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Date	November 2021	Marks available	3	Reference code	21N.2.SL.TZ0.3
Level	Standard level	Paper	Paper 2	Time zone	TZ0 / no time zone
Command term	Explain	Question number	3	Adapted from	N/A

Question

Compare and contrast the mode of nutrition of detritivores and saprotrophs.

[2]

The image shows an example of a soil food web.

[Source: Anon., n.d. The Soil Food Web. [image online] Available at: https://www.nrcs.usda.gov/wps/portal/nrcs/
photogallery/soils/health/biology/gallery/?cid=1788&position=Promo [Accessed 11 March 2020].]

Draw a food chain from this food web, showing at least three organisms.

[1]

b.i.

Explain the reasons for food chains rarely containing more than four or five trophic levels.

[3]

b.ii.

The amount of food passing into food chains can be affected by the rate of photosynthesis. Explain the effect of one limiting factor on photosynthesis.

[3]

All of the leaves in the image are from Solanum, a wild genus of tomato.

[Source: Courtesy: National Science Foundation, Credit Leonie Moyle.]

State one cause of variation in a plant such as the tomato.

[1]

Markscheme

Accept not autotrophic/not photosynthetic instead of heterotrophic.

Do not accept that both groups are decomposers or consumers for the similarity.

food chain of three or more organisms starting with plants;

b.i.

a. energy is lost between the trophic levels;
b. transfer between levels is only usually 10% efficient
OR
energy transformations take place in living organisms / the process is never 100% efficient;
c. energy is lost by the organism/used in respiration / released as heat/movement;
d. energy is lost as waste/feces/urine/undigested food/uneaten parts;
e. as energy is lost between trophic levels and so (higher ones) have less biomass / less biomass available for next level;

b.ii.

a. the rate of reaction will be limited by the limiting factor that is nearest to its minimum value;

temperature:
b. enzymes that control photosynthesis are influenced by temperature;
c. as temperature increases, reaction rate will increase;
d. above a certain temperature, the rate of photosynthesis will decrease;
e. (where temperature is limiting) essential enzymes begin to denature/not working to optimum;

light intensity:
f. light is source of energy / converted into chemical energy;
g. as light intensity increases reaction rate will increase;
h. at a certain light intensity, rate of photosynthesis will plateau;
i. another factor becomes limiting;

CO₂ concentration:
j. CO₂ is fixed to form organic molecules;
k. as CO₂ concentration increases, reaction rate will increase;
l. at a certain concentration of CO₂, rate of photosynthesis will plateau;
m. another factor becomes limiting;

Accept answers using an annotated graph to explain

Only accept the first factor described

Do not accept pH as a limiting factor

a. mutations;
b. meiosis/crossing over/random assortment of homologous pairs;
c. sexual reproduction/recombination/random fertilisation;

Examiners report

Knowledge of detritivores and saprotrophs was good in 3a, with many of the poorer answers describing them as feeding on inorganic material.

Many took the word ‘draw’ literally in the food chain, and the drawings of the nematodes and fungi amused the examiners. A food chain must start with a plant. The stem did say “three organisms”. Detritus was not taken as an organism, so negated the mark.

b.i.

Most were able to gain at least two marks for the explanation of energy loss through the trophic levels.

b.ii.

In 3c, most knew that temperature/CO₂ concentration/light affect photosynthesis but could not explain the idea of acting as a limiting factor. Only the better students gained all 3 marks.

In 3d there was a great deal of variety in the leaf shape, so an answer in terms of sexual reproduction was looked for.

Syllabus sections

Core » Topic 2: Molecular biology » 2.9 Photosynthesis

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