OCR A2 Biology, some excretion exam questions with answers!

2. Explain how changes in the
concentrations of substances in the blood
are brought about by the proximal
convoluted tubule.
 Selective reabsorption occurs from the proximal
convoluted tubule (PCT), where 85% of the filtrate is
reabsorbed. All of the glucose, amino acids and
some salts are reabsorbed along with some of the
waste. The membrane contains co-transporter
proteins which transport glucose and amino acids by
facilitated diffusion. Microvilli on the cell surface
membrane increase the surface area for
reabsorption.

3. Fill in the spaces…
 ADH is a hormone that is produced by specialised nerve cells
known as osmoreceptor hormones. These cells detect
changes in the water potential of the blood flowing through the
hypothalamus. If the water potential of the blood is too low,
then ADH is released. ADH is not secreted immediately into
the blood but passes along the axon of specialised nerve cells
to the posterior pituitary gland, where it is released into the
blood. ADH acts on the cells of the collecting duct. The ADH
molecule attaches to receptors on the membrane of these cells
and causes protein channels known as aquaporins to insert
themselves into the membrane. Water passes through these
channels by osmosis and a smaller concentration of more
concentrated urine is produced.

4. Suggest where ADH is removed from
the blood and what happens to it.
 As a protein: ADH undergoes deamination
(removing an amine group from amino acids to form
ammonia), and the ornithine (ammonia is converted
to urea) in the hepatocytes.
 As a small molecule, ADH is dealt with in the kidney.
It is ultrafiltrated from the blood because it is small. It
is not reabsorbed but excreted.

5. Why will a diet high in protein result in a
high concentration of urea in urine?
 A high intake of protein will result in a high level of
amino acids. These amino acids can’t be stored so
they are deaminated, converting them to ammonia.
The ammonia enters the ornithine cycle to be
converted to urea. The increased blood plasma
concentration of urea leads to more urea in urine.

6. What is the effect on the composition
of the blood if the kidneys fail?
 If the kidneys can’t filter, substances remain in the
blood. So, there will be high levels of urea and
water.

7. Explain the need for close matching
of the donated kidney to the
recipient.
 If the kidney isn’t closely matched, it will be
recognised as foreign which causes rejection by the
immune system. The use of immuno-suppressant
drugs will help to reduce the risk of rejection.
 Also, a specific size could be needed (for example, if
the recipient is a small child)

8. Explain how a pregnancy
test indicates pregnancy.
The stick (which you urinate on), tests for hCG. hCG is
a small hormone so it can pass from the blood into the
filtrate (Bowmans Capsule). There are already
monoclonal antibodies on the stick, which are marked
with blue dye. The hormone binds to the
complementary antibody, and a line becomes visible.
The first line is for ‘control’/comparison, and the second
line indicates pregnancy.

9. Describe the features of the glomerulus and the
Bowmans Capsule which allow them to perform
their function effectively.
 Their function is ultrafiltration.
 The afferent arteriole is wider in diameter than the efferent
arteriole.
 There is a higher hydrostatic pressure in the glomerulus.
 The endothelium has gaps to allow/prevent the passage of
substances (e.g. fenestrations prevent red blood cells exiting).
 The basement membrane only allows small molecules to pass
through.
 Podocytes of the Bowmans Capsule have finger like
projections which ensure gaps to allow the passage of
substances.

10. How does the structure of an aquaporin
prevent the passage of ions?
 The ions are too long to pass through the channel.
 The shape isn’t compatible.
 The positive charge in the channel repel the
positively charged ions.

11. Explain the role of the loop of Henle in the
production of urine.
 The loop of Henle causes a decrease in the water
potential in the medulla. In the ascending limb,
sodium and chlorine ions are actively transported
outwards, but the descending limb is permeable to
water so water is removed from here. As a result,
the water potential of the tissues surrounding the
collecting duct is lower than fluid inside it, so water is
removed from urine.

12. Describe the sequence of events that
results in the water potential of the blood
plasma returning to normal.
 The osmoreceptors in the hypothalamus detect the low water
potential in the blood. So, ADH is produced by the
hypothalamus. The ADH passes to and from the pituitary
posterior gland before being released into the blood. ADH then
acts on the collecting duct and binds to receptors in the
plasma membrane of collecting duct cells. This activates the
phosphorylase enzyme and causes vesicles with aquaporins
which bind with the plasma membrane. This increases the
permeability to water, so water is reabsorbed by osmosis. We
then begin to feel thirsty, so the water potential of the blood
rises, switching off the release of ADH.

13. Explain how ultrafiltration
takes place in the kidney.
 Blood flows into the glomerulus from the afferent arteriole,
which is wider in diameter than the efferent arteriole. This
difference in diameter ensures the blood in the capillaries is
under increased pressure. The high pressure forces liquid and
small molecules out into the Bowmans capsule. The
endothelium of the capillaries has small gaps to ensure
substances can pass through. The basement membrane is
made of glycoproteins which ensures large proteins cannot
pass through – it is a selective barrier. Epithelial cells of the
Bowmans capsule called podocytes have finger like
projections which ensure fluid can pass into the lumen of the
Bowmans capsule.

14. Describe the homeostatic mechanisms tht
would normally prevent glucose
appearing in the urine.
 It The rise in plasma glucose concentration is
detected by cells in the pancreas. The beta cells in
the islets of Langerhans consequently produce more
insulin. The insulin is secreted into the blood, so the
cells take up more glucose. The glucose is
converted to glycogen. This increases the rate of the
use of glucose in respiration, but the glucose
concentration remains below threshold value in the
glomerular filtrate. It is reabsorbed at the PCT.