D3.3 Homeostasis

The diagram shows a heterotrophic, unicellular, freshwater organism that has been placed in distilled water. The short arrows show movement of water and the long arrows show a sequence of steps.

What life function is illustrated? [1]

A. Nutrition

B. Homeostasis

C. Endocytosis

D. Response

Answer: B

A. Incorrect: Nutrition involves taking in and processing food or nutrients to provide energy, not the control of water balance. The diagram shows movement of water, not ingestion of food particles.

B. Correct: The diagram shows a unicellular freshwater organism using a contractile vacuole to remove excess water that continuously enters the cell by osmosis. Since the organism is placed in distilled water, the surrounding environment is hypotonic, causing water to move into the cell. To prevent the cell from bursting, the contractile vacuole collects and expels the excess water through the process labeled X. This regulation of the internal water balance maintains the organism’s internal conditions within narrow limits, demonstrating homeostasis

C. Incorrect: Endocytosis refers to the uptake of materials (like food or other substances) by engulfing them with the cell membrane. Here, the vacuole is expelling water, not taking substances in.

D. Incorrect: Response involves reacting to stimuli (like light, chemicals, or touch). The process shown is a continuous physiological regulation, not a reaction to an external stimulus.

The graph shows human body temperature variation in a daily rhythm of 24 hours.

Which hormone controls this variation? [1]

A. Leptin

B. Insulin

C. Glucagon

D. Thyroxin

Answer: D

A. Incorrect: Leptin is secreted by fat cells (adipose tissue) and mainly controls appetite and energy balance, not body temperature. It signals the brain to reduce hunger when energy stores are sufficient, but it does not directly regulate the metabolic heat production shown in the graph.

B. Incorrect: Insulin, secreted by the pancreas, regulates blood glucose levels by promoting glucose uptake into cells and glycogen formation. Although metabolism is indirectly linked to heat production, insulin does not control the daily variation in body temperature.

C. Incorrect: Glucagon, also from the pancreas, works opposite to insulin by increasing blood glucose through glycogen breakdown in the liver. Its main function is glucose homeostasis, not the regulation of metabolic rate or body temperature.

D. Correct: Thyroxin, a hormone secreted by the thyroid gland, plays a crucial role in controlling the metabolic rate of body cells. Metabolism produces heat as a byproduct, so changes in thyroxin levels directly affect body temperature. The daily rhythm (circadian variation) in the graph - showing body temperature rising during the day and dropping at night - reflects changes in metabolic activity throughout a 24 hour cycle. Thus, thyroxin is the hormone responsible for maintaining and adjusting basal metabolic rate and, consequently, body temperature within normal limits.

Which hormone is part of a negative feedback control system and acts on cells in the hypothalamus? [1]

A. Insulin

B. Glucagon

C. Malatonin

D. Leptin

Answer: D

A. Incorrect: Insulin is secreted by the pancreas and regulates blood glucose levels, not appetite. Although it is involved in a negative feedback loop for glucose control, it acts mainly on liver, muscle, and fat cells, not on the hypothalamus for appetite regulation.

B. Incorrect: Glucagon, also secreted by the pancreas, works in opposition to insulin to raise blood glucose levels by stimulating glycogen breakdown in the liver. It does not act on the hypothalamus and is not part of the feedback system controlling appetite or fat storage.

C. Incorrect: Melatonin is secreted by the pineal gland and helps regulate sleep-wake cycles (circadian rhythm), not hunger or energy balance. Although it acts on the brain, it is not part of a negative feedback loop involving the hypothalamus to control food intake.

D. Correct: Leptin is a hormone secreted by adipose (fat) tissue and is part of a negative feedback control system that helps regulate body weight and appetite. When fat stores increase, leptin levels rise and act on receptors in the hypothalamus to reduce appetite and increase energy expenditure. Conversely, when fat stores are low, leptin levels fall, which stimulates hunger. This feedback loop helps maintain a stable body mass and energy balance. Therefore, leptin is the correct answer because it directly acts on the hypothalamus as part of a negative feedback system to control food intake and fat storage.

