Biology 9700/21 — May/June 2024

Background Concept

The fluid-mosaic model describes the cell surface membrane as a phospholipid bilayer in which proteins float. A typical cell surface membrane is approximately $7\ \text{nm}$ ($\approx 70\ \text{Å}$) thick, although the value varies from about $5$ to $10\ \text{nm}$ depending on cell type and on which components (cholesterol, glycolipids, integral proteins) are particularly abundant.

The membrane is so thin that it is below the resolution of the light microscope and can only be visualised with the transmission electron microscope (TEM). For scale, $1\ \text{nm} = 10^{-9}\ \text{m}$, so $7\ \text{nm}$ is roughly the diameter of a small globular protein — about $70{,}000$ times thinner than a millimetre.

Understanding the Question

In Fig. 1.1 the bracket $G$–$H$ on the right-hand side spans the full vertical thickness of the membrane, from the outer surface (top) to the inner surface (bottom). The command word is "state" — a single value with the correct unit, no explanation required.

Approach

Recall the standard textbook thickness of a cell surface membrane, write it with the correct unit (nanometres), and check that it lies inside the mark-scheme range.

Step-by-Step Reasoning

1. The membrane shown in Fig. 1.1 is a phospholipid bilayer drawn at very high magnification so that its two layers are clearly visible.
2. The $G$–$H$ bracket measures the entire bilayer, top to bottom.
3. The standard textbook value is $7\ \text{nm}$.
4. The mark scheme accepts any value in the range $5$–$10\ \text{nm}$, so candidates who write, say, $6\ \text{nm}$ or $8\ \text{nm}$ also score the mark.
5. The unit must be given. Writing the number alone, or with the wrong unit ($\text{µm}$, $\text{mm}$ or $\text{Å}$), would lose the mark because the unit is explicitly required.

Key Takeaways

• A cell surface membrane is approximately $7\ \text{nm}$ thick.
• Always include the unit when asked to state a measurement.
• Useful conversions: $1\ \text{mm} = 10^3\ \text{µm} = 10^6\ \text{nm}$.

Common Mistakes

• Writing the value without a unit (e.g. just "7") — the mark scheme insists the unit is shown.
• Writing the value in micrometres by mistake — that is $1{,}000$ times too large.
• Writing it in Ångströms — the mark scheme specifically asks for nanometres.

Things to Be Careful About

• The question asks for the thickness of the membrane as a whole, not the length of a phospholipid tail or the diameter of a single protein. Stay with the bilayer as one object.
• If you cannot recall the precise value, a range of $5$–$10\ \text{nm}$ is accepted, so do not leave the answer blank.

Background Concept

The fluid-mosaic model has the following key molecular components, each with a recognisable appearance in a diagram such as Fig. 1.1:

• Phospholipid bilayer — two rows of phospholipids arranged tail-to-tail. The hydrophilic phosphate heads face the aqueous environments on either side; the hydrophobic fatty-acid tails meet in the middle. The bilayer is the structural backbone and the main selective barrier.
• Channel proteins — integral proteins that form a water-filled pore through the bilayer, allowing specific ions or small polar molecules to cross by facilitated diffusion down their concentration gradient.
• Carrier proteins — integral proteins that bind a specific molecule on one side, change shape, and release it on the other; used in both facilitated diffusion and active transport.
• Glycoproteins and glycolipids — proteins or lipids with a short carbohydrate chain on the outer surface. They act as cell-signalling receptors and as cell-recognition markers (e.g. ABO blood-group antigens).
• Cholesterol — small lipid molecules wedged between phospholipid tails in animal-cell membranes. At warm temperatures they reduce fluidity and stiffen the membrane; at cool temperatures they prevent the fatty-acid tails from packing too closely and so maintain fluidity.

Understanding the Question

The candidate is given a partly completed table with three columns: component, function and letter on Fig. 1.1. Two rows have the component already given; one row has the function already given; one row has the letter already given. The candidate must fill in the missing cells so that, in each row, the name, function and letter all agree.

The mark scheme notes that the answer is marked row by row, with one mark for each row where all three entries are consistent. As a fallback, a candidate who gets columns rather than rows correct can still score up to two marks column-wise.

Approach

For each row, look at the part already given, use Fig. 1.1 to identify which structure has that label, and complete the row so that name, function and letter are all consistent with the same structure.

Step-by-Step Reasoning

Row 1 — channel protein (component already given)

• A channel protein forms an open pore, so it allows ions / water / small polar / water-soluble molecules to pass by facilitated diffusion down their concentration gradient (no ATP required).
• In Fig. 1.1 the tall cylindrical protein spanning the bilayer at the far left of the diagram is labelled A.

Row 2 — phospholipid (component already given)

• A phospholipid's main job is to form the bilayer and act as the barrier to water-soluble / polar substances / ions, while still allowing small non-polar (fat-soluble) substances to diffuse across. The bilayer also provides the membrane's basic fluidity and stability.
• The bracket E on the right-hand side of Fig. 1.1 points to a single phospholipid in the lower leaflet (note that the upper layer of heads is labelled D and the whole bilayer thickness is G–H).

Row 3 — function already given: receptor for cell signalling

• The cell-signalling receptor on the outer surface is a glycoprotein (a protein bearing a short branched carbohydrate chain). Glycolipids (carbohydrate on a phospholipid head) also act as receptors, and "glycolipid" is accepted as an alternative.
• In Fig. 1.1 the long carbohydrate chain attached to a protein on the upper (outer) surface is labelled B. A similar carbohydrate chain attached to a phospholipid head in the upper layer is labelled D — accept either for this row, but B is the more usual answer because the carbohydrate is clearly on a protein.

Row 4 — letter already given: F

• F points to a small hexagonal/ring-shaped molecule sitting among the phospholipid tails in the hydrophobic core. This is cholesterol.
• Cholesterol's function: it gives mechanical stability to the membrane and regulates its fluidity — at higher temperatures it decreases fluidity, while at lower temperatures it prevents the fatty-acid tails from packing too closely and so keeps the membrane from solidifying.

Key Takeaways

• The fluid-mosaic diagram has a small set of iconic features; you should be able to read each one at sight.
• Glycoprotein vs glycolipid: same short carbohydrate chain, but anchored to a protein (glycoprotein) or a phospholipid head (glycolipid).
• Channel vs carrier: a channel is a continuous open pore; a carrier binds its substrate and changes shape. In a typical diagram, channels are drawn as open cylinders, carriers as a closed blob with a clear shape-change illustrated.
• Cholesterol has a bidirectional effect on fluidity: it buffers fluidity against temperature change.

