Water Transport

Question 1

Why is water important to plants?

Answer 1

• Hydrostatic pressure created by osmosis provides a hydrostatic skeleton of support for stems and leaves.
• This pressure makes root cells strong enough to force their way through hard surfaces.
• Evaporation of water keeps plant cool.
• Mineral ions and products of photosynthesis are transported in solution.
• Water is required for photosynthesis.

Question 2

Movement of water into root

Answer 2

• Root hair cells are exchange surfaces of water. Root hairs are long thin extensions and they are well adapted.
• Microscopic in size so they can grow between soil particles.
• Large SA:V so more space for diffusion.
• Surface layer is 1 cell thick so osmosis is quick.
• Concentration of solutes in cytoplasm of R.H cells maintains a water potential gradient between soil and cell.
• Soil water has v low conc. of solutes so high water potential.
• Cytoplasm and vacuolar sap of R.H cell contains many solutes so water potential is lower.

Question 3

Movement of water across the root: Symplast

Answer 3

• Water moves from root to xylem through cortex of packed cells.
• In symplast pathway, water moves through the symplast: continuous cytoplasm connected by plasmodesmata.
• Each root hair cell has a higher water potential than the next cell along. So water moves across cells by osmosis.
• As water leaves root hair cells by osmosis, water pot. of cytoplasm falls and this maintains steep water pot. gradient to ensure as much water continues to move from soil to cell.

Question 4

Movement of water across the root: Apoplast

Answer 4

• Water moves through the apoplast: the cell walls and intercellular spaces.
• Water moves between the loose network of cellulose fibres in cell wall.
• Water molecules are pulled along the pathway due to cohesive forces between molecules.
• So as 1 molecule at the beginning of the chain is pulled into the xylem, water molecules at the end of the chain are pulled along into the apoplast.

Question 5

Movement of water into the xylem

Answer 5

• Water moves across root through the pathways until it reaches endodermis: the layer of cells surrounding the vascular tissue (xylem+phloem) of the roots.
• Casparian strip is a band of waxy material called suberin that runs around each of the endodermal cells, forming a waterproof layer.
• This forces water to go through symplast pathway therefore water moves through selectively permeable cell surface membrane which would filter any toxic solutes from soil water.
• Endodermal cells actively transport ions into xylem which decreases the water pot. of xylem cells compared to endodermal cells.
• This increases rate of water moving into xylem via osmosis.
• Active transport creates root pressure which helps push water and solutes up the xylem.

Question 6

Evidence of role of active transport

Answer 6

• When cyanide is added to root cells, root pressure drops. This is because cyanide kills mitochondria, and therefore stops the production of ATP, so there is no energy for active transport.
• Root pressure increases w/ temperature until certain point which suggests chemical reactions are involved.
• If levels of oxygen or respiratory substrates fall, root pressure falls, suggesting it’s active.
• Xylem sap is forced out of ends of cut stems suggesting that it’s not just cohesive forces of water. In nature, Guttation can occur for example overnight where water is forced out of plants through pores when transpiration is low.