Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
318
The hormone adrenaline also increases the concentration
of blood glucose. It does this by binding to different receptors
on the surface of liver cells that activate the same enzyme
cascade and lead to the same end result – the breakdown
of glycogen by glycogen phosphorylase. Adrenaline also
stimulates the breakdown of glycogen stores in muscle
during exercise. The glucose produced remains in the muscle
cells where it is needed for respiration.
QUESTIONS
14.8 The control of blood glucose concentration involves a
negative feedback mechanism.
a What are the stimuli, receptors and effectors in
this control mechanism?
b Explain how negative feedback is involved in this
homeostatic mechanism. (You may have to look
back to page 301.)
14.9 a Name the process by which glucose enters and
leaves cells.
b Explain why:
i muscle cells do not have receptors for
glucagon
ii there are second messengers for insulin
and glucagon
ii insulin and glucagon have different second
messengers.
Diabetes mellitus
Sugar diabetes, or diabetes mellitus, is one of the most
common metabolic diseases in humans. In 2013, the
International Diabetes Federation estimated that
382 million people, or approximately 8.3% of the world’s
adult population, had this disease. Although the
percentages are higher among some ethnic groups and
in some countries than others, it is a disease that is
increasing steeply everywhere.
There are two forms of sugar diabetes. In insulindependent
diabetes, which is also known as type 1
diabetes, the pancreas seems to be incapable of secreting
sufficient insulin. It is thought that this might be due to
a deficiency in the gene that codes for the production
of insulin, or because of an attack on the β cells by the
person’s own immune system. Type 1 diabetes is sometimes
called juvenile-onset diabetes, because it usually begins
very early in life.
The second form of diabetes is called non-insulindependent
diabetes or type 2 diabetes. In this form of
diabetes, the pancreas does secrete insulin, but the liver
and muscle cells do not respond properly to it. Type 2
diabetes begins relatively late in life and is often associated
with diet and obesity.
The symptoms of both types of diabetes mellitus are
the same. After a carbohydrate meal, glucose is absorbed
into the blood, and the concentration increases and stays
high (Figure 14.25). Normally there is no glucose in urine,
but if the glucose concentration in the blood becomes
very high, the kidney cannot reabsorb all the glucose, so
that some passes out in the urine. Extra water and salts
accompany this glucose, and the person consequently feels
extremely hungry and thirsty.
In a diabetic person, uptake of glucose into cells is
slow, even when there is plenty of glucose in the blood.
Thus cells lack glucose and metabolise fats and proteins
as alternative energy sources. This can lead to a buildup
of substances in the blood called keto-acids (or
ketones). These are produced when the body switches
to metabolising fat and they decrease the blood pH. The
combination of dehydration, salt loss and low blood pH
can cause coma in extreme situations.
Between meals, the blood glucose concentration of
a person with untreated diabetes may decrease steeply.
This is because there is no glycogen to mobilise, as it was
not stored when there was plenty of glucose. Once again,
coma may result, this time because of a lack of glucose for
respiration.
Blood glucose concentration / mg 100 cm –3
220
200
blood glucose in a
diabetic person
180
160
140
blood glucose in a
non-diabetic person
120
100
80
60
40
20
blood insulin in a
non-diabetic person
blood insulin in a diabetic person
0 1 2 3
glucose ingested
Time / hours
Figure 14.25 Concentrations of blood glucose and insulin
following intake of glucose in a person with normal control of
blood glucose and a person with type 1 diabetes.
Blood insulin concentration