Common Mistakes

• Writing "phospholipid" but giving only a vague function ("forms the membrane") without specifying bilayer, barrier role or fluidity. The mark scheme requires a role within the bilayer.
• Putting B and D in the wrong row. B is the glycoprotein (carbohydrate on a protein), D is the glycolipid (carbohydrate on the upper phospholipid head). Both can act as signalling receptors, but the mark scheme prefers B for the standard answer.
• Confusing F (cholesterol, in the hydrophobic core) with one of the phospholipid labels. F has a distinctive shape — a small ring of three fused hexagons — not a phospholipid with a head and two tails.
• For cholesterol, writing only "stability" without any reference to fluidity. The mark scheme rewards stability and a fluidity role, or a temperature-dependent effect.

Things to Be Careful About

• "Glycolipid" is offered as an alternative to "glycoprotein" for the receptor row — pick the one that matches the letter you are using.
• The mark scheme's column-fallback means a candidate who gets every component right but the wrong letter on a row still scores nothing for that row; the letter has to be correct in the same row.
• The lipid labelled D is in the outer leaflet, so it is unambiguously a glycolipid; the lipid labelled E is in the inner leaflet and is just a phospholipid (no carbohydrate shown).

Background Concept

Three endomembrane-system organelles appear in plant cells and have distinctive appearances in a transmission electron micrograph (TEM):

• Mitochondrion — a double-membrane organelle about the size of a bacterium. The outer membrane is smooth; the inner membrane is thrown into folds called cristae that project into the matrix. Cristae are the TEM-visible feature that confirms the structure is a mitochondrion rather than a vesicle or a plastid.
• Rough endoplasmic reticulum (RER) — a network of flattened membrane sacs (cisternae) whose cytoplasmic face is studded with ribosomes, giving the membrane a "rough" appearance in the TEM. RER is the site of synthesis of membrane-bound and secretory proteins.
• Smooth endoplasmic reticulum (SER) — a network of branching tubules (not flattened sacs) with no ribosomes. SER is the site of lipid and steroid synthesis, and (in some cell types) detoxification or Ca²⁺ storage.

The only visual difference between RER and SER at TEM level is the presence of ribosomes on the cytoplasmic face of the membrane, and the fact that RER is typically drawn as flattened sacs while SER is drawn as smooth tubes.

Understanding the Question

Fig. 1.2 is a biological drawing of a palisade mesophyll cell. It already shows the cell wall, a large central vacuole, a nucleus with nucleolus, several chloroplasts and a Golgi body (X). The candidate must add three more organelles to the empty cytoplasm: a mitochondrion, RER and SER.

The mark scheme requires:

• Each organelle drawn in the cytoplasm and clearly labelled (max 2 marks if drawn correctly but not labelled, or labelled incorrectly).
• Mitochondrion: two membranes and at least one crista.
• RER: at least one cisterna (two close lines) with ribosomes on the outside; ribosomes inside the membrane are rejected.
• SER: at least one tube with no ribosomes.
• 3-D drawings of any organelle are rejected — flat 2-D outlines are required.
• The abbreviations "RER" and "SER" are accepted as labels.
• The RER or SER may be drawn attached to the nuclear envelope.

Approach

1. Find an empty region of cytoplasm in Fig. 1.2 — somewhere clear of the vacuole, nucleus, chloroplasts and the existing Golgi (X).
2. Draw the mitochondrion first, then the RER, then the SER, in distinct areas so that none of them is hidden behind another structure.
3. Add a clean, ruled label line to each one.

Step-by-Step Reasoning

Mitochondrion

• Draw an oval roughly the size of one of the chloroplasts already on the page.
• Sketch the outer membrane as a smooth, continuous curve.
• Inside, draw the inner membrane as a second parallel curve, leaving a thin intermembrane space.
• Add at least one crista: a finger-like infolding of the inner membrane that projects into the matrix.
• Do not add any external ribosomes (mitochondria have their own ribosomes in the matrix, but at this scale and for this mark scheme they are not part of the feature being tested).
• Add a label "mitochondrion".

Rough endoplasmic reticulum

• Draw a flattened cisterna: two parallel lines a short distance apart, with closed, rounded ends, like a deflated balloon. (The membrane appears in the TEM as two dark lines because the section cuts through both leaflets of the bilayer; the candidate should mimic that.)
• Stud the outer (cytoplasmic) face of these two lines with small solid dots — these are the ribosomes. They should sit on the surface, not be embedded in the membrane or floating in the lumen.
• Adding several cisternae stacked loosely improves recognition; one cisterna is the minimum to score.
• The RER can also be drawn continuous with the nuclear envelope, which reflects the in-cell reality.
• Add a label "rough endoplasmic reticulum" (or "RER").

Smooth endoplasmic reticulum

• Draw a branching, tubular network: a single line that splits and re-forms curves, with no parallel partner.
• Make sure the line has no ribosomes anywhere on it. This is the key visual contrast with the RER.
• Place it near the RER so the eye can compare the two.
• Add a label "smooth endoplasmic reticulum" (or "SER").

Drawing conventions to follow

• Use a sharp HB pencil and continuous, clean lines — no sketchy or broken outlines.
• Keep drawings two-dimensional — no 3-D perspective or shading, as the mark scheme rejects 3-D.
• Use a ruler for label lines; the line should end exactly on the organelle, with no arrowhead.
• Labels should be written horizontally in lower case (a capital first letter is acceptable).

Key Takeaways

• The TEM feature that distinguishes RER from SER is the presence of ribosomes on the cytoplasmic face. This is the only feature the examiner needs to see in a small drawing.
• A crista is the diagnostic TEM feature of a mitochondrion — without it, the organelle could be confused with a vesicle or an amyloplast.
• Drawings on a TEM-style question should be flat, 2-D and unlabelled-by-anything-but-text. No shading, no 3-D perspective.

