Secondary 4 Combined Science Biology Human Physiology Quiz

Secondary 4 Combined Science Biology Quiz - Human Physiology

Answer Key

Section A: Multiple Choice & Short Answer (Questions 1–10)

1. C — Aorta [1]
The aorta is the main artery that carries oxygenated blood from the left ventricle to the rest of the body. Common mistake: selecting the pulmonary vein, which carries oxygenated blood but only from the lungs to the heart.

2. Diffusion [1]
Oxygen moves from a high concentration in the alveoli to a low concentration in the blood capillaries by diffusion, down the concentration gradient.

3. (a) Y (left ventricle) [1]
(b) The left ventricle has the thickest wall because it needs to pump blood at high pressure to the entire body (systemic circulation), whereas the right ventricle only pumps blood to the lungs (pulmonary circulation) over a shorter distance. [1]
Marking note: Award 1 mark for identifying the left ventricle and 1 mark for a valid explanation involving pumping blood to the whole body / higher pressure.

4. Bicuspid valve (or mitral valve) [1]
This valve prevents the backflow of blood from the left ventricle into the left atrium during ventricular contraction.

5. Any two of the following: [2]

• Increased carbon dioxide concentration in the blood
• Decreased blood pH (increased acidity)
• Signals from the brain (medulla oblongata)
• Release of adrenaline
• Increased body temperature
  Marking note: 1 mark each, maximum 2 marks.

6. (a) Carbon dioxide [1]
CO₂ concentration increases from 40 to 46 units, indicating it is produced by muscle cells during respiration.

(b) Working:
Percentage decrease = [(95 − 40) / 95] × 100
= (55 / 95) × 100
= 57.9% (accept 57.89% or 58%) [2]
Marking note: 1 mark for correct working, 1 mark for correct answer. Accept answers in the range 57–58%.

7. The sinoatrial node (SAN) acts as the pacemaker of the heart. It generates electrical impulses that initiate and regulate the heartbeat, causing the atria to contract. [2]
Marking note: 1 mark for identifying SAN as the pacemaker, 1 mark for stating that it generates electrical impulses / initiates contraction.

8. Any one of the following: [1]

• Arteries have thicker walls than veins
• Arteries have a smaller lumen than veins
• Veins have valves; arteries do not (except the semi-lunar valves at the heart)
• Arteries have more elastic tissue than veins

9. (a) The cartilage rings keep the trachea open / prevent it from collapsing during breathing. [1]
(b) Any two of the following: [2]

• Large surface area (many alveoli)
• Thin walls (one cell thick) for rapid diffusion
• Moist surface for gases to dissolve
• Surrounded by a dense network of blood capillaries
  Marking note: 1 mark each, maximum 2 marks.

10. The left ventricle pumps blood to the entire body (systemic circulation), which requires higher pressure to overcome the greater resistance of the extensive network of blood vessels. The right ventricle only pumps blood to the lungs (pulmonary circulation), which is a shorter distance with less resistance, so lower pressure is sufficient. [2]
Marking note: 1 mark for identifying the difference in destination (body vs. lungs), 1 mark for linking this to pressure requirement.

Section B: Structured Response (Questions 11–15)

11. (a) 7.8 mmol/L [1]
(b) Between 1 hour and 2 hours [1]
(c) Insulin [1]
(d) Insulin causes the liver and muscle cells to convert excess glucose into glycogen for storage. It also increases the uptake of glucose by body cells for respiration. [2]
Marking note: 1 mark for stating glucose → glycogen conversion, 1 mark for stating increased glucose uptake by cells.
(e) Pancreas [1]

12. (a) A = Stomach, B = Liver, C = Small intestine [3]
Marking note: 1 mark each.
(b) Bile emulsifies fats — it breaks large fat globules into smaller droplets, increasing the surface area for lipase to act on. Bile also neutralises the acidic food from the stomach, providing an alkaline pH for intestinal enzymes. [2]
Marking note: 1 mark for emulsification, 1 mark for neutralisation / providing alkaline conditions.
(c) The small intestine has a very large surface area due to the presence of villi and microvilli. The villi have thin walls (one cell thick) and are richly supplied with blood capillaries, which allow for rapid absorption of nutrients into the bloodstream. [2]
Marking note: 1 mark for large surface area (villi/microvilli), 1 mark for thin walls / rich blood supply.

