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;