Why Homeostasis Matters: Enzymes and Osmosis

1. 1 State [1 mark]

   State the optimum temperature of most enzymes in the human body.

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   Model answer

   About 37 °C.

   Mark scheme

   • 37 °C — 1 mark
2. 2 Explain [2 marks]

   Explain why a low temperature reduces the rate of an enzyme-controlled reaction.

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   Model answer

   The enzyme and substrate molecules have less energy and move more slowly. There are fewer collisions between them, so the rate of the reaction is lower.

   Mark scheme

   • Molecules move more slowly — 1 mark
   • Fewer collisions — 1 mark
3. 3 Explain [3 marks]

   Explain why a very high body temperature can be fatal.

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   Model answer

   A very high temperature breaks the bonds that hold the enzymes in shape, so the enzymes are denatured. The substrate no longer fits in the active site, so the reactions in the cells, such as respiration, stop.

   Mark scheme

   • Enzymes are denatured — 1 mark
   • Active site changes shape, so the substrate does not fit — 1 mark
   • Reactions in the cells stop — 1 mark
4. 4 Describe [3 marks]

   Describe what happens to a red blood cell that is placed in pure water.

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   Model answer

   Water enters the cell by osmosis, because the water outside is more dilute than the cytoplasm. The cell swells, and it may burst, because animal cells have no cell wall.

   Mark scheme

   • Water enters by osmosis — 1 mark
   • The cell swells — 1 mark
   • May burst, no cell wall — 1 mark
5. 5 Calculate [3 marks]

   A cylinder of potato has a mass of 5.0 g. After one hour in a concentrated salt solution, its mass is 4.2 g. Calculate the percentage change in mass.

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   Model answer

   The change is 0.8 g, and 0.8 divided by 5.0, multiplied by 100, is 16%, so the percentage change is a decrease of 16%.

   Mark scheme

   • Change in mass is 0.8 g — 1 mark
   • 0.8 divided by 5.0, multiplied by 100 — 1 mark
   • 16% decrease — 1 mark
6. 6 Evaluate [6 marks]

   Explain the importance of controlling the temperature of the body, and the water content of the blood.

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   Model answer

   The body temperature is kept at about 37 °C, which is the optimum temperature for the enzymes in the cells. If the temperature falls, the enzyme and substrate molecules move more slowly, there are fewer collisions, and the reactions slow down. If the temperature rises too high, the enzymes are denatured, because the bonds that hold their shape break, and the active site changes, so the substrate no longer fits and the reactions stop. The water content of the blood must also be controlled. Animal cells have no cell wall. If the blood is too dilute, water enters the cells by osmosis and they swell and may burst. If the blood is too concentrated, water leaves the cells by osmosis, and they shrink, so they cannot work properly.

   Mark scheme

   • Optimum temperature, about 37 °C — 1 mark
   • Too cold, slower reactions — 1 mark
   • Too hot, enzymes denatured — 1 mark
   • Active site changes shape — 1 mark
   • Too dilute, cells swell or burst by osmosis — 1 mark
   • Too concentrated, cells shrink — 1 mark

Quick check

1. 1

   What is the optimum temperature of human enzymes?

   1. A20 °C
   2. B30 °C
   3. C37 °C
   4. D50 °C
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   C: 37 °C

   About 37 °C.

2. 2

   What happens to an enzyme at a very high temperature?

   1. AIt works faster for ever
   2. BIt is denatured
   3. CIt is converted into a substrate
   4. DIt starts to divide
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   B: It is denatured

   It is denatured, because its shape changes.

3. 3

   Why does a low temperature slow down an enzyme?

   1. AThere are fewer collisions
   2. BThe enzyme is denatured
   3. CThe active site is destroyed
   4. DThe substrate is destroyed
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   A: There are fewer collisions

   There are fewer collisions between enzyme and substrate molecules.

4. 4

   What happens to a red blood cell in a concentrated solution?

   1. AIt bursts
   2. BIt swells
   3. CIt stays the same
   4. DIt shrinks
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   D: It shrinks

   Water leaves by osmosis, and the cell shrinks.

5. 5

   What happens to a red blood cell in a dilute solution?

   1. AIt shrinks
   2. BIt swells and may burst
   3. CIt makes more haemoglobin
   4. DIt divides
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   B: It swells and may burst

   Water enters by osmosis, and the cell swells.

6. 6

   Why can animal cells burst in a dilute solution?

   1. AThey have no nucleus
   2. BThey are too small
   3. CThey have no cell wall
   4. DThey have no membrane
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   C: They have no cell wall

   They have no cell wall.

7. 7

   What is osmosis?

   1. AThe net movement of water from a dilute to a more concentrated solution across a partially permeable membrane
   2. BThe movement of solutes from low to high concentration
   3. CThe movement of oxygen into cells
   4. DThe digestion of food
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   A: The net movement of water from a dilute to a more concentrated solution across a partially permeable membrane

   The net movement of water across a partially permeable membrane from a dilute to a concentrated solution.

8. 8

   A potato cylinder gains mass in a solution. What does this show?

   1. AWater left the cells
   2. BThe cells divided
   3. CThe potato respired
   4. DWater entered the cells by osmosis
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   D: Water entered the cells by osmosis

   Water entered the cells by osmosis, so the solution was more dilute than the cytoplasm.

9. 9

   Which term means the control of the water content of the body?

   1. AThermoregulation
   2. BOsmoregulation
   3. CDigestion
   4. DExcretion
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   B: Osmoregulation

   Osmoregulation.