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3.3.4 Mass transport in animals and plants

AQA A-level Biology 3.3.4 practice on Mass transport in animals and plants, with free MCQs, clues and worked explanations drawn from the Organisms exchange substances with their environment section of specification 7402.

Specification route
3.3.4
Question bank
34 questions
Course stage
AS / Year 12

Sample questions

QUESTION 1 · 3.3.4.1 · LEVEL 2

Haemoglobins are a group of chemically similar molecules. What level of protein structure allows haemoglobin to consist of multiple interacting polypeptide chains?

  • Quaternary structure
  • Tertiary structure
  • Secondary structure
  • Primary structure
Show clue

This level of structure applies exclusively to proteins with more than one subunit.

Show answer and explanation

Answer: Quaternary structure

Haemoglobin is a protein with a quaternary structure. A frequent misconception is that 3D folded proteins are only tertiary; the presence of multiple chains defines quaternary structure.

QUESTION 2 · 3.3.4.1 · LEVEL 3

The oxyhaemoglobin dissociation curve is sigmoidal (S-shaped). What biological phenomenon causes this specific shape?

  • The cooperative nature of oxygen binding, where binding the first $O_{2}$ alters the molecule's shape, making subsequent binding easier.
  • The competitive inhibition of oxygen by carbon dioxide at the haemoglobin active site.
  • The denaturation of the haemoglobin protein at high partial pressures of oxygen.
  • The constant, linear rate of oxygen diffusing into red blood cells from the alveoli.
Show clue

The molecule changes its affinity dynamically as it loads up.

Show answer and explanation

Answer: The cooperative nature of oxygen binding, where binding the first $O_{2}$ alters the molecule's shape, making subsequent binding easier.

The loading, transport and unloading of oxygen is related to the oxyhaemoglobin dissociation curve and the cooperative nature of oxygen binding.

QUESTION 3 · 3.3.4.1 · LEVEL 3

Which statement best describes the Bohr effect?

  • A high concentration of carbon dioxide causes oxyhaemoglobin to release oxygen more readily, shifting the dissociation curve to the right.
  • A high concentration of oxygen increases the cooperative binding of carbon dioxide to red blood cells.
  • Carbon monoxide permanently binds to haemoglobin, preventing the transport of oxygen.
  • The active transport of oxygen into actively respiring tissues lowers the local pH.
Show clue

This effect ensures that actively respiring tissues (which produce $CO_{2}$) get more oxygen.

Show answer and explanation

Answer: A high concentration of carbon dioxide causes oxyhaemoglobin to release oxygen more readily, shifting the dissociation curve to the right.

The effects of carbon dioxide concentration on the dissociation of oxyhaemoglobin is known as the Bohr effect. The misconception that $CO_{2}$ binds competitively to the oxygen binding site is common; it actually alters the protein's overall shape by changing the pH.

QUESTION 4 · 3.3.4.1 · LEVEL 4

How might the haemoglobin of an animal adapted to a high-altitude (low oxygen) environment differ from that of a sea-level animal?

  • It would have a higher affinity for oxygen, shifting its oxyhaemoglobin dissociation curve to the left.
  • It would have a lower affinity for oxygen, allowing it to unload oxygen much more easily to tissues.
  • It would lack haemoglobin entirely and rely on purely dissolved oxygen in the blood plasma.
  • It would bind irreversibly to oxygen to prevent any loss during transport.
Show clue

If oxygen is scarce in the lungs, the haemoglobin must be very 'greedy' to pick it up.

Show answer and explanation

Answer: It would have a higher affinity for oxygen, shifting its oxyhaemoglobin dissociation curve to the left.

Many animals are adapted to their environment by possessing different types of haemoglobin with different oxygen transport properties. A misconception is that high-altitude animals need lower affinity to unload easier; actually, they need higher affinity to load scarce $O_{2}$ in the first place.

QUESTION 5 · 3.3.4.1 · LEVEL 2

In the general pattern of mammalian blood circulation, which blood vessel carries deoxygenated blood away from the right ventricle towards the lungs?

  • Pulmonary artery
  • Pulmonary vein
  • Aorta
  • Vena cava
Show clue

Arteries carry blood Away from the heart.

Show answer and explanation

Answer: Pulmonary artery

The general pattern of blood circulation in a mammal. A major misconception is that all arteries carry oxygenated blood; the pulmonary artery carries deoxygenated blood to the lungs.

QUESTION 6 · 3.3.4.1 · LEVEL 3

During the cardiac cycle, what specific physical event causes the atrioventricular (AV) valves to close?

  • Ventricular pressure exceeds atrial pressure, forcing the valve flaps shut.
  • Atrial pressure exceeds ventricular pressure as the atria contract.
  • The valve muscles themselves actively contract to seal the opening.
  • The spontaneous electrical firing of the sinoatrial node signals them to close.
Show clue

Valves in the heart are passive flaps driven entirely by fluid gradients.

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Answer: Ventricular pressure exceeds atrial pressure, forcing the valve flaps shut.

Pressure and volume changes and associated valve movements during the cardiac cycle maintain a unidirectional flow of blood. A common misconception is that valves have muscles that open/close them; they act purely based on pressure differences.

QUESTION 7 · 3.3.4.1 · LEVEL 2

How does the structure of a typical artery relate directly to its function?

  • It has a thick elastic layer to stretch and recoil, maintaining high continuous blood pressure.
  • It has pocket valves throughout its length to prevent the backflow of high-pressure blood.
  • It has extremely thin walls made of a single endothelium layer for rapid gas exchange.
  • It possesses a very wide lumen to minimize resistance to slow, low-pressure blood flow.
Show clue

Arteries must smooth out the pulsing pressure generated by the ventricles.

Show answer and explanation

Answer: It has a thick elastic layer to stretch and recoil, maintaining high continuous blood pressure.

The structure of arteries, arterioles and veins in relation to their function. A widespread misconception is that arteries possess valves throughout; valves are characteristic of veins to prevent backflow under low pressure.

QUESTION 8 · 3.3.4.1 · LEVEL 2

Why are capillary beds critical as exchange surfaces in mammalian mass transport?

  • They possess a vast total cross-sectional area and a wall only one cell thick, providing a short diffusion distance.
  • They have thick muscular walls that actively pump nutrients and oxygen into the surrounding tissue fluid.
  • They are the only blood vessels capable of carrying fully oxygenated blood.
  • They contain specialized carrier proteins that actively transport whole red blood cells into the tissues.
Show clue

Exchange relies on passive diffusion, which requires thin boundaries and large surface areas.

Show answer and explanation

Answer: They possess a vast total cross-sectional area and a wall only one cell thick, providing a short diffusion distance.

The structure of capillaries and the importance of capillary beds as exchange surfaces. The misconception that capillaries 'pump' or actively move substances is incorrect; exchange is driven by diffusion and hydrostatic pressure across a thin endothelium.

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