Question
a. 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]
b. Describe the role of oxygen in aerobic cell respiration. [5]
c. 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]
▶️Answer/Explanation
a.
- Active transport is when a substance is moved from a low concentration to a high concentration, which is the opposite direction of diffusion.
- This movement needs energy from ATP and uses special carrier proteins or pumps in the cell membrane.
- It allows the cell to absorb more calcium, even when calcium levels are already higher inside the cell than outside. This is important when calcium in food is low — the body can still take in enough.
b.
- Oxygen is the final electron acceptor in the electron transport chain inside the mitochondria.
- It accepts electrons and hydrogen ions (protons) at the end of the chain, which helps form water (O₂ + 2e⁻ + 2H⁺ → H₂O).
- This step is important because it keeps the chain going, which helps the cell continue making ATP.
- By removing hydrogen ions, oxygen helps maintain the proton gradient, which is needed to power ATP production through chemiosmosis.
- It also allows NADH and FADH₂ to be recycled back to NAD⁺ and FAD, so they can be reused in earlier stages of respiration.
c.
- The body keeps gas exchange working well by constantly bringing in fresh air (with oxygen) and removing carbon dioxide through breathing.
- This happens because muscles like the diaphragm and rib muscles contract and relax to change chest volume, pulling air in and out.
- During inhaling, the diaphragm and external intercostal muscles contract, increasing chest space and drawing in air.
- The lungs are filled with tiny air sacs called alveoli, and each one is surrounded by many capillaries, which keeps the blood moving.
- Oxygen diffuses from the alveoli into the blood, and carbon dioxide moves from the blood into the alveoli to be breathed out.
- This gas movement is fast because the walls of the alveoli and capillaries are very thin (just one cell thick).
- There are millions of alveoli, giving a huge surface area, and they are kept moist so gases can dissolve and diffuse easily.
—————————————————————————————————-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. terminal/final electron acceptor ✔
b. at the end of electron transport chain ✔
c. oxygen also accepts protons/hydrogen ions ✔
d. water produced/
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 ✔