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Date	November 2020	Marks available	3	Reference code	20N.2.HL.TZ0.4
Level	Higher level	Paper	Paper 2	Time zone	TZ0 / no time zone
Command term	Describe	Question number	4	Adapted from	N/A

Question

Distinguish between the transfers of energy and inorganic nutrients in ecosystems.

[2]

Outline the role of methanogenic archaeans in the movement of carbon in ecosystems.

[2]

Describe how autotrophs absorb light energy

[3]

Markscheme

energy is lost (between trophic levels) / not all passed on / not reused / must be supplied;

nutrients are recycled/reused;

a. methane produced from organic matter;

b. in anaerobic conditions;

c. methane diffuses into atmosphere/accumulates in ground/soil;

d. oxidized/converted to carbon dioxide (in atmosphere);

a. light absorbed by (photosynthetic) pigments;

b. chlorophyll absorbs blue and red / drawing of absorption spectrum for chlorophyll;

c. photosystems are groups of pigment molecules/are light harvesting complexes;

d. photosystems are located in thylakoid membranes;

e. electrons excited/raised to higher energy level;

Examiners report

A sizeable number of candidates seemed to miss the point here, in what was intended to be an easy question. The command term ‘distinguish’ is an instruction to give the key differences. In this case, the distinction was that energy flows through an ecosystem and is ultimately lost whereas inorganic nutrients can be recycled.

Answers were rather polarised, with candidates either understanding the role of methanogens or stumbling round in a mire of uncertainty. Although this may seem an obscure part of biology, methanogenesis is of great significance in terms of climate change.

As this question was near the end of Section A, the mark scheme was constructed so as to reward stronger answers, rather than simple ideas such as ‘light is absorbed by chlorophyll’. Answers were very varied. Few candidates including any ideas about photosystems.

Syllabus sections

Core » Topic 4: Ecology » 4.3 Carbon cycling

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