Cambridge International A Level Biology Revision Guide

Chapter 7: Transport in plants
centre. Once inside the xylem vessels, the water moves
upwards through the root to the stem and from there into
the leaves.
From leaf to atmosphere – transpiration
Figure 7.15 shows the internal structure of a
dicotyledonous leaf. The cells in the mesophyll (‘middle
leaf’) layers are not tightly packed, and have many spaces
around them filled with air. The walls of the mesophyll
cells are wet, and some of this water evaporates into the air
spaces (Figure 7.16), so that the air inside the leaf is usually
saturated with water vapour.
The air in the internal spaces of the leaf has direct
contact with the air outside the leaf, through small pores
called stomata. If there is a water potential gradient
between the air inside the leaf (higher water potential) and
the air outside (lower water potential), then water vapour
will diffuse out of the leaf down this gradient.
Although some of the water in the leaf will be used, for
example, in photosynthesis, most eventually evaporates
and diffuses out of the leaf by the process of transpiration.
The distribution of stomata in the epidermis of leaves
can be seen by examining ‘epidermal peels’ or ‘epidermal
impressions’ (Box 7.2).
Transpiration is the loss of water vapour from a plant
to its environment, by diffusion down a water potential
gradient; most transpiration takes place through the
stomata in the leaves.
QUESTION
7.2 Most stomata are usually found in the lower
epidermis of leaves. Suggest why this is the case.
Privet leaf
upper epidermis
vascular bundle
xylem
phloem
palisade
mesophyll
spongy
mesophyll
135
cuticle
stoma
upper
epidermis
palisade
mesophyll
lower epidermis
spongy
mesophyll
lower
epidermis
guard cell
stoma
air space
chloroplast
Figure 7.15 The structure of a dicotyledonous leaf. Water enters the leaf as liquid water in the xylem vessels and diffuses out as
water vapour through the stomata.