1. 1
   A biologist compares two organisms of similar shape. Organism P is a single spherical cell 20 micrometres in diameter that relies entirely on diffusion across its surface to obtain oxygen and nutrients and remove waste. Organism Q is a complex multicellular animal several centimetres in diameter with a working circulatory system. Both organisms need a continuous supply of oxygen, dissolved food molecules and mineral ions, and must remove carbon dioxide and urea.

   (a)

   Which of the following statements best explains why Organism Q could not survive relying on diffusion alone across its outer surface, unlike Organism P?

   [1 mark]

   • AAs size increases, the surface area to volume ratio increases, making diffusion faster over long distances
   • BAs size increases, the surface area to volume ratio decreases, so diffusion alone cannot supply cells deep inside the body quickly enough
   • COrganism Q's cells require less oxygen per unit volume than Organism P's cell
   • DDiffusion distance has no effect on the rate at which substances move into cells

   (b)

   Which pair of substances listed must both be transported out of Organism Q's cells as waste products of metabolism?

   [1 mark]

   • AOxygen and mineral ions
   • BDissolved food molecules and water
   • CCarbon dioxide and urea
   • DMineral ions and dissolved food molecules

   (c)

   Using ideas about surface area to volume ratio and diffusion distance, explain why Organism Q needs a transport system to supply its cells with oxygen and dissolved food molecules.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A doctor takes a small blood sample from a patient during a routine check-up. Under the microscope, the sample shows several types of cell suspended in a pale yellow liquid: biconcave cells with no nucleus that appear to be carrying oxygen, larger cells with a lobed nucleus that engulf bacteria, and small cell fragments involved in clotting.

   (a)

   What is the function of the biconcave cells with no nucleus described in the blood sample?

   [1 mark]

   • AThey engulf and destroy pathogens by phagocytosis
   • BThey transport oxygen bound to haemoglobin around the body
   • CThey produce antibodies against specific pathogens
   • DThey release clotting factors to seal damaged blood vessels

   (b)

   Why do red blood cells lack a nucleus?

   [1 mark]

   • ATo decrease their surface area to volume ratio
   • BTo enable them to engulf bacteria more easily
   • CTo allow them to divide rapidly by mitosis
   • DTo leave more space for haemoglobin, increasing their oxygen-carrying capacity

   (c)

   Describe two ways in which the structure of red blood cells is adapted to their function of transporting oxygen.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student examines three blood vessels removed from a mammal during a dissection. Vessel X has a thick muscular and elastic wall, a narrow central space (lumen) and no valves, and carries blood away from the heart under high pressure. Vessel Y has a thin wall only one cell thick and a lumen just wide enough for red blood cells to pass through in single file. Vessel Z has a much thinner wall than Vessel X, a wider lumen than Vessel X, and contains valves at intervals along its length.

   (a)

   Explain how the structure of Vessel X is related to its function.

   [3 marks]

   (b)

   Explain how the structure of Vessel Y is related to its function.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A patient is diagnosed with a fault in the heart in which blood entering the right side of the heart mixes with blood in the left side before being pumped to the body. Normally, deoxygenated blood returns from the body to the right atrium, passes to the right ventricle and is pumped to the lungs, while oxygenated blood returns from the lungs to the left atrium, passes to the left ventricle and is pumped around the body through the aorta. At rest, the patient's heart has a stroke volume of 60 cm3 and a heart rate of 90 beats per minute.

   (a)

   Explain how the structure of the heart, including its chambers, valves and associated blood vessels, allows blood to be pumped separately to the lungs and to the rest of the body, and explain the effect this patient's fault would have on the efficiency of oxygen delivery to the body.

   [6 marks]

   (b)

   Calculate the patient's resting cardiac output and use it, together with ideas about the structure and function of the circulatory system, to explain why maintaining an adequate cardiac output is essential for supplying body cells with oxygen and removing waste products.

   [6 marks]

   Total for question 4: 12 marks

End of questions