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Date May 2018 Marks available 3 Reference code 18M.2.HL.TZ2.6
Level Higher level Paper Paper 2 Time zone Time zone 2
Command term Describe Question number 6 Adapted from N/A

Question

All living organisms depend on a continuous supply of energy.

Explain the stages of aerobic respiration that occur in the mitochondria of eukaryotes.

[8]
a.

Outline how ventilation in humans ensures a supply of oxygen. 

[4]
b.

Describe the reasons for the shape of a pyramid of energy.

[3]
c.

Markscheme

a. «cell» respiration is the «controlled» release of energy from organic compounds to produce ATP 

b. «cell respiration» involves the oxidation and reduction of electron carriers 

c. in link reaction pyruvate is converted into acetyl coenzyme A, CO2 is released and NAD is reduced 

d. in the Krebs cycle, a 4 C molecule combines with acetyl CoA 

e. decarboxylation releases 2 CO2 molecules for each pyruvate / conversion of 6C to 5C/5C to 4C releases CO2 

f. «3» reduced NAD and «1» reduced FAD are produced 

g. ATP generated in the Krebs cycle 

h. reduced molecules/FAD/NAD are carried to the cristae/inner membrane of the mitochondria 

i. transfer of electrons between carriers in the electron transport chain in the membrane of the cristae is coupled to proton pumping 

j. protons accumulate in intermembrane space/ between cristae/inner membrane and outer membrane
OR
proton / electrochemical gradient between intermembrane space and matrix is established 

k. protons diffuse through ATP synthase to generate ATP 

l. chemiosmosis is the use of a proton/electrochemical gradient to generate ATP 

m. oxygen is the final electron acceptor

Accept any of the points in a correctly annotated diagram.

a.

a. ventilation is exchange of gases between lungs and air. 

b. during inhalation diaphragm contracts AND lowers. 

Both needed.

c. external intercostal muscles contract, raising ribs upwards and outwards 

d. increase in volume AND decrease in pressure within thoracic cavity 

e. air drawn into alveoli bringing fresh supply of oxygen 

f. oxygen concentration in alveolar sacs is higher than in blood capillaries 

g. «oxygen concentration gradient» causes oxygen to diffuse out of alveoli into red blood cells in capillaries 

b.

a. pyramid of energy has stepped shape with largest bottom step being producers, then first consumer, second consumer, etc 

b. light energy «from sun» converted to chemical energy in carbon compounds by photosynthesis 

c. energy released by respiration is used in living organisms AND converted to heat 

d. heat «energy» is lost from ecosystems 

e. approximately 10 % of energy in trophic level converted into new material for next level 

f. energy also lost as undigested material/uneaten material/feces/excretion 

c.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.

Syllabus sections

Core » Topic 4: Ecology » 4.2 Energy flow
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