State the approximate thickness of an animal cell surface membrane.

• 1. Values between 5 nm and 10 nm are generally accepted.
• 2. A typical value is 7 nm (nanometers).
• 3. The unit nm stands for micrometer.
• 4. The unit (nm) is essential for the measurement.

Name the component described as ‘a transmembrane protein forming a pore’ typically found in an animal cell surface membrane and state its function.

• 1. Name: This component is known as a Channel protein.
• 2. Function: It requires cellular energy (ATP) to pump substances against their concentration gradient.
• 3. Function: It allows specific substances like water or ions to move across the membrane down their concentration gradient (facilitated diffusion).

Name the component described as ‘a lipid molecule forming the bilayer’ typically found in an animal cell surface membrane and state its function.

• 1. Function: It actively transports ions across the membrane using ATP.
• 2. Function: It contributes to membrane fluidity and stability.
• 3. Name: This molecule is a Phospholipid.
• 4. Function: It forms the basic bilayer structure, acting as a barrier to water-soluble substances and allowing lipid-soluble substances to pass.

Name the component described as ‘a protein with an attached carbohydrate chain’ typically found in an animal cell surface membrane and state its function.

• 1. Function: It acts as a receptor for cell signalling (e.g., binding hormones) and is involved in cell recognition and adhesion.
• 2. Function: It is primarily involved in storing genetic information within the membrane.
• 3. Name: This component is a Glycoprotein.

Name the component described as ‘a lipid with an attached carbohydrate chain’ typically found in an animal cell surface membrane and state its function.

• 1. Function: It acts as a receptor for cell signalling and is involved in cell recognition (e.g., blood group antigens) and adhesion.
• 2. Function: It directly hydrolyzes ATP to provide energy for membrane transport.
• 3. Name: This component is a Glycolipid.

Name the component described as ‘a lipid molecule that regulates fluidity’ typically found in an animal cell surface membrane and state its function.

• 1. Function: It forms the primary water-impermeable barrier of the membrane.
• 2. Name: This molecule is Cholesterol.
• 3. Function: It reduces the permeability of the membrane to water-soluble substances.
• 4. Function: It reduces membrane fluidity at high temperatures and increases fluidity at low temperatures (by preventing tight packing).

Describe the key structural features of a mitochondrion as seen in a transmission electron micrograph of a plant palisade mesophyll cell.

• 1. It is typically oval-shaped.
• 2. It has a double membrane; the outer is smooth, the inner is folded into cristae.
• 3. The internal space enclosed by the inner membrane is the matrix.
• 4. It contains stacks of flattened sacs called grana involved in photosynthesis.

Describe the key structural features of rough endoplasmic reticulum (RER) as seen in a transmission electron micrograph of a plant palisade mesophyll cell.

• 1. Its outer surface is studded with ribosomes.
• 2. It consists of a network of interconnected, flattened membrane-bound sacs (cisternae).
• 3. It is primarily involved in lipid synthesis and detoxification.
• 4. It often appears continuous with the outer nuclear membrane.

Describe the key structural features of smooth endoplasmic reticulum (SER) as seen in a transmission electron micrograph of a plant palisade mesophyll cell.

• 1. Its surface appears ‘smooth’ due to the absence of ribosomes.
• 2. It is often connected to the RER.
• 3. Its primary role is the synthesis of proteins for export from the cell.
• 4. It consists of a network of interconnected, membrane-bound tubules.

Identify the organelle composed of a stack of flattened membrane-bound sacs (cisternae), often located near the endoplasmic reticulum, and state one function related to processing proteins or lipids.

• 1. Function: Modifies proteins/lipids (e.g., glycosylation), sorts and packages them into vesicles.
• 2. Name: Golgi body / Golgi apparatus / Golgi complex / dictyosome.
• 3. Function: Synthesizes ATP through cellular respiration.
• 4. Function: Forms lysosomes containing digestive enzymes.

Explain how the properties of water make it suitable as the main component of blood.

• 1. Excellent Solvent: Its polarity allows dissolution of many ionic and polar substances (salts, glucose, urea) for transport.
• 2. Transport Medium: Its liquid state at body temperature allows suspension of cells and transport of dissolved solutes.
• 3. High Specific Heat Capacity: Helps blood absorb metabolic heat with minimal temperature increase, contributing to stable body temperature.
• 4. Nonpolar Nature: Allows it to easily dissolve lipids and fats for efficient transport.

Name the major artery carrying oxygenated blood from the left ventricle to the systemic circulation and the major vein carrying deoxygenated blood from the systemic circulation to the right atrium.

