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AS OCR Biology Revision Pack
UNIT f211 Cells, exchange, transport
Module 1 Cells
Cell Structure
1. State the resolution and magnification that can be achieved by a light microscope, a transmission electron microscope
and a scanning electron microscope.
Light Microscope TEM SEM
MaximumResolution
0.2 micrometres 0.0001 micrometres 0.005 micrometres
MaximumMagnification
X 1500 Over x 1,000,000 Under x 1, 000, 000
2. Explain the difference between magnification and resolution
Magnification How much bigger the image is than the specimen.
Magnification = Length of Image / Length of specimen
Resolution How well a microscope distinguishes between two points that are close
together.
3. Explain the need for staining samples for use in light and electron microscopy
In Light microscopes and TEMs the beam of lights/electrons pass through the object,
and there is an image produced as some parts of the specimen absorb more
light/electrons than others, but sometimes the specimen is transparent so it will look
white because light/electrons pass through so the object is stained
Light Microscope Electron Microscope
Dye- usually methylene blue/eosin Specimen dipped in metal like lead, themetal ions scatter electrons to contrast.
4. Calculate the linear magnification of an image
Magnification = Length of Image / Length of specimen
5. Outline the functions of the structures.
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Picture Description Function
Large and containschromatin. Enclosed by anuclear envelope double membrane.
Nuclear pores go throughthe envelope. Nucleolus
inside.
Nucleus contains thecells genetic material.Chromatin contains DNA
and proteins whichregulate cell activities.Instructions for making
proteins.
Flattened membranoussacs called cisternae,rough is studded with
ribosomes, smooth is not.
RER transports proteinsand SER is involved in
lipid synthesis.
Stack of flat, membranebound stacks. [Pitta
bread!]
Golgi body receivesproteins from ER and
modifies them.Packages proteins into
vesicles to transportthem exocytosis
Sausage shaped. Doublemembrane separated byfluid filled space. Innermembrane is folded toform cristae and themiddle part of themitochondria is called thematrix.
Site of aerobicrespiration, ATP is
produced.
In plant cells. Doublemembrane. Membranous
sacs called thylakoids,plural=granum.Plural=grana.
Site of photosynthesis,carbohydrate molecules
made.
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Spherical sacssurrounded by a single
membrane, with no clearinternal structure.
Contains enzymes.
Enzymes break downcells. E.g. white bloodcell lyosomes break
down invadingmicroorganisms and
lyosome in the spermshead breaks down thematerial surrounding the
egg.
TINY.Bound to ER to make
RER and also incytoplasm. Consist of two
subunits.
Site of protein synthesis,they are like an
assembly line wheremRNA from the nucleusis used to make proteins
from amino acids.
Eukaryotic- 80SProkaryotic- 70S
Small tubes ofmicrotubules. A pair can
be found next to thenucleus in animal cells.
Also in some protocytists.
Involved in cell divisionto make spindles whichmove chromosomes in
nuclear division.
Membrane bound sac
found in plants filled withcell sap.
Keep the plant
supported, rigid andturgid. Also like a
garbage disposal forplants.
Network of protein fibresSupport, movement.E.g. Chromosome
movement in mitosis.
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Thick layer, in plants.Made of cellulose ineukaryotic cells andmurein in prokaryotic
cells.
Gives the cell strengthand rigidity
Thin, flexible layer aroundall eukaryotic cells. Made
of phospholipids andproteins.
It separates the cellcontents from externalenvironment and evencontrols movement of
substances in and out ofthe membrane with
receptor cells.
Enclosed jelly likesubstance within the cell
membrane.
In eukaryotic cells itcontains organelles, in
prokaryotic cells itcontains enzymesneeded for metabolic
reactions.
Circular and loose.Unprotected, unlike in
eukaryotic cells.
Genetic instructions
PlasmidSmall circle of DNA Exchange DNA easily
and quickly betweeneukaryotic cells. Used in
genetic engineering.
A thick polysaccharidelayer outside of the cell
wall
Useful for sticking cellstogether, and as a food
reserve. Protectsagainst phagocytosis
and chemicals.
Rigid tail that rotates.The motor is embeddedin the cell membrane andis driven by a H+ gradient
across the membrane.Clockwise rotation drivesthe cell forwards, while
Propels the cell
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1. A bud forms at the cell
surface
2. The cell undergoes interphase
3. The cell undergoes mitosis
4. Nuclear division is complete
budding cells nucleus has anidentical copy of parent cell
dna
5. The bud separates off from
the parent cell with a
genetically identical yeast cell
Meiosis:
1. Gametes are found in all
sexually reproducing
organisms
2. Male & Female join at
fertilisation forming a zygote
dividing into a new organism
3. (Sperm and Egg)
4. (Pollen grains and ovules)
5. Normal body cells of plants
and animals have diploid (2n)number of chromosomes,
each cell contains two of each
chromosome from each
parent
6. Gametes have the haploid
number of chromosomes (n)
theres one copy of each
chromosome
7. At fertilisation the haploid
male gamete and female fuse
to make a cell with the diploid
number of chromosomes, half
from sperm half from egg.