Common Mistakes

• Drawing ribosomes on the inside of the RER membrane (i.e. inside the cisterna). The mark scheme rejects this because ribosomes on the cytoplasmic face synthesise secretory and membrane proteins; ribosomes inside the lumen would be incorrect.
• Drawing the SER as flattened sacs rather than tubes. The textbook/electron-microscope convention is that SER is tubular and RER is cisternal, and the mark scheme explicitly mentions a tube for SER.
• Drawing the mitochondrion with a single membrane, or with no cristae. Without a crista, the organelle does not score.
• Putting a label line through the cell wall, the vacuole or another organelle, or using a label that is hard to read. The mark scheme allows only two of the three marks if labels are missing or unclear.
• Drawing in 3-D (e.g. shading or perspective). The mark scheme rejects 3-D drawings of any of the three organelles.

Things to Be Careful About

• Make sure the RER and SER are clearly separated in the drawing. If they overlap so much that the eye cannot tell which is which, the SER mark (no ribosomes) may be lost.
• The mitochondrion should be roughly the size of a chloroplast, not enormous and not tiny — proportions matter for a recognisable drawing.
• "RER" and "SER" abbreviations are accepted by the mark scheme, so time-pressed candidates can save writing by using the abbreviations.
• Drawings are scored strictly on observable features, so a correctly drawn but unlabelled organelle can still earn a mark, but the ceiling drops to two marks out of three.

Background Concept

The Golgi apparatus (also called the Golgi body, Golgi complex, or — in plant cells, where it often appears as a set of discrete stacks — dictyosome) is a stack of flattened membrane sacs (cisternae) that receives proteins and lipids from the rough endoplasmic reticulum, modifies them, and dispatches them in membrane-bound vesicles to other destinations.

In a TEM, a Golgi stack looks like a small pile of smooth, curved cisternae with small round vesicles budding off the edges — exactly the appearance of the organelle labelled X in Fig. 1.2.

The Golgi is the cell's "post office": proteins arrive on its cis (forming) face in vesicles from the RER, are passed through the stack, and are modified along the way — for example by glycosylation (addition of sugar groups), by trimming of polypeptides, or by assembly into multi-subunit complexes. On the trans (maturing) face, finished products are packaged into vesicles for secretion, for delivery to lysosomes, or for insertion into the plasma membrane.

Understanding the Question

In Fig. 1.2 the organelle labelled X is a small stack of curved, smooth cisternae with associated small round vesicles — a textbook picture of a Golgi body. The candidate must give its name and one function.

The command word is "identify … and state" — one mark for the name, one mark for the function.

Approach

1. Look at the shape: a stack of curved, smooth cisternae (no ribosomes, no cristae) with small round vesicles budding off the rims.
2. Match it to the organelle with that appearance — the Golgi body.
3. Pick one standard function that can be stated in a short sentence.

Step-by-Step Reasoning

Name

• The shape is diagnostic: a stack of smooth flattened sacs with vesicles, with no ribosomes on the surface. This is the Golgi body (also accepted: Golgi apparatus, Golgi complex, or, in plant cells, dictyosome).
• A mitochondrion would be a single oval with cristae — not this shape.
• Smooth ER would be a branching tubular network with no stack structure — not this shape either.

Function

• The Golgi's main jobs are:
  1. Modifying proteins (and lipids) — for example, adding or trimming sugar chains (glycosylation), adding phosphate groups, or cutting polypeptides.
  2. Packaging finished products into vesicles that bud off the trans face and travel to the plasma membrane (for secretion), to lysosomes, or back to the ER.
  3. Forming lysosomes (in animal cells) by packaging hydrolytic enzymes.
  4. Producing carbohydrates for the plant cell wall (in plant cells).
• Any one of these is acceptable. The mark scheme's preferred answers are "modifies / processes proteins (or polypeptides or lipids)" or "packaging of proteins into Golgi vesicles".

Key Takeaways

• The Golgi's appearance — a stack of smooth curved cisternae with budding vesicles — is unmistakable in a TEM.
• "Modify and package" is a useful one-line summary of its function.
• "Transports" alone, without a destination, is too vague and is ignored by the mark scheme.

Common Mistakes

• Writing "Golgi body" with the wrong function — e.g. "makes proteins" (that is the ribosome) or "makes ATP" (that is the mitochondrion).
• Writing "transports proteins" without saying where or how. The mark scheme ignores "transports" on its own.
• Mis-identifying X as smooth ER or as a mitochondrion. The stack of cisternae with budding vesicles is the giveaway for Golgi, not the membrane-less tube of SER or the cristae of a mitochondrion.
• Spelling "Golgi" correctly but giving only a vague function such as "processes things" — the mark scheme wants a specific modification or a specific packaging role.

Things to Be Careful About

• The plant-cell Golgi is also called a dictyosome. Either name is accepted.
• The mark scheme gives an AVP (any valid point) option, so a candidate who writes "forms lysosomes" or "synthesises polysaccharides for the cell wall" can also score, even though these are not the textbook first answer.
• The function mark is awarded independently of the name mark — but a clearly wrong function for the organelle named will not earn the function mark.

Background Concept

The mammalian heart has four chambers: right atrium, right ventricle, left atrium, left ventricle. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs, and the pulmonary vein returns oxygenated blood from the lungs to the left atrium. The aorta carries oxygenated blood from the left ventricle to the rest of the body (systemic circulation), and the vena cava returns deoxygenated blood from the body to the right atrium.

When a heart is drawn in a textbook, the right side of the body is conventionally shown on the LEFT of the diagram (as if you are looking at the person facing you). So the right ventricle is on the left of the diagram, and the left ventricle is on the right of the diagram.

Understanding the Question

The question shows a diagram of the double circulation with two labels: P and Q. P is on the right side of the diagram (corresponding to the left side of the body), and Q is on the left side of the diagram (corresponding to the right side of the body). You need to identify each vessel by name.

Approach

Trace the direction of blood flow in the diagram. The vessel on the right of the diagram leaves the heart and goes to the body — this must be the AORTA (left ventricle → body). The vessel on the left of the diagram enters the heart from the body — this must be the VENA CAVA (body → right atrium).

Step-by-Step Reasoning

• The diagram shows the pulmonary circulation (heart ↔ lungs) at the top and the systemic circulation (heart ↔ body) at the bottom/sides.
• P is on the right of the diagram, connected to the systemic circulation, with blood leaving the heart in this direction. This is the AORTA — the largest artery in the body, which branches from the left ventricle to deliver oxygenated blood at high pressure to the systemic circulation.
• Q is on the left of the diagram, connected to the systemic circulation, with blood entering the heart in this direction. This is the VENA CAVA — which returns deoxygenated blood from the body to the right atrium. It can be either the superior vena cava (from above the diaphragm) or the inferior vena cava (from below), but the mark scheme does not require this distinction.

