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

100

80

60

40

20

0

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

100

80

60

40

20

0

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

100

80

60

40

20

0

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)