Produces cells genetically different-
genetic variation, it creates variation.
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Define the term stem cell
Stem cells are cells that are not specialized and can differentiate into specialized cells
with mitosis and the correct stimulation.
Define the term differentiation, with reference to the production of erythrocytes (red blood cells) and neutrophils derived from stem
cells in bone marrow, and the production of xylem vessels and phloem sieve tubes from cambium.
Bones are living organs containing nerves and blood vessels, and the main bones have
marrow in the middle, adult stem cells divide and differentiate to replace worn out
erythrocytes and
neutrophils to fight
infection.
In plant cells stem cells
are in the cambium. In the
root and stem the stem
cells of the vascular
cambium divide to
differentiate into the xylem
and phloem, the vascular
cambium then forms a
ring inside the root and shoots. These cells divide and grow from the ring differentiating
and moving away from the cambium.
Describe and explain, with the aid of diagrams and photographs, how cells of multicellular organisms are specialised for particular
functions, with reference to erythrocytes (red blood cells), neutrophils, epithelial cells, sperm cells, palisade cells, root hair cells and
guard cells.
Neutrophills protect the body against illness, they are flexible so they can engulf
pathogens and they have lots of lysosomes with digestive enzymes that can break
down the pathogens.
Erythrocytes carry oxygen in the blood and they have a
biconcave disc shape to give a large surface area to
volume ratio for gaseous exchange, they dont have a
nucleus so they have more room for haemoglobin.
Epithelial cells cover organ surfaces and cilia can beat to
move particles, and other like microvilli can fold in the cell
membrane to increase surface area to volume ratio
Sperm cells have a flagellum that enables them to swim
to the egg and they have lots of mitochondria to provide
energy to swim, the acrosome contains digestive enzymes so the sperm can penetrate
the egg surface.
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Explain the meaning of the terms tissue, organ and organ system.
A tissue is a group of similar cells that are specialized to work together to carry out a
particular function.
E.g. Ciliated epithelium, xylem tissue, squamous epithelium tissue, phloem tissue
Organs are groups of different tissues that work together to form a function.
E.g. Lungs squamous epithelium, ciliated epithelium, elastic connective tissue and
vascular tissue.
Organ systems are different organs working together for a different function, e.g. the
respiratory system is made of all of the organs, tissues and cells involved in breathing
like the lungs, trachea, larynx, nose, the diaphragm and mouth.
Discuss the importance of cooperation between cells, tissues, organs and organ systems.
Mulitcellular organisms work efficiently as they have different cells that are specialized
for various functions
It is beneficial because every different cell can carry out a specialized function in a more
efficient way than unspecialized cells could.
Each cell depends on the other cells for the functions it cannot carry out
So cells, tissues and organs in multicellular organisms cooperate to keep the organism
alive and working well.
E.g. Muscle cells can move well but to do so they need oxygen, so they need
erythrocytes to carry oxygen to them from lungs.
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Transport in plants
Explain the need for transport systems in multicellular plants in terms of size and surface area to volume ratio.
Plants need water, CO2 minerals like nitrates and potassium, and sugars to live and
they need to get rid of waste substances. They are multicellular and have a small
surface area to volume ratio so need transport systems to move substances to and from
cells quickly as diffusion alone is too slow.
Describe, with the aid of diagrams and photographs, the distribution of xylem and phloem tissue in roots, stems and leaves of
dicotyledonous plants.
Leaf Cross Section
Root Cross Section
Describe, with the aid of diagrams and photographs, the structure and function of xylem vessels, sieve tube elements and
companion cells.
In a root the xylem and phloem are
in the centre to give support to the
root as it pushes through the soil.
In stems the xylem and phloem are
near the outside to provide stability
that reduces bending.
In a leaf the xylem and phloem make
up a vein network to support the
thin leaves.
Xylem vessels are long tube
structures formed from vessel
elements joined end to end. There
arent end walls so they are not
interrupted tubes, and allow water
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Phloem tissue transports solutes like sucrose around plants, it is only a transport tissue.
Sieve tube elements are living cells that form the tube for transportation of solutes
around the plant, they are joined end-end to make sieve tubes. The sieves are end
walls with holes in them for solutes to pass through, although they have no nucleus, a
thin layer of cytoplasm and few organelles. The cytoplasm of nearby cells is joined
through holes in sieve plates.
Companion cells are there for each sieve tube element to carry out metabolic processes
for the sieve tube elements that cannot survive on their own as they have no nucleus,
etc., and itself- e.g. they provide energy for active transport of solutes
.
Define the term transpiration.
The loss of water from the plants surface
Explain why transpiration is a consequence of gaseous exchange.
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A plant must open its stomata for absorption of carbon dioxide for photosynthesis,
which as a consequence allows water to escape because there is a higher water
potential inside the leaf than outside. So water moves out of the leaf by osmosis down
the water potential gradient.
Describe, with the aid of diagrams, how a potometer is used to estimate transpiration rates.