Key Takeaways

• Aorta = left ventricle → body, oxygenated, high pressure.
• Vena cava = body → right atrium, deoxygenated, low pressure.
• Pulmonary artery = right ventricle → lungs, deoxygenated.
• Pulmonary vein = lungs → left atrium, oxygenated.
• Arteries and veins are defined by direction of flow (away from / towards the heart), not by oxygen content.

Common Mistakes

• Confusing the pulmonary artery with the aorta (because of the 'artery = oxygenated' misconception).
• Naming P as the pulmonary artery because the right side of the heart is sometimes (incorrectly) thought of as 'right' of the body.
• Writing 'superior vena cava' or 'inferior vena cava' instead of 'vena cava' — both are accepted, but the mark scheme ignores the distinction.

Things to Be Careful About

• Arteries and veins are defined by direction of flow (away from / towards the heart), not by oxygen content.
• The diagram convention: right of body = left of diagram.

Background Concept

The function of any blood vessel is determined by where it comes from, where it goes to, what it carries, and the pressure at which it carries it.

• Arteries (including the aorta) have thick, muscular, elastic walls to withstand and maintain high blood pressure. They generally carry blood away from the heart.
• Veins (including the vena cava) have thinner walls and lower pressure; they often have valves to prevent backflow. They generally carry blood back to the heart.

Understanding the Question

The question asks you to describe the function of P (aorta) and Q (vena cava) — i.e. what each vessel does. Two marks are available, one per vessel.

Approach

For each vessel, state: (a) what is being transported (oxygenated/deoxygenated OR at high/low pressure) AND (b) where it comes from and goes to. The mark scheme accepts either the oxygen content OR the pressure, plus the destination, as one complete mark.

Step-by-Step Reasoning

• Aorta (P): The aorta is the largest artery in the body. It leaves the LEFT VENTRICLE and carries OXYGENATED BLOOD at HIGH PRESSURE to the organs and respiring tissues of the systemic circulation. The high pressure is generated by the contraction of the thick left ventricular wall.
• Vena cava (Q): The vena cava is the largest vein in the body. It carries DEOXYGENATED BLOOD at LOW PRESSURE from the organs of the systemic circulation back to the RIGHT ATRIUM of the heart. (There are actually two venae cavae — superior and inferior — but the question is asking about the vessel in general.)

Key Takeaways

• Aorta = oxygenated blood, high pressure, left ventricle → body.
• Vena cava = deoxygenated blood, low pressure, body → right atrium.
• Function follows structure: thick-walled arteries carry blood at high pressure; thin-walled veins carry blood at low pressure.

Common Mistakes

• Saying 'the aorta carries blood to the body' without specifying OXYGENATED or HIGH PRESSURE.
• Confusing the destinations (e.g. saying the aorta goes to the lungs).
• Saying the vena cava carries blood to the LEFT atrium (this is the pulmonary vein).

Things to Be Careful About

• Be specific about which chamber of the heart blood enters or exits.
• Aorta = left ventricle; vena cava = right atrium.
• Either oxygen content or pressure is accepted by the mark scheme — but stating both makes the answer clearer and harder to mis-mark.

Background Concept

The mammalian circulatory system is described as CLOSED and DOUBLE. These two terms are independent and refer to different features:

• Closed: blood is always contained inside blood vessels and never leaves them to flow freely in body cavities. This is in contrast to an OPEN circulation (found in insects), where blood (haemolymph) flows directly over the tissues in an open body cavity.
• Double: blood passes through the heart twice during one complete circuit of the body. The two passes are the pulmonary circulation (right side of heart → lungs → left side of heart) and the systemic circulation (left side of heart → body → right side of heart). A single circulation (found in fish) has blood passing through the heart only once per circuit.

Understanding the Question

The question asks you to explain WHY mammalian circulation is described as both 'closed' and 'double'. You need to give one mark-worthy point for each term.

Approach

For each adjective, state a defining feature of the mammalian circulation that matches the adjective.

Step-by-Step Reasoning

• Closed: The blood is contained within the network of blood vessels (arteries → arterioles → capillaries → venules → veins). The blood never directly bathes the body tissues; exchange happens only by diffusion across the thin walls of the capillaries. The blood cells and plasma remain inside the vessels at all times.
• Double: In one complete circuit of the body, blood passes through the heart twice. The first pass is the pulmonary circulation (deoxygenated blood from the right ventricle → lungs → oxygenated blood returns to the left atrium), and the second pass is the systemic circulation (oxygenated blood from the left ventricle → body → deoxygenated blood returns to the right atrium). The two circuits operate in series.

Key Takeaways

• Closed = blood stays inside blood vessels.
• Double = blood goes through the heart twice per complete circuit.
• Pulmonary circulation: heart ↔ lungs.
• Systemic circulation: heart ↔ rest of body.
• The two circuits are in series, not in parallel.

Common Mistakes

• Saying 'blood goes through the heart twice' without explaining what this means (e.g. not naming the two circuits).
• Saying 'blood is closed' (which makes no sense).
• Confusing the two circuits (e.g. saying the pulmonary circulation goes to the body).
• Stating only that the heart has 'two sides' without explaining that blood passes through the heart TWICE per circuit.

Things to Be Careful About

• The pulmonary and systemic circulations are in series, not in parallel — blood must go through one before the other.
• The mark scheme accepts the alternative wording 'pulmonary circulation and systemic circulation' as a description of 'double'.

Background Concept

Blood pressure is the force that blood exerts on the walls of the vessels. Pressure is highest in the arteries (especially the aorta) and decreases as blood moves through arterioles, capillaries, venules, and veins. The values given in the question — $12.7\ \text{kPa}$ in the muscular artery and $2.7\ \text{kPa}$ at the end of the arteriole — illustrate this large pressure drop.

The walls of capillaries are very thin (one cell thick, formed of endothelial cells) to allow rapid exchange of substances between the blood and the surrounding tissues by diffusion and through pores.

Understanding the Question

The question gives the pressure values in the muscular artery and at the end of the arteriole, and asks why the decrease in pressure is important. You need two mark-worthy points.

Approach

Think about what would happen if capillaries received blood at the same high pressure as arteries, and what the capillaries need in order to function correctly.

