Question
The diagram shows the original drawing of the cell membrane made in 1972 by Singer and Nicolson.
(a) Label with the letter H part of a phospholipid molecule that is hydrophilic.
(b) Distinguish between this model of the cell membrane and the Davson–Danielli model.
(c) Explain the role of molecules such as M in sodium–potassium pumps.
▶️Answer/Explanation
(a)
(b) proteins in the phospholipid bilayer/embedded versus sandwich of phospholipids and
proteins/proteins on outside/proteins on top and bottom of membrane (in Davson-Danielli);
(c) a. active transport / ATP used/hydrolyzed to provide energy for pumping/conformation change;
b. transfers across membrane against concentration gradient / creates concentration gradient;
c. sodium ions/Na+ out and potassium ions/K+ in;
d. in axons;
e. creates a resting potential/membrane potential so that action potentials can take place;
Question
▶️Answer/Explanation
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 ✔