Cambridge International A Level Biology Revision Guide

Chapter 14: Homeostasis
The control of blood glucose
Carbohydrate is transported through the human
bloodstream in the form of glucose in solution in the
blood plasma. Glucose is converted into the polysaccharide
glycogen, a large, insoluble molecule made up of many
glucose units linked together by 1–4 glycosidic bonds with
1–6 branching points (page 33). Glycogen is a short-term
energy store that is found in liver and muscle cells and is
easily converted to glucose (Figure 14.24, page 317).
In a healthy human, each 100 cm 3 of blood
normally contains between 80 and 120 mg of glucose.
If the concentration decreases below this, cells may not
have enough glucose for respiration, and may be unable
to carry out their normal activities. This is especially
important for cells that can respire only glucose, such as
brain cells. Very high concentrations of glucose in the
blood can also cause major problems, again upsetting
the normal behaviour of cells. The homeostatic control
of blood glucose concentration is carried out by two
hormones secreted by endocrine tissue in the pancreas.
This tissue consists of groups of cells, known as the
islets of Langerhans, which are scattered throughout
the pancreas. The word islet means a small island, as you
might find in a river. The islets contain two types of cells:
■■
α cells secrete glucagon
■■
β cells secrete insulin.
The α and β cells act as the receptors and the central
control for this homeostatic mechanism; the hormones
glucagon and insulin coordinate the actions of the
effectors.
Figure 14.21 shows how the blood glucose concentration
fluctuates within narrow limits around the set point,
which is indicated by the dashed line.
After a meal containing carbohydrate, glucose from
the digested food is absorbed from the small intestine
and passes into the blood. As this blood flows through
the pancreas, the α and β cells detect the increase in
glucose concentration. The α cells respond by stopping
the secretion of glucagon, whereas the β cells respond by
secreting insulin into the blood plasma. The insulin is
carried to all parts of the body, in the blood.
Insulin is a signalling molecule. As it is a protein, it
cannot pass through cell membranes to stimulate the
mechanisms within the cell directly. Instead, insulin binds
to a receptor in the cell surface membrane and affects
the cell indirectly through the mediation of intracellular
messengers (Figure 14.22 and pages 77–79).
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set point
Figure 14.21 The control mechanism for the concentration of glucose in the blood.