Step-by-Step Reasoning

• Capillary structure cannot withstand high pressure: Capillaries have very thin walls — only ONE (endothelial) cell thick — to allow rapid exchange of substances. If blood at high pressure (e.g. $12.7\ \text{kPa}$) were to enter the capillaries, the thin walls would burst or be damaged. By the time the blood reaches the capillaries, the pressure has dropped to $2.7\ \text{kPa}$, which the thin walls can safely withstand.
• Slow flow for exchange: At low pressure, blood flows slowly through the capillaries. This slow flow gives sufficient time for the exchange / diffusion of substances (oxygen, carbon dioxide, glucose, urea, etc.) between the blood and the tissues. High pressure would push blood through too quickly for efficient exchange.
• (Optional credit) At the venous end of the capillaries, the pressure is low enough to allow reabsorption of tissue fluid back into the blood (by osmosis, due to the plasma proteins remaining in the blood). If pressure were still high, fluid would continue to be forced out and tissue fluid would build up (oedema).

Key Takeaways

• Capillaries are one cell thick — they cannot withstand high pressure.
• Slow blood flow through capillaries is necessary for efficient exchange of substances.
• Arterioles act as a 'pressure buffer' to protect the capillaries.

Common Mistakes

• Saying 'capillaries are thin because they have to be' without linking it to why high pressure would damage them.
• Not mentioning exchange / diffusion.
• Saying capillaries have 'cell walls' (which they do not — this is a common error that the mark scheme explicitly REJECTS).
• Giving only one point when two are needed.

Things to Be Careful About

• Capillaries are composed of ONE layer of endothelial cells, not plant-style 'cell walls'.
• The link between low pressure and slow flow is important — the rate of flow, not just the pressure, is what allows exchange.
• Pressure drop is not just 'to slow the blood' but also 'to protect the thin capillary walls'.

Background Concept

Blood vessels have three main layers:

• Tunica intima (endothelium): the inner layer, made of a single layer of thin squamous endothelial cells, in direct contact with the blood.
• Tunica media: the middle layer, made of smooth muscle and elastic fibres. The amount varies with the function of the vessel.
• Tunica externa (adventitia): the outer layer, made mainly of collagen fibres.

The proportions of these layers vary with function. Muscular arteries have a thick tunica media with lots of smooth muscle to regulate blood flow by vasoconstriction / vasodilation, and lots of elastic fibres to withstand and maintain the high blood pressure generated by the heart. Arterioles are smaller vessels that lead into capillary networks; they have proportionally less smooth muscle and elastic tissue than muscular arteries but still regulate flow into capillary beds.

Understanding the Question

The question shows a TEM of an arteriole in cross-section (Fig. 2.2) and asks you to compare its structure with that of a muscular artery. You need to identify both similarities and differences. Three marks are available — the mark scheme accepts any three from a combined list of similar and different features.

Approach

First, list the structural features common to BOTH vessels. Then list the differences — always making it clear which is which (the mark scheme accepts ORA, or reverse argument, for the arteriole). Do not describe functions — only structures earn marks here.

Step-by-Step Reasoning

Similarities (features in BOTH):

1. Both have an endothelium (tunica intima) lining the lumen — a single layer of squamous epithelial cells in direct contact with the blood.
2. Both have a tunica media containing smooth muscle in the wall.

Differences (muscular artery vs arteriole):
3. The muscular artery has a thicker wall (in absolute terms, and especially relative to the size of its lumen). The mark scheme explicitly REJECTS vague statements like 'the artery is bigger' or 'thicker' without specifying the structure.
4. The muscular artery has a wider lumen (in absolute terms).
5. The muscular artery has more (layers of) smooth muscle / a thicker tunica media.
6. The muscular artery has more elastic fibres / a clear elastic lamina (which is absent or not visible in the arteriole).
7. The muscular artery has more collagen fibres (mainly in the tunica externa / adventitia).

Key Takeaways

• Both muscular arteries and arterioles have an endothelium and a smooth muscle layer.
• Muscular arteries are larger, with a relatively thicker wall, more smooth muscle, more elastic fibres, and more collagen fibres.
• These structural differences reflect the muscular artery's role in withstanding and regulating the high blood pressure of the systemic circulation.

Common Mistakes

• Comparing 'size' without specifying the relative thickness of wall to lumen.
• Saying the artery is 'bigger' or 'thicker' without specifying in what way (wall vs lumen).
• Describing FUNCTIONS instead of structures (e.g. 'the artery carries blood at high pressure') — these do not earn marks.
• Saying capillaries have 'cell walls' (which they do not — this is a common error that the mark scheme explicitly REJECTS).
• Forgetting the similarities (only listing differences).

Things to Be Careful About

• The mark scheme rejects vague statements like 'arteries are bigger' without specifying the structure (wall thickness, lumen size, smooth muscle content, etc.).
• The ORA (or reverse argument) is allowed for the differences — you can describe the arteriole as 'smaller lumen', 'less smooth muscle', 'fewer elastic fibres', etc.
• Functions do not earn marks here; only structural descriptions do.
• The TEM shows the arteriole in a constricted state (lining is folded), so the wall appears even thicker relative to the lumen than it would in a relaxed arteriole.

Background Concept

A percentage expresses one quantity as a proportion of 100 of another. Here we are asked what fraction of the UN's 2020 target (3 million PrEP users) was actually reached by 2019 (605 000 users).

Understanding the Question

The data are given in Table 3.1. The 2019 value is 605 000 and the UN target is 3 000 000. We must give the answer to the nearest whole number — the mark scheme specifically rejects answers given to decimal places (e.g. 20.2 %).

Approach

Use the standard percentage formula:

Step-by-Step Reasoning

1. Read the 2019 figure: 605 000.
2. Identify the UN 2020 target: 3 000 000.
3. Substitute: $\frac{605\,000}{3\,000\,000} \times 100 = 20.166\dots\,\%$.
4. Round to the nearest whole number: 20 %.

Key Takeaways

• Percentage calculations require the part to be divided by the whole, then multiplied by 100.
• Always check whether the question requires rounding and to what precision.
• Be alert to data-table questions where the target value is in the question text rather than in the table.

Common Mistakes

• Giving 20.2 % (or similar) — the mark scheme rejects decimal-place answers.
• Using 2018's value (340 000) by mistake.
• Dividing 3 000 000 by 605 000 and quoting 496 % — the wrong way round.

Things to Be Careful About

• The question says 'percentage of people who received PrEP in 2019 as a percentage of the target' — so 2019 is the part and 3 000 000 is the whole.
• Round to the nearest whole number, not to one decimal place.

