Homeostasis and feedbackEdexcel A-Level Biology B: Subtopic test

1. 1
   A student records the core body temperature of a healthy volunteer every hour for 24 hours. The temperature fluctuates between 36.4 °C and 37.2 °C, with an average of about 37 °C, even though the volunteer is active during the day and rests at night.

   (a)

   Which term describes how the volunteer's temperature fluctuates around a set point of 37 °C?

   [1 mark]

   • ADynamic equilibrium
   • BStatic equilibrium
   • CPositive feedback
   • DDenaturation

   (b)

   Why is it important that the volunteer's core temperature does not rise far above 37 °C?

   [1 mark]

   • AThe kinetic energy of the substrates falls, so there are fewer collisions with enzymes
   • BEnzymes are converted into substrates
   • CHydrogen bonds in the enzymes break, which changes the shape of the active sites
   • DMore peptide bonds form, which makes the enzymes more active

   (c)

   Explain why a fall in core temperature well below 37 °C would slow the rate of enzyme-controlled reactions in cells.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   On a hot day, thermoreceptors in the hypothalamus detect a rise in core body temperature. Nerve impulses lead to the sweat glands secreting more sweat and to the arterioles supplying the skin capillaries dilating. Core body temperature falls back towards 37 °C and the sweating then decreases.

   (a)

   Which of these are the effectors in this response?

   [1 mark]

   • AThe thermoreceptors in the hypothalamus
   • BThe sweat glands and the muscle in the walls of the arterioles
   • CThe hypothalamus
   • DThe sensory neurones

   (b)

   Which statement describes negative feedback?

   [1 mark]

   • AThe response increases the original change
   • BThe response only occurs when hormones are secreted
   • CThe receptors stop detecting the stimulus
   • DThe response reverses the change and returns the condition towards the set point

   (c)

   Explain how the dilation of the arterioles in the skin helps to reduce core body temperature.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   During labour, the stretching of the cervix by the baby's head stimulates the release of the hormone oxytocin from the pituitary gland. Oxytocin causes stronger contractions of the uterus muscle. The stronger contractions push the baby's head harder against the cervix, causing more stretching and more oxytocin release. This continues until the baby is born.

   (a)

   Explain why this is an example of positive feedback.

   [3 marks]

   (b)

   Compare positive feedback and negative feedback in the control of body conditions.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A runner completing a marathon in hot conditions produces large amounts of metabolic heat and loses water in sweat. Her core temperature rises from 37.0 °C to 38.5 °C. Lactate produced in her muscles makes her blood slightly more acidic.

   (a)

   Explain why it is important that the body maintains its temperature, pH and water potential close to set points.

   [6 marks]

   (b)

   Explain how negative feedback would restore the runner's core temperature, and why positive feedback would not be suitable for controlling body temperature.

   [6 marks]

   Total for question 4: 12 marks

End of questions