▶️ Answer/Explanation
Defence by Body Cells:
Skin cells act as a physical barrier, blocking pathogens.
- They also produce acids and enzymes (like lysozyme) to kill microbes.
- Mucous membranes trap pathogens in mucus.
- Stomach cells make hydrochloric acid to kill swallowed microbes.
Platelets help form blood clots, sealing cuts and preventing pathogen entry.
White Blood Cells (Leukocytes):
There are two main types:
1. Phagocytes (Non-specific immunity):
- These cells engulf and digest any pathogen (via phagocytosis).
They give general protection, not specific to one type of microbe.
2. Lymphocytes (Specific immunity):
- These cells recognize specific antigens on pathogens.
- B cells (a type of lymphocyte) make antibodies to fight specific pathogens.
- Each B cell makes one type of antibody.
Memory Cells:
- Some lymphocytes become memory cells.
- If the same pathogen infects again, they respond faster by quickly producing the right antibody.
This is how immunity is developed after an infection or vaccination.
Markscheme:
a. Cells of skin provide a physical barrier/produce fatty acids/lactic acid/lysozyme which stops entry of microbes
OR
Mucous membranes produce mucus to trap pathogens
OR
Stomach cells produce hydrochloric acid which kills microbes;
b. Platelets start the clotting process preventing access of pathogens;
c. (Two types of) white blood cells fight infections in the body;
d. Phagocytes ingest pathogens (by endocytosis/phagocytosis);
e. Gives non-specific immunity to diseases / ingest any type of pathogen;
f. Production of antibodies by lymphocytes/B cells;
g. In response to particular pathogens/antigens;
h. Gives specific immunity;
i. Lymphocyte/B cell makes only one type of antibody;
j. Plasma cells produce large quantity of (one type of) antibody;
k. Some lymphocytes act/remain as memory cells;
l. Can quickly reproduce to form a clone of plasma cells if a pathogen carrying a specific antigen is re-encountered;
m. Results in faster defence against second exposure to specific antigen/pathogen/disease;
▶️ Answer/Explanation
a.
Parents:
One parent: IAi (blood type A)
Other parent: IBi (blood type B)
Punnett Grid:
| IB | i | |
|---|---|---|
| IA | IAIB (AB) | IAi (A) |
| i | IBi (B) | ii (O) |
Phenotypes of children:
A, B, AB, and O — all four blood groups possible.
Ratio: 1 : 1 : 1 : 1
b.
| Feature | Arteries | Veins |
|---|---|---|
| Wall thickness | Thick, muscular walls | Thinner walls |
| Valves | No valves | Valves to prevent backflow |
| Elastic layer | Thick elastic layer | Thin elastic layer |
| Lumen size | Narrow (smaller lumen) | Wide (larger lumen) |
c.
- Platelets detect injury and release clotting factors.
- A cascade of reactions begins.
- This produces an enzyme called thrombin.
- Thrombin converts fibrinogen (soluble) into fibrin (insoluble).
- Fibrin forms a mesh over the wound, trapping blood cells and sealing the cut.
Markscheme:
Answer to part (a):
a. Correct gametes of one parent as IA AND i in header line/column AND correct gametes of other parent as IB AND i in header column/line
b. Correct corresponding genotypes in inner squares as IAIB, IAi, IBi, ii
c. Corresponding phenotypes of children identified as AB, A, B and O
d. Ratio of phenotypes is 1:1:1:1 (OWTTE)
Allow ECF.
Answer to part (b):
a. Arteries have thicker (muscular) walls/layer/tunica media OR veins have thinner (muscular) walls/layer/tunica media
b. Arteries have no valves OR veins have valves
c. Arteries have thicker elastic layer OR veins have thinner elastic layer
d. Arteries have a smaller lumen/bore OR veins have a larger lumen/bore
Accept answers presented in a table.
Answer to part (c):
a. Clotting factors released from platelets
b. Clotting process involves a cascade/series of reactions
c. Produces thrombin
d. Causes rapid conversion of fibrinogen into fibrin
e. Fibrin makes a mesh to seal the wound (OWTTE)
▶️ Answer/Explanation
a.i.
a.ii.
b.
- Valves open and close based on pressure changes as the heart beats.
- They prevent backflow, keeping blood moving in one direction.
- This helps the heart fill and empty properly during each beat.
c.
- Clots in coronary arteries cause coronary artery disease (CAD).
