DSE Biology Diagnostic: Human Physiology

Unit Test 1: Digestion and Villi Adaptations

Question

(a) Describe the role of bile in the digestion of fats. Explain why bile is described as an emulsifier rather than an enzyme. [3 marks]

(b) The small intestine has numerous finger-like projections called villi. Describe three structural features of villi that increase the efficiency of absorption, and explain how each feature aids absorption. [6 marks]

(c) A person has a condition in which the villi of the small intestine are flattened and shortened (e.g. coeliac disease). Explain the consequences of this condition on the absorption of (i) fatty acids and glycerol, and (ii) amino acids. [4 marks]

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Worked Solution

(a) Bile is produced in the liver and stored in the gall bladder. It contains bile salts that emulsify fats -- they break large fat globules into smaller droplets, increasing the surface area for the action of lipase. This speeds up fat digestion.

Bile is not an enzyme because it does not catalyse a biochemical reaction; it does not break chemical bonds in fats. It physically breaks up fat droplets (a physical, not chemical, process). Enzymes are biological catalysts that lower activation energy of specific reactions; bile does not do this.

(b) Three structural features of villi:

1. Large surface area: Villi greatly increase the surface area of the small intestine for absorption. More surface area means more space for transport proteins and a greater area over which diffusion and active transport can occur.
2. Thin epithelial layer (one cell thick): The villus epithelium is only one cell thick, creating a short diffusion distance. This increases the rate of diffusion of digested molecules from the gut lumen into the blood and lacteals.
3. Dense capillary network: Each villus contains a network of blood capillaries that carry absorbed glucose and amino acids away from the villus. This maintains a steep concentration gradient between the gut lumen and the blood, facilitating continuous diffusion.

(Alternative acceptable feature: presence of microvilli on the epithelial cells further increasing surface area; presence of lacteals (lymphatic capillaries) for fat absorption; mitochondria in epithelial cells providing ATP for active transport.)

(c) In coeliac disease, the flattened villi result in a greatly reduced surface area for absorption.

(i) Fatty acids and glycerol: Absorption is reduced. Normally, fatty acids and glycerol diffuse into epithelial cells, are re-esterified into triglycerides, and enter lacteals as chylomicrons. With fewer and shorter villi, fewer lacteals are available, and the reduced surface area limits lipid absorption. This can lead to fatty stools (steatorrhoea) and weight loss.

(ii) Amino acids: Absorption is also reduced. Amino acids are normally absorbed by active transport and facilitated diffusion across the villus epithelium into blood capillaries. With flattened villi, there are fewer epithelial cells and fewer transport proteins, reducing the capacity for amino acid uptake. This can lead to protein deficiency and malnutrition.

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Unit Test 2: Haemoglobin and Oxygen Transport

Question

The oxygen dissociation curve shows the relationship between the partial pressure of oxygen ($pO_{2}$) and the percentage saturation of haemoglobin with oxygen.

(a) Explain why the oxygen dissociation curve is S-shaped (sigmoidal) rather than a straight line. [3 marks]

(b) Fetal haemoglobin (HbF) has a higher affinity for oxygen than adult haemoglobin (HbA). Explain the adaptive advantage of this difference. [3 marks]

(c) During vigorous exercise, the oxygen dissociation curve of haemoglobin shifts to the right (the Bohr effect). Explain the mechanism causing this shift and its significance during exercise. [4 marks]

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Worked Solution

(a) The S-shaped (sigmoidal) curve reflects cooperative binding of oxygen to haemoglobin:

1. When the first oxygen molecule binds to one of the four haem groups, it induces a conformational change in the haemoglobin molecule (from the tense/T state to the relaxed/R state).
2. This change increases the affinity of the remaining three haem groups for oxygen, making it easier for subsequent oxygen molecules to bind.
3. At low $pO_{2}$, the curve rises slowly (binding of the first oxygen is difficult). In the middle range, the curve rises steeply (cooperative binding makes additional oxygen uptake rapid). At high $pO_{2}$, the curve flattens as haemoglobin approaches full saturation.

(b) Fetal haemoglobin has a higher affinity for oxygen, meaning it binds oxygen more readily at a given $pO_{2}$. This is essential because the $pO_{2}$ in the maternal blood in the placenta is relatively low (oxygen has already been partially used by the mother's tissues). The higher affinity of HbF allows it to effectively load oxygen from the maternal blood even at this lower $pO_{2}$, ensuring the fetus receives an adequate oxygen supply. At the same $pO_{2}$, HbF has a higher percentage saturation than HbA.

(c) Mechanism of the Bohr effect: During exercise, respiring tissues produce more carbon dioxide ($CO_{2}$), which reacts with water to form carbonic acid: $CO_{2} + H_{2}O \rightleftharpoons H_{2}CO_{3} \rightleftharpoons H^{+} + HCO_{3}^{-}$. This increases the concentration of $H^{+}$ ions (lowering pH). The increased $H^{+}$ concentration and increased $CO_{2}$ partial pressure cause haemoglobin to change its shape, reducing its affinity for oxygen and shifting the dissociation curve to the right.

