Size matters II Bill Indge Size matters Water

Size matters II Bill Indge

Size matters Water and inorganic ions transported in xylem Products of photosynthesis transported in phloem

Size matters Cohesion–tension ψ = – 30 MPa Water evaporates ψ = – 3 MPa

Size matters Cohesion–tension ψ = – 30 MPa Water evaporates ψ = – 3 MPa Pulls water up xylem Water molecules stick together — cohesion Column under tension

Size matters Source Phloem Carbohydrate produced by photosynthesis in leaves Sink

Size matters Source Phloem Carbohydrate actively transported into phloem Sink

Size matters Source Phloem Water moves into phloem down water potential gradient Sink

Size matters Source Phloem Increase in pressure forces contents down phloem Sink

Size matters Source Phloem Sucrose removed from phloem in roots and other parts of the plant Sink

Size matters Source Phloem Water follows by osmosis Sink

Size matters Transport in 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 Transport in 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 Percentage of sap feeders 100 Phloem feeders 80 Xylem feeders 60 40 20 0 1 3 5 9 14 22 34 53 81 124 Body length/mm (i) Describe the results shown in the graph. (1 mark)

Size matters Percentage of sap feeders 100 80 • Most phloem feeders are small; • Percentage of sap feeders feeding from phloem falls with increasing body length 60 40 20 0 1 3 5 9 14 22 34 53 Body length / mm (i) Describe the results shown in the graph. 81 124 (1 mark)

Size matters Percentage of sap feeders 100 • Most xylem feeders are large; • 80 Percentage of sap feeders feeding from xylem increases as body length increases; 60 40 20 0 1 3 5 9 14 22 34 53 Body length / mm (i) Describe the results shown in the graph. 81 124 (1 mark)

Size matters (ii) The mass of the muscles concerned with the 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 (ii) The mass of the muscles concerned with the 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) 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;