Transport in plants

Why plants have a transport system

Small surface area to volume ratio, so longer diffusion distance, so diffusion alone would be too slow

A) surface is too far from the centre

B) long distance so there would be no sufficient supply of nutrients needed

So plant need xylem vessels to transport water and minerals as well as Phloem to translocate sucrose and amino acids (assimilates)

Describe Xylem vessels

1. Elongated cells to form elongated tube for transport

2. No end walls to form a continuous tube to minimise resistance to water flow

3. Hollow dead cells with no cytoplasm , no nucleus , and no cell organelles allowing a greater volume of water to flow with minimal resistance .

4. Cellulose lining which has a hydrophilic nature allowing the adhesion of water molecules to maintain a continuous water column .

5. Lignified walls (secondary thickening) which prevent water loss from xylem (water impermeable)

6. Lignified walls (secondary thickening) which prevent collapse that might be caused by hydrostatic pressure gradient in xylem vessels caused by transpiration

7. Pits which are non lignified parts in xylem vessels allow lateral movement of water .

8. Narrow diameter of lumen for adhesion, in other words more water molecules will be in contact with walls of xylem vessels .

9. Relatively large diameter ( large surface area ) : greater volume of water to flow

Describe transpiration

loss of water vapour from the leaf

Evaporation of water on the cell wall surface of mesophyll cells into air spaces of spongy mesophyll cells followed by diffusion of water vapour out of the leaf through stomata down water potential gradient

Factors affecting transpiration

1. Temperature

2. Wind speed

3. Humidity

4. Light intensity

Describe how factor 1 affects transpiration

The increase in temperature will increase rate of transpiration because

An increase in temperature Increase rate of evaporation of water in cell wall of spongy mesophyll cells. Thus increasing the water potential gradient of water vapour with the atmosphere So increase rate of diffusion of water vapour out of stomata.

Also at very high temperature the stomata close so transpiration slows.

Describe how factor 2 affects transpiration

Higher wind speeds can enhance transpiration by moving moist air away from the leaf's surface, maintaining a steeper water potential gradient. This results in a higher rate of diffusion of water vapour out through the open stomata. Conversely, at very high wind speeds, the stomata may close, which also slows transpiration

Describe how factor 3 affects transpiration

Increased humidity decreases the rate of transpiration because it reduces the steepness of the water potential gradient between the air spaces in the leaf and the outside environment, leading to a lower rate of diffusion of water vapour.

Describe how factor 4 affects transpiration

Higher light intensity increases the rate of transpiration as it causes the stomata to open more widely to allow gas exchange for photosynthesis. However, at very high light intensity, the stomata may close, which slows down transpiration.

Describe the link between transpiration and a certain process

Transpiration is an inevitable consequence of gas exchange

Explain why transpiration is linked to that function in that specific way

As the stomata opens for gas exchange to allow carbon dioxide to diffuse into the leaf (as Carbon dioxide is a raw material for photosynthesis) and oxygen out of the leaf so more water vapour diffuse out through open stomata down water potential gradient

Importance of transpiration

1. Cools down the leaves in hot conditions as the water from the walls of spongy mesophyll cells evaporate, thus preventing over heating of cells and reducing their temperature

2. To create transpiration stream needed to help movement of water with dissolved minerals up the plant

State the two pathways water can take to move between plant cells

Apoplast pathway and Symplast pathway

Describe the apoplast pathway

Water moves down water potential gradient

Water enters the cell through cell wall

Water moves through cell wall

Water moves from one cell to another through cell wall and through intercellular spaces

It is a non living pathway so it allows a faster rate of water flow due to less resistance due to adhesion of water molecules to the cell wall due to hydrophilic nature of cellulose cell wall

Describe the Symplast pathway

Water moves down the water potential gradient

Water enters the cytoplasm through cell surface membrane by osmosis (By cytoplasmic pathway)

And water moves into vacuole through tonoplast by osmosis (Vacuolar pathway)

Water moves from one cell to another through plasmodesmata by diffusion

It is a living pathway, allowing a slower rate of water flow with more resistance to allow selectivity through cell surface membrane