Background Concept

HIV is transmitted via exchange of certain body fluids — blood, semen, vaginal secretions and breast milk. The main routes are unprotected sexual intercourse, contaminated needles (intravenous drug use, unsafe medical injections), contaminated blood products, and mother-to-child transmission (in utero, at birth, or via breast milk). Public-health measures that interrupt any of these routes reduce transmission. The question stresses that PrEP alone does not prevent transmission, so additional behavioural and medical interventions are needed.

Understanding the Question

The question asks candidates to state measures that health authorities can use and explain how each reduces HIV transmission. The mark scheme pairs each measure with its explanation and awards one mark per statement and one mark per explanation. Crucially, the mark scheme caps marks at 3 unless the response includes at least one statement and at least one explanation — so a list of bare statements, or a list of bare explanations, will earn a maximum of 3 marks even if more than four points are made.

Approach

Think of HIV transmission routes and pick a measure for each:

1. Sexual route → condoms / femidoms / dental dams.
2. Needle route → needle exchange, sterile injecting equipment.
3. Blood-transfusion route → screening donated blood, excluding HIV-positive donors, heat-treating blood products.
4. Mother-to-child route → test pregnant women, give antiretrovirals, provide powdered milk.
5. Case-finding → contact tracing and HIV testing so that infected people can be diagnosed early.
6. Reducing infectiousness → supply antiretrovirals (e.g. tenofovir) to HIV-positive people, lowering viral load so they are less likely to transmit.
7. Behavioural → public-health education about safer sex and transmission routes.

For each, give both the action (statement) and the reason (explanation).

Step-by-Step Reasoning

• Condoms are physical barriers that stop the exchange of semen, vaginal secretions and blood during intercourse, so they directly block the sexual route.
• Needle-exchange schemes give injecting drug users sterile equipment, so contaminated needles are not shared, removing a key blood-borne route.
• Screening donated blood (and excluding HIV-positive donors) prevents any HIV present in donated blood from reaching a recipient during surgery or transfusion.
• Testing pregnant women identifies those who are HIV-positive so they can be given antiretrovirals during pregnancy and powdered milk instead of breast milk, preventing mother-to-child transmission.
• Contact tracing finds the sexual and needle-sharing contacts of newly diagnosed individuals so they too can be tested, breaking chains of silent transmission.
• Supplying antiretrovirals to HIV-positive people suppresses viral replication, lowering viral load so the patient is less infectious to others (treatment-as-prevention).
• Public education raises awareness of how HIV is spread and how it can be avoided, leading to behaviour change such as condom use or fewer sexual partners.

Key Takeaways

• HIV control depends on interrupting every transmission route, not just one.
• Public-health responses combine biomedical (condoms, antiretrovirals, blood screening) and behavioural/educational (information, contact tracing) measures.
• PrEP is for uninfected at-risk individuals, not a transmission-blocker on its own — so it must be combined with other measures.
• 'Statement + explanation' is a recurring Cambridge command structure; marks are often lost by giving only one half of the pair.

Common Mistakes

• Writing only statements and no explanations (capped at 3 marks even if many statements are made).
• Vague phrases like 'use clean needles' — the mark scheme rejects 'clean needles' and rewards 'new / sterilised needles' or 'needle-exchange schemes'.
• Mentioning 'a vaccine' — no effective HIV vaccine exists.
• Saying 'abstain from sex' as a measure without explaining how authorities could implement it.
• Listing individual behaviour changes (which a person can do) rather than what health authorities can do.

Things to Be Careful About

• Match each statement to a specific explanation of how it cuts transmission; partial credit is given for the explanation even if the statement is weak, but you need at least one of each to access the fourth mark.
• Use precise terms: 'condoms' (not 'protection'), 'needle-exchange schemes' (not 'clean needles'), 'antiretrovirals' (not 'drugs' alone).
• The mark scheme accepts 'screen donated blood' or 'heat-treat donated blood' as alternatives to 'prevent HIV-positive people from being donors' — pick whichever is most precise for the explanation given.

Background Concept

The gas exchange system (trachea, bronchi and larger bronchioles) is lined by a pseudostratified ciliated columnar epithelium. Within this epithelium sit two principal cell types that work together as the mucociliary escalator. Ciliated epithelial cells bear many motile cilia on their apical (free) surface. Goblet cells are mucus-secreting cells interspersed among the ciliated cells; their apical cytoplasm is packed with mucin-containing secretory vesicles that bulge the membrane outwards, giving them a characteristic 'goblet' or wine-glass shape.

Understanding the Question

This part is a straight identification. Fig. 4.1 is a scanning electron micrograph (SEM) of the lining of a bronchus, magnified ×3000. The surface bristles are cilia (label X). Label A points to a tall cell whose apex bears these cilia; label B points to a shorter, rounded cell that bulges above its neighbours and has no cilia on its surface. You must name each cell type using the correct biological term.

Approach

Use the visible morphology of each cell together with your knowledge of which two cell types populate the respiratory epithelium.

Step-by-Step Reasoning

• Cell A is column-shaped and its apical surface carries many of the hair-like cilia. This is the classic appearance of a ciliated epithelial cell of the respiratory lining.
• Cell B has a rounded, bulging apical surface and no cilia visible on top. This is typical of a goblet cell, whose apical cytoplasm is swollen with mucin vesicles ready for secretion.
• The mark scheme requires the exact terms 'ciliated epithelial cell' and 'goblet cell' (singular cell, not the tissue). 'Ciliated epithelium' is rejected because the question asks for a cell, not a tissue.

Key Takeaways

The bronchial epithelium is dominated by ciliated cells and goblet cells; you should know both by name and by their appearance in an SEM.

Common Mistakes

• Writing 'ciliated epithelium' for A — the mark scheme rejects this because it names a tissue, not a cell.
• Calling B a 'mucus cell', 'gland cell' or 'secretory cell' — the specific term 'goblet cell' is required.

Things to Be Careful About

Use the singular 'cell'. Note that goblet cells in the SEM look smooth and rounded because their cilia-free apex is full of secretory vesicles, in contrast to the cilia-covered ciliated cells.

