A Level H1 Biology Human Physiology Quiz

A-Level Biology H1 Quiz - Human Physiology (Answer Key)

Total Marks: 40

Section A: Multiple Choice Answers

1. B
Reasoning: Antibodies bind to antigens, causing agglutination (clumping) which facilitates phagocytosis. They do not engulf pathogens (phagocytes do), release histamine (mast cells do), or differentiate into memory cells (B-cells do).

2. C
Reasoning: Vaccines stimulate the primary immune response, leading to the formation of memory B-cells and T-cells. This ensures a rapid and robust secondary response upon actual infection.

3. D
Reasoning: Innate immunity is non-specific and present at birth. Adaptive immunity is specific, has a lag time, involves memory, and develops after exposure.

4. B
Reasoning: Helper T-cells release cytokines that are essential for activating B-cells (for antibody production) and cytotoxic T-cells (for killing infected cells). Their depletion cripples the entire adaptive immune response.

5. C
Reasoning: Active immunity involves the host’s immune system producing its own antibodies and memory cells. Passive immunity involves receiving pre-formed antibodies without memory cell production.

6. B
Reasoning: Vasodilation increases blood flow and permeability, allowing more phagocytes and plasma proteins (like antibodies and clotting factors) to reach the infected tissue.

7. B
Reasoning: Memory B-cells (and Memory T-cells) persist after the primary response and are responsible for the rapid secondary response. Plasma cells are effector cells that produce antibodies but are short-lived.

8. B
Reasoning: Antibiotics target specific bacterial structures (e.g., cell walls, ribosomes) or metabolic pathways. Viruses lack these structures and rely on host cell machinery, making them unaffected by antibiotics.

9. A
Reasoning: Herd immunity occurs when a high percentage of the population is immune, breaking the chain of transmission and protecting those who are not immune (e.g., immunocompromised individuals).

10. B
Reasoning: APCs (like macrophages and dendritic cells) ingest pathogens, process them, and display antigen fragments on MHC molecules to activate helper T-cells, initiating the adaptive response.

Section B: Structured Questions Answers

11.
(a) Variable region / Hypervariable region / Tip of the Y-shape. [1]
(b) The variable region has a specific tertiary structure (shape) [1] that is complementary to a specific antigen’s epitope [1].
(c) Antibodies bind to antigens on the pathogen surface [1]. This causes agglutination (clumping), making it easier for phagocytes to engulf and destroy many pathogens at once [1]. (Alternatively: Opsonization marks pathogens for phagocytosis).

12.
(a) Airborne droplets / Inhalation of droplets from coughing/sneezing. [1]
(b) Overcrowding facilitates close contact between individuals [1], increasing the likelihood of inhaling infectious droplets expelled by an infected person [1].
(c) Plasmodium has a complex life cycle / Antigenic variation / The bacterium hides inside host cells (macrophages) making it hard for antibodies to reach it. [1] (Any valid biological reason).

13.
(a) Antigen: A molecule (usually protein) on the surface of a pathogen that triggers an immune response [1]. Antibody: A protein produced by plasma cells (B-cells) that specifically binds to an antigen [1].
(b) Helper T-cells bind to antigen-presenting cells [1]. They release cytokines [1] which stimulate B-cells to divide by mitosis and differentiate into plasma cells and memory cells [1]. (Max 2 marks: need link between T-cell activation and B-cell response).

14.
(a) The introduction of a harmless form of a pathogen (or its antigens) into the body to stimulate an immune response and produce memory cells [1].
(b) Memory cells may decline in number over time [1]. Booster doses restimulate the immune system, increasing the number of memory cells and antibody levels to ensure long-term protection [1].

15.
(a) RNA. [1]
(b) HIV infects and destroys helper T-cells [1]. As the number of helper T-cells drops, the immune system cannot activate B-cells or cytotoxic T-cells effectively [1], leaving the body vulnerable to opportunistic infections (AIDS).

Section C: Data Response and Extended Answer

16.
(a) Curve B. [1]
(b) The secondary response has a shorter lag phase because memory B-cells are already present [1]. These cells recognize the antigen immediately and differentiate rapidly into plasma cells, unlike naive B-cells which require activation and clonal selection in the primary response [1].
(c) There are more memory B-cells available to respond than naive B-cells in the primary response [1]. This leads to a faster rate of division and differentiation into plasma cells, producing a higher concentration of antibodies more quickly [1].

17.
(a) Infectious diseases are caused by pathogens entering the body [1]. Contagious diseases are transmitted directly from person to person. Malaria requires a vector (mosquito) for transmission and cannot be passed directly from human to human [1].
(b) 1. Use of insecticide-treated nets to prevent mosquito bites (blocks transmission). [1]
2. Drainage of stagnant water to remove mosquito breeding sites (reduces vector population). [1]
(Other valid answers: Indoor residual spraying, biological control of larvae).

18.
(a) Variation exists in bacterial populations due to mutation [1]. Some bacteria possess genes for antibiotic resistance. When antibiotics are used, non-resistant bacteria die, while resistant bacteria survive [1]. Resistant bacteria reproduce and pass the resistance gene to offspring [1]. Over time, the population becomes dominated by resistant strains [1]. (Max 3 marks for clear logical sequence).
(b) Strict hygiene practices (hand washing) / Isolation of infected patients / Rotating antibiotics / Reducing unnecessary prescription. [1]

19.
(a) 1. Physical barrier: Keratinized layer of dead cells prevents pathogen entry. [1]
2. Chemical barrier: Sebaceous glands secrete sebum/oils with low pH that inhibits bacterial growth. [1]
(b) Clotting seals the wound [1], preventing pathogens from entering the bloodstream and underlying tissues [1]. It also traps any pathogens that may have entered at the site of injury.

20. Essay Marking Guide (6 Marks)

Level 3 (5-6 marks): Comprehensive answer. Clearly distinguishes primary and secondary responses. Explains the mechanism of memory cell formation and function. Uses correct terminology (clonal selection, differentiation, lag phase, antibody titre).

Level 2 (3-4 marks): Good answer. Describes primary and secondary responses but may lack detail on the mechanism of memory cells. Some terminology errors.

Level 1 (1-2 marks): Basic answer. Mentions memory cells but fails to compare responses effectively. Limited biological detail.

Key Points to Include:

1. Primary Response:
   • First exposure to antigen.
   • Lag phase is long (days/weeks) as naive B-cells must be activated.
   • Clonal selection and expansion occur.
   • Differentiation into plasma cells (short-lived, produce antibodies) and memory B-cells.
   • Antibody concentration rises slowly and peaks at a lower level.
2. Role of Memory Cells:
   • Memory B-cells persist in the blood/lymph for years/decades.
   • They are specific to the antigen.
   • They do not produce antibodies immediately but are "primed" for rapid response.
3. Secondary Response:
   • Upon re-exposure to the same antigen.
   • Memory cells recognize the antigen immediately.
   • Rapid division and differentiation into plasma cells.
   • Lag phase is very short.
   • Antibody concentration rises rapidly to a much higher peak.
   • Antibodies remain in circulation longer.
4. Significance:
   • Prevents disease symptoms by neutralizing pathogens before they can cause significant harm.
   • Basis of vaccination and long-term immunity.