The Transport of Substances in Plants

TRANSPORTTRANSPORTTransport of Substances in PlantsTransport of Substances in Plants

Necessity• Transport substances to all parts of plant

Water – for photosynthesis and cellular metabolism Mineral ions – for healthy growth and development Organic food materials – for cellular metabolism or

storage

• Problem Small TSA/V ratio Long distances

• Overcome Internal system of tubes = VASCULAR SYSTEM

The Vascular System

• The vascular system involved in transporting substances in plants– except movement of gases (diffusion)

• Consist of 2 types of tissues: xylem and phloem– which function as two separate

transport system.

Monocot plant

leaf:

stem: root:

Dicot plant

Xylem

• Transports water and dissolved mineral ions from the roots to the upper parts of the plant– Against gravitational pull

• Gives mechanical support to woody plants

Xylem adaptation • Consists of 4 types of cells:

Xylem vessels Long, hollow, continuous tube Dead cell, no protoplasm Thickened by lignin (patterns: pits, spiral, ring) Numerous pits (holes), end walls open

Tracheids Dead cells, hollow Longer and smaller than xylem vessels Pointed end and pitted, end walls breaks

down in pits Parenchyma – food storage Fibres - support

Phloem

• Transports organic substances (e.g. sucrose, amino acids) synthesised mainly in the leaves to other parts of plant– Used in cellular metabolism

• Growth (shoot, root, buds, fruits)• Respiration

– Stored in in storage organ • e.g. tuber

Phloem adaptation • Consists of

Sieve tubes• Living cells (nucleus disintegrates during

maturation, thin layer of cytoplasm, small mitochondria)

• Cylindrical, elongated, end-to-end• End walls perforated to form sieve plates

Companion cells (transport food into tubes)

• Shorter than sieve tubes• Living cells (with nucleus, many mitochondria)• Adjacent to and support the function of sieve tube

Parenchyma – food storage Fibres - support

Transport of substances:Food (Translocation)

• The movement of sucrose and other organic materials within the plant body

• Primarily through phloem (sieve tube)• Drives by concentration (pressure)

gradients• Importance:

– Plant’s survival– Store and convert sucrose to other sugars

Bark Ringing

• Removal of a ring of tissue external to the xylem from around the trunk of a woody plant.– Removal of phloem

Observation Inference

Tissue above the ring swells up Tissue below the ring withers

•Tissue removed is phloem•Disrupt flow of food to root•Accumulation causes

swelling at the upper part• Insufficient food cause the

lower part to wither

The upper part of plants lives normally

•Xylem is not removed •Flow of water is not

disrupted•The upper part do not wilt

Transport of substances:Water and minerals

• Water by osmosis • Minerals by active transport• Aided by three factors:

– Root pressure – Capillary action

• Cohesion and adhesion

– Transpiration pull

1) Root Pressure

• The process that forces the water absorbed from the soil to move through the roots and up the stem of a plant

• Caused by osmosis of water and active transport of mineral ions into the root xylem

The mechanism: • Cell sap of root hair cells is hypertonic to soil

water• Water enter by osmosis• Cell sap is diluted, hypotonic to adjacent cells• Water moves to the next cells until crosses

cortex and endodermis• Creates root pressure to push water into xylem• Helped by the active transport of minerals into

xylem that increase osmotic pressure

Guttation

• In small plants, water may be pushed out of special pores at leaf (hydathodes)

• Usually seen early morning

2) Capillary action

• Chemical forces that move water as a continuous column rather than as individual molecules.

• Combination of 2 forces due to hydrogen bonding: cohesion & adhesion

• The forces can be overcome by gravity.

Cohesion and Adhesion• Cohesion:

attraction between water molecules, prevents break of water column

• Adhesion: attraction between water molecules and xylem walls, prevents slipping of molecules

3) Transpiration pull

• When water evaporates, it creates an empty space which is filled continuously.

• As the water moves, more water is absorbed to replace the evaporated water (transpiration pull)

Con

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Transpiration • The process of water loss (as water

vapour) from plants to the atmosphere due to evaporation (99% of water intake)– Through the stomata in leaves (90%)– Through cuticle (5%) or lenticels (5%)

• Importance – Creates transpiration pull – Provide cooling effect and turgor pressure– Maintain osmotic pressure

The process:

• Water absorbed by roots to mesophyl cells

• Heat from Sun cause water to evaporate to air spaces

• Water vapour diffuse through stomata (traspiration)

Regulation by stomata:Open

• During day– Photosynthesis

produce glucose– K+ pumped into

guard cells – Accumulation of

glucose and K+

increase osmotic pressure

– Water enters by osmosis

– Guard cells become turgid, curved outwards, stoma opens

– Transpiration occurs

Regulation by stomata:Close

• At night/dark– No

photosynthesis, no glucose produced

– K+ moves out to epidermal cells

– Osmotic pressure decrease

– Water diffuses out by osmosis

– Guard cells become flaccid, stoma closes

– Transpiration stops

Factors affecting transpiration rate:Air Movement

• More air movement, easier evaporation Tra

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Air movement

Factors affecting transpiration rate: Temperature

• Higher temperature, higher rate of transpiration Tra

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Temperature

Factors affecting transpiration rate: Light intensity

• Higher intensity, higher rate of transpiration

Lig

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Time

Tra

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Light intensity

Transpiration rate

Factors affecting transpiration rate: Relative humidity

• Higher surrounding humidity, lower rate of transpirationTra

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Relative humidity