A Level H2 Biology Human Physiology Quiz

A-Level Biology H2 Quiz - Human Physiology (Answer Key)

1. Define the term homeostasis. [2]

• The maintenance of a constant/stable internal environment [1]
• Within narrow limits / despite changes in the external environment [1]

2. (a) Explain the mechanism by which insulin lowers blood glucose concentration in liver cells. [3]

• Insulin binds to specific complementary receptors on the cell surface membrane of liver cells [1]
• This triggers the translocation of GLUT4 transporter proteins to the membrane / increases permeability to glucose [1]
• Glucose enters the cell via facilitated diffusion and is converted to glycogen (glycogenesis) / oxidized in respiration [1]

(b) Suggest why blood glucose concentration does not fall below the normal resting level... [2]

• As blood glucose falls, insulin secretion decreases / stops [1]
• Glucagon is secreted (by alpha cells), stimulating glycogenolysis / gluconeogenesis to raise blood glucose back to normal [1]

3. Describe the role of the hypothalamus in thermoregulation when body temperature rises above the set point. [3]

• Thermoreceptors in the hypothalamus detect an increase in blood temperature [1]
• The heat loss center in the hypothalamus is activated [1]
• It sends impulses via the sympathetic nervous system to effectors (e.g., skin arterioles vasodilate, sweat glands secrete sweat) [1]

4. Explain how the kidney contributes to the regulation of blood water potential via the production of ADH. [4]

• Osmoreceptors in the hypothalamus detect low blood water potential (high osmolarity) [1]
• This stimulates the posterior pituitary gland to release ADH (Antidiuretic Hormone) [1]
• ADH increases the permeability of the collecting duct walls to water by inserting aquaporins [1]
• More water is reabsorbed from the filtrate into the blood by osmosis, producing a smaller volume of concentrated urine [1]

5. Distinguish between negative feedback and positive feedback... [2]

• Negative feedback: The response counteracts the change, returning the system to the set point (e.g., thermoregulation) [1]
• Positive feedback: The response amplifies the change, moving the system further away from the set point (e.g., blood clotting / childbirth) [1]

6. (a) State the function of the Schwann cells. [1]

• They produce the myelin sheath / insulate the axon [1]

(b) Explain how myelination increases the speed of nerve impulse transmission. [2]

• Myelin acts as an electrical insulator, preventing ion exchange across the membrane except at the Nodes of Ranvier [1]
• The impulse "jumps" from node to node (saltatory conduction), which is faster than continuous conduction [1]

7. Describe the events that occur at a cholinergic synapse... [4]

• Arrival of action potential causes voltage-gated Ca²⁺ channels to open; Ca²⁺ enters the presynaptic knob [1]
• Vesicles containing acetylcholine (ACh) fuse with the presynaptic membrane and release ACh by exocytosis [1]
• ACh diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane [1]
• This opens Na⁺ channels, causing depolarization; if threshold is reached, a new action potential is generated [1]

8. Explain why the transmission of impulses across a synapse is unidirectional. [2]

• Neurotransmitter receptors are only present on the postsynaptic membrane [1]
• Neurotransmitter vesicles are only present in the presynaptic knob [1]

9. (a) Explain the cause of the depolarization phase. [2]

• Voltage-gated Na⁺ channels open [1]
• Na⁺ ions rush into the axon down their electrochemical gradient, making the inside positive relative to the outside [1]

(b) Explain the importance of the refractory period. [2]

• It ensures that action potentials are discrete / separate events [1]
• It limits the frequency of impulse transmission / ensures unidirectional travel [1]

10. Compare the roles of the sympathetic and parasympathetic nervous systems in the control of heart rate. [3]

• Sympathetic nervous system releases noradrenaline, which binds to receptors in the SAN, increasing heart rate (during stress/exercise) [1.5]
• Parasympathetic nervous system releases acetylcholine, which binds to receptors in the SAN, decreasing heart rate (during rest) [1.5]

