Cambridge International A Level Biology Revision Guide

Chapter 7: Transport in plants
continuous columns. It is just like sucking water up a
straw. When you suck a straw, you reduce the pressure
at the top of the straw, causing a pressure difference
between the top and bottom. The higher pressure at the
bottom pushes water up the straw. Note that the lower
the hydrostatic pressure, the lower the water potential, so
a hydrostatic pressure gradient is also a water potential
gradient.
The water in the xylem vessels, like the liquid in a
‘sucked’ straw, is under tension. If you suck hard on a
straw, its walls may collapse inwards as a result of the
pressure differences you are creating. Xylem vessels have
strong, lignified walls to stop them from collapsing in
this way.
The movement of water up through xylem vessels is
by mass flow. This means that all the water molecules
(and any dissolved solutes) move together, as a body of
liquid, like water in a river. This is helped by the fact that
water molecules are attracted to each other by hydrogen
bonding (page 36); this attraction is called cohesion.
They are also attracted to the cellulose and lignin in the
walls of the xylem vessels, and this attraction is called
adhesion. Cohesion and adhesion help to keep the water
in a xylem vessel moving as a continuous column. The
vessels are full of water. The fact that the cells are dead is
an advantage, because it means there is no protoplasm to
get in the way of transport.
If an air bubble forms in the column, the column of
water breaks and the difference in pressure between the
water at the top and the water at the bottom cannot be
transmitted through the vessel. We say there is an air lock.
The water stops moving upwards. The small diameter of
xylem vessels helps to prevent such breaks from occurring.
Also, the pits in the vessel walls allow water to move
QUESTIONS
7.8 In plants, transport of water from the environment
to cells occurs in several ways. State the stages in the
following types of transport:
a osmosis
b mass flow.
7.9 Explain how each of the following features adapts
xylem vessels for their function of transporting water
from roots to leaves.
a total lack of cell contents
b no end walls in individual xylem elements
c a diameter of between 0.01 mm and 0.2 mm
d lignified walls
e pits
out into neighbouring vessels and so bypass such an air
lock. Air bubbles cannot pass through pits. Pits are also
important because they allow water to move out of xylem
vessels to surrounding living cells.
The xylem tissue in dicotyledonous stems is arranged
in a series of rods around the centre of the stem, as shown
in Figure 7.24. These strong rods help to support the stem.
vascular
bundle
pith
epidermis
collenchyma
tissue
parenchyma
phloem
lignified fibres
xylem
cortex
Figure 7.24 TS of a young sunflower (Helianthus) stem to show
the distribution of tissues. The sunflower is a dicotyledonous
plant.
Root pressure
You have seen how transpiration reduces the water
(hydrostatic) pressure at the top of a xylem vessel
compared with the pressure at the base, so causing the
water to flow up the vessels. Plants may also increase the
pressure difference between the top and bottom by raising
the water pressure at the base of the vessels.
The pressure is raised by the active secretion of solutes,
for example mineral ions, into the water in the xylem
vessels in the root. Cells surrounding the xylem vessels use
energy to pump solutes across their membranes and into
the xylem by active transport (page 86). The presence of
the solutes lowers the water potential of the solution in the
xylem, thus drawing in water from the surrounding root
cells. This influx of water increases the water pressure at
the base of the xylem vessel.
Although root pressure may help in moving water
up xylem vessels, it is not essential and is probably not
significant in causing water to move up xylem in most
plants. Water can continue to move up through xylem
even if the plant is dead. Water transport in plants is
largely a passive process, driven by transpiration from the
leaves. The water simply moves down a continuous water
potential gradient from the soil to the air.
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