What is most likely to increase in the presence of insulin? [1]

A. The rate of anaerobic respiration

B. The chances of Type I diabetes

C. The uptake of glucose by muscles

D. The concentration of glucagon

Answer: C

A. Incorrect: Insulin promotes aerobic respiration because it increases glucose availability and oxygen-based energy production in cells. Anaerobic respiration occurs mainly when oxygen is insufficient (e.g., during intense exercise), not because of insulin activity. Hence, insulin does not increase anaerobic respiration

B. Incorrect: Type I diabetes is caused by the destruction of pancreatic β cells, leading to a lack of insulin production. The presence of insulin would actually reduce the chances of Type I diabetes symptoms, not increase them. Therefore, this option is incorrect.

C. Correct: Insulin is a hormone secreted by the β cells of the pancreas when blood glucose levels are high, such as after eating. Its main function is to lower blood glucose concentration by stimulating cells, especially muscle and liver cells to absorb glucose from the blood. In muscle cells, insulin increases the number of glucose transport proteins (GLUT4) on the cell membrane, allowing more glucose to enter the cells. The absorbed glucose is then used for cellular respiration to produce energy or stored as glycogen for later use. Therefore, in the presence of insulin, the uptake of glucose by muscles increases, helping maintain blood glucose homeostasis.

D. Incorrect: Glucagon is a hormone with opposite effects to insulin, secreted when blood glucose levels are low to raise them. When insulin levels are high, glucagon secretion is inhibited, so its concentration decreases, not increases.

Which hormone helps control levels of glucose in blood? [1]

A. Insulin secreted by the liver cells

B. Thyroxin secreted by the thyroid gland

C. Glucagon secreted by the α cells of the pancreas

D. Melatonin secreted by the β cells of the pancreas

Answer: C

A. Incorrect: Insulin is not secreted by the liver, but by the β cells of the pancreas. The liver only responds to insulin by storing glucose as glycogen - it does not produce or secrete insulin. Therefore, this statement is factually incorrect.

B. Incorrect: Thyroxin regulates metabolic rate and body temperature, not directly the blood glucose concentration. While metabolism can influence glucose use, thyroxin does not directly control or maintain blood glucose levels. Hence, it’s not the correct hormone for this function.

C. Correct: Glucagon is a hormone produced by the α cells in the islets of Langerhans of the pancreas. It plays a key role in maintaining blood glucose homeostasis by increasing blood glucose levels when they fall too low (for example, between meals or during fasting). Glucagon acts mainly on the liver, stimulating the breakdown of glycogen into glucose (a process called glycogenolysis) and the synthesis of glucose from non-carbohydrate sources (gluconeogenesis). The glucose produced is then released into the bloodstream, restoring normal blood glucose concentration. Thus, glucagon works in opposition to insulin through a negative feedback mechanism to keep blood glucose levels stable.

D. Incorrect: Melatonin is produced by the pineal gland in the brain, not the pancreas. It regulates sleep-wake cycles (circadian rhythms), not blood glucose. Additionally, β cells of the pancreas secrete insulin, not melatonin. Therefore, this option is doubly incorrect (wrong hormone and wrong gland).

Where in the nephron is most glucose reabsorbed? [1]

Answer: B

A. Incorrect: This is where ultrafiltration occurs - blood is filtered under pressure, and glucose, water, and ions pass into the filtrate. However, no reabsorption occurs here, so glucose simply enters the filtrate but is not reabsorbed.

B. Correct: Most glucose reabsorption occurs in the proximal convoluted tubule (PCT) of the nephron. After blood is filtered in the glomerulus and enters Bowman’s capsule, the filtrate contains glucose, amino acids, salts, and water. The cells lining the PCT have microvilli that increase surface area for reabsorption and contain specific carrier proteins that actively transport glucose from the filtrate back into the blood capillaries through co-transport with sodium ions (Na⁺). Nearly 100% of glucose is reabsorbed here under normal conditions, ensuring that glucose is not lost in the urine. This process is an example of selective reabsorption, which requires energy (active transport) and maintains proper blood glucose levels.

C. Incorrect: The loop of Henle mainly functions in water conservation and the maintenance of the osmotic gradient in the medulla. It reabsorbs water and sodium chloride, not glucose. Glucose reabsorption has already occurred earlier in the nephron.

D. Incorrect: The distal convoluted tubule adjusts pH and ion balance (e.g., sodium and potassium), but by this stage, no glucose remains in the filtrate because it has already been completely reabsorbed in the proximal convoluted tubule.

The image shows a transverse section through a collecting duct in a vertebrate kidney.

