Translocation

Question 1

Describe the function of phloem tissue.

Answer 1

Transports organic substances eg. sucrose in plants.

Question 2

Suggest how phloem tissue is adapted for its function.

Answer 2

1. Sieve tube elements
   - No nucleus / few organelles → maximise space for / easier flow of organic substances
   - End walls between cells perforated (sieve plate) -> allows for a continuous flow of the sugar solution.
2. Companion cells
   - Many mitochondria → high rate of respiration to make ATP for active transport of solutes

Question 3

What is the phloem tissue made up of?

Answer 3

Companion cells
Sieve tube elements

Question 4

What is translocation?

Answer 4

Movement of solutes such as sucrose from source cells to sink cells by mass flow.

Question 5

Explain the mass flow hypothesis for translocation in plants.

Answer 5

1. At source, sucrose is actively transported into phloem sieve tubes / cells
2. By companion cells
3. This lowers water potential in sieve tubes so water enters (from xylem) by osmosis
4. This increases hydrostatic pressure in sieve tubes (at source) / creates a hydrostatic pressure gradient
5. So mass flow occurs - movement from source to sink
6. At sink, sucrose is removed by active transport to be used by respiring cells or stored in storage organs

Question 5

Describe the use of tracer experiments to investigate transport in plants.

Answer 5

1. Leaf supplied with a radioactive tracer eg. CO2 containing radioactive isotope
   14C
2. Radioactive carbon incorporated into organic substances during photosynthesis
3. These move around plant by translocation
4. Movement tracked using autoradiography or a Geiger counter.

Question 6

Describe the use of ringing experiments to investigate transport in plants.

Answer 6

1. Remove / kill phloem eg. remove a ring of bark
2. Bulge forms on source side of ring
3. Fluid from bulge has higher conc. of sugars than below - shows sugar is transported in phloem
4. Tissues below ring die as cannot get organic substances

Question 7

Suggest some points to consider when interpreting evidence from tracer & ringing experiments and evaluating evidence for / against the mass flow hypothesis.

Answer 7

● Is there evidence to suggest the phloem (as opposed to the xylem) is involved ?
● Is there evidence to suggest respiration / active transport is involved?
● Is there evidence to show movement is from source to sink? What are these in the experiment?
● Is there evidence to suggest movement is from high to low hydrostatic pressure?
● Could movement be due to another factor eg. gravity?

Question 8

What do cells need to respire?

Answer 8

Sugar

Question 9

What is one of the key functions of the leaves?

Answer 9

Photosynthesis.