- Following a period of intense exercise resulting in dehydration, the pituitary gland releases antidiuretic hormone (ADH). What is the direct effect of ADH on the nephron to conserve water? [HL]
Answer: It triggers the insertion of aquaporins into the membranes of collecting duct cells.
ADH acts on the collecting ducts and the distal convoluted tubules. Its primary mechanism is to increase their permeability to water by promoting the insertion of aquaporin-2 channels into the apical membrane of the cells. This allows more water to move by osmosis from the filtrate into the hypertonic interstitial fluid of the medulla, thus conserving water. (A) is incorrect; ADH does not directly affect salt transport in the loop of Henle. (B) describes a different homeostatic mechanism. (D) is incorrect; glucose reabsorption occurs in the PCT and is not regulated by ADH.
- What is a primary reason for the difference in composition between the glomerular filtrate and the blood plasma? [HL]
Answer: The basement membrane and podocytes act as a barrier to large molecules like plasma proteins.
Ultrafiltration is a non-selective process based on size. The filtration barrier in the glomerulus (fenestrated endothelium, basement membrane, and podocyte filtration slits) prevents large molecules (like albumin and fibrinogen) and formed elements (like red blood cells) from passing into Bowman's capsule. Therefore, the glomerular filtrate is essentially blood plasma minus the large proteins and cells. (A) is incorrect; urea is filtered passively. (C) is incorrect; small solutes like glucose and salts are freely filtered. (D) is incorrect; red blood cells are too large to be filtered.
- The final stage of protein digestion in the small intestine occurs at the surface of the epithelial cells. Which class of enzymes is responsible for this?
Answer: Exopeptidases, which cleave peptide bonds at the ends of peptide chains.
Initial protein digestion by endopeptidases (like trypsin) in the lumen breaks large polypeptides into smaller ones. The final breakdown into amino acids or di/tripeptides is completed by exopeptidases (like dipeptidases and aminopeptidases) which are located in the membranes of the microvilli (the brush border). These enzymes cleave amino acids from the ends (exo-) of the peptide fragments. (A) describes enzymes like trypsin and pepsin which act in the lumen. (C) Lipases digest lipids. (D) Zymogens are inactive precursors, not the final active enzymes at the brush border.
- The loop of Henle is essential for creating a salt gradient in the medulla of the kidney. What is the role of the descending limb in this process? [HL]
Answer: It is highly permeable to water, which moves out via osmosis into the hypertonic interstitial fluid.
The counter-current multiplier system relies on the different permeabilities of the limbs. The descending limb is highly permeable to water but relatively impermeable to salt. As filtrate flows down into the hypertonic medulla (created by the ascending limb), water leaves the descending limb by osmosis, concentrating the filtrate. (A) describes the function of the thick ascending limb. (C) is incorrect; it is permeable to water. (D) describes the function of the proximal convoluted tubule.
- Following absorption from the small intestine, where would the highest concentration of amino acids be found shortly after a protein-rich meal?
Answer: In the blood of the hepatic portal vein, travelling from the intestine to the liver.
Water-soluble nutrients like amino acids are absorbed into the capillaries of the villi, which converge into the hepatic portal vein. This vessel carries nutrient-rich blood directly to the liver for processing before it enters general circulation. Therefore, its concentration of amino acids will be highest immediately after absorption. (A) is where lipids are absorbed. (C) is incorrect because the liver will have processed and metabolized some of the amino acids, so the concentration will be lower than in the hepatic portal vein. (D) represents blood that has already passed through the liver and general circulation.
- If a person has a genetic defect that results in fewer mitochondria in the cells of the proximal convoluted tubule (PCT), which substance would likely have the highest concentration in their urine compared to a healthy individual? [HL]
Answer: Sodium ions
The PCT is responsible for the majority of selective reabsorption, a process that heavily relies on active transport powered by ATP from mitochondria. Sodium ions are actively transported out of the filtrate, and many other substances (like glucose and amino acids) are co-transported with them. A lack of mitochondria would impair this active transport, causing less sodium to be reabsorbed. This would lead to a higher concentration of sodium ions remaining in the filtrate and ultimately in the urine. (A) Urea is passively reabsorbed, so it would be less affected. (B) Less solute reabsorption would osmotically keep more water in the tubule, but the concentration of sodium would be the most direct indicator of failed active transport. (D) Plasma proteins are not normally filtered.
- In severe liver failure (cirrhosis), a patient may develop ascites (fluid accumulation in the abdomen) and impaired blood clotting. Which two functions of the liver are most directly related to these symptoms? [HL]
Answer: Synthesis of albumin and fibrinogen.
The liver synthesizes most plasma proteins. Albumin is crucial for maintaining colloid osmotic pressure in the blood; its deficiency leads to fluid leaking into tissues (edema/ascites). Fibrinogen is a key clotting factor; its deficiency impairs blood clotting. (A) Detoxification failure causes neurological symptoms, and cholesterol is related to transport, not these specific symptoms. (B) Erythrocyte breakdown is related to jaundice. (C) Bile production relates to digestion, and glycogen storage to glucose regulation.
- A patient with a blocked pancreatic duct is found to have fatty, undigested stools (steatorrhea) and shows signs of malnutrition despite eating a balanced diet. Which statement best explains this observation?
Answer: The absence of pancreatic lipase and bicarbonate ions prevents fat digestion and the creation of an optimal pH for other intestinal enzymes.