• 1. Major Artery: Pulmonary Artery.
• 2. Major Vein: Vena cava.
• 3. Major Artery: Aorta.
• 4. Major Vein: Pulmonary Vein.

Describe the functions of the aorta and the vena cava.

• 1. Vena Cava: Returns deoxygenated blood from body tissues to the right atrium under low pressure.
• 2. Aorta: Carries oxygenated blood under low pressure from the left ventricle to the systemic circulation.
• 3. Aorta: Supplies body tissues (except lungs) with oxygen and nutrients under high pressure.
• 4. Vena Cava: Carries deoxygenated blood from body tissues back to the right atrium.

Explain why the mammalian circulation is described as a closed, double circulation.

• 1. Systemic Circuit: Oxygenated blood is pumped from the left side of the heart to the rest of the body and returns deoxygenated to the right side.
• 2. Double: Blood passes through the heart twice during one complete circuit (once through the right side for pulmonary, once through the left side for systemic).
• 3. Closed: Blood directly bathes the tissues in open cavities before returning to vessels.
• 4. Pulmonary Circuit: Blood is pumped from the right side of the heart to the lungs and returns oxygenated to the left side.
• 5. Closed: Blood is always contained within a continuous network of vessels (heart, arteries, capillaries, veins).

Explain why it is important that the pressure of blood decreases as it passes through arterioles before entering capillaries, considering the structure of capillaries.

• 1. Efficient Exchange: Reduced pressure leads to slower flow velocity in capillaries, allowing sufficient time for diffusion across thin capillary walls.
• 2. Capillary Protection: Prevents damage or rupture of the fragile, thin-walled (often one cell thick) capillaries by high pressure.
• 3. Increased Flow Rate: Lower pressure allows blood to accelerate into the wider total cross-sectional area of the capillary network.
• 4. Fluid Balance: Appropriate lower pressure towards the venous end of capillaries facilitates the reabsorption of tissue fluid back into the bloodstream.

Compare the typical structure of a muscular artery with the typical structure of an arteriole.

• 1. Similarity: Both possess an inner lining of endothelial cells (tunica intima).
• 2. Difference – Tunica Media: Arterioles have a thicker tunica media relative to their overall wall diameter compared to muscular arteries.
• 3. Difference – Size: Muscular arteries are larger in overall diameter and generally have thicker walls than arterioles.
• 4. Difference – Elastic Tissue: Muscular arteries contain more prominent elastic laminae (internal and external) and more elastic fibres within the media than arterioles.
• 5. Similarity: Both possess an outer tunica externa made of connective tissue, although it’s thicker in muscular arteries.
• 6. Similarity: Both contain smooth muscle within their tunica media, which allows for regulation of diameter.

Identify the pathogen, disease, and mode of transmission for the scenario: Non-cellular pathogen transmitted in body fluids.

• 1. Transmission: Body fluids (e.g., blood, semen).
• 2. Classification: Non-cellular (Virus).
• 3. Pathogen: HIV (Human Immunodeficiency Virus).
• 4. Disease: Malaria.

Identify the pathogen, disease, and mode of transmission for the scenario: Eukaryotic cellular pathogen transmitted by an insect vector.

• 1. Classification: Eukaryotic cellular pathogen (Protist).
• 2. Transmission: Insect vector / Anopheles mosquito.
• 3. Example Pathogen: Plasmodium falciparum (or other Plasmodium species).
• 4. Disease: Cholera.
• 5. Disease: Malaria.

Identify the pathogen, disease, and mode of transmission for the scenario: Prokaryotic cellular pathogen transmitted by the faecal-oral route.

• 1. Disease: Cholera.
• 2. Pathogen: Vibrio cholerae.
• 3. Classification: Prokaryotic cellular pathogen (Bacterium).
• 4. Disease: Tuberculosis.
• 5. Transmission: Faecal-oral route.

Identify the pathogen, disease, and mode of transmission for the scenario: Prokaryotic cellular pathogen transmitted by airborne droplets.

• 1. Disease: Tuberculosis.
• 2. Classification: Prokaryotic cellular pathogen (Bacterium).
• 3. Transmission: Airborne droplets / Aerosol infection.
• 4. Pathogen: Plasmodium falciparum.
• 5. Pathogen: Mycobacterium tuberculosis (or M. bovis).

The drug tenofovir… inhibits HIV reverse transcriptase. Suggest the mechanism of action of tenofovir.