13. (a) The level of activity (or type of exercise) [1]
(b) Any two of the following: [2]

• Same person / same student
• Same duration of each activity
• Same environmental conditions (temperature, humidity)
• Same method of measuring breathing rate
• Same rest period between activities
  Marking note: 1 mark each, maximum 2 marks.
  (c) As the level of activity increases, the breathing rate increases. [1]
  (d) During exercise, muscle cells respire more rapidly, producing more carbon dioxide. The increased CO₂ concentration in the blood is detected by the brain (medulla oblongata), which sends signals to increase the breathing rate to take in more oxygen and remove excess carbon dioxide. [2]
  Marking note: 1 mark for linking increased respiration to CO₂ production, 1 mark for describing the detection and response mechanism.

14. (a) Ultrafiltration [1]
(b) Any two of the following: [2]

• Glucose
• Amino acids
• Water
• Mineral ions (e.g., sodium ions)
  Marking note: 1 mark each, maximum 2 marks. Urea is NOT reabsorbed (it is a waste product).
  (c) The concentration of urea in the urine would decrease. When a person drinks a large amount of water, more water is reabsorbed from the collecting duct (due to increased water potential in the blood), resulting in more dilute urine. The urea is still excreted but in a larger volume of water, so its concentration is lower. [2]
  Marking note: 1 mark for stating the concentration decreases, 1 mark for explaining the mechanism (more water reabsorbed / more dilute urine).

15. (a) The relay neuron transmits the nerve impulse from the sensory neuron to the motor neuron within the central nervous system (spinal cord). [1]
(b) Reflex actions are important for survival because they are rapid, automatic responses that protect the body from harm. They do not require conscious thought, so the response is faster, minimising damage (e.g., pulling away from a hot object before the brain registers pain). [2]
Marking note: 1 mark for stating rapid/automatic response, 1 mark for linking to protection from harm.
(c) The pathway is:

1. Heat (stimulus) is detected by temperature receptors in the skin.
2. A nerve impulse is generated and travels along the sensory neuron to the spinal cord.
3. In the spinal cord, the impulse passes through a relay neuron.
4. The relay neuron transmits the impulse to the motor neuron.
5. The motor neuron carries the impulse to the effector (arm muscle).
6. The muscle contracts, pulling the hand away from the hot object. [3]
   Marking note: 1 mark for receptor → sensory neuron, 1 mark for relay neuron in spinal cord, 1 mark for motor neuron → effector → response.

Section C: Extended Response (Questions 16–20)

16. (a) Working:
Percentage decrease = [(21.0 − 16.0) / 21.0] × 100
= (5.0 / 21.0) × 100
= 23.8% (accept 23.81% or 24%) [2]
Marking note: 1 mark for correct working, 1 mark for correct answer.
(b) Carbon dioxide is produced as a waste product of cellular respiration in body cells. It diffuses from the cells into the blood and is transported to the lungs, where it diffuses into the alveoli and is exhaled. [2]
Marking note: 1 mark for stating CO₂ is a product of respiration, 1 mark for describing transport to lungs and exhalation.
(c) Nitrogen is an inert gas and is not used in the body's metabolic processes, so it is neither absorbed nor released during breathing. [1]
(d) As air passes through the moist lining of the respiratory tract (trachea, bronchi, and alveoli), it becomes saturated with water vapour. [1]

