Biology 9700/21 — May/June 2012

Background Concept

The mammalian circulatory system is a closed double circulation consisting of two circuits: the pulmonary circuit (heart → lungs → heart) and the systemic circuit (heart → body tissues → heart). Each circuit requires an artery (carries blood away from the heart) and a vein (carries blood back to the heart).

The key unusual feature of the pulmonary circuit is that its artery — the pulmonary artery — carries deoxygenated blood, while its vein — the pulmonary vein — carries oxygenated blood. This is the opposite of the systemic pattern, where arteries carry oxygenated blood and veins carry deoxygenated blood.

Understanding the Question

The stem supplies two diagnostic clues:

1. The vessel is a blood vessel (so a named artery, vein or capillary).
2. It carries deoxygenated blood AND the blood is being carried from the heart.

A vessel that carries blood from the heart is, by definition, an artery. We must then identify the artery that carries deoxygenated blood — the only artery in the body that does so in the post-natal circulation is the pulmonary artery (which leaves the right ventricle and goes to the lungs).

Approach

Combine the two clues: "from the heart" → artery; "deoxygenated" → pulmonary artery. The venae cavae also carry deoxygenated blood, but they carry it to the heart, so they do not fit.

Step-by-Step Reasoning

• Vessels leaving the heart are arteries; vessels entering the heart are veins.
• In the systemic circulation, arteries carry oxygenated blood and veins carry deoxygenated blood.
• The pulmonary circulation reverses this: the pulmonary artery carries deoxygenated blood from the right ventricle to the lungs, where it is oxygenated.
• Therefore, the blood vessel described is the pulmonary artery.

Key Takeaways

• Arteries carry blood away from the heart; veins carry blood towards the heart — regardless of oxygen content.
• The pulmonary artery is the only artery in the post-natal mammalian circulation that carries deoