Question
a. Outline how cuts in the skin are sealed to prevent blood loss. [4]
b. Outline how two parents could have a child with any of the four ABO blood groups. [4]
c. Explain how ventilation and lung structure contribute to passive gas exchange. [7]
▶️Answer/Explanation
a.
- Platelets detect damage and become activated.
- Platelets release clotting factors.
- Clotting factors trigger a cascade that forms thrombin.
- Thrombin converts fibrinogen to fibrin, which forms a clot to seal the wound.
b.
- One parent is IAi, the other is IBi.
- Each parent can pass on IA, IB, or i alleles.
- Offspring genotypes: IAIB, IAi, IBi, ii.
- Blood groups: AB, A, B, O—all four possible.
c.
- Air travels through trachea → bronchi → bronchioles → alveoli.
- Alveoli have a large surface area and thin walls.
- Type I pneumocytes carry out gas exchange.
- Type II pneumocytes secrete surfactant to reduce surface tension.
- Moist surface allows gases to dissolve for diffusion.
- Ventilation maintains steep concentration gradients.
- Oxygen diffuses into capillaries; CO₂ diffuses into alveoli.
—————————————————————————————————–Markscheme———-
a. platelets respond to/detect skin/blood vessel damage ✔
b. platelets release clotting factors ✔
c. clotting factors trigger a chain/cascade of reactions ✔
d. «leading to» formation of thrombin ✔
e. thrombin causes fibrinogen conversion into fibrin ✔
f. blood clot seals the wound due to fibrin network of fibres ✔
Accept answers presented as a flow chart.
a. «first set of» gametes/parental genotype IA, i ✔
b. «other set of» gametes/parental genotype IB, i ✔
c. «genotypes of offspring are respectively» IAIB, IBi, IAi, ii ✔ All four correct required.
d. «phenotypes of offspring are respectively» AB, B, A, O ✔ All four correct required linked to genotypes
Award marks only for the first grid if more than one drawn;
e.g. of Punnett grid
Answers can be given in a Punnett grid or in prose.
Accept the four possible blood groups of the offspring anywhere in the answer.
a. air carried through trachea AND bronchi/bronchioles AND alveoli ✔ All three required in correct order.
b. alveoli increase the surface area/thin walled for gas exchange ✔
c. gas exchange carried out through type I pneumocytes ✔
d. type II pneumocytes secrete surfactant to reduce surface tension ✔
e. moist surface/surfactant allows gases to diffuse in solution ✔
f. ventilation/moving blood maintains concentration gradients of oxygen and carbon dioxide ✔
g. between air in alveoli and blood in «adjacent» capillaries
OR
oxygen diffuses from alveoli to capillaries and carbon dioxide from capillaries to alveoli ✔ OWTTE
h. external intercostal muscles/diaphragm contract during inspiration ✔
i. lowering air pressure «in lungs»/increasing thorax volume ✔
j. relaxation of external intercostal muscles/diaphragm enable «passive» expiration ✔
k. internal intercostal «and abdominal muscles» contract «to force» expiration ✔
l. expiration due to increasing air pressure «in lungs»/decreasing thorax volume ✔
Accept correctly annotated diagram.
Question
Multicellular organisms benefit from cell specialization and division of labour.
(a) Outline the processes occurring during interphase in the cell cycle.[4]
(b) Describe what occurs in a neuron when an action potential is propagated along the axon. [4]
(c) Explain how cells in the bloodstream cause a specific immune response.[7]
Answer/Explanation
(a)
- The cell grows and increases in size.
- It produces more organelles like mitochondria and ribosomes.
- DNA replication happens, so each chromosome is copied.
- The cell also makes proteins and mRNA through transcription to prepare for division.
- ATP is produced through respiration to provide energy for all these processes.
(b)
- When a neuron is stimulated, sodium (Na⁺) channels open and sodium ions rush into the axon.
- This causes depolarization the inside of the axon becomes more positive.
- Then, potassium (K⁺) channels open and potassium ions move out, causing repolarization.
- As sodium ions spread along the axon, they trigger nearby sodium channels to open, allowing the action potential to move down the axon like a wave.
(c)
- A specific immune response means making antibodies against a particular pathogen.
- Macrophages (a type of white blood cell) engulf pathogens and present their antigens on their surface.
- These antigens activate T lymphocytes (T cells).
- Activated T cells stimulate B lymphocytes (B cells) that are specific to the antigen.
- Only B cells with the matching antibody are selected and activated.
- These B cells divide by mitosis to form plasma cells.
- Plasma cells make large amounts of antibodies that bind to the pathogen.
- Some B cells become memory cells, which help the body respond faster if the same pathogen appears again.
——————————————————————————————————Markscheme——————————————————————————————————————
a a. growth/increase in cell size;
b. division of mitochondria/chloroplasts/production of more organelles/number of organelles doubled;
c. replication of DNA/amount of DNA is doubled;
d. transcription of genes/production of mRNA;
e. protein synthesis;
f. cell respiration/production of ATP;
b a. sodium ions/Na+ enter/diffuse in;
b. depolarization/membrane potential/voltage changes from negative to positive;
c. potassium channels open AND potassium ions/K+ exit/diffuse out;
d. repolarization/membrane potential/voltage changes back from positive to negative;
e. local current due to diffusion of sodium ions along the neuron;
f. (local currents) cause next sodium channels to open/next part of axon to depolarize;
g. opening of sodium channels triggered when threshold potential/-50mV reached;
c a. (specific immune response is) production of antibodies in response to a particular pathogen;
b. antibody is specific to/binds to a specific antigen;
c. macrophages/phagocytes engulf/present antigens from pathogens/viruses/bacteria;
d. T lymphocytes activated by antigens/antigen presentation/antigens presented by macrophage;
e. (activated) T lymphocytes activate B lymphocytes;
f. only B lymphocytes that produce antibodies against the antigen/pathogen are activated;
g. (activated) B lymphocytes clone/divide by mitosis to form plasma cells;
h. plasma cells then secrete (large quantity) of an antibody/secrete antibodies of same type;
i. some B lymphocytes/plasma cells form memory cells;
j. memory cells give long lasting immunity/faster response to a disease/pathogen;