Agriculture Reference
In-Depth Information
Plants, like people, are dependent on the transport
of substances around their structure. Sugars made
in the leaves by photosynthesis must be transported
to roots, stems, shoots and flowers where they can
be used for immediate growth or stored until they
are required for future growth. Water is essential for
many processes in the plant and, since it is absorbed
through the roots, it has to be moved to cells in
other plant organs where it is essential for support,
as a medium for chemical reactions and as a raw
material for processes such as photosynthesis. The
two 'superhighways' along which sugars and water
flow, the phloem and xylem, respectively, are also
responsible for moving other vital substances such as
essential mineral nutrients and plant hormones around
the plant body. In this chapter we examine how water
is moved both through the plant and from cell to cell
and how it is lost from the leaves. The transport of
sugars and mineral nutrients is also described.
The structure of xylem and phloem is discussed in
Chapter 6.
(a) Diffusion
Direction of movement of diffusing substance
(b) Osmosis
Selectively
permeable
membrane
Outside cell
Inside cell
Water
Direction of movement of water
As
water
is the major constituent of any living
organism, then the maintenance of a plant with
optimum water content is a very important part of
plant growth and development. There is a tendency
sometimes to overwater, but probably more plants die
from lack of water than from any other cause (see p.
151).
Solute
Figure 10.2
Diagrammatic representations of: (a)
diffusion; (b) osmosis
concentration (higher solute concentration),
through a
selectively permeable membrane
,
such as the cell membrane (Figure 10.2b).
Osmosis is in effect the diffusion of water across
a membrane and is the method by which water
enters cells. A selectively permeable membrane
allows passage of some dissolved substances
but not others and the term 'solute' refers to the
substances dissolved in the water.
Movement of substances in the
plant
Diffusion and osmosis
Two ways in which substances move in the plant are
described below:
X
Diffusion
is a process whereby molecules of a liquid
or a gas move from an area of high concentration
to an area of lower concentration of the diffusing
substance. For example, sugar in a cup of tea will
diffuse through the tea without being stirred -
eventually! (Figure 10.2a) Examples of diffusion
in the plant include the movement of gases such
as water vapour (see transpiration p. 123), carbon
dioxide and oxygen (see photosynthesis and
respiration, Chapter 9) into and out of the leaf.
Osmosis is a special kind of diffusion where water
is the diffusing substance (see below).
X
Osmosis
is defi ned as the movement of
water
from an area of high water (low solute)
concentration to an area of lower water
Diffusion
is the movement of a substance from
a high concentration to a lower concentration.
Osmosis
is the movement of water from a high
water (low solute) concentration to a low water
(high solute concentration) across a selectively
permeable membrane.
When water moves into the cell by osmosis it swells
like a balloon. This inner pressure, caused by the water
pushing outwards, is called
turgor pressure
and is
very important in providing support to young plants
and non-woody herbaceous plants. The plant and
