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

Describe the role of oxygen in aerobic cell respiration.

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

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. terminal/final electron acceptor ✔

b. at the end of electron transport chain ✔

c. oxygen also accepts protons/hydrogen ions ✔

d. water produced/ $\frac{1}{2}$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 ✔

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 ✔