

temporary or permanent.

Permanent wilting causes the death of cells and eventually the whole plant

dies.


Uptake of minerals by plant roots

In contrast to water, minerals are taken up actively by the cells in the roots.

Plants have the ability to concentrate minerals; i.e. minerals are taken up

until the concentation is much higher in the plant than in the surrounding

environment.

Some minerals are up to 10 000 times more concentrated inside the roots

than in the surrounding soil.

After minerals are taken up by the epidermis cells of the root hairs, they

move to the xylem, together with the absorbed water. They are

transported upwards in the plant with the water to the leaves.

Minerals can leave the xylem at any place in the plant and go to the cells

where they are needed.

In the following slides the process of active uptake of minerals by roots will

be described.


Uptake of minerals by plant roots:Adaptations of roots for the uptake of minerals

Two types of uptake are found:

Roots absorb minerals without any external assistance.

Roots absorb minerals in a symbiotic relationship with micro organisms,

e.g.:

Rhizobium symbiosis

Mikorrhiza species


Uptake of minerals by plant roots:Absorption without external assistance

Symplastic route:

The mechanism of membrane transport of mineral ions

through the plasma membrane of the root hairs:

Energy is supplied by ATP.

1. A membrane pump hydrolyses ATP to pump H+ -ions from the root hairs (epidermis cells) to the soil outside the cell (active transport).

2. Cathions, e.g. K+-ions occur in close association with many types of soil particles.

3. Negative ions (e.g. Cl– or I–) in association with H+ are transported into the endodermis cell (facilitated diffusion).

4. The electro-chemical gradient that arises due to the pumping out of H+ -ions drives these positive ions (e.g. K+),

5. through channel proteins into the cell.


Uptake of minerals by plant roots:Rhizobium-symbiosis for the absorption of nitrogen

Legumes, e.g. soy beans and lucerne, may be involved in this type of symbiosis.

The bacteria (Rhizobium spp.) colonise the roots and grow inside the root cells.

The plants form special structures (nodules) to house these bacteria.

The bacteria obtain organic and inorganic nutrients from the plant.

While the bacteria grow inside the nodules, they metabolise nitrogen as follows: they break the covalent bonds (N≡N) between the nitrogen atoms and reduce the nitrogen atoms to form NH4

+ .

NH4+ can be built into organic compounds by the Rhizobium.

A part of the NH4+ and organic compounds are provided to the host plant.

Legumes can grow without the nodules, but the association with the nodules is to their advantage.


Translocation of food in plants:

Organic substances

The largest upward or downward movement of carbohydrates in plants

(usually as sucrose), takes place by the phloem.

In most plants nitrogen is absorbed by the roots as nitrate and transported

upwards in the plant to the leaves by the xylem.

After biosynthesis soluble amino acids and amides are transported by the

phloem.

Some plants convert nitrogen in the roots into organic substances that are

transported by the xylem or the phloem.

Upward movement of organic substances does not necessarily take place

by the phloem, but downward movement does.


Translocation of food in plants:

Inorganic substances

Minerals are transported through the entire plant by the xylem vessels.

Some minerals sometimes move downwards in the sieve tubes of the phloem.

Experiments with radio active forms of minerals indicated the following:

Inorganic ions like calcium, sulphate and phosphate ions are transported upwards from the roots to the leaves by the xylem.

Some minerals, like phosphate, is very mobile in plants and move rapidly along the phloem through the plant.

Other minerals, like boron and manganese move out of the phloem of older leaves and into young, growing leaves where they are used again.

Certain minerals, like calcium, is not mobile in the phloem and plants must continuously absorb these ions from the soil.

Many inorganic nutrients can be directly absorbed by the leaves by spraying them with artificial plant food.


Translocation of food in plants:Pressure-flow model for phloem transport

Sugars and amino acids are mainly transported by the phloem in plants.

The movement of fluids in the phloem (translocation) may take place

upwards or downwards.

The direction of flow in the phloem is typically from source cells (cells that

produce sugars and amino acids) to sink cells (cells that need sugars and amino acids).

Sugars are pumped actively from the source cells into the sieve tubes of

the phloem.

That increases the osmotic potential of the sieve tubes and that results in

water moving into the sieve tubes by osmosis.

The influx of water into the sieve tubes increases the pressure in the sieve tubes near the source cells.


