The rat that never drinks
327 words over 30 numbered lines, then 6 parts worth 12 marks (Modular Unit 2 (4WBI2) style, 12–13 marks). Read the article, answer each part, then open its mark scheme and set your mark.
Illustrative article written for Scholar Lab, not a Pearson text. The biology is real; the studies, people and figures are invented or rounded for practice.
The rat that never drinks
Kangaroo rats live in the deserts of North America, where it may not
rain for months. They hop on their long back legs and eat mostly dry
seeds. Remarkably, many kangaroo rats never drink water at all. To
survive, they must lose as little water as possible. A kangaroo rat is
about the size of a small orange, so it has a large surface area
compared with its volume and could easily dry out in the hot, dry air.
In mammals, the kidneys control the water content of the blood. This
is called osmoregulation. In each kidney, blood is filtered under high
pressure in the glomerulus, and the filtrate passes along a nephron.
Useful substances such as glucose are reabsorbed into the blood. Water
is reabsorbed by osmosis as the filtrate flows down the collecting
duct.
The amount of water reabsorbed is controlled by a hormone called ADH,
which is released by the pituitary gland. When the blood is too
concentrated, more ADH is released. ADH makes the walls of the
collecting ducts more permeable to water, so more water is reabsorbed
and a small volume of concentrated urine is produced.
The kidneys of kangaroo rats have unusually long loops of Henle and
can make extremely concentrated urine. Figure 1 compares the most
concentrated urine that different mammals can produce. Their faeces
are also very dry, because water is absorbed from them in the large
intestine. A human who drank nothing at all would survive only a few
days in the same conditions.
Kangaroo rats also gain water in another way. The seeds they eat
contain starch and fat. When these are used in aerobic respiration,
water is one of the products. For a kangaroo rat, this water from
respiration provides most of the water it needs.
Behaviour helps as well. During the hot day, kangaroo rats stay in
burrows, where the air is cooler and more humid, and they come out to
feed only at night.
Figure 1 Most concentrated urine produced by four mammals
Read the article and study the figure. Use the information in the article and your own knowledge to answer the questions. The lines in brackets show where to look; select them to highlight the text.
Use the information in the article and your own knowledge to answer the questions. The lines in brackets show you where to look; select them to highlight the text.
State what is meant by the term osmoregulation.
(1)Name the hormone that controls water reabsorption in the kidney, and the gland that releases it.
(2)Explain how ADH reduces the volume of water lost in urine.
(3)Use Figure 1 to calculate how many times more concentrated the urine of a kangaroo rat can be than the urine of a human.
(2)Explain how eating dry seeds can provide a kangaroo rat with water.
(2)Suggest how staying in a burrow during the day helps a kangaroo rat to reduce water loss.
(2)Total for this question: 12 marks
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