Background Concept

The mucociliary escalator is the principal defence of the conducting airways. Goblet cells secrete mucin glycoproteins which, mixed with water, salts and antibacterial proteins, form a sticky layer of mucus over the epithelium. This mucus traps inhaled particles (dust, smoke, microorganisms, pollen). Ciliated cells then beat their cilia in a coordinated metachronal rhythm so that the mucus sheet, with everything trapped in it, is propelled up the airways towards the pharynx and swallowed.

Understanding the Question

The question asks for a description of how the tissue seen in Fig. 4.1 is adapted to its function. The tissue is the ciliated pseudostratified columnar epithelium of the bronchi, and its function in the gas exchange system is to keep the airways clean by trapping and removing inhaled debris. Three marks require three linked points, covering both cell types and the way they cooperate.

Approach

Structure your answer in three clearly separated points: (1) what goblet cells do, (2) what the mucus does, and (3) what the cilia do. This order mirrors the sequence of events — secretion, entrapment, removal.

Step-by-Step Reasoning

• Goblet cells are full of mucin-containing secretory vesicles. They release their contents by exocytosis onto the apical surface of the epithelium. The mucin swells with water to form the mucus layer that sits on top of the cilia.
• The mucus is sticky and covers the ciliated epithelium. Inhaled particles (dust, microbes, smoke particles) stick to it, so the mucus acts as a physical barrier preventing pathogens and debris from reaching the delicate gas exchange surface of the alveoli.
• The cilia on the ciliated epithelial cells beat in a coordinated, wave-like pattern (power stroke followed by recovery stroke). The tips of the cilia grip the mucus and propel it, with its trapped contents, upwards along the airway towards the pharynx, where it is swallowed and destroyed in the stomach. The direction must be specified (upwards / towards the mouth / away from the alveoli / away from the lungs / away from the gas exchange surface).

Key Takeaways

The mucociliary escalator is a clear structure–function example: goblet cells provide the trap, cilia move the trap. Together they keep the gas exchange surface clean.

Common Mistakes

• 'Cilia filter the air' — cilia do not filter; they move the mucus. The mucus itself does the filtering.
• 'Mucus moves out of the lungs' unqualified — this does not specify the direction of ciliary beating; the mark scheme requires 'towards the mouth/throat/pharynx' or 'away from the alveoli/lungs/gas exchange surface'.
• 'To be swallowed' on its own — not credited without the direction of ciliary beating.
• Failing to mention the role of goblet cells at all — at least one mark is tied to the mucus-secreting function.

Things to Be Careful About

Always give the direction of mucus movement. The mark scheme explicitly rejects vague phrasings such as 'out of the lungs', 'out of the gas exchange system' or 'out of the respiratory system' as the destination for ciliary beating. Equally, do not give the function of mucus to cilia or the function of cilia to mucus.

Background Concept

A motile cilium has a highly conserved internal ultrastructure — the axoneme — that is only resolvable by electron microscopy. In transverse section the axoneme shows nine outer doublets of microtubules arranged in a ring around two central single microtubules. This is the '9 + 2' pattern. Each outer doublet bears dynein arms, which hydrolyse ATP and slide adjacent doublets past one another, producing the bending motion that drives ciliary beating. Radial spokes, nexin links and a central sheath hold the structure together; the whole axoneme is enclosed by an extension of the plasma membrane.

Understanding the Question

Fig. 4.2 is a TEM (×80 000) of a horizontal section cut at the position of the arrows in Fig. 4.1 — that is, a cross-section through the cilia (X) on top of the ciliated cells. Two marks are available; you must describe the internal appearance of one cilium as seen in this TEM.

Approach

Look at any one cilium cross-section in Fig. 4.2. Count the dark structures around the outside and in the middle; identify what they are made of; and note any extra detail that is visible (the dynein arms or the surrounding membrane).

Step-by-Step Reasoning

• Each cilium shows a ring of nine paired dark structures around its edge with two lighter single structures in the middle. This is the canonical 9 + 2 microtubule pattern of a motile cilium.
• All of these structures are microtubules — hollow cylinders made of tubulin protein. The '9' are doublets (two fused microtubules), the '2' are single microtubules.
• On closer inspection, the outer doublets carry short dynein 'arm' projections, which are the motor proteins responsible for ciliary beating. The whole axoneme is bounded by a thin line — the plasma membrane (see part (b)(ii)). Any one of these additional correct observations earns the AVP mark.

Key Takeaways

The motile cilium has a 9 + 2 microtubule axoneme with dynein arms, all wrapped in an extension of the plasma membrane.

Common Mistakes

• Writing only 'microtubules' without the 9 + 2 pattern — the 9 + 2 is the specific feature worth a mark.
• Writing '9 + 2 arrangement of cilia' instead of '9 + 2 arrangement of microtubules' — the doublets themselves are microtubules.
• Confusing this with the unrelated structure of a bacterial flagellum.

Things to Be Careful About

You are describing a transverse (cross) section through one cilium, not its length. Refer to microtubules, not 'fibres' or 'filaments'.

Background Concept

A cilium grows outwards from the cell as an extension of the apical plasma membrane. The plasma membrane is continuous with, and an extension of, the cell's surface membrane, so anything inside that membrane is by definition inside the cell — i.e. intracellular. (Contrast: a secreted mucus droplet lying in the airway lumen would be outside this membrane, and therefore extracellular.)

Understanding the Question

Part (b)(ii) asks you to use Fig. 4.2 as visual evidence that the cilia (X) are intracellular. Only one mark is available, and the mark scheme is specific: you must refer to the cell (surface) membrane that you can see wrapping each cilium in the TEM.

Approach

Look at Fig. 4.2 again. Notice that every 'ring' of microtubules has a thin dark line around it. Identify that line as the cell's plasma membrane and use it as evidence that the cilium lies inside that membrane.

Step-by-Step Reasoning

• In the TEM, each cilium cross-section is enclosed by a thin dark boundary. That boundary is the cell's plasma membrane, which extends up and around the cilium as the cilium projects from the cell surface.
• Because the microtubules of the axoneme lie inside this membrane, they are inside the cell — or, more precisely, inside an extension of the cell.
• Therefore the cilium is an intracellular structure: a surface projection of the cell, not a free-floating extracellular filament.

Key Takeaways

A cilium is intracellular because the plasma membrane wraps around it; the TEM makes this directly visible.

Common Mistakes

• Saying 'the microtubules are inside the cell' without mentioning the membrane — the membrane observation is the specific evidence the mark scheme requires.
• Writing 'inside the cell body' — true but imprecise; the mark scheme wants the membrane observation.
• Confusing 'intracellular' with 'within the nucleus' — a cilium is intracellular but lies in the cytoplasm, not inside the nucleus.

