Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
Figure 15.35 The leaves of the Venus fly trap, Dionaea muscipula, have a group of stiff, sensitive hairs in their centres. When these
are touched, the leaves respond by closing, trapping whatever was crawling over them. Digestive juices are then secreted, and the
soluble products absorbed into the leaf cells.
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lobes to cells underneath. Instead, it is likely that the rapid
change occurs as a result of a release of elastic tension in
the cell walls.
However, the trap is not completely closed at this
moment. To seal the trap, it requires ongoing activation
of the trigger hairs by the trapped prey. Unless the prey is
able to escape, it will further stimulate the inner surface of
the lobes, thereby triggering further action potentials. This
forces the edges of the lobes together, sealing the trap to
form an external ʻstomachʼ in which prey digestion occurs.
Further deflections of the sensory hairs by the trapped
insect stimulate the entry of calcium ions into gland cells.
Here, calcium ions stimulate the exocytosis of vesicles
containing digestive enzymes in a similar way to their role
in synapses (page 341). The traps stay shut for up to a week
for digestion to take place. Once the insect is digested, the
cells on the upper surface of the midrib grow slowly so
the leaf reopens and tension builds in the cell walls of the
midrib so the trap is set again.
Venus fly traps have two adaptations to avoid closing
unnecessarily and wasting energy. First, the stimulation
of a single hair does not trigger closure. This prevents the
traps closing when it rains or when a piece of debris falls
into the trap. Second, the gaps between the stiff hairs that
form the ‘bars’ of the trap allow very small insects to
crawl out. The plant would waste energy digesting a very
small ‘meal’.
Chemical communication in plants
Chemicals known as plant hormones or plant growth
regulators are responsible for most communication within
plants. Unlike animal hormones, plant growth regulators
are not produced in specialised cells within glands, but in
a variety of tissues. They move in the plant either directly
from cell to cell (by diffusion or active transport) or are
carried in the phloem sap or xylem sap. Some may not
move far from their site of synthesis and may have their
effects on nearby cells.
Here we consider two types of plant growth regulator:
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auxins, which influence many aspects of growth
including elongation growth which determines the
overall length of roots and shoots
gibberellins, which are involved in seed germination
and controlling stem elongation.
Abscisic acid (ABA) is another plant hormone, which
controls the response of plants to environmental stresses
such as shortage of water (Chapter 14, page 323).
Plant hormones interact with receptors on the surface
of cells or in the cytoplasm or nucleus. These receptors
usually initiate a series of chemical or ionic signals that
amplify and transmit the signal within the cell in much
the same way that we saw in Chapters 4 and 14.