Transport of organic substances in the phloem

Question 1

What is translocation?

Answer 1

Where organic molecules e.g. sucrose, amino acids and some mineral ions e.g. K, Cl are transported from one part to another part of the plant.

Question 2

What is the tissue which transports biological molecules?

Answer 2

Phloem, it is made up of sieve tube elements and companion cells.

Question 3

Where do plants transfer their sugar to and from?

Answer 3

Sugar which is produced during photosynthesis is transported from the site of production - (source) to where it’s used or stored - (sink).

Question 4

How is sucrose transferred into sieve tube elements from photosynthesising tissue?

How does the mass flow of sucrose through sieve tube element work?

How is sucrose transferred from the sieve tube elements into storage or other sink cells?

Answer 4

• Sucrose is made from photosynthesis in cells with chloroplast.
• Sucrose diffuses into the companion cell down the concentration gradient by facilitated diffusion.
• H+ ions are actively transported from companion cells into the spaces within cell walls using ATP.
• The H+ ions are diffused down a concentration gradient through carrier proteins into the sieve tube element.
• Sucrose molecules are transported along with H+ ions due to co-transport. The protein is co-transport proteins.
• Due to the transportation of the sucrose, it causes sieve tube elements to have a lower water potential.
• As xylem has a much higher water potential, water moves from the xylem into the sieve tube element by osmosis, creating a high hydrostatic pressure within them.
• At the sink cell, sucrose is either used up for respiration or converted into starch for storage.
• These cells, therefore, would have a low sucrose content so sucrose is actively transported from the sieve tube element which lowers the water potential in the sink cells.
• Due to this lowered water potential, water moves into those respiring cells from the sieve tube by osmosis.
• The hydrostatic pressure in the sieve tube is therefore lowered.
• As a result, water enters the sieve tube at the source and leaves at the sink, there’s high hydrostatic pressure at source and low at the sink.
• Therefore there is a mass flow of sucrose solution down the hydrostatic gradient in the sieve tubes.
• The sucrose is actively transported by companion cells at the sink from the sieve tube element.

Question 5

Is mass flow a passive or active process?

Answer 5

Passive but it occurs due to active transport of sugars so the overall process is active and can be affected by temperature and metabolic factors.