Things to Be Careful About

'Intracellular' means inside the cell (i.e. enclosed by the plasma membrane). Cilia project above the apical surface but remain wrapped in plasma membrane, so they remain part of the cell.

Background Concept

An antibody (immunoglobulin) is a Y-shaped protein made of four polypeptide chains: two identical heavy chains and two identical light chains, held together by disulfide bonds. Each chain has a constant region (the stem and lower arms of the Y) and a variable region (the tips of the two arms). The variable region is where the antigen binds, and because each antibody can bind only one specific antigen, the variable region must be uniquely shaped for that antigen. The shape of any protein is determined by its amino-acid sequence (primary structure), because different amino acids have different R-groups / side chains that fold in different ways.

Understanding the Question

Part (a)(i) asks you to explain the link between the structure of the antigen-binding site and its specificity. The word "explain" means you must give the reason, not just describe. You need to connect the shape of the binding site to the underlying amino-acid sequence and R-group chemistry that produces that shape. Fig. 6.1 shows two antigens bound at the tips of the Y — those tips are the variable regions.

Approach

Reach for the structure–function relationship: shape comes from sequence, sequence comes from differing R-groups. Combine these to produce a complementary fit between binding site and antigen (an extension of the lock-and-key / induced-fit principle you met in enzymes).

Step-by-Step Reasoning

1. The variable regions at the tips of the Y carry the antigen-binding sites, and Fig. 6.1 shows them in direct contact with the antigen.
2. The first marking point is that the binding site has a tertiary structure whose shape is complementary to the antigen — just as an enzyme active site is complementary to its substrate. "Tertiary" or "quaternary" structure is acceptable wording.
3. The second marking point is the underlying reason: the variable region has a specific amino-acid sequence (primary structure), and because different amino acids have different R-groups, they interact differently during folding.
4. Hence different sequences fold into different tertiary structures, producing binding sites of unique shape that fit only their specific antigen — the molecular basis of antibody specificity.

Key Takeaways

• Antibody specificity arises from the tertiary/quaternary structure of the variable region.
• The shape is ultimately dictated by the primary sequence of amino acids, and each amino acid contributes its R-group to that final shape.
• The principle is the same as enzyme–substrate specificity: complementary shape.

Common Mistakes

• Saying the antibody "is the right shape" without saying complementary — "shape" on its own is too vague; the examiner needs complementary.
• Confusing constant and variable regions — only the variable region at the tips binds antigen.
• Saying "the antigen has a specific shape that fits the antibody" — this is reverse; it is the binding site that is complementary to the antigen.

Things to Be Careful About

• "Explain" needs a reason, not just a description — pair every observation with a mechanism.
• Acceptable alternative wording per the mark scheme includes reference to tertiary or quaternary structure, and to epitopes.

Background Concept

The hinge region is the short flexible section of polypeptide between the Fab arms and the Fc stem of an antibody. It contains proline-rich sequences that make it less rigid than the surrounding globular domains, so the two arms of the Y can swivel independently. This is what allows a single antibody to bind two antigens that may be spaced differently on the surface of a pathogen.

Understanding the Question

The question simply asks for the function of the hinge region labelled in Fig. 6.1. One mark — one short, focused point.

Approach

Think about what the hinge does mechanically: it permits movement between the two binding arms and the stem.

Step-by-Step Reasoning

1. The Y-shape of the antibody in Fig. 6.1 has two antigen-binding tips that need to contact antigens on the surface of a pathogen.
2. Antigens on a real pathogen surface are not always the same distance apart — the spacing varies.
3. The hinge region gives the two Fab arms flexibility so they can move independently and bind antigens that are at different distances or angles from each other.

Key Takeaways

• Hinge region = flexibility of the antibody arms.
• This flexibility maximises the chance that both binding sites can engage their antigens simultaneously.

Common Mistakes

• Saying "holds the two chains together" — that is the disulfide bonds, not the hinge.
• Saying "helps bind to the antigen" — too vague; the marks want explicit mention of flexibility / movement.

Things to Be Careful About

• The mark scheme rewards "allows flexibility for binding" or "allows variable regions to move"; do not invent a function that is not on the mark scheme.

Background Concept

Macrophages are phagocytic cells that engulf and digest pathogens. They have Fc receptors on their plasma membrane that recognise the constant (Fc) region of antibodies. When antibodies coat a pathogen, the Fc regions stick outwards; when a macrophage's Fc receptors grab these Fc regions, the entire antibody-pathogen complex is pulled into the macrophage for destruction. This is called opsonisation — the antibody acts as an opsonin, "marking" the pathogen for phagocytosis.

Understanding the Question

The question is a "suggest" type — you are asked for a reasonable biological advantage. The mark scheme accepts opsonisation or "easier for the macrophage to engulf antibody-bound antigens". The word suggests allows reasonable reasoning rather than demanding recall of a single fact.

Approach

Think about what antibodies do when they bind to a pathogen, and what happens when a macrophage meets that antibody-coated pathogen.

Step-by-Step Reasoning

1. Antibodies have two Fab arms that bind antigens and an Fc stem (constant region).
2. When antibodies coat a pathogen, their Fc regions project outward from the pathogen surface.
3. Macrophages have Fc receptors on their membranes that bind these exposed Fc regions.
4. This attachment "flags" or opsonises the pathogen, making it much easier for the macrophage to recognise, engulf and destroy it. Without opsonisation, the macrophage might not recognise the pathogen as readily.

Key Takeaways

• Antibody Fc regions bind to Fc receptors on macrophages.
• This is opsonisation, and it greatly enhances phagocytic destruction of pathogens.
• The antibody is therefore both a neutraliser and a signal to other immune cells.

Common Mistakes

• Writing "stimulates phagocytosis of the pathogen" without mentioning that the antibody–antigen complex is what is engulfed — the mark scheme explicitly ignores unqualified "stimulates phagocytosis".
• Saying antibodies "kill" pathogens — antibodies do not kill; they mark pathogens for destruction by phagocytes or activate complement.
• Confusing this with B-cell receptor binding, which is a different receptor–antibody interaction.

Things to Be Careful About

• Make clear it is the antibody–antigen complex that is engulfed, not the pathogen on its own.
• "Opsonisation" is the technical term that the examiner will recognise immediately.