Transport in Plants

Question 1

What is mass transport?

Answer 1

The movement of materials over large distances due to pressure differences.

Question 2

What are the uses of water by a plant?

Answer 2

• Photosynthesis.
• Maintaining turgid cells.

Question 3

What are the features of xylem vessels?

Answer 3

• Cell walls contain lignin which strengthens the xylem walls against the tension within them and makes them waterproof.
• Lignified vessel walls cause the cell contents to die, so leaves a hollow lumen with no cytoplasm that offers little resistance to mass flow of water and minerals.
• Walls contain tiny holes called pits, so if a vessel becomes blocked or damaged, the water can be diverted laterally, so the towards movement of water can continue in an adjacent vessel.
• Vessels also lose their end walls, so they form a continuous column for water movement from root to leaves.

Question 4

What is the process of transpiration?

Answer 4

• Transpiration occurs in the leaves.
• Water is evaporated from the mesophyll cells of the leaf, reducing their water potential.
• Water vapor forms in the air spaces which then diffuses out through open stomata.
• A water potential gradient is formed across the leaf. Water leaves the xylem vessels in the leaf and diffuses into mesophyll cells by osmosis, replacing the lost water.
• This creates negative pressure, or tension, at the top of the xylem vessels.
• The remaining water in the xylem is under tension and is pulled up towards the leaves.
• Continuous columns of water are maintained due to cohesion between water molecules.
• There is also adhesion of water molecules to the walls of the xylem vessels. This creates an inward pull on the vessel walls as the water is pulled up, causing the xylem vessels to decrease in diameter.

Question 5

What is the evidence for transpiration?

Answer 5

• Tension has been measured in xylem as plants respire.
• If a column of water in the xylem is broken, air bubbles in the xylem form and this stops any further upward movement of water in that xylem vessel as the air bubbles prevent cohesion.
• Respiratory inhibitors such as cyanide or a lack of oxygen do not inhibit this process.
• The diameter of trees decrease when they are transpiring as tension pulls xylem walls in and more so when temperatures and light intensities are higher.
• This can be measured using a dendrometer.

Question 6

What a potometer?

Answer 6

• The potometer can be used to investigate the rate of transpiration.
• The apparatus must be airtight so all joins are sealed with vaseline.
• The shoot is cut underwater to prevent air entering xylem vessels.
• The distance moved by the air bubble in a known length of time can be used to estimate the rate of transpiration.
• The diameter of the capillary tube will be needed to calculate the volume.

Question 7

What are some adaptations of xerophytes?

Answer 7

• Include:
- Reducing the water potential gradient.
- Increasing the length of the diffusion pathway.
- So decreasing the rate of water from the leaf.

• Examples:
- Hairy leaves.
- Stomata sunken in pits.
- Thick waxy cuticle.

Question 8

What are features of the phloem tissue?

Answer 8

• Each sieve tube element links to the next via a sieve plate which is perforated with pores.
• The sieve tube has little cytoplasm, no nucleus, no vacuole and few organelle other than a small number of mitochondria.
• The sieve tubes are alive because of cytoplasmic connections (plasmodesmata) with the companion cell.
• Each companion cell has a nucleus, many mitochondria and other organelles.

Question 9

What is the process of translocation?

Answer 9

• At the source, sucrose is actively transported from mesophyll cells to the phloem via companion cells.
• This lowers the water potential of the phloem, so water enters the phloem by osmosis from the xylem down a water potential gradient.
• This increases the volume which increases the hydrostatic pressure.
• Sucrose solution moves by mass flow down a pressure gradient to the sink.
• At the sink, sucrose is absorbed and hydrolysed into glucose to be used in respiration.
• This increases the water potential of the phloem, so water moves back to the xylem from the phloem by osmosis.
• This decreases the hydrostatic pressure.