USABO - USA Biology Olympiad Animal Physiology and Homeostasis Questions and Answers — Questions and Answers
Question 1: In the flippers of a seal swimming in icy water, which of the following best describes the function of the countercurrent heat exchanger?
- It concentrates solutes in the interstitial fluid to prevent the flipper from freezing.
- It maximizes the diffusion of oxygen from the water into the blood vessels of the flippers.
- Warm arterial blood flowing to the flipper transfers heat to the cold venous blood returning to the body core, reducing overall heat loss. (Correct answer)
- It shunts cold blood directly back to the heart to be rapidly rewarmed by metabolic processes.
Correct answer: Warm arterial blood flowing to the flipper transfers heat to the cold venous blood returning to the body core, reducing overall heat loss.
The countercurrent heat exchanger in a seal's flipper consists of arteries and veins that are in very close proximity. Warm blood in the arteries flowing out to the cold flipper transfers its heat to the cold blood in the veins returning to the body. This warms the venous blood before it re-enters the body core, thereby conserving a significant amount of heat that would otherwise be lost to the environment.
Question 2: A healthy individual consumes a large meal rich in carbohydrates. Which of the following represents the primary homeostatic response to the resulting increase in blood glucose?
- Alpha cells of the pancreas secrete glucagon, stimulating glycogenolysis.
- Beta cells of the pancreas secrete insulin, promoting glucose uptake by cells and glycogenesis. (Correct answer)
- The adrenal medulla secretes epinephrine, stimulating gluconeogenesis.
- The posterior pituitary releases ADH, increasing water retention to dilute the blood glucose.
Correct answer: Beta cells of the pancreas secrete insulin, promoting glucose uptake by cells and glycogenesis.
After a carbohydrate-rich meal, blood glucose levels rise, stimulating the beta cells in the islets of Langerhans of the pancreas to release insulin. Insulin acts on body cells (especially muscle and adipose tissue) to increase their uptake of glucose and stimulates the liver to convert excess glucose into glycogen for storage (glycogenesis). Both actions lower blood glucose levels, returning them to the normal range.
Question 3: An action potential is propagated unidirectionally along an axon, from the axon hillock to the terminal. What is the primary molecular mechanism that prevents the action potential from propagating backward?
- The hyperpolarization of the membrane following repolarization.
- The immediate pumping of Na+ ions out of the cell by the Na+/K+ pump.
- The refractory period caused by the inactivation of voltage-gated sodium channels. (Correct answer)
- The selective permeability of the membrane to K+ ions during the resting state.
Correct answer: The refractory period caused by the inactivation of voltage-gated sodium channels.
The unidirectional nature of the action potential is due to the refractory period of the voltage-gated Na+ channels. After these channels open to cause depolarization, they enter an inactivated state for a short period and cannot be reopened, regardless of the stimulus. This inactivation prevents the wave of depolarization from re-exciting the portion of the membrane it has just passed, ensuring the signal moves in only one direction.
Question 4: During a period of intense exercise, the affinity of hemoglobin for oxygen changes to facilitate delivery to muscle tissues. Which of the following factors causes a 'right shift' in the oxygen-hemoglobin dissociation curve?
- An increase in blood pH and a decrease in temperature.
- A decrease in the partial pressure of CO2 and an increase in 2,3-BPG.
- An increase in the partial pressure of CO2 and a decrease in blood pH. (Correct answer)
- A decrease in temperature and a decrease in blood pH.
Correct answer: An increase in the partial pressure of CO2 and a decrease in blood pH.
A right shift in the oxygen-hemoglobin dissociation curve indicates a decreased affinity of hemoglobin for oxygen, which promotes oxygen unloading in tissues. During exercise, muscle metabolism increases the production of CO2 and lactic acid. CO2 increases the partial pressure of CO2 (PCO2) and also forms carbonic acid, both of which lower blood pH. This phenomenon, known as the Bohr effect, facilitates the release of oxygen where it is most needed.
Question 5: A patient is suffering from severe dehydration. How would the state of their urine and the level of Antidiuretic Hormone (ADH) compare to a normally hydrated individual?
- High volume of dilute urine; low levels of ADH.
- Low volume of dilute urine; high levels of ADH.
- High volume of concentrated urine; low levels of ADH.
- Low volume of concentrated urine; high levels of ADH. (Correct answer)
Correct answer: Low volume of concentrated urine; high levels of ADH.
During dehydration, the osmolarity of the blood increases, which is detected by osmoreceptors in the hypothalamus. This stimulates the posterior pituitary to release high levels of Antidiuretic Hormone (ADH). ADH increases the water permeability of the collecting ducts in the kidneys by promoting the insertion of aquaporins. This allows for greater reabsorption of water from the filtrate back into the blood, resulting in the excretion of a small volume of highly concentrated urine to conserve water.
Question 6: In the sliding filament model of muscle contraction, which of the following events directly triggers the 'power stroke' where the myosin head pivots and pulls the actin filament?
- The binding of a new ATP molecule to the myosin head.
- The release of inorganic phosphate (Pi) from the myosin head. (Correct answer)
- The hydrolysis of ATP into ADP and Pi by the myosin head.
- The binding of Ca2+ to the myosin head.
Correct answer: The release of inorganic phosphate (Pi) from the myosin head.
The cross-bridge cycle begins when the myosin head, already energized by ATP hydrolysis, binds to actin. The power stroke itself—the pivoting motion that pulls the actin filament—is directly initiated by the release of the inorganic phosphate (Pi) that was bound to the myosin head. Following the power stroke, ADP is released, and the binding of a new ATP molecule is required for the myosin head to detach from actin.
USABO - USA Biology Olympiad Animal Physiology and Homeostasis Questions and Answers
In the flippers of a seal swimming in icy water, which of the following best describes the function of the countercurrent heat exchanger?