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
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 ✔
Question
All living organisms depend on a continuous supply of energy.
a. Explain the stages of aerobic respiration that occur in the mitochondria of eukaryotes. [8]
b. Outline how ventilation in humans ensures a supply of oxygen. [4]
c. Describe the reasons for the shape of a pyramid of energy. [3]
▶️Answer/Explanation
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. 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
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