Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
146
water and nutrients, especially phosphate, from the soil
and transport them into the plant. Some trees, if growing
on poor soils, are unable to survive without these fungi.
In return, the fungi receive organic nutrients from the
plant. The name given to a relationship such as this, in
which two organisms of different species both benefit,
is mutualism.
a
b
thin, permeable,
cellulose cell wall
partially
permeable
cell surface
membrane
dilute soil
solution – higher
water potential
more concentrated
solution in cytoplasm
and vacuole – lower
water potential
osmosis –
water
diffusing
down the
water
potential
gradient
Transport of mineral ions
Apart from the carbohydrates made in photosynthesis,
plants need a supply of mineral ions to complete their
nutrition. Examples are nitrate, phosphate, sulfate,
potassium, magnesium and calcium.
Mineral ions in solution are absorbed along with water
by the roots, particularly by the root hairs. Their route
through the plant is the same as that for water, crossing
the root by apoplastic and symplastic pathways before
moving in the mass flow of xylem sap up the xylem to the
rest of the plant. From the xylem they enter the apoplastic
and symplastic pathways again.
As well as moving by mass flow through the apoplastic
pathway and xylem, mineral ions can also move by
diffusion and active transport. For example, they can
diffuse into the apoplastic pathway of the root from the
soil and once in the apoplastic pathway can diffuse in any
direction according to concentration gradients. They can
also enter cells by the methods described in Chapter 4,
namely diffusion, facilitated diffusion and active transport.
Facilitated diffusion and active transport allow cells to
control what ions enter or leave cells. One important
control point is the root endodermis, where the Casparian
strip forces ions to pass through living cells before they
can enter the xylem, as discussed on page 145.
Translocation
The term translocation can be applied to transport in
both xylem and phloem – it means literally moving from
one place to another. It tends to be used more commonly
to describe the transport of soluble organic substances
within a plant. These are substances which the plant
itself has made – for example, sugars which are made
by photosynthesis in the leaves. These substances are
sometimes called assimilates.
Assimilates are transported in sieve elements.
Sieve elements are found in phloem tissue, along with
several other types of cells including companion cells,
parenchyma and fibres (Figures 7.30 and 7.31). For phloem,
as with xylem, understanding the structure of the tissue
is essential for understanding how transport within the
tissue occurs, so we will study its structure first.
Figure 7.29 a A root of a young radish (Raphanus) plant
showing the root cap and the root hairs. b Water uptake by a
root hair cell. Mineral ions are also taken up, mostly by active
transport against the concentration gradient via carrier
proteins, but also by diffusion.
QUESTION
7.10 Give an example of an organic molecule containing:
a nitrogen
b phosphorus
c sulfur.