Mass Transport In Plants

Question 1

Transpiration factors?

Answer 1

• light intensity, the higher the light intensity the faster the rate of transpiration. When it is light plants undergo photosynthesis so the stomata is open, for carbon dioxide to move into the leaf via diffusion. The stomata being open enables water to be evaporated, this has a positive correlation.
• temperature, the higher the temperature the faster the rate of transpiration. Warmer water molecules have more kinetic energy therefore evaporate quicker, increasing the WP gradient, this has a positive correlation.
• humidity, the higher the humidity the slower the transpiration rate, dry air surrounding the leaf results in a high WP gradient, this has a negative correlation.
• wind, the winder it is the faster the rate of transpiration. Air blows water molecules away from the stomata which increase the WP.

Question 2

What are the two mass transport mechanisms in plants?

Answer 2

Xylem, for the transport of water and inorganic ions from the roots to the leaves.

Phloem which transports organic substances (sugars) up and down the plant.

Question 3

Xylem?

Answer 3

Water and mineral salts are carried up from the root.

Question 4

How is the xylem adapted?

Answer 4

• cells are elongated and arranged end to end to form a continuous column called vessels.
• cells die when they mature and are hollow due to lignification, no cytoplasm or nucleus to slow water down. Proliferation plates / end walks can break down so there is no barrier to prevent water flow.
• cell walls are thick with lignin this waterproofs the xylem to prevent water loss. It is rigid and less likely to collapse under water tension and have pits to allow lateral water movement.
• a narrow lumen increase the height that water can rise due to capillary action

Question 5

How does water move through the xylem?

Answer 5

Water moves up the xylem vessels via transpiration the water evaporates/transpires from leaves via the stomata which reduces WP in the cell and osmotic gradient across the cell. Water is drawn up and out of the xylem which creates tension. Cohesive forces between water molecules pulls water up and the water lost is replace by osmosis in the roots.

Question 6

Why is transpiration rate not the same as water uptake rate?

Answer 6

Transpiration rate is not the same as water uptake rate as some water is used during photosynthesis so doesn’t transpire, some water is also used to make the plant turgid.

Question 7

Phloem?

Answer 7

Transports solutes e.g. Glucose and amino acids around the plant.

Question 8

Phloem structure?

Answer 8

• formed from columns of tube like cells.
• sieve tube elements, living cells that forms the tube for transporting solutes, they have no nucleus.
• companion cells, is a companion for the sieve tube element and carries out living functions for the sieve tube cell and respiration got the active transport of solutes.

Question 9

Translocation?

Answer 9

The process by which organic molecules and some minerals are transported from one part of the plant to another. Phloem transports sugars made via photosynthesis from sources: places sugars are made and stored e.g. Leaves. To an area white the sugars will be used directly or stored: sinks I.e. The site of high respiration such as where the roots are growing.

Question 10

The source?

Answer 10

Places where sugars are made and stored e.g. leaves. Sugars made by photosynthesis move into the sieve tubes by active transport. This lovers the WP and water moves in via osmosis form the xylem and companion cells this increases water pressure and the source

Question 11

The sink?

Answer 11

The solutes are removed form the phloem which increases the WP inside the sieve tube element. Water leaves the sieve tube by osmosis which lowers the pressure inside the sieve tube.

Question 12

How does the source and sink result in the flow of sugars through the phloem?

Answer 12

There is a pressure gradient from the source to sink end solutes are pushed along the sieve tube form the source end creating high pressure to the sink end which has low pressure. The higher the concentration of sucrose at eye source the higher the rate of translocation

Question 13

Evidence for mass flow?

Answer 13

• metabolic inhibitor stops ATP and translocation showing active transport is needed.
• a radioactive tracer can be used to track the movement of organic molecules.

Question 14

Evidence against mass flow?

Answer 14

• sugars travel to different sinks not just ones with the highest WP.
• sieve plates create a barrier to mass flow, lots of pressure would be need for solutes to get through.