Control systemsEdexcel A-Level Biology B: Topic test

1. 1
   A healthy adult's core body temperature is kept close to 37 °C, although she produces metabolic heat all the time and the air temperature changes through the day.

   (a)

   Which statement best describes homeostasis?

   [1 mark]

   • AKeeping core temperature and other internal conditions absolutely constant at all times
   • BMaintaining internal conditions within narrow limits as a dynamic equilibrium, with small changes being corrected
   • CResponding to changes in the external environment by moving to a more suitable habitat
   • DReducing the metabolic rate so that internal conditions do not need to be controlled

   (b)

   Why is she described as an endotherm?

   [1 mark]

   • AHer body temperature changes with her surroundings because she gains heat only from the environment
   • BShe has no thermoreceptors, so her temperature is controlled only by behaviour
   • CShe produces heat from metabolic processes and can keep her core temperature largely independent of the surroundings
   • DShe controls her temperature only by changing her behaviour, such as moving into shade

   (c)

   Explain how negative feedback keeps her core temperature close to 37 °C.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A researcher adds adrenaline to cultured skeletal muscle cells, and within seconds the cells begin to break down glycogen. When she repeats the experiment with adrenaline attached to large beads that cannot enter the cells, the response is identical.

   (a)

   What does the second experiment show about the adrenaline receptor?

   [1 mark]

   • AIt is on the outer surface of the cell surface membrane
   • BIt is inside the nucleus, bound to DNA
   • CIt is free in the cytoplasm
   • DIt is in the inner mitochondrial membrane

   (b)

   What is the role of the second messenger in this response?

   [1 mark]

   • AIt enters the nucleus and binds to transcription factors
   • BIt carries adrenaline across the cell surface membrane into the cytoplasm
   • CIt breaks down adrenaline once it has bound to the receptor
   • DIt is released inside the cell after the receptor is activated and activates specific enzymes

   (c)

   Explain why adrenaline is able to cause a response inside the cell without entering it.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Potato tubers stored in a dark cupboard produce long, pale, thin shoots with tiny unexpanded leaves. When tubers are moved into a lit room, the new shoots are shorter, green and have expanded leaves. In both conditions the shoot with the apical bud grows, while buds lower down the shoot stay dormant until the apical bud is removed.

   (a)

   Explain how phytochrome brings about the different shoot growth in the lit room.

   [3 marks]

   (b)

   Explain how auxin and cytokinin together control the growth of the dormant buds lower down a shoot, including what happens when the apical bud is removed.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Multiple sclerosis (MS) is a disease in which the immune system damages the myelin sheaths of axons in the central nervous system. A patient with MS has slow reflexes, an unsteady walk with poor coordination, and cannot tolerate hot weather. A scan shows areas of myelin damage in her spinal cord, cerebellum and hypothalamus.

   (a)

   Explain why damage to myelin sheaths slows the transmission of impulses along axons.

   [6 marks]

   (b)

   Use your knowledge of the nervous system to explain the patient's poor coordination, her intolerance of heat and her slow reflexes.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   An astronomer finds that a very faint star is easiest to see when she looks slightly to one side of it rather than straight at it. When she looks straight at a bright planet she can see its colour clearly, but faint stars look grey.

   (a)

   Why is the faint star easier to see when she looks slightly to one side of it?

   [1 mark]

   • ACones, which are more sensitive than rods, are found mainly around the edge of the retina
   • BThere are no photoreceptors at the edge of the retina, so less light is scattered
   • CRods in the fovea are connected one-to-one to bipolar cells, so low light is detected there
   • DRods, which are more sensitive to low light, are absent from the fovea but common elsewhere in the retina

   (b)

   What happens to rhodopsin in a rod cell when a photon of light is absorbed?

   [1 mark]

   • AIt is broken down into retinal and opsin, which leads to Na⁺ channels in the rod cell membrane closing
   • BIt is resynthesised rapidly, which opens Na⁺ channels in the rod cell membrane
   • CIt binds to receptors on the bipolar cell and causes release of acetylcholine
   • DIt combines with calcium ions, which opens Na⁺ channels in the rod cell membrane

   (c)

   Explain why the faint stars look grey but the bright planet looks coloured.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A chef accidentally eats a meal containing pufferfish liver, which contains tetrodotoxin. This toxin enters nerve axons and blocks the voltage-gated sodium ion channels in the axon membrane. Within minutes the chef has numb lips, and later his muscles weaken and he struggles to breathe.

   (a)

   What is the direct effect of blocking voltage-gated Na⁺ channels in an axon?

   [1 mark]

   • ANa⁺ ions cannot be pumped out of the axon, so the resting potential becomes more positive
   • BK⁺ ions cannot leave the axon, so the membrane cannot repolarise
   • CNa⁺ ions cannot diffuse in, so the membrane cannot depolarise to threshold and no action potential is propagated
   • DAcetylcholine cannot bind to receptors on the muscle, so the muscle membrane is not depolarised

   (b)

   Which statement explains why the chef's breathing becomes difficult?

   [1 mark]

   • AThe toxin stimulates the parasympathetic nerve to the sinoatrial node
   • BImpulses from the medulla oblongata cannot travel along motor neurones to the diaphragm and intercostal muscles, so they do not contract
   • CThe toxin mimics acetylcholine, so the breathing muscles stay contracted
   • DThe cerebellum, which controls breathing, is destroyed by the toxin

   (c)

   The toxin has little effect on the resting potential of an axon. Explain why.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A doctor treats a patient whose heart is beating at 160 beats per minute at rest by gently massaging the neck over the carotid artery. This stimulates the baroreceptors in the carotid artery wall, and within seconds the patient's heart rate falls. Later the same patient exercises on a bicycle and her heart rate rises again.

   (a)

   Explain how stimulating the carotid baroreceptors causes the heart rate to fall.

   [3 marks]

   (b)

   Explain how the autonomic nervous system and a hormone bring about the rise in heart rate during exercise.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A patient with cranial diabetes insipidus produces up to 15 dm³ of very dilute urine a day because her pituitary gland releases very little ADH. Treatment with a synthetic form of ADH reduces her urine volume and makes it more concentrated. Her kidneys, loops of Henle and osmoreceptors are otherwise normal.

   (a)

   Explain how ADH normally keeps the water potential of the blood plasma constant in a healthy person who has not drunk for several hours.

   [6 marks]

   (b)

   Explain how the loop of Henle makes it possible for the synthetic ADH to produce concentrated urine.

   [6 marks]

   Total for question 8: 12 marks

End of questions