Topic 3.4.2 Mass transport in plants

Question 1

Describe the function of xylem tissue

Answer 1

Transports water (and mineral ions) through the stem, up the plant to leaves of plants

Question 2

Suggest how xylem tissue is adapted for its function

Answer 2

● Cells joined with no end walls forming a long continuous tube → water flows as a continuous column
● Cells contain no cytoplasm / nucleus → easier water flow / no obstructions
● Thick cell walls with lignin → provides support / withstand tension / prevents water loss
● Pits in side walls → allow lateral water movements

Question 3

Explain the cohesion-tension theory of water transport in the xylem

Answer 3

1. Leaf :
2. Water lost from leaf by transpiration - water evaporates from mesophyll
   cells into air spaces and water vapour diffuses through (open) stomata
3. Reducing water potential of mesophyll cells
4. So water drawn out of xylem down a water potential gradient
5. Xylem :
6. Creating tension (‘negative pressure’ or ‘pull’) in xylem
7. Hydrogen bonds result in cohesion between water molecules (stick
   together) so water is pulled up as a continuous column
8. Water also adheres (sticks to) to walls of xylem
9. Root :
10. Water enters roots via osmosis

Question 4

Describe how to set up a potometer

Answer 4

1. Cut a shoot underwater at a slant →
   prevent air entering xylem
2. Assemble potometer with capillary tube
   end submerged in a beaker of water
3. Insert shoot underwater
4. Ensure apparatus is watertight / airtight
5. Dry leaves and allow time for shoot to
   acclimatise
6. Shut tap to reservoir
7. Form an air bubble - quickly remove end
   of capillary tube from water

Question 5

Describe how a potometer can be
used to measure the rate of transpiration

Answer 5

estimates transpiration rate by measuring water uptake:

1. Record position of air bubble
2. Record distance moved in a certain amount of time (eg. 1 minute)
3. Calculate volume of water uptake in a given time:
   ○ Use radius of capillary tube to calculate cross-sectional area of water (πr2)
   ○ Multiply this by distance moved by bubble
4. Calculate rate of water uptake - divide volume by time taken

Question 6

Describe how a potometer can be used to investigate the effect of a named
environmental variable on the rate of transpiration

Answer 6

● Carry out the above, change one variable at a time (wind, humidity, light or temperature)
○ Eg. set up a fan OR spray water in a plastic bag and wrap around the plant OR change
distance of a light source OR change temperature of room
● Keep all other variables constant

Question 7

Suggest limitations in using a potometer to measure rate of transpiration

Answer 7

● Rate of water uptake might not be same as rate of transpiration
○ Water used for support / turgidity
○ Water used in photosynthesis and produced during respiration

● Rate of movement through shoot in potometer may not be same as
rate of movement through shoot of whole plant
○ Shoot in potometer has no roots whereas a plant does
○ Xylem cells very narrow

Question 8

Suggest how different environmental variables affect transpiration rate

Answer 8

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Question 9

Describe the function of phloem tissue

Answer 9

Transports organic substances eg. sucrose in plants

Question 10

Suggest how phloem tissue is adapted for its function

Answer 10

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

Question 11

What is translocation?

Answer 11

● Movement of assimilates / solutes such as sucrose
● From source cells (where made, eg. leaves) to sink cells (where used / stored, eg. roots) by mass flow

Question 12

Explain the mass flow hypothesis for translocation in plants

Answer 12

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 13

Describe the use of tracer experiments to investigate transport in plants

Answer 13

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 14

Describe the use of ringing experiments to investigate transport in plants

Answer 14

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 15

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

Answer 15

● Could movement be due to another factor eg. gravity?
● Is there evidence to suggest the phloem (as opposed to the xylem) is involved ?
● Is there evidence to suggest respiration / active transport is involved?