Mass transport in plants

Question 1

Describe the cohesion-tension theory of water transport in the
xylem.

Answer 1

Water evaporates from the leaves via the (open) stomata due to transpiration

Reducing water potential in the cell and increasing water potential gradient

Water drawn out of xylem

Creating tension

Cohesive forces between water molecules pull water up as a column

Water lost enters the roots via osmosis

Water is moving up, against gravity

Water is also cohesive so sticks to the edges of the column

Question 2

What is translocation

Answer 2

Movement of solutes from source to sink/ one place to another

Question 2

Describe the mass flow theory in the phloem

Answer 2

At the source:
- High concentration of solute
- Active transport loads solutes from companion cells to sieve tubes of the phloem
- Lowering the water potential inside the sieve tubes
- Water enters sieve tubes by osmosis from xylem and companion cells
- Increasing pressure inside sieve tubes at the source end

At the sink:
- Low concentration of solute
- Solutes removed to be used up e.g. enzymes hydrolyse
- Increasing the water potential inside the sieve tubes
- Water leaves tubes via osmosis
- Lowering pressure inside sieve tubes

Question 3

Importance of movement in cells

Answer 3

Allows for organic compounds to be moved from the source to the sink. For example photosynthesis happens at the leaf so it needs to transport the glucose it makes to whole plant. Without the phloem the sieve tubes would not be able to load all the solutes to take it to the sink.