Cambridge International A Level Biology Revision Guide

Chapter 7: Transport in plants
Solute
Concentration / mol dm −3
sucrose 250
potassium ions 80
amino acids 40
chloride ions 15
phosphate ions 10
magnesium ions 5
sodium ions 2
ATP 0.5
nitrate ions 0
plant growth substances
(e.g. auxin, cytokinin)
small traces
Table 7.3 Composition of phloem sap.
QUESTION
7.11 Which of the substances listed in Table 7.3 have been
synthesised by the plant?
stylet penetrates a
single sieve tube
in a vascular
bundle
aphid stylet has
moved between
cortical cells
aphid
mouthparts
How translocation occurs
Phloem sap, like the contents of xylem vessels, moves
by mass flow, as the aphid experiment shows. Mass flow
moves organic solutes about 1 m h −1 on average, about
10 000 times faster than diffusion would. Whereas in
xylem vessels, the difference in pressure that causes mass
flow is produced by a water potential gradient between
soil and air, requiring no energy input from the plant,
this is not so in phloem transport. To create the pressure
differences needed for mass flow in phloem, the plant has
to use energy. Phloem transport is therefore an active
process, in contrast to the passive transport in xylem.
The pressure difference is produced by active loading
of sucrose into the sieve elements at the place from which
sucrose is to be transported. Any area of a plant in which
sucrose is loaded into the phloem is called a source. This
is usually a photosynthesising leaf or a storage organ. Any
area where sucrose is taken out of the phloem is called a
sink – for example, the roots.
Loading a high concentration of sucrose into a sieve
element greatly decreases the water potential in the sap
inside it. Therefore, water enters the sieve element, moving
down a water potential gradient by osmosis. This causes
a correspondingly high build up in pressure (equivalent
to about six times atmospheric pressure). The pressure
is referred to as hydrostatic pressure, turgor pressure or
pressure potential. A pressure difference is therefore created
between the source and the sink. This pressure difference
causes a mass flow of water and dissolved solutes through
the sieve tubes, from the high pressure area to the low
pressure area (Figure 7.33). At the sink, sucrose may be
149
vascular
bundle
cortex
epidermis
anaesthetised
aphid removed;
phloem sap
exudes from
broken stylet
Figure 7.32 Using an aphid to collect phloem sap.
Figure 7.33 The phloem sap of the sugar maple (Acer
saccharum) contains a high concentration of sugar and can be
harvested to make maple syrup. Taps are inserted into each
tree and the sap runs out under its own pressure through the
plastic pipelines.