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
The diagram shows part of two neurons.
a.i. Outline how the amphipathic properties of phospholipids play a role in membrane structure. [2]
a.ii. State the role of cholesterol in animal cell membranes. [1]
b. Describe what happens to the membranes of an animal cell during mitosis. [2]
c.i. State the name of the structure shown. [1]
c.ii. X indicates the movement of a structure in the neuron. Explain what events trigger this movement and what happens next. [3]
▶️Answer/Explanation
Markscheme
a. part hydrophobic/not attracted to water/non-polar AND part hydrophilic/attracted to water/polar;
b. bilayer formed (formed naturally by phospholipids in water);
c. hydrophilic heads/parts face outwards and hydrophobic tails/parts face inwards.
Do not allow water loving/hating in mpa or mpc.
a. controls/regulates/reduces fluidity of membrane / prevents crystallization of phospholipids;
b. reduces permeability to some substances.
Do not accept ‘stabilizes membrane’.
a. nuclear membrane breaks down/disappears (in prophase/at start of mitosis);
b. nuclear membrane reforms around two new nuclei (in telophase/at end of mitosis);
c. plasma membrane pulled inwards at equator / cleavage furrow formed;
d. membrane pinches apart to form two cells / cytoplasm divided / cytokinesis.
synapse/synaptic
Allow any answer including either of these terms unless out of context.
a. depolarization of pre-synaptic membrane / action potential/nerve impulse arrives;
b. uptake of calcium / calcium ions diffuse in / calcium channels open;
c. structures containing neurotransmitter/vesicles move to/fuse with membrane;
d. neurotransmitter/acetylcholine released by exocytosis into cleft/binds to postsynaptic membrane/receptors;
Must see exocytosis.