Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
those of phloem sieve elements form sieve plates. It
is calculated that without the resistance of the sieve
plates along the pathway, the steep positive pressure
gradient inside the sieve tubes would quickly be lost,
with the different pressures at source and sink quickly
equilibrating. Xylem on the other hand has to withstand
high negative pressure (tension) inside its tubes and
buckling is prevented by its lignified walls.
Sieve plates also allow the phloem to seal itself up
rapidly with callose if damaged – for example, by a grazing
herbivore – rather as a blood vessel in an animal is sealed
by clotting. Phloem sap has a high turgor pressure because
of its high solute content, and would leak out rapidly if the
holes in the sieve plate were not quickly sealed. Phloem
sap contains valuable substances such as sucrose, which
the plant cannot afford to lose in large quantity. The
‘clotting’ of phloem sap may also help to prevent the entry
of microorganisms which might feed on the nutritious sap
or cause disease.
QUESTION
7.13 Draw up a comparison table between xylem vessels
and sieve tubes. Some features which you could
include are: cell structure (walls, diameter, cell
contents, etc.), substances transported, methods and
direction of transport. Include a column giving a brief
explanation for the differences in structure.
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Summary
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Multicellular organisms with small surface area : volume
ratios need transport systems. Flowering plants do
not have compact bodies like those of animals. They
spread and branch above and below ground to obtain
the carbon dioxide, light energy, water and mineral
ions needed for nutrition. Plants do not need systems
for transporting carbon dioxide or oxygen – diffusion is
sufficient.
Water and mineral salts are transported through a plant
in xylem vessels. Movement of water through a plant
is a passive process in which the water moves down
a water potential gradient from soil to air. The energy
for this process comes from the Sun, which causes
evaporation of water from the wet walls of mesophyll
cells in leaves. Water vapour in the air spaces of the leaf
diffuses out of the leaf through stomata, in a process
called transpiration. This loss of water sets up a water
potential gradient throughout the plant.
Transpiration is an inevitable consequence of gaseous
exchange in plants. Plants need stomata so that
carbon dioxide and oxygen can be exchanged with
the environment. The rate of transpiration is affected
by several environmental factors, in particular
temperature, light intensity, wind speed and humidity.
It is diffcult to measure rate of transpiration directly, but
water uptake can be measured using a potometer.
Plants that are adapted to live in places where the
environmental conditions are likely to cause high
rates of transpiration, and where soil water is in short
supply, are called xerophytes. Xerophytes have evolved
adaptations that help to reduce the rate of loss of water
vapour from their leaves.
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Water enters the plant through root hairs by osmosis.
Water crosses the root either through the cytoplasm of
cells (the symplastic pathway) or via their cell walls (the
apoplastic pathway), and enters the dead, empty xylem
vessels. Water also moves across the leaf by symplast
and apoplast pathways.
Water and mineral ions (xylem sap) move up xylem
vessels by mass flow, as a result of pressure differences
caused by loss of water from leaves by transpiration.
Root pressure can also contribute to this pressure
difference. Movement in the xylem is in one direction
only, from roots to the rest of the plant.
Translocation of organic solutes such as sucrose occurs
through living phloem sieve tubes. Phloem sap moves
by mass flow from a region known as the source to a
region known as the sink.
Sucrose is produced at the source (e.g. photosynthesising
leaves) and used at the sink (e.g. a flower or a storage
organ). Mass flow occurs as a result of pressure
differences between the source and the sink. Active
loading of sucrose into the sieve tubes at the source
results in the entry of water by osmosis, thus creating a
high hydrostatic pressure in the sieve tubes. Phloem sap
can move in different directions in different sieve tubes.
Both xylem vessels and phloem sieve tubes show
unique structural features and distributions which are
adaptations to their roles in transport.