How is the movement of materials across the wall of the collecting duct affected by the release of ADH from the pituitary gland? [1]

A. There is increased movement of water in the direction of arrow I.

B. There is increased movement of sodium in the direction of arrow I.

C. There is increased movement of water in the direction of arrow II.

D. There is increased movement of sodium in the direction of arrow II.

Answer: A

A. Correct: ADH (antidiuretic hormone), also known as vasopressin, is released from the pituitary gland when the body needs to conserve water (for example, during dehydration). ADH acts on the collecting duct cells in the kidney. It binds to receptors on these cells, triggering the insertion of aquaporins (water channel proteins) into their membranes. These aquaporins increase the permeability of the collecting duct walls to water. As a result, water moves out of the collecting duct (arrow I) by osmosis into the hypertonic medulla (where solute concentration is high). This reabsorbed water returns to the bloodstream, concentrating the urine and conserving body water. Thus, the movement of water increases in the direction of arrow I, from inside the duct 🡪 into the surrounding medulla tissue 🡪 blood capillaries.

B. Incorrect: ADH mainly affects water permeability, not sodium transport. Sodium movement in the nephron is mainly regulated by aldosterone, not ADH. Therefore, ADH does not increase sodium movement in this direction.

C. Incorrect: Arrow II points in the opposite direction (from medulla 🡪 into the duct). ADH reduces water movement in that direction because it enhances water reabsorption out of the duct, not into it. So, less water enters the duct from the surrounding tissue.

D. Incorrect: ADH has no direct effect on sodium reabsorption or secretion in the collecting duct. Sodium transport is controlled by aldosterone, which increases sodium reabsorption (but not under ADH control). Therefore, this statement is irrelevant to ADH function.

What property of water accounts for its usefulness as a coolant in sweat? [1]

A. High specific heat capacity

B. High latent heat of vaporization

C. High boiling point

D. High melting point

Answer: B

A. Incorrect: This means water can absorb or release a large amount of heat with only a small change in temperature. While this helps regulate body temperature and stabilize environmental temperatures, it does not directly explain how sweating cools the body. Sweating depends on evaporation, not temperature change.

B. Correct: Water has a high latent heat of vaporization, meaning it requires a large amount of energy to change from a liquid to a vapor without a change in temperature. When you sweat, your body releases water onto the skin’s surface. As the sweat evaporates, it absorbs a significant amount of heat energy from your body to break the hydrogen bonds between water molecules. This energy transfer removes heat from the skin, effectively cooling the body down. Because of this property, even a small amount of evaporating sweat can remove a large amount of heat - making water an excellent biological coolant.

C. Incorrect: Water’s high boiling point means it remains liquid over a wide temperature range, which is useful for life but doesn’t explain cooling during sweating. Cooling through sweat occurs well below the boiling point.

D. Incorrect: This property means water stays liquid at common Earth temperatures instead of freezing easily, but it has no role in the cooling mechanism of sweat.

Explain how blood solute concentrations are kept within narrow limits in the human body. [7]

Any seven of the following:

a. solute concentration of blood monitored by the brain/hypothalamus;

b. pituitary gland secretes ADH;

c. ADH secreted when solute concentration/osmolarity is too high/a person is dehydrated/OWTTE;

d. collecting duct more permeable to water;

e. «more» aquaporins/opens aquaporins «in the plasma membrane of collecting duct cells»;

f. «more» water reabsorbed «into the medulla»;

g.medulla is hypertonic/hyperosmotic «so water can be reabsorbed from filtrate»;

h. small volume of urine/concentrated urine produced «with ADH»;

i. no/little/less ADH secreted if «blood» solute concentration is too low;

j. collecting duct less permeable to water/less water reabsorbed/large volume of urine produced/dilute urine produced «with low/no ADH»;

k. insulin causes blood glucose «concentration» to be reduced;

l. glucose stored as glycogen in the liver;

m. glucagon causes blood glucose «concentration» to be increased;

n. negative feedback;

Sample answer:

The solute concentration of blood is monitored by the hypothalamus [1]. When blood becomes too concentrated, the pituitary gland releases ADH (antidiuretic hormone) [2]. ADH makes the collecting ducts of the kidney more permeable to water by causing aquaporins to open in their plasma membranes [2]. As the medulla of the kidney is hypertonic, water is reabsorbed by osmosis from the filtrate back into the blood [1]. This results in a smaller volume of concentrated urine, helping lower blood solute concentration [2]. When blood solute concentration becomes too low, less or no ADH is secreted, making the collecting ducts less permeable to water so more dilute urine is produced [1]. These processes work through negative feedback [1], maintaining blood solute concentrations within narrow limits.

