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Excretion Exam Q's and A's

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OCR A2 Biology, some excretion exam questions with answers!

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Excretion Exam Q's and A's

  1. 1. 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.
  2. 2. 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.
  3. 3. 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.
  4. 4. 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.
  5. 5. 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.
  6. 6. 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)
  7. 7. 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.
  8. 8. 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.
  9. 9. 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.
  10. 10. 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.
  11. 11. 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.
  12. 12. 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.
  13. 13. 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.

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