The increase of pressure, forces the solution in the sieve tubes to parts with

a lower pressure (upwards or downwards).

In parts with a lower pressure the sink cells withdraw sucrose from the sieve

tubes.

As sucrose is withdrawn from the sieve tubes, the osmotic potential

decreases (and water potential increases) and water also flows from the

sieve tubes to nearby cells with a lower water potential.

The pressure in this region of the sieve tubes thus decreases and the solution

in the sieve tubes is continuously forced from regions with a higher pressure

in the direction of parts with the lower pressure.

During different times of the year tissues may act as source or sink.



Examples:

Young growing leaves require more nutrients than they can produce and then

act as sinks. When these leaves are mature they produce more nutrients by

photosynthesis than they need and then they act as source.

During autumn plants translocate much of their excess sugars to storage organs,

like stems and roots, that act as sinks. Here sugars are converted to starch before

they are stored in special cells. In spring starch is converted to sugars again and

the cells in the roots and stems act as source that supplies sugars to the plant to

form new leaves and flowers.

The following slide illustrates the pressure-flow model for phloem transport.



xylem

vessel

water

molecule

sieve-tube

element

companion

cell

mature cells

of leaf

phloem

sucrose

molecule

sieve plate

Source:

Sucrose is actively

transported into

sieve tubes and

water follows by

osmosis

This creates a

positive pressure

that causes sap to

flow within phloem

Sink:

Sucrose is actively

transported out of

sieve tubes, and

water follows by

osmosis


Transpiration

During the processes of photosynthesis and respiration plants constantly

exchange gases to the atmosphere.

Gaseous exchange takes place by diffusion where thin walled plant cells

are in contact with the atmosphere, like aerial roots, pores in stems

(lenticels) and several small openings, mainly in leaves (stomata).

Stomata are mainly found in the epidermis of leaves and young stems.

The stomata are essential for the exchange of CO2 and O2 and is

responsible for the loss of water vapour to the atmosphere (transpiration).


Transpiration

More than 90% of the water that moves into the leaf is lost by transpiration.

Some of the water transpires directly from the epidermis cells.

Epidermal transpiration depends on the thickness of the cuticle (waxy layer that covers the aerial parts of the plant and limits the rate of water loss).

Epidermal transpiration takes place continuously, except when the air is saturated by water and there is no wind.

Many plants grow in dry environments with a limited water supply.

These plants possess a thick cuticle to conserve water, but a thick cuticle also limits the rate of diffusion of carbon dioxide into the leaves.

Transpiration and the absorption of carbon dioxide mainly take place through the stomata.

Transpiration through the stomata is the loss of water vapour from the intercellular spaces of the plant through the stomata.


Transpiration: The mechanism of the

opening and closing of the stomata

Two specialised, bean-shaped guard cells, border the pore of each stoma.

The walls of the guard cells are unevenly thickened – the inner walls, bordering the pore, are thicker and are thus less elastic than the remainder of the walls of the guard cells.

Guard cells also contain radial arranged cellulose micro-fibrils that cause the lengthening of the cells when turgor pressure increases.

The uneven wall thickenings and the micro-fibrils result in the change of the shape of the guard cells due to turgor pressure. The guard cells are pulled away from each other and the pore opens.

Guard cells contain chloroplasts that enable them to photosynthesise.

During the day the light dependent reactions of photosynthesis increases and ATP is produced in the guard cells.

The blue spectrum of day light is absorbed by phototropins in the guard cells. That activates a proton pump in the plasma membranes of the guard cells.

ATP, produced by photosynthesis, drives this pump.



opening and closing of stomata

As protons (H+) are pumped out of the guard cells an electro-chemical

gradient originates over the plasma membrane – increasingly negative

inside and increasingly positive outside the guard cells.

This gradient results that potassium ions (K+) move through channel proteins

into the guard cells.

The osmotic potential of the guard cells increases resulting in water flowing

into the cells.

The guard cells become turgid and the stomata open.

The open stomata result in an increase in the rate of transpiration.

When the uptake of soil water cannot keep up with the water loss by

transpiration, abscisic acid (a plant hormone) is secreted.

Abscisic acid binds to receptors on the surface of the plasma membrane of

the guard cells.



opening and the closing of the stomata

The receptors activate several mutually related routes that result in the

following in the guard cells:

the pH of the cytosol increases;

calcium ions (Ca2+)are transported from the vacuoles to the cytosol.

The increased Ca2+ concentration in the cytosol blocks any further

absorption of K+ into the guard cells.

The increased pH stimulates the loss of chlorine ions (Cl–) and other organic

ions (e.g. malate2–) by the guard cells.

The loss of these ions lowers the osmotic potential as well as the turgor

pressure in the guard cells.

The stomata close.




The influence of water absorption by the guard cells and the shape of the guard cells

during the opening and closing of the stomata




The influence of potassium ions (K+) in the opening and closing of stomata


Transpiration: Environmental conditions

that influence the rate of transpiration

Relative humidity of the atmosphere: The water potential gradient between the intercellular spaces and the immediate environment depends on the humidity of the atmosphere. The drier the air all round the plant, the greater is the water potential gradient and the higher the rate of transpiration.

Air movement (wind): Wind removes the moist air near the leaf surface. The water potential of the air surrounding the leaves thus stays lower than that inside the intercellular spaces and transpiration increases.

Light and temperature: Photosynthesis takes place during the day (light), the carbon dioxide concentration in the guard cells drops, stomata open and transpiration takes place. During the night when the stomata close, transpiration stops to a great extend. Increased temperature accelerates metabolic reactions, as well as diffusion. Temperature and light together influence the rate of transpiration. Increased light intensity and temperature increase the transpiration rate. In the same way a high temperature and wind together increase the rate of transpiration and the wilting of leaves.


Transpiration: Environmental conditions


Soil conditions: Water lost by transpiration can only be replaced by water

taken up from the soil by the roots.

A lack of soil water results to a shortage of water in the plant and the

transpiration rate decreases.

Superfluous water in the soil can also result in the witing and death of plants,

because roots need oxygen for normal metabolic reactions. Superfluous water

leads to the drowning of plants when they cannot obtain oxygen.

Soil temperature influence the kinetic energy of water molecules. If the soil is very cold absorption is delayed and mesophytes may wilt and even die if the

turgidity is not restored, although there may be enough water in the soil.

Soil type also has an influence on the absorption of water by the roots. Clayey

soil retains water much better that sandy soil, thus plants that grow in sandy soil

can absorb water more easily and grow better.


Transpiration: Internal physiological factors


Any metabolic process that disturbs the osmotic balance between the leaf

cells and the remainder of the plant, will have an influence on transpiration:

Sugar, formed during photosynthesis, lowers the water potential in the mesophyll

cells.

Starch (insoluble) can be converted to sugar molecules (soluble) that can lower

the water potential.

Growth hormones change the elasticity of young, growing cell walls. Due to

turgor pressure the cell walls expand, cells become bigger and turgor pressure

drops.


Transpiration: Structural adaptations of

plants to decrease the transpiration rate Plants are adapted in various ways to keep the rate of photosynthesis as high as possible

while the transpiration rate is limited at the same time.

These adaptations are related to the environmental conditions.

Both carbon dioxide for photosynthesis and water vapour during transpiration move through the stomata.

The adaptations therefore are mainly concerned with the number, size and functioning of the stomata.

Some of the following structural adaptations are observed in plants:

small, few sunken stomata;

stomata only on the lower surface (shady side) of leaves;

thick cuticle;

stomata close during the day and open during the night, or close when the temperature and light intensity is very high;

epidermal hairs;

reduced leaf surface (small leaves);

lenticular transpiration.


Transpiration: Lenticular transpiration

Lenticular transpiration is the evaporation of water though lenticells on the

stems of deciduous plants.

In older plants both gaseous exchange and evaporation of water take

place through the lenticells of stems.


Transpiration: Advantages and

disadvantages of transpiration

Advantages:

Evaporation of water from the leaves cool down plants.

Pulling force of transpiration assists to transport water from the roots to the leaves

All essential minerals in the xylem sap are transported upwards to the leaves in

the transpiration stream.

Disadvantage:

Transpiration may lead to excessive water loss that may cause wilting and death

of plants, when there is not enough soil water to replace the lost water.

Animal tissues - Hoërskool Patriot - ONS GLO...Animal tissues We concluded the previous lesson with plant tissues. Just as plant tissues, animal tissue is a collection of cells and

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