Cambridge International A Level Biology Revision Guide

Chapter 14: Homeostasis
People with type 1 diabetes receive regular injections of
insulin, which they learn to do themselves. They must also
take blood samples to check that the insulin is effective
(Figure 14.26). Some people have mini-pumps which
deliver the exact volumes of insulin that they need when
they need them. A carefully controlled diet also helps to
maintain a near-constant concentration of glucose in
the blood.
People with type 2 diabetes rarely need to have
insulin injections; instead they can use diet and regular
and frequent exercise to keep their blood glucose within
normal limits (Figure 14.21, page 315). Diabetics now
receive insulin made by genetically engineered cells
(page 466).
Figure 14.26 A nurse teaches a girl with type 1 diabetes to
inject insulin. The girl may have to receive injections of insulin
throughout her life.
Urine analysis
It is much easier to collect a urine sample from someone
than a blood sample. Simple tests on urine can give
early indications of health problems, which can then be
investigated more thoroughly.
The presence of glucose and ketones in urine indicates
that a person may have diabetes. If blood glucose
concentration increases above a certain value, known as
the renal threshold, not all of the glucose is reabsorbed
from the filtrate in the proximal convoluted tubule of the
kidney and some will be present in the urine.
The presence of protein in the urine indicates that
there is something wrong with the kidneys. Most protein
molecules are too large to be filtered. However, some
protein molecules are filtered (Table 14.1), but these are
reabsorbed by endocytosis in the proximal convoluted
tubule, broken down and the amino acids absorbed into
the blood. It is not unusual for some protein to be present
in the urine for short periods of time, such as during a
high fever, after vigorous exercise and during pregnancy.
However, a large quantity or the long-term presence of
protein in the urine indicates that there may be a disease
affecting the glomeruli or there is a kidney infection.
Protein in the urine is also associated with high blood
pressure, which is a risk factor in heart disease.
Dip sticks and biosensors
Dip sticks (also known as test strips) can be used to test
urine for a range of different factors including pH, glucose,
ketones and protein. Dip sticks for detecting glucose
contain the enzymes glucose oxidase and peroxidase.
These two enzymes are immobilised on a small pad
at one end of the stick. The pad is immersed in urine
and if it contains glucose, glucose oxidase catalyses a
chemical reaction in which glucose is oxidised into a
substance called gluconolactone. Hydrogen peroxide is
also produced. Peroxidase catalyses a reaction between
hydrogen peroxide and a colourless chemical in the pad
to form a brown compound. The resulting colour of the
pad is matched against a colour chart. The chart shows the
colours that indicate different concentrations of glucose.
The more glucose that is present, the darker the colour
(Figure 14.27).
One problem with urine tests is that they do not
indicate the current blood glucose concentration, but
rather whether the concentration was higher than the
renal threshold in the period of time while urine was
collecting in the bladder.
A biosensor like the one in Figure 14.28 allows people
with diabetes to check their blood to see how well they
are controlling their glucose concentration. Like the
negative
mg / cm 3 0 1.0 2.5 5.0 10.0 20.0
or more
Figure 14.27 A dip stick can be used to test for the presence
of glucose in urine. These dip sticks are used by people with
diabetes to check whether their urine contains any glucose.
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