17. When blood glucose concentration rises above the normal level:

1. The beta cells in the islets of Langerhans in the pancreas detect the increase in blood glucose concentration.
2. The pancreas secretes the hormone insulin into the bloodstream.
3. Insulin travels in the blood to target organs, primarily the liver and muscle cells.
4. Insulin causes the liver and muscle cells to convert excess glucose into glycogen (glycogenesis) for storage.
5. Insulin also increases the permeability of cell membranes to glucose, allowing more glucose to enter body cells for respiration.
6. As glucose is taken up by cells and converted to glycogen, the blood glucose concentration falls back to the normal level. [5]
   Marking note: 1 mark for pancreas detecting high glucose, 1 mark for insulin secretion, 1 mark for liver/muscle as target organs, 1 mark for glycogen conversion, 1 mark for glucose uptake by cells / return to normal level.

18. (a) A double circulatory system separates oxygenated and deoxygenated blood, ensuring that oxygen-rich blood is delivered to body tissues at high pressure. This is more efficient than a single circulatory system, where oxygenated and deoxygenated blood mix, resulting in lower oxygen delivery to tissues. The systemic circuit delivers blood at high pressure to the entire body, while the pulmonary circuit sends blood to the lungs at lower pressure to avoid damaging the delicate lung capillaries. [3]
Marking note: 1 mark for separation of oxygenated/deoxygenated blood, 1 mark for high-pressure delivery to body, 1 mark for efficiency advantage over single circulation.
(b) The pulmonary artery carries deoxygenated blood, while the aorta carries oxygenated blood. [1]
(c) The ventricles pump blood out of the heart to the lungs and body, which requires more force than the atria, which only pump blood into the ventricles. The ventricles therefore have thicker muscular walls to generate the higher pressure needed. [2]
Marking note: 1 mark for ventricles pumping blood out of the heart, 1 mark for thicker walls generating higher pressure.

19. In Type 1 diabetes, the beta cells in the pancreas are destroyed and cannot produce insulin. Without insulin:

• Blood glucose concentration remains high after meals because glucose cannot be converted to glycogen in the liver and muscles.
• Body cells cannot take up glucose efficiently, leading to a lack of energy.
• Excess glucose is excreted in the urine.
• Prolonged high blood glucose can damage blood vessels, nerves, and organs.

Two ways to manage the condition:

1. Insulin injections — The person must regularly inject insulin to replace the hormone their body cannot produce. The dosage is adjusted based on blood glucose monitoring and food intake.
2. Dietary management — The person should eat a balanced diet with controlled carbohydrate intake, avoiding foods that cause rapid spikes in blood glucose. Regular meals help maintain stable blood glucose levels.
   Additional acceptable answer: Regular monitoring of blood glucose levels using a glucose meter. [5]
   Marking note: Up to 2 marks for explaining the effect on blood glucose regulation, up to 3 marks for describing two management methods (at least 1 mark per method with detail).

20. (a) The myelin sheath insulates the axon and increases the speed of nerve impulse transmission. [1]
(b) When a nerve impulse reaches the end of the presynaptic neuron:

1. It triggers the release of neurotransmitter chemicals (e.g., acetylcholine) from synaptic vesicles into the synaptic cleft.
2. The neurotransmitters diffuse across the synaptic cleft.
3. The neurotransmitters bind to specific receptors on the membrane of the postsynaptic neuron.
4. This binding generates a new electrical impulse in the postsynaptic neuron, which continues along the next neuron. [3]
   Marking note: 1 mark for neurotransmitter release, 1 mark for diffusion across cleft, 1 mark for binding to receptors / generation of new impulse.
   (c) A voluntary action is a conscious, deliberate action controlled by the cerebrum (e.g., raising your hand to answer a question). An involuntary (reflex) action is an automatic, rapid response that does not involve conscious thought and is controlled by the spinal cord or lower brain (e.g., pulling your hand away from a hot object). Reflex actions are faster because the impulse does not need to travel to the brain for processing. [2]
   Marking note: 1 mark for defining voluntary action (conscious/cerebrum), 1 mark for defining reflex action (automatic/spinal cord/faster).

End of Answer Key