Translocation

Question 1

Explain the mass flow hypothesis for translocation in plants

Answer 1

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

Question 2

Describe the function of the phloem tissue

Answer 2

Transports organic substances e.g sucrose in plants

Question 3

Suggest how the phloem tissue is adapted to its function

Answer 3

Sieve tube elements contain no nucleus / few organelles, maximises space for transport of organic substances
- End walls between cells perforated

Companion Cells
- Many mitochondria - high rate of respiration to make ATP for active transport of solutes

Question 4

What is translocation?

Answer 4

Movement of assimilates / solutes such as sucrose
From source cells (where synthesized/made) to sink cells (where used)

Question 5

Describe the use of ringing experiments to investigate transport in plants

Answer 5

1) Remove / kill phloem
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 6

Describe the use of tracer experiments to investigate transport in plants

Answer 6

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

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?