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

Question

The electron micrograph shows part of a cell including a mitochondrion.

[Source: Used with permission of McGraw-Hill Education, from Harrison’s Principles of Internal Medicine,
J L Jameson et al., 16th edition, 2004; permission conveyed through Copyright Clearance Center, Inc.]

Outline how the structures labelled X and Y are adapted to carry out the function of the mitochondrion.

X:

 

Y:

[2]
a.

Explain how ATP is generated in mitochondria by chemiosmosis.

[4]
b.

Markscheme

X: large/increased SA area for ATP production/electron transport/oxidative phosphorylation/proton pumping
OR
X: small/narrow intermembrane space for generating proton gradient (rapidly/steeply);

Y: contains enzymes for Krebs cycle/link reaction;

a.

a. protons pumped across inner membrane of mitochondria/into intermembrane space;

b. using energy released by flow of electrons/by electron transport/by electron carriers;

c. proton gradient established/maintained / proton motive force generated;

d. protons pass/diffuse back through inner membrane/membrane of cristae/to matrix;

e. through ATP synthase;

f. ATP production coupled to flow of protons / ATP from ADP and Pi using energy from protons;

Marks can be awarded in an annotated diagram.

b.

Examiners report

The mean mark for this question was only 0.6 – less than half marks. Many students did not recognise they had to describe adaptations so, for example, the large surface area of the cristae or the small volume of the intermembrane space had to be included together with the function carried out. Weaker candidates had forgotten basic ideas of what happens in the mitochondrion.

a.

This was answered more successfully by most candidates and there were some impressively detailed and accurate answers.

b.

Syllabus sections

Core » Topic 1: Cell biology » 1.2 Ultrastructure of cells
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