11. Describe the structure of a sarcomere... [3]

• A-band: Contains myosin filaments (and overlapping actin); does not change length during contraction [1]
• I-band: Contains only actin filaments; shortens during contraction [1]
• H-zone: Central region of A-band with only myosin; shortens/disappears during contraction [1]
• (Note: Z-line marks the boundary of the sarcomere)

12. Explain the role of calcium ions in the contraction of a skeletal muscle fiber. [3]

• Ca²⁺ binds to troponin on the actin filament [1]
• This causes tropomyosin to move away from the myosin-binding sites on actin [1]
• This exposes the binding sites, allowing myosin heads to attach to actin [1]

13. Describe the sliding filament theory of muscle contraction. [3]

• Myosin heads bind to actin forming cross-bridges [1]
• Myosin heads pivot (power stroke), pulling actin filaments toward the center of the sarcomere [1]
• ATP binds to myosin, causing it to detach from actin and reset for the next cycle [1]

14. Explain the role of ATP in muscle contraction and relaxation. [3]

• Hydrolysis of ATP provides energy for the power stroke of the myosin head [1]
• Binding of ATP to the myosin head causes it to detach from actin [1]
• ATP is required for the active transport of Ca²⁺ back into the sarcoplasmic reticulum (for relaxation) [1]

15. Differentiate between slow-twitch (Type I) and fast-twitch (Type II) muscle fibers... [4]

• Slow-twitch: High myoglobin content, many mitochondria, rich blood supply, relies on aerobic respiration, resistant to fatigue, suited for endurance (e.g., marathon running) [2]
• Fast-twitch: Low myoglobin, fewer mitochondria, relies on anaerobic respiration/glycolysis, fatigues quickly, generates rapid powerful contractions, suited for sprinting/weightlifting [2]

16. (a) Explain the increase in reaction rate between 10°C and 40°C. [2]

• Kinetic energy of enzyme and substrate molecules increases [1]
• Frequency of successful collisions between enzyme and substrate increases, forming more enzyme-substrate complexes [1]

(b) Explain the decrease in reaction rate between 40°C and 60°C. [2]

• High temperature breaks hydrogen/ionic bonds maintaining the tertiary structure of the enzyme [1]
• The active site changes shape (denaturation), so the substrate no longer fits/complementary shape is lost [1]

17. (a) Explain why individuals with Type 1 diabetes must inject insulin rather than take it as a tablet. [2]

• Insulin is a protein/polypeptide [1]
• It would be digested/hydrolyzed by protease enzymes in the stomach/small intestine if taken orally [1]

(b) Suggest one long-term consequence of poorly controlled blood glucose levels... [1]

• Damage to blood vessels (atherosclerosis) / Kidney failure / Retinopathy (blindness) / Neuropathy [1]

18. Explain how this toxin leads to muscle paralysis. [3]

• Acetylcholine is not released into the synaptic cleft [1]
• ACh cannot bind to receptors on the muscle fiber membrane [1]
• Na⁺ channels do not open, so no depolarization/action potential is generated in the muscle, preventing contraction [1]

19. (a) Explain how lactic acid is produced in muscle cells. [2]

• During intense exercise, oxygen supply is insufficient for aerobic respiration [1]
• Pyruvate is converted to lactate (lactic acid) via anaerobic glycolysis to regenerate NAD+ [1]

(b) Describe how the body removes lactic acid after exercise has ceased. [2]

• Lactic acid is transported to the liver via the blood [1]
• It is converted back to pyruvate/glucose (Cori cycle) using oxygen (oxygen debt) [1]

20. Describe the sequence of events that occurs when blood pressure rises significantly. [4]

• Baroreceptors in the carotid sinus/aortic arch detect increased stretch/pressure [1]
• They send increased frequency of impulses to the cardiovascular center in the medulla oblongata [1]
• The center sends more impulses via the parasympathetic nervous system (vagus nerve) to the SAN [1]
• Acetylcholine is released, decreasing the heart rate, which lowers blood pressure [1]