Principles of homeostasis and negative feedbackAQA A-Level Biology: Subtopic test

1. 1
   In a healthy human, core temperature is held at about 37 °C and blood pH at about 7.4. Both stay within very narrow limits, even though the environment and the person's activity change from hour to hour.

   (a)

   Which statement best explains why core temperature must be kept stable?

   [1 mark]

   • ACold temperatures denature enzymes, and heat slows enzyme activity
   • BEnzymes work faster at higher temperatures, so a high temperature is always better
   • CTemperature affects only the rate of diffusion, not enzyme activity
   • DLow temperatures slow reactions as there is less kinetic energy, and high temperatures break bonds and change the shape of the active site

   (b)

   Why must blood pH be kept close to 7.4?

   [1 mark]

   • AChanges in pH alter the pH of the cytoplasm only and do not affect proteins
   • BChanges in hydrogen ion concentration interfere with the bonds that hold an enzyme's tertiary structure, so the active site changes
   • CA change in pH affects the kinetic energy of enzyme molecules
   • DpH controls the concentration of substrate in the blood

   (c)

   Explain what is meant by negative feedback.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Hormone X is secreted by an endocrine gland into the blood. It circulates to every organ in the body, but only liver cells show a response to it. The response begins within minutes and lasts for several hours.

   (a)

   Why do only liver cells respond to hormone X?

   [1 mark]

   • AOnly liver cells have receptors with a tertiary structure complementary to hormone X
   • BHormone X is carried only to the liver by the blood
   • CHormone X is a small molecule that can enter only liver cells
   • DLiver cells are the only cells with a nucleus

   (b)

   Which row describes the usual characteristics of a response to a hormone, compared with a nervous response?

   [1 mark]

   • ARapid, short-lived and localised
   • BRapid, long-lasting and localised
   • CSlow, long-lasting and widespread
   • DSlow, short-lived and widespread

   (c)

   Explain why a response to a hormone is usually slower than a response to a nerve impulse.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   During childbirth, the baby's head stretches the cervix. Stretch receptors send impulses to the hypothalamus, which causes oxytocin to be released from the posterior pituitary gland into the blood. Oxytocin stimulates the muscle of the uterus to contract more strongly, which pushes the baby's head harder against the cervix.

   (a)

   Explain why this is an example of positive feedback.

   [3 marks]

   (b)

   Explain why negative feedback would be unsuitable for controlling childbirth.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In humans, thermoreceptors in the hypothalamus detect the temperature of the blood. When blood temperature rises above 37 °C the hypothalamus sends impulses to sweat glands, which secrete more sweat, and to skin arterioles, which dilate. When blood temperature falls below 37 °C the hypothalamus sends impulses that cause shivering and constriction of skin arterioles. Blood glucose concentration is also kept near 5 mmol dm⁻³ in a healthy adult.

   (a)

   Use the information to explain how negative feedback returns body temperature to normal after exercise has raised it, and why separate mechanisms for a rise and a fall give better control.

   [6 marks]

   (b)

   Explain why it is important to keep blood glucose concentration stable in a healthy adult.

   [6 marks]

   Total for question 4: 12 marks

End of questions