Outline the role of ADH in osmoregulation. [4]

Any four of the following:

a. ADH (secreted by pituitary) if body/blood is dehydrated/hypertonic/has high solute concentration;

b. more aquaporins / aquaporins open (in collecting duct);

c. collecting duct more permeable to water/reabsorbs more water (from filtrate/urine);

d. water reabsorbed by osmosis/water reabsorbed because medulla is hypertonic;

e. (reabsorbed) water passes (from filtrate) to blood / blood solute concentration reduced;

f. less water lost in urine / smaller volume of (more concentrated) urine;

g. negative feedback / less/no ADH secreted when blood solute concentration returns to normal;

Sample answer:

When the body or blood becomes dehydrated or hypertonic (having a high solute concentration), the pituitary gland secretes antidiuretic hormone (ADH) [1].

ADH travels to the kidneys and causes aquaporins in the collecting duct to open, making the duct more permeable to water [2].

As a result, more water is reabsorbed by osmosis from the filtrate back into the blood because the medulla is hypertonic [1].

This reduces water loss in urine, producing a smaller volume of more concentrated urine [1].

When blood solute concentration returns to normal, less or no ADH is secreted, showing a negative feedback control mechanism [1].

Discuss the control of blood glucose levels and the consequences if they are not maintained. [8]

Any eight of the following:

control: [6 max] 

a. homeostasis is the maintenance of a constant internal environment;

b. the pancreas produces hormones that control the levels of glucose;

c. if glucose levels in blood are high, beta-cells «of the pancreas» produce insulin;

d. «insulin» causes the cells to take up /absorb glucose;

e. liver stores excess glucose as glycogen;

f. if glucose levels in blood are low, alpha-cells «of the pancreas» produce glucagon;

g. «glucagon» causes the liver to break down glycogen into glucose;

h. «glucagon» increase levels of glucose in the blood;

i. negative feedback controls the glucose levels;

consequences: 

j. if the pancreas produces little/no insulin a person can develop type I diabetes;

k. a person with type I diabetes «usually» needs/is dependent on injections of insulin;

l. type II diabetes occurs when the body becomes resistant to insulin/cells do not respond to insulin;

m. type II diabetes can «sometimes» be controlled by diet and exercise;

n. named consequence of having diabetes «eg: eye damage»;

Sample answer:

The control of blood glucose levels is a key example of homeostasis, which is the maintenance of a constant internal environment in the body [1]. Blood glucose concentration must be kept within narrow limits to ensure a steady supply of energy to cells while preventing damage caused by excessively high or low glucose levels.

The pancreas plays a central role in regulating blood glucose through the secretion of two main hormones: insulin and glucagon [1]. When blood glucose levels rise after eating, β cells in the pancreas detect this increase and release insulin into the bloodstream. Insulin stimulates body cells, particularly in the liver, muscles, and fat tissue, to absorb glucose from the blood [1]. In the liver, insulin promotes the conversion of excess glucose into glycogen, which is stored for later use [1]. This process lowers blood glucose concentration back to normal levels.

Conversely, when blood glucose levels fall such as between meals or during exercise, α cells in the pancreas secrete glucagon [1]. Glucagon acts mainly on the liver to break down stored glycogen into glucose, which is then released into the bloodstream [1]. This raises the blood glucose level back to the normal range [1]. The interaction between insulin and glucagon operates through a negative feedback mechanism [1], where a deviation in glucose levels triggers corrective hormonal responses that restore balance.

Failure to maintain proper blood glucose regulation can lead to serious health consequences. If the pancreas produces little or no insulin, the person develops Type I diabetes [1], an autoimmune disease where insulin injections are required for survival [1]. If the body becomes resistant to insulin or cells fail to respond effectively, Type II diabetes develops [1]. This condition is often linked to lifestyle factors such as poor diet and lack of exercise and can sometimes be managed through dietary control and regular physical activity.

Both types of diabetes can lead to long-term complications if untreated, including damage to the eyes (retinopathy), kidneys (nephropathy), nerves (neuropathy), and cardiovascular disease [1]. Thus, maintaining blood glucose levels within a narrow range is vital for metabolic balance, energy supply, and long-term health.