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

Transpiration : part 1 (leaves)

1. Water vapour diffuse out of the leaf through stomata down WATER POTENTIAL GRADIENT .

2. More water will evaporate from cell wall of mesophyll cells into air spaces

3. So More water will move out of the mesophyll cells

• osmosis through cell surface membrane , by Symplast pathway

• To the cell wall of the spongy mesophyll , by apoplast pathway

4. Water leaves the xylem vessels through PITS into mesophyll cells in the leaf DOWN WATER POTENTIAL GRADIENT

• By SYMPLAST PATHWAY : when water enters the cytoplasm of the cell

through cell surface membrane by OSMOSIS ( cytoplasmic pathway ) and enters the vacuole through tonoplast by OSMOSIS ( VACUOLAR PATHWAY )

• BY APOPLAST PATHWAY: from xylem vessels to CELL WALL of mesophyll cells by diffusion

Transpiration part 2 (stem)

1. Reduces the hydrostatic pressure at the top of xylem vessels

2. Causing a negative pressure in the xylem vessels

3. Creating a transpiration pull

4. Creating tension on the walls of xylem vessels

5. Resulting in a transpiration stream (mass flow down the hydrostatic pressure gradient)

6. So water with dissolved minerals move in a continuous water column from roots to leaves

Maintained by cohesion and adhesion forces (adhesion due to hydrophilic nature of cellulose)

Transpiration part 3 (root)

Water moves down the water potential gradient
1. Across cortex by symplast and apoplast pathway
2. From cortex into endodermis where endodermis has a casparian strip made from subrein which blocks the apoplast pathway and water is diverted to the cytoplasm through cell surface membrane and vacuole through tonoplast by symplast pathway
3. This allows selection of ions to pass into xylem vessels (which plays a role in root pressure) and prevents entry of pathogens
4. Water moves from endodermis to xylem through pits by apoplast pathway

Why cell membrane of endodermal cells has many transport proteins ?

1. to allow the active transport of minerals into xylem vessels

2. this lowers the water potential in the xylem than the endodermis

3. so more water enters the xylem through the pits down the water potential gradient by apoplast pathway

4. This increases the hydrostatic pressure at the base of the xylem

5. Causing root pressure