9.2- Water Transport In Multicellular Plants

Question 1

Why is water vital for plants?

Answer 1

• raw material for photosynthesis.
• loss off water by evaporation acts as cooling mechanism.
• mineral ions and photosynthesis products transported in aqueous solutions.
• turgor pressure as a result of osmosis provides a hydrostatic skeleton to support stems and leaves.
• turgor drives cell expansion.

Question 2

What are root hair cells?

Answer 2

The exchange surface in plants where water is taken into the body of the plant from the soil.
- it is a specialised epidermal cell found near the growing tip with a long thin extension called the root hair.

Question 3

How are root hairs adapted as exchange surfaces?

Answer 3

• microscopic size means they can penetrate easily between soil particles.
• each hair as a large SA:V and there are thousands on each root tip.
• thin surface layer- short diffusion/osmosis distances.
• conc of solutes in cytoplasm of cells maintains a water potential gradient between the soil water and cell.

Question 4

Explain the symplast pathway.

Answer 4

• water moves through the symplast by osmosis.
• root hair cell has higher water potential than the next cell due to water diffusing in from soil.
• so water moves from the root hair cell to the next cell by osmosis via plasmodesmata.
• continues until it reaches the xylem.
• as water leaves the root hair cell, the water potential of the cytoplasm falls again, causing water to move in from the soil. This maintains a steep water potential gradient.

Question 5

What is the symplast and apoplast?

Answer 5

Symplast - the continuous cytoplasm of the living plant cells that is connected through the plasmodesmata.

Apoplast- the cell walls and intercellular spaces.

Question 6

Explain the apoplast pathway.

Answer 6

• water moves through the apoplast.
• water fills between loose, open network of fibres in cellulose cell wall.
• as water molecules move into xylem, more molecules behind are pulled through apoplast due to cohesive forces between the water molecules.
• the pull of water through plant to xylem along with cohesive forces creates a tension meaning there is continuous flow of water through the open structure of cellulose wall.

Question 7

Explain the movement of water into the xylem.

Answer 7

1. Water moves across the root in apoplast and symplast pathways until it reaches endodermis.
2. Water in apoplast pathway cant go further due to casparian strip so is forced into cytoplasm, joining symplast pathway.
3. Solute conc in endodermal cells is dilute compared to cells in the xylem.
4. Endodermal cells move mineral ions into xylem by active transport.
5. As a result, endodermal cells have higher water potential than xylem which increases rate of water moving into xylem down water potential gradient.
6. Once inside vascular bundle, water returns to apoplast pathway to enter xylem itself.
7. Root pressure gives water a push up the xylem

Question 8

Explain root pressure.

Answer 8

Root pressure is created by the active pumping of minerals into the xylem to produce movement of water by osmosis.
- it gives water a push up the xylem but isn’t the major factor in movement of water from roots to leaves.

Question 9

What is the role of active transport in root pressure?

Answer 9

• root pressure increases with rise in temp and decreases with decrease in temp. Shows chemical reactions are involved.
• if levels of oxygen or respiratory substrates fall, root pressure falls.
• cyanide (poison) affects mitochondria so prevents ATP production. If applied to root cells, root pressure disappears.