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Size matters IIBill Indge
Size matters
Water and inorganic ions transported
in xylem
Products of photosynthesis
transportedin phloem
Size matters
Water evaporates
ψ = –3 MPa
ψ = –30 MPa
Cohesion–tension
Size matters
Water evaporates
ψ = –3 MPa
ψ = –30 MPa
Pulls water up xylem
Water molecules stick together — cohesion
Column under tension
Cohesion–tension
Size matters
Source
Sink
PhloemCarbohydrate produced by
photosynthesis in leaves
Size matters
Source
Sink
PhloemCarbohydrate actively
transported into phloem
Size matters
Source
Sink
PhloemWater moves into
phloem down water potential gradient
Size matters
Source
Sink
Phloem
Increase in pressure forces contents down
phloem
Size matters
Source
Sink
Phloem
Sucrose removed from phloem in roots and other parts of the
plant
Size matters
Source
Sink
PhloemWater follows by
osmosis
Size matters
Transportin phloem
(a) Aphids feed on phloem sap. If a feeding aphid is anaesthetised and its body cut off, phloem sap continues to flow out through the mouthparts. Use your knowledge of the way in which phloem transports sap to explain why. (2 marks)
Answer (a)•Sap in phloem is under pressure/at high pressure;•Does not require aphid muscle contraction so continues to flow;
(2 marks)
Size matters
Transportin xylem
(b) When a stem is cut under a solution of coloured dye, dye enters the xylem in the cut ends of the stem. Use your knowledge of the way in which sap travels through xylem to explain why. (3 marks)
Answer (b)•Sap in xylem is under tension;•Tension released when stem is cut;•Sap column contracts;•Pulls dye into xylem; (3 marks)
Size matters
Some insects feed on plant sap. These insects differ in size. Some, such as aphids, are very small.Adult willow aphids, as shown here, are approximately 4 mm long.
Size matters
Others, such as the cicadas, are larger. This species is 3–4 cm long.
Size matters
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Phloem feeders
Xylem feeders
Body length/mm 1 3 5 9 14 22 34 53 81 124
(i) Describe the results shown in the graph. (1 mark)
Size matters
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60
40
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Body length / mm 1 3 5 9 14 22 34 53 81 124
• Most phloem feeders are small;
• Percentage of sap feeders feeding from phloem falls with increasing body length
(i) Describe the results shown in the graph. (1 mark)
Size matters
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80
60
40
20
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Body length / mm 1 3 5 9 14 22 34 53 81 124
• Most xylem feeders are large;• Percentage of sap feeders
feeding from xylem increases as body length increases;
(i) Describe the results shown in the graph. (1 mark)
Size matters
(ii) The mass of the muscles concerned withthe mouthparts is directly proportional to body length in these insects. Use this information and your knowledge of the way in which sap is transported to suggest an explanation for the difference in body length of phloem feeders and xylem feeders. (4 marks)
Answer (ii)•Sap under pressure in phloem;•Therefore will flow along mouthparts/into gut of insect without muscle action;•Muscles associated with feeding small allowing for small body size;•But sap under tension in xylem;
Size matters
Alternative answer (ii)•Sap under tension in xylem;•Needs muscle action to overcome this tension;•Muscles associated with feeding are large, suggesting large body size;•But sap under pressure in phloem;
(ii) The mass of the muscles concerned withthe mouthparts is directly proportional to body length in these insects. Use this information and your knowledge of the way in which sap is transported to suggest an explanation for the difference in body length of phloem feeders and xylem feeders. (4 marks)