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Date November 2018 Marks available 7 Reference code 18N.2.HL.TZ0.6
Level Higher level Paper Paper 2 Time zone TZ0 / no time zone
Command term Explain Question number 6 Adapted from N/A

Question

Calcium is absorbed from food in the human gut by both active and passive processes. Outline active transport, including the benefits of the process.

[3]
a.

Describe the role of oxygen in aerobic cell respiration.

[5]
b.

Adult humans may absorb more than five hundred litres of oxygen per day. Explain how gas exchange is maintained in the human respiratory system.

[7]
c.

Markscheme

a. moved against a concentration gradient/lower to higher concentration ✔

b. energy/ATP required/used ✔

c. pump/carrier «protein» «carries out active transport» ✔

d. absorption «by active transport» into a cell is possible even if exterior concentrations are «very» low
OR
allows all/nearly all of/more of the substance/calcium to be absorbed «whereas diffusion can only even out concentrations»
OR
unidirectional/allows the direction of movement to be controlled
OR
allows a concentration gradient to be built up/potential energy to be stored/membrane potential to be generated/maintained
OR
allows a specific concentration to be maintained «in a cell» ✔

a.

a. terminal/final electron acceptor ✔

b. at the end of electron transport chain ✔

c. oxygen also accepts protons/hydrogen ions ✔

d. water produced/  1 2 O2 + 2 electrons + 2H+ → H2O ✔

e. helps to maintain proton gradient «across inner mitochondrial membrane by removal of protons from the stroma» ✔

f. oxygen is highly electronegative/electrons strongly attracted to oxygen ✔

g. avoids anaerobic respiration/buildup of lactic acid ✔

h. allows more electrons to be delivered to the electron transport chain
OR
allows NADFAD to be regenerated/reduced NAD/FAD converted back to NAD/FAD ✔

i. oxygen allows maximum yield of energy «from glucose» allows complete oxidation of glucose/allows fats to be used in respiration ✔

b.

a. ventilation/inhaling brings fresh air/air with high oxygen concentration to the lungs
OR
ventilation/exhaling gets rid of stale air/air with high concentration of carbon dioxide ✔

b. ventilation due to muscle contractions causing pressure/volume changes in the thorax ✔

c. contraction of external intercostal muscles AND diaphragm occurs during inspiration
OR
contraction of internal intercostal muscles/abdomen wall muscles during «forced» expiration ✔

d. alveoli surrounded by «many» capillaries ✔

e. blood flow/pumping of heart «brings blood to/takes blood away from alveoli/lungs» ✔

f. concentration gradients «of oxygen/ CO2» maintained «by ventilation/blood flow» ✔

g. O2 AND CO2 diffuse ✔

h. CO2 from capillaries/blood/vessel to alveolus/air AND O2 from alveoli into capillaries/blood/vessel ✔

i. large numbers of alveoli increase surface area ✔

j. short distance so rapid diffusion/gas exchange ✔

k. type I pneumocytes/alveolus wall/capillary walls are one cell thick/very thin ✔

l. alveoli «lining» moist for dissolving of gases/rapid diffusion
OR
type II pneumocytes keep the «lining of» the alveolus moist ✔

m. type II pneumocytes secrete surfactant to reduce surface tension/prevents alveoli from collapsing ✔

c.

Examiners report

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

Syllabus sections

Core » Topic 6: Human physiology » 6.4 Gas exchange
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