- This can start from a fatty diet or high LDL cholesterol → forms plaque.
- If plaque breaks, it damages the artery wall and triggers a blood clot.
- A clot may block the artery, stopping oxygen from reaching the heart → leads to a heart attack.
- In sickle cell disease, misshaped red cells can also cause clots that block arteries.
d.
- Lymphocytes produce antibodies to attack pathogens.
- They also make memory cells, which give long-term immunity.
- Each lymphocyte responds to a specific antigen (specific immunity).
- They recognize and destroy infected or foreign cells, including cancer cells.
Markscheme:
a.i.
Both labelled clearly as in diagram ✔
a.ii.
Both labelled clearly as in diagram ✔
b.
a. valves open and close in response to changes in blood pressure/heart contraction/pumping ✔
b. valve prevents backflow/maintains direction of blood flow ✔
c. valves allow heart chambers to fill/to empty ✔
c.
a. coronary heart disease/CHD/coronary artery disease/CAD occurs when there is reduction of oxygen to the heart muscle ✔
b. high ratio of LDL to HDL/fatty diet leads to plaque formation in arteries ✔
c. plaque breaks off causing damage that activates blood clot formation ✔
d. clots «in the bloodstream» may block a coronary artery/coronary thrombosis reducing blood flow/oxygen
OR
clots can cause heart attack/muscle death ✔
e. sickle cell anemia «crisis» produces blood clots «that can cause coronary/arterial blockage» ✔
d.
a. produce antibodies ✔
b. memory cells confer immunity ✔
c. specific immunity results from production of antibodies specific to a particular antigen ✔
d. recognize pathogens ✔
e. destroy foreign cells/cancer cells ✔
▶️ Answer/Explanation
a.
- Platelets detect damage to the skin or blood vessels.
- They release clotting factors.
- This starts a clotting cascade that produces thrombin.
- Thrombin changes fibrinogen into fibrin.
- Fibrin forms a mesh that traps blood cells and seals the wound as a clot.
b.
- Parent 1 genotype: IAi (blood type A)
- Parent 2 genotype: IBi (blood type B)
Gametes:
- Parent 1 gives IA or i
- Parent 2 gives IB or i
Possible combinations in child:
- IAIB = AB
- IAi = A
- IBi = B
- ii = O
So, all four blood groups are possible.
c.
Lung structure:
- Air moves from trachea → bronchi → bronchioles → alveoli.
- Alveoli have a huge surface area and very thin walls, perfect for gas exchange.
- Type I pneumocytes help gases pass through easily.
- Type II pneumocytes make surfactant to keep alveoli open and reduce surface tension.
- The alveoli are moist, so gases can dissolve and diffuse.
Ventilation:
- Inhalation: diaphragm and external intercostals contract → chest volume increases → pressure drops → air in.
- Exhalation (passive): muscles relax → chest volume decreases → pressure rises → air out.
- Forced exhalation uses internal intercostals and abdominal muscles.
Gas exchange:
- Constant airflow keeps oxygen high in alveoli and CO₂ high in blood, so diffusion continues.
Markscheme:
a.
1. Platelets detect skin/blood vessel damage
2. Platelets release clotting factors
3. Clotting cascade produces thrombin
4. Thrombin converts fibrinogen to fibrin
5. Fibrin network forms clot to seal wound
Accept flow chart format
b.
• Parental genotypes: IAi × IBi
• Possible gametes: IA, i (from Parent 1) and IB, i (from Parent 2)
• Offspring genotypes:
– IAIB (AB)
– IAi (A)
– IBi (B)
– ii (O)
Accept answers in Punnett grid or prose
c.
Lung Structure:
1. Air pathway: trachea → bronchi → bronchioles → alveoli
2. Alveoli provide large SA with thin walls for diffusion
3. Type I pneumocytes facilitate gas exchange
4. Type II pneumocytes secrete surfactant to reduce surface tension
5. Moist surface allows gas dissolution
Ventilation Mechanism:
6. Inspiration: external intercostals/diaphragm contract → ↑thorax volume → ↓pressure
7. Passive expiration: muscle relaxation → ↓volume → ↑pressure
8. Forced expiration: internal intercostals/abdominal muscles contract
Gas Exchange:
9. Maintains concentration gradients (O2 alveoli→blood; CO2 blood→alveoli)
Accept annotated diagrams