• 1. Mechanism – Allosteric Inhibition: Tenofovir binds to a site distinct from the active site, causing a conformational change that inhibits the enzyme.
• 2. Mechanism – Competitive Inhibition: Phosphorylated tenofovir competes with the natural substrate (dATP) for the enzyme’s active site due to structural similarity.
• 3. Mechanism – Chain Termination: Incorporation into the growing viral DNA prevents further elongation because the required 3′-OH group is missing.

In 2019, approximately 605,000 people worldwide received pre-exposure prophylaxis (PrEP)… The UN had set a target of 3 million PrEP users by 2020. Calculate the percentage of the UN target achieved in 2019.

• 1. Answer (nearest whole number): 20%.
• 2. Calculation: (605,000 / 3,000,000) * 100%
• 3. Result: 20.166…%
• 4. Calculation: (3,000,000 – 605,000) / 3,000,000 * 100%

PrEP does not prevent transmission of HIV. State and explain four distinct ways health authorities can reduce the transmission of HIV.

• 1. Method: Screening Blood Donations. Explanation: Prevents transmission through infected blood products.
• 2. Method: Promote Condom Use. Explanation: Physical barrier prevents exchange of infected body fluids during sex.
• 3. Method: Antiretroviral Therapy (ART). Explanation: Reduces viral load in infected individuals, lowering transmission risk (TasP – Treatment as Prevention).
• 4. Method: Mandatory PrEP Use. Explanation: Forcing high-risk populations to take PrEP eliminates transmission.
• 5. Method: Needle Exchange Programs. Explanation: Providing sterile needles reduces transmission via contaminated blood shared among IV drug users.
• 6. Method: Prevention of Mother-to-Child Transmission (PMTCT). Explanation: Testing and ART for pregnant women reduce transmission during pregnancy/birth/breastfeeding.

Name the two main epithelial cell types lining the bronchi responsible for producing mucus and moving it along the airway surface.

• 1. Mucus production: Goblet cell.
• 2. Mucus movement: Ciliated epithelial cell.
• 3. Mucus movement: Smooth muscle cell.

Describe how the ciliated epithelium and goblet cells lining the bronchi are adapted to their function in the gas exchange system.

• 1. Adaptation – Mucus Secretion: Goblet cells contain numerous vesicles filled with mucin glycoproteins, ready for secretion to form mucus.
• 2. Adaptation – Trapping Debris: The secreted sticky mucus layer efficiently traps inhaled particles like dust, pollen, bacteria, and viruses.
• 3. Function – Mucociliary Escalator: The numerous cilia on ciliated cells beat in a coordinated, upward wave-like motion, propelling the mucus layer towards the pharynx.
• 4. Adaptation – Cilia Structure: Cilia contain specialized contractile proteins (similar to muscle actin and myosin) allowing them to actively engulf and phagocytose trapped pathogens.

Describe the characteristic internal arrangement of microtubules found within cilia.

• 1. Arrangement: Nine outer doublet microtubules surrounding two central single microtubules (referred to as the “9 + 2” array).
• 2. Arrangement: A random meshwork of microtubules providing structural support and flexibility.
• 3. Components: Motor proteins like dynein arms are attached to the outer doublets and interact with adjacent doublets to generate bending movement.

Explain why cilia are considered intracellular structures, based on their relationship with the cell membrane.

• 1. Reason: Cilia are formed entirely within membrane-bound vesicles inside the cytoplasm before being actively secreted or exocytosed from the cell.
• 2. Reason: The contents of the cilium (the ciliary matrix surrounding the axoneme) are continuous with the cell’s cytoplasm, indicating it is an extension of the cell.
• 3. Reason: The entire internal microtubule structure (axoneme) and its surrounding matrix are enclosed by an outward extension, or outpouching, of the cell’s own plasma membrane.

Describe the function of centrioles and explain how they are involved in the cell cycle of an animal stem cell undergoing mitosis.

• 1. Involvement: They duplicate during the Interphase stage (specifically S/G2 phases) of the cell cycle.
• 2. Function: They are the core components of the centrosome, which acts as the main Microtubule Organizing Center (MTOC) in animal cells, organizing the mitotic spindle.
• 3. Involvement: As part of the duplicated centrosomes, they migrate to establish opposite poles of the cell during Prophase.
• 4. Function: They directly synthesize the DNA polynucleotide chains required for the new chromosomes during S phase.
• 5. Involvement: The centrosomes (containing centrioles) serve as anchoring points from which spindle microtubules grow and attach to chromosomes, facilitating their alignment and separation.
• 6. Function: They are essential for the formation of basal bodies, which anchor cilia and flagella.

