When reefs turn white
369 words over 33 numbered lines, then 7 parts worth 13 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 surveys, people and figures are invented or rounded for practice.
When reefs turn white
Coral reefs cover less than one per cent of the ocean floor, yet about
a quarter of all marine species depend on them at some stage of their
lives. A reef is built by tiny animals called coral polyps, which take
calcium ions and carbonate ions from sea water to make hard skeletons.
Over thousands of years, these skeletons build up into huge structures
that can be seen from space. The reefs provide food and hiding places
for fish, crabs, sea snails, sea urchins and sharks.
Inside the cells of each polyp live millions of single-celled algae.
The algae photosynthesise and pass up to 90 per cent of the sugars
they make to the coral. In return, the coral provides the algae with
shelter and with the carbon dioxide and nitrogen-containing waste
produced by its own cells. The algae also give the coral its colour.
When the sea becomes too warm, this partnership breaks down. If the
water stays just 1 °C above the usual summer maximum for several
weeks, the algae start to produce harmful substances and the coral
expels them. Without its algae the coral looks white, because its
skeleton can be seen through its clear tissues. This is called
bleaching. A bleached coral is not dead, but it is starving, and if
the water does not cool within a few weeks it may die.
Mass bleaching was almost unknown before the 1980s. In an illustrative
survey of one reef, 4 out of 50 sites were bleached in 1998, but 41
out of 50 sites were bleached in 2024. Scientists link the increase to
global warming, caused by rising concentrations of greenhouse gases
such as carbon dioxide and methane in the atmosphere.
When corals die, the reef is soon covered in seaweed. Fish that feed
on coral polyps disappear, and so do many of the larger fish that feed
on them. Coastal communities lose fishing grounds and tourism, and
their shorelines lose protection from storm waves.
There is some hope. Some corals survive higher temperatures than
others, and researchers are breeding these heat-tolerant corals and
planting them on damaged reefs. Most scientists agree, however, that
this can only buy time unless emissions of greenhouse gases are
reduced.
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.
State what is meant by the term greenhouse gas.
(1)Explain how the coral is able to use the sugars it receives from the algae.
(2)Explain how the waste from the coral’s cells helps the algae to grow.
(2)Calculate the percentage increase in bleached sites on the reef between 1998 and 2024.
(2)Explain how rising concentrations of greenhouse gases cause the temperature of the sea to increase.
(2)Explain why the number of larger fish on the reef also decreases when corals die.
(2)Suggest why planting heat-tolerant corals "can only buy time".
(2)Total for this question: 13 marks
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