O Level Revision : Integrated Science - Transport in Plants

Root and stem structure

Transverse sections of dicotyledonous stem & root

Cross sections of root and stem

 Functions of the parts of a root

 Locating the position of xylem vessels in roots and stems using dyes

Red dye on cross-sections of roots and stems shows position of xylem vessels 

• A potted bean plant is watered using (coloured water) water mixed with a dye e.g. eosin, methylene blue etc.
• Sections of stems and roots are cut and observed under a microscope.
• The dyed parts show positions of xylem vessels as the coloured water dyes them during its movement.

 Diffusion and osmosis

• Diffusion is movement of particles from a region of their higher concentration to a region of their lower concentration i.e. a movement down a concentration gradient.
• Diffusion is fastest in gases and slowest in solids.
• Osmosis is the movement of water molecules from a region of their higher concentration to a region of their lower concentration across a partially/differentially/selectively permeable membrane. It is a special case of diffusion of water.
• Osmosis can be demonstrated easily using visking tubing, eggshell membrane or living tissues such as potatoes.

Demonstrating osmosis using visking tubing or eggshell membrane

Observations

(i)   In the set up with viskings tubing, the level of solution in capillary tube A rises while the level capillary tube B falls.

(ii)  In the eggshell membrane set up, the level of solution in membrane A rises while the level of solution in B falls.

(iii) ln eggshell membrane set up level of solution in beaker A falls while in B rises.

Conclusion

Osmosis occurs across viskings tubing and across egg shell membrane.

Explanation

(i)   Visking tubing set up:

►     In set up A, water moves by osmosis from beaker into capillary tube.

►     In B water moves from viskings tube into beaker. (ii)  Egg shell membrane:

►     Water moves from beaker in A into egg shell membrane.

►     Water moves from egg shell membrane into beaker.

Demonstrating  osmosis  using  potatoes  (living  tissue) Alternative 1

Conclusion

Osmosis occurs across living plant tissue.

 To demonstrate osmosis using potatoes (living tissue) Alternative 2

Results

• Osmosis occurs from beaker into the potato and up to the salt thereby dissolving the salt.

Conclusion

• Osmosis occurs across living plant tissue.

Osmosis does not occur across boiled plant tissue

Water and ion uptake

• Water enters plant roots and moves to the xylem vessels by osmosis.
• Energy is used to move mineral salts against their concentration gradient in a process called active uptake.

Movement of water through a plant

Definition: Transpiration is the loss of water in plants through stomata as vapour.

Transpiration

Transpiration occurs from the leaves

• The colourless liquid that collects in the plastic bag turns white anhydrous copper (II) sulphate blue or blue cobalt chloride paper pink showing that the liquid is water.
• Nothing collects in the plastic bag where leaves have been removed.
• Vaseline prevents loss of water from open xylem vessels.

Functions of transpiration

(i)   Water transport action-loss of water from leaves creates a ‘transpiration pull’ on water in stem and roots. This increases the rate of water and nutrient absorption by plants.

(ii)  Coolingeffect-aliquidevaporates whenheated.The waterevaporatingfrom leavesabsorbs heat from leaves leaving the leaves cool and this effect called cooling by evaporation.

 Investigating factors affecting transpiration using a potometer

The funnel with the tap is the reservoir whose tap is opened to reset the bubble in the burette/capillary tube.

Findings from the experiment

Adaptations of leaves to minimize water loss

1. Reduction of leaf surface area – smaller leaves absorb less heat energy causing lower evaporation rates – so less transpiration.
2. Thickness of cuticle – Thick waxy cuticle reduces water loss by evaporation.
3. Distribution of stomata – Less stomata on the lower leaf surfaces reduces water loss.
4. Presence of hairs – Hairs trap the moisture around the leaf reducing the concentration gradient

– lower transpiration rate.

Wilting

Definition: Wilting is the loss of turgidity in plants due to excessive water loss during transpiration.

• This occurs when plants lose more water than they absorb from the soil.
• If the water lost from a plant is not replaced this can result in the death of the plant.