5.15 Water, urea and osmosis in the body
6 exam questions worth 16 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Give two ways that water leaves the body that are not controlled.
Mark scheme
- through the lungs / in exhaled breath / as water vapour (1)
- through the skin / in sweat (1)
Model answer
Through the lungs when we breathe out, and through the skin in sweat.
Explain why it is important to keep the water content of the blood constant.
Mark scheme
- if the blood / plasma is too dilute, water moves into cells by osmosis (1)
- cells swell / burst (lysis) (1)
- if too concentrated, water moves out of cells (by osmosis) and they shrink (1)
- cells cannot function (efficiently) (1)
Max 3
Model answer
If the blood becomes too dilute, water moves into cells by osmosis, so they swell and may burst. If it becomes too concentrated, water moves out of the cells by osmosis and they shrink. In both cases the cells cannot function properly.
Red blood cells are placed in pure water.
Describe and explain what happens to them.
Mark scheme
- water has a higher water potential outside / the cytoplasm is more concentrated (1)
- water moves into the cells by osmosis (1)
- cells swell and burst (lysis) (1)
Model answer
Pure water has a higher water potential than the cytoplasm of the red blood cells, so water moves into the cells by osmosis through the partially permeable membrane. The cells swell and burst, because they have no cell wall.
Higher tier. Describe how excess amino acids from the diet are made safe and removed from the body.
Mark scheme
- amino acids are deaminated (1)
- in the liver (1)
- forming ammonia (1)
- ammonia is toxic and is converted to urea (1)
- urea is (carried in the blood and) filtered / removed by the kidneys in urine (1)
Max 4
Model answer
Excess amino acids are deaminated in the liver: the amino group is removed, forming ammonia. Ammonia is toxic, so the liver immediately converts it into urea. Urea is carried in the blood to the kidneys, which remove it in the urine.
Where is urea made?
Choose one answer.
Correct Not quite: the answer is Liver
Mark scheme
B (liver)
Model answer
B: the liver.
On a hot day, a person sweats a lot.
Suggest how this would affect the urine they produce. Explain your answer.
Mark scheme
- less urine / smaller volume (1)
- more concentrated urine (1)
- because water has been lost in sweat, so the kidneys reabsorb more water (to keep the blood water content constant) (1)
Model answer
They would produce a smaller volume of more concentrated urine, because they have lost water in sweat, so the kidneys reabsorb more water back into the blood to keep its water content constant.
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