Compare the transpiration responses of Nerium oleander and Helianthus annuus based on the provided information about their response to increasing leaf vapour pressure deficit (LVPD).

• 1. Initial Trend (0-2.5 kPa): The rate of transpiration increases in both species as LVPD rises from 0 to 2.5 kPa.
• 2. Rate of Increase (0-2.5 kPa): Helianthus annuus shows a steeper increase in transpiration rate compared to Nerium oleander in this LVPD range.
• 3. Starting Point: Both species exhibit negligible or zero transpiration when the LVPD is zero.
• 4. Later Trend (2.5-3.0 kPa): Both species show a continued steep increase in their transpiration rates between 2.5 and 3.0 kPa LVPD.
• 5. Overall Rate Comparison: The transpiration rate for Helianthus annuus is consistently higher than that of Nerium oleander at any given LVPD above zero (within the described range).

Describe and explain two structural adaptations commonly found in the leaves of xerophytes (plants adapted to hot, dry conditions) like Nerium oleander, focusing on features visible in leaf cross-sections.

• 1. Adaptation: Hairs (trichomes) located within stomatal pits/crypts. Explanation: These trap a layer of moist, still air near the stomata, reducing the water potential gradient and thus the rate of transpiration.
• 2. Adaptation: Thin cuticle covering the epidermis. Explanation: Allows the leaf to maximize absorption of atmospheric water vapour during infrequent periods of high humidity or dew.
• 3. Adaptation: Stomata located in sunken pits or crypts in the leaf surface. Explanation: Protects stomata from drying winds and traps humid air, reducing water loss by diffusion.
• 4. Adaptation: Thick, waxy cuticle covering the epidermis. Explanation: Provides a significant barrier to uncontrolled water loss directly through the epidermal cells (cuticular transpiration).
• 5. Adaptation: Thick-walled or multi-layered epidermis. Explanation: Increases the physical barrier and the diffusion distance for water escaping through the cuticle.

Explain how the structure of an antibody’s antigen-binding site makes it specific to a particular antigen.

• 1. Mechanism: The unique 3D shape formed by the folding of the variable regions of the heavy and light chains is precisely complementary to the shape of a specific antigenic determinant (epitope).
• 2. Mechanism: The specific sequence of amino acids in the variable regions (primary structure) dictates how these regions fold into their final, unique 3D conformation (tertiary/quaternary structure).
• 3. Mechanism: All antibodies produced by a B cell clone have the same basic antigen-binding site structure derived from germline genes, but they gain specificity by binding different co-factors.

State the function of the hinge region in a typical antibody molecule.

• 1. Function: Allows the two Fab arms (containing antigen-binding sites) to move independently and adopt various angles relative to the Fc stem.
• 2. Function: Connects the antibody directly to T-helper cells, facilitating B cell activation.
• 3. Function: Provides flexibility, enabling the antibody to bind simultaneously to two epitopes that may be spaced differently on an antigen’s surface.

Suggest an advantage of antibodies binding to receptors on macrophages.

• 1. Advantage: Allows antibodies to directly enter the macrophage’s cytoplasm and nucleus to reprogram its immune functions.
• 2. Advantage: Significantly enhances the efficiency of phagocytosis, as the macrophage can more easily recognize and bind to the antibody-coated particle (opsonisation).
• 3. Advantage: Acts as a bridge, making it easier for macrophages (which have Fc receptors) to identify, engulf, and ultimately destroy pathogens or foreign particles coated with antibodies.

It is estimated that the immune system can produce antibodies binding to over 10¹² different antigens… This diversity arises partly from modifying primary mRNA transcripts… Suggest how this modification occurs.

• 1. Mechanism: Non-coding sequences called introns are removed from the initial pre-mRNA transcript by the spliceosome.
• 2. Mechanism: Through alternative splicing, different combinations of exons (coding segments, e.g., those coding for constant regions or transmembrane domains) can be joined together in the mature mRNA.
• 3. Outcome: This allows a single rearranged antibody gene locus to produce different mRNA molecules, leading to variants like secreted vs. membrane-bound antibodies, or potentially switching constant region types (though class switching involves DNA changes).
• 4. Process Name: Alternative splicing is a key post-transcriptional modification contributing to protein diversity from a limited number of genes.
• 5. Mechanism: The entire primary mRNA transcript, including introns, is translated multiple times by ribosomes, with slight reading frame shifts introduced each time to generate protein variations.