A plant that counts
401 words over 35 numbered lines, then 8 parts worth 15 marks (Linear Paper 2 (4BI1/2B) style, 14–18 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.
A plant that counts
The Venus flytrap grows wild in only one small area of the world: the
wet, sandy bogs of North and South Carolina in the United States. The
soil there is acidic and contains very few nitrate ions, because
decomposers work slowly in the waterlogged ground. Most plants grow
poorly in these conditions, but the flytrap has found another source
of nitrogen. Charles Darwin, who studied the plant in the 1870s,
called it one of the most wonderful plants in the world.
Each leaf ends in a pair of hinged lobes lined with stiff teeth. On
the inner surface of each lobe there are three tiny trigger hairs.
When an insect touches a hair, an electrical signal passes across the
leaf, but the trap does not close. Only if a second hair is touched
within about 20 seconds does the trap snap shut, in about a tenth of a
second. Botanists say that the plant can count to two. Each trap can
only close a few times before it dies, and reopening it takes about a
day.
Once the insect is trapped, its struggles touch the hairs again and
again. The lobes press tightly together and glands on the inner
surface release digestive enzymes, including proteases. Over about a
week, the soft parts of the insect are digested and the products are
absorbed into the leaf. Only the hard outer skeleton is left, and the
trap opens again ready for its next meal. A trap that closes on
something that cannot be digested, such as a small stone, opens again
within a day.
The flytrap still has green leaves and photosynthesises. In an
illustrative investigation, scientists grew two groups of young
flytraps in a greenhouse for eight weeks. One group was fed one small
cricket each week; the other group was not fed. The mean mass of the
fed plants increased from 1.6 g to 4.2 g, while the mean mass of the
unfed plants increased from 1.6 g to 2.8 g.
In the wild, the number of flytraps has fallen sharply. Many bogs have
been drained to build houses and roads, and plants are dug up and sold
illegally. Natural fires used to clear away shrubs that would shade
the flytraps, but fires are now usually put out quickly. Today it is
illegal to take a flytrap from the wild, and the plants sold in garden
centres are grown in nurseries.
Read the article. 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.
The touch of an insect is a stimulus. State what is meant by the term stimulus.
(1)Which substance is produced when proteases digest the proteins in the insect?
(1)Explain how catching insects helps the flytrap to grow in soil with very few nitrate ions.
(2)Calculate how many times greater the increase in mean mass of the fed plants was than the increase in mean mass of the unfed plants.
(2)Suggest why it is an advantage to the flytrap that the trap only closes if two hairs are touched within about 20 seconds.
(3)Describe how the scientists could make sure that the difference in mass was caused only by feeding the plants.
(2)Explain why the flytrap still needs light even though it catches insects.
(2)Suggest why putting out fires quickly could reduce the number of flytraps.
(2)Total for this question: 15 marks
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