The pancreas secretes lipase for fat digestion and bicarbonate to neutralize stomach acid, creating an alkaline environment necessary for most small intestine enzymes to function. The lack of both directly causes steatorrhea and impairs overall digestion. (A) is incorrect as carbohydrate digestion begins in the mouth and is not the primary cause of fatty stools. (C) is a real function, but the most pronounced symptom described (fatty stools) points directly to lipid malabsorption. (D) is incorrect because insulin and glucagon are endocrine hormones released into the blood, not exocrine secretions into the intestine.
- A new weight-loss drug is found to destroy the protein channels required for the co-transport of sodium ions and glucose into the epithelial cells of the small intestine. What would be the most likely effect of this drug?
Answer: Decreased absorption of glucose due to the disruption of active transport, and subsequent osmotic loss of water.
Glucose absorption from the intestinal lumen is an example of secondary active transport, relying on a sodium gradient maintained by the Na+/K+ pump. The SGLT1 co-transporter moves both sodium and glucose into the cell. Blocking this channel would severely impair glucose absorption. The unabsorbed glucose would increase the solute potential of the chyme, causing water to be retained in the lumen via osmosis, leading to diarrhea. (B) is incorrect as the gradient for glucose is not favorable for significant passive diffusion. (C) is incorrect as fatty acid absorption is primarily via simple diffusion. (D) is incorrect because co-transport is the main mechanism, not facilitated diffusion from the lumen.
- Which comparison between the blood in the renal artery and the renal vein of a healthy person is correct? [HL]
Answer: Renal vein has lower urea concentration and lower oxygen concentration.
The kidneys filter urea from the blood, so the blood leaving in the renal vein will have a lower urea concentration. The kidney cells themselves are metabolically active and perform aerobic respiration; therefore, they consume oxygen and produce carbon dioxide. They also consume glucose for energy. Thus, the renal vein has lower urea, lower oxygen, and lower glucose compared to the renal artery. Option (B) correctly identifies two of these changes.
- Which statement correctly describes the integrated roles of the liver and kidneys in processing the products of protein metabolism? [HL]
Answer: The liver deaminates excess amino acids to form urea, which is then transported by the blood to the kidneys for filtration and excretion.
This question tests the integration of the liver and kidney in nitrogenous waste handling. The liver is the site of deamination (removal of the amine group from amino acids) and the urea cycle, which converts the resulting toxic ammonia into less toxic urea. The urea is then released into the blood, transported to the kidneys, filtered from the blood, and excreted in urine. The other options misrepresent the roles and locations of these processes.
- Based on this information, which long-term physiological consequence would be most expected in an individual with untreated celiac disease?
Answer: Weight loss and deficiencies in minerals like iron and calcium due to malabsorption.
The primary function of villi is to maximize surface area for the absorption of all digested nutrients, including minerals. A reduction in this surface area leads to malabsorption, causing weight loss and specific deficiencies (e.g., iron deficiency anemia, calcium deficiency osteoporosis). (A) is the opposite of what would happen; glucose absorption would be impaired, leading to lower blood glucose. (C) is incorrect; digestion might be normal, but absorption is impaired. (D) is plausible, as reduced protein absorption would lead to less deamination and urea production, but the most direct and wide-ranging consequence is general malabsorption and weight loss.
- Villi and microvilli in the small intestine are adaptations that increase surface area. How does this structural adaptation directly improve the efficiency of the digestive system?
Answer: It provides more space for the embedding of transport proteins and channels for absorption.
The primary function of greatly increasing the surface area is to maximize the rate of absorption. A larger surface area allows for a greater number of membrane transport proteins (for active transport and facilitated diffusion) and a larger area for simple diffusion to occur across. This directly enhances the efficiency of moving digested nutrients from the lumen into the body. (A) is incorrect; folds increase surface area without significantly increasing volume. (B) is also a function of intestinal motility, but the surface area's primary role is absorption, not transit time. (D) relates to the large intestine, not the small intestine's villi.
- The digestion of starch involves hydrolysis by enzymes in two different locations. Which row correctly identifies the enzyme, its location, and the primary product of its action?
Answer: Amylase; Small intestine; Maltose
Starch digestion begins in the mouth with salivary amylase and is completed in the small intestine by pancreatic amylase. Both enzymes break down starch (a polysaccharide) into the disaccharide maltose and other small oligosaccharides. (A) is incorrect as the acidic pH of the stomach denatures amylase. (B) is incorrect as maltase (which breaks maltose into glucose) is found in the small intestine, not the mouth. (D) is incorrect as pepsin is a protease found in the stomach, and amylose is a component of starch.
- How does the liver regulate blood glucose concentration in the hours immediately following a carbohydrate-rich meal? [HL]
Answer: It converts excess glucose into glycogen for storage and into fatty acids for transport to adipose tissue.
After a carbohydrate-rich meal, blood glucose levels rise, stimulating the pancreas to release insulin. Insulin signals the liver to take up glucose from the blood. The liver then converts this glucose into glycogen for short-term storage (glycogenesis) and, once glycogen stores are full, converts the remaining excess glucose into fatty acids, which are assembled into triglycerides and transported to adipose tissue. (A) describes the response to low blood sugar (fasting). (B) is incorrect; the kidneys only excrete glucose if blood levels are pathologically high. (D) is incorrect; glucagon raises blood sugar, and it is a hormone, not something 'stored' and released by the liver.