Really it measures the water upta
by the plant, but we assume that
water uptake is directly related to
water loss by leaves.
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1. Cut a shoot under water to stop air from going into the xylem at a slant to
increase surface area to volume ratio for water uptake
2. Check that the apparatus has no air bubbles and is full with water
3. Put the shoot into the apparatus underwater to prevent air entering
4. Remove the photometer from the water and make it air and water tight
5. Dry the leaves, let the shoot acclimatize and shut the tap
6. Keep conditions constant throughout the experiment
7. Record the starting position of the air bubble
8. Start a stopwatch and record the distance moved by the bubble per unit time
Explain, in terms of water potential, the movement of water between plant cells, and between plant cells and their environment.
Light Lighter= faster rate of transpiration as thestomata open for photosynthesis
Temperature Higher= faster rate as water molecules havehigher kinetic energy so they evaporate from
cells quicker, increasing the water potentialgradient between inside and outside of leafmaking water diffuse out quicker.
Humidity Lower= faster, if the air around the plant is drythe water potential gradient between the leafand air is steeper
Wind Higher= faster, air movement blows the watermolecules from the stomata, steepening thewater potential gradient
Describe, with the aid of diagrams and photographs, how the leaves of some xerophytes are adapted to reduce water loss by
transpiration.
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Describe, with the aid of diagrams, the pathway by which water is transported from the root cortex to the air surrounding the leaves,
with reference to the Casparian strip, apoplast pathway, symplast pathway, xylem and the stomata.
Water travels through the roots via the root cortex into the xylem by two ways
The Symplast Pathway The Apoplast Pathway
Goes through living parts of the cells, thecytoplasm. The cytoplasm of nearby cellsconnect through plasmodestmata, which aresmall spaces in cell walls.
Goes through non living parts of the cells, thecell walls, the walls are absorbent and watercan diffuse by osmosis through them and passthrough spaces between them.
When water is in the Apoplast pathway it goes to the endodermis cells in the root, but the path
is blocked by the Casparian strip- which is just a waxy strip. The water then must take the
Symplast pathway.
This is not a hindrance because the water than has to go through the cell membrane which
controls substances entering/leaving.
If the water goes past the barrier it moves into the Xylem.
The main pathway used is the Apoplast pathway as it provides the least resistance.
Explain the mechanism by which water is transported from the root cortex to the air surrounding the leaves, with reference to
adhesion, cohesion and the transpiration stream.
Cohesion and tension move water up from roots to the leaves against gravity, water
evaporates from the leaves at the top of the xylem via transpiration
This creates suction/tension which pulls more water into the leaf
Water molecules are cohesive, meaning they stick together, so if one is pulled into the
leaf so are more. The whole column of water in the xylem moves upwards, and it enters
the stem through the roots.
Adhesion is the water molecules being attracted to the walls of the xylem vessels,
helping water rise up.
Explain translocation as an energy-requiring process transporting assimilates, especially sucrose,
between sources (e.g. leaves) and sinks (e.g. roots, meristem).
Translocation is the movement of dissolved substances like sucrose
and amino acids when they are needed in a plant- called assimilates.
This requires energy and happens in the phloem.
Translocation moves substances from sources (where it is produced-
higher concentration) to sinks (where it is used- lower concentration)
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E.g. The source for sucrose is the leaves and the sinks are mainly food storage organs and the
meristems (growth areas) in the roots, stems and leaves.
Enzymes maintain the concentration from the source to the sink by changing the dissolved
substances at the sink, like by breaking them down or changing them into something else, to
make sure there is a lower concentration at the sink than the source to keep a steep
concentration gradient.
Describe, with the aid of diagrams, the mechanism of transport in phloem involving active loading at the source and removal at the
sink, and the evidence for and against this mechanism.
y At the source active transport is said to actively load the dissolved solutes into sieve
tubes of the phloem.
y Lowering the water potential inside sieve tubes and water enters them via osmosis.
y Creating a high pressure inside the sieve tubes at the source end of the phloem.
y At the sink the solutes are removed from the phloem to be used
y Increasing water potential inside the sieve tubes so water leaves by osmosis
y Lowering pressure inside the sieve tubes
y Creating a pressure gradient from the source to the sink
y This gradient is responsible for pushing solutes along the sieve tubes to where they are
required in the plant.
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For Against
Removing a ring of bark from a tree takingthe phloem not the xylem from a woodystem a bulge will form above the ring. Onanalysis of the fluid in the bulge, there willbe a higher sugar concentration above the
ring than below- so there must be adownward sugar flow.
Sugar travels to many sinks not one withthe highest water potential, as the modelindicates
Aphids pierce the phloem with theirmouthparts and sap flows into them, thesap flows out quicker nearer the leavesthan further down the stem, so there mustbe a pressure gradient.
Sieve plates would make a barrier to massflow, a lot of pressure would be needed forsolutes to pass at a reasonably quick rate
A metabolic inhibitor stopping ATPproduction in the phloem stopstranslocation, proving it is active transport.
There are experimental mass flow models