Significance: The rightward shift means that at any given tissue $pO_{2}$, haemoglobin releases more oxygen to the tissues. During exercise, muscles have a higher oxygen demand. The Bohr effect ensures that haemoglobin unloads oxygen more readily in the actively respiring muscles, meeting their increased metabolic requirements.

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Unit Test 3: Neurone Structure and Synaptic Transmission

Question

(a) Describe the structural differences between a motor neurone and a sensory neurone. [3 marks]

(b) Explain how a nerve impulse is transmitted along a myelinated neurone, including the role of the myelin sheath and the nodes of Ranvier. [4 marks]

(c) At a cholinergic synapse, acetylcholine is released from the presynaptic neurone. Describe the sequence of events from the arrival of the nerve impulse at the presynaptic membrane to the generation of an action potential in the postsynaptic membrane. [5 marks]

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Worked Solution

(a) Structural differences:

Feature	Motor Neurone	Sensory Neurone
Cell body position	At the end of the neurone (in the CNS)	In the middle of the neurone (in a ganglion)
Dendrites	Short dendrites at the cell body end	Long dendron carrying impulse from receptor to cell body
Axon direction	Carries impulse away from the CNS to the effector	Carries impulse towards the CNS from the receptor

(b) In a myelinated neurone, the axon is surrounded by a myelin sheath formed by Schwann cells. The myelin sheath acts as an electrical insulator, preventing the flow of ions across the membrane in the myelinated regions.

Between the Schwann cells are gaps called nodes of Ranvier where the axon membrane is exposed and contains voltage-gated sodium ion channels.

The nerve impulse jumps from one node of Ranvier to the next -- this is called saltatory conduction. At each node, sodium ions rush in, regenerating the action potential. Saltatory conduction is much faster than continuous propagation in unmyelinated neurones because the impulse effectively "skips" the insulated regions, and less ion movement is required overall, saving energy.

(c) Sequence of events at a cholinergic synapse:

1. The action potential arrives at the presynaptic membrane (synaptic knob).
2. Depolarisation causes voltage-gated calcium ion channels to open, and $Ca^{2+}$ ions diffuse into the synaptic knob down their concentration gradient.
3. The influx of $Ca^{2+}$ causes synaptic vesicles (containing the neurotransmitter acetylcholine) to move to and fuse with the presynaptic membrane (exocytosis).
4. Acetylcholine is released into the synaptic cleft and diffuses across to the postsynaptic membrane.
5. Acetylcholine binds to receptor proteins on the postsynaptic membrane, causing ligand-gated sodium ion channels to open.
6. Sodium ions diffuse into the postsynaptic neurone, causing depolarisation. If the depolarisation reaches the threshold potential, an action potential is generated in the postsynaptic neurone.
7. Acetylcholine is rapidly broken down by the enzyme acetylcholinesterase into choline and ethanoic acid (acetate). This prevents continuous stimulation and allows the synapse to be ready for the next impulse.

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Integration Test 1: Ventilation Mechanism and Gas Exchange

Question

(a) Describe the changes that occur in the intercostal muscles, diaphragm, ribcage, and lung pressure during inspiration. Explain how these changes cause air to enter the lungs. [5 marks]

(b) In the alveoli, oxygen diffuses from the alveolar air into the blood and carbon dioxide diffuses from the blood into the alveolar air. Explain two features of the alveolar membrane that facilitate efficient gas exchange. [4 marks]

(c) A person with emphysema has damaged alveolar walls, resulting in fewer but larger alveoli. Explain the effect of this condition on the rate of gas exchange, using the concept of surface area to volume ratio. [3 marks]

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Worked Solution

(a) During inspiration:

1. The external intercostal muscles contract, pulling the ribcage upwards and outwards.
2. The diaphragm contracts and flattens (changes from a dome shape to a flatter shape).
3. These movements increase the volume of the thoracic cavity (chest).
4. According to Boyle's law, increasing volume decreases the pressure inside the lungs (intrapulmonary pressure drops below atmospheric pressure).
5. Air flows into the lungs from the atmosphere, moving down the pressure gradient (from higher atmospheric pressure to lower intrapulmonary pressure).

(b) Two features of the alveolar membrane:

1. Extremely thin diffusion pathway: The alveolar wall is only one cell thick (squamous epithelium), and the capillary wall is also one cell thick. The total diffusion distance is very short (approximately 1 $\mu$m), allowing rapid diffusion of gases.

2. Large surface area: The millions of alveoli provide a very large total surface area (approximately 70 m$^{2}$) for gas exchange, increasing the rate of diffusion.

(Alternative: dense capillary network maintains a steep concentration gradient; good ventilation and blood flow maintain the concentration gradient; moist surface helps dissolve gases.)

(c) In emphysema, the total number of alveoli is reduced but individual alveoli are larger. This means the total surface area available for gas exchange is significantly reduced. At the same time, the larger alveoli have a smaller surface area to volume ratio, meaning the volume of air in each alveolus is greater relative to the surface area through which gas exchange can occur.

The rate of diffusion is proportional to surface area and inversely proportional to diffusion distance. The reduced surface area leads to a lower rate of oxygen uptake and carbon dioxide removal, causing shortness of breath and reduced exercise tolerance. The person may need to breathe faster and deeper to compensate.

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Integration Test 2: Circulatory System Structure and Function

Question

(a) Compare the structure of an artery, a vein, and a capillary. Your answer should include differences in wall thickness, lumen size, presence of valves, and the composition of the wall. [5 marks]

(b) Explain why the double circulatory system in mammals is more efficient than the single circulatory system found in fish. [3 marks]

(c) A person has a resting heart rate of 75 beats per minute, a stroke volume of 70 cm$^{3}$, and a blood pressure of 120/80 mmHg. Calculate the cardiac output. If the person begins vigorous exercise and the cardiac output increases to 15 dm$^{3}$/min while the stroke volume increases to 120 cm$^{3}$, calculate the new heart rate. [3 marks]

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Worked Solution

(a) Comparison of blood vessel structures:

Feature	Artery	Vein	Capillary
Wall thickness	Thick wall (muscle and elastic tissue)	Thin wall (less muscle, less elastic tissue)	Wall is one cell thick (endothelium only)
Lumen size	Narrow (relatively)	Wide (relatively)	Very narrow (just wide enough for red blood cells)
Valves	No valves (except semilunar valves in heart)	Valves present to prevent backflow	No valves
Wall composition	Thick layer of smooth muscle and elastic fibres; folds when empty	Thin layer of smooth muscle; collagen and elastic fibres; remains open when empty	Single layer of endothelial cells; basement membrane; pores between cells

(b) In the single circulatory system of fish, blood passes through the heart once in a complete circuit. Blood is pumped from the heart to the gills (where it loses pressure as it passes through narrow capillaries) and then directly to the rest of the body. By the time blood reaches the body tissues, the blood pressure is low, resulting in a slow rate of blood flow.

In the double circulatory system of mammals, blood passes through the heart twice in a complete circuit:

• Pulmonary circulation: Heart to lungs and back. Blood is pumped at relatively low pressure to the lungs (to avoid damage to delicate lung tissue).
• Systemic circulation: Heart to body and back. The heart pumps blood again at high pressure to the body tissues.

This allows blood to be delivered to body tissues at high pressure, ensuring a rapid blood flow and efficient delivery of oxygen and nutrients. Mammals maintain a higher metabolic rate supported by this more efficient system.

(c) Cardiac output = heart rate $\times$ stroke volume

At rest: Cardiac output $= 75 \times 70 = 5250$ cm$^{3}$/min $= 5.25$ dm$^{3}$/min

During exercise: Heart rate $= \frac{\text{cardiac output}}{\text{stroke volume}} = \frac{15000}{120} = 125$ beats per minute

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Integration Test 3: Nervous System and Reflex Arc

Question

A person accidentally touches a hot object and quickly withdraws their hand. This is a reflex action involving a withdrawal (withdrawal) reflex arc.

(a) Name the three types of neurones involved in this reflex arc, in the order in which the nerve impulse travels. [2 marks]

(b) Explain why reflex actions are important for survival, giving two advantages of reflex arcs. [4 marks]

(c) Describe the pathway of the nerve impulse in this reflex arc, from the receptor in the skin to the effector muscle. [5 marks]

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Worked Solution

(a) The three neurones in order:

1. Sensory neurone (transmits impulse from receptor to the CNS)
2. Relay neurone (intermediate neurone in the CNS)
3. Motor neurone (transmits impulse from CNS to effector)

(b) Importance of reflex actions:

1. Speed: Reflex arcs are fast because they involve only three neurones and typically only one synapse (or a few). The short pathway means the response is rapid, minimising tissue damage (e.g. quickly removing the hand from a hot object before serious burns occur).

2. Involuntary (automatic): Reflex actions do not require conscious thought or decision-making by the brain. This means the protective response occurs automatically and immediately, without any delay that would be caused by processing in the conscious part of the brain.

(Alternative: protective function; innate (inborn), not learned.)

(c) Pathway of the nerve impulse in the withdrawal reflex:

1. Receptor: Heat receptors in the skin detect the high temperature and generate a nerve impulse (generator potential that reaches threshold, triggering an action potential).
2. Sensory neurone: The impulse travels along the sensory neurone (dendron then axon) towards the spinal cord. The impulse is transmitted as a wave of depolarisation due to sodium ion influx.
3. Relay neurone: In the grey matter of the spinal cord, the impulse passes across a synapse (chemical transmission via neurotransmitter) to a relay neurone. The relay neurone may also transmit the impulse to the brain (so the person becomes consciously aware of the pain), but the reflex response does not wait for this.
4. Motor neurone: The impulse passes across another synapse to a motor neurone, which carries the impulse out of the spinal cord via the ventral root (anterior root) of the spinal nerve.
5. Effector: The motor neurone stimulates a muscle (effector) in the arm via a neuromuscular junction, causing the muscle to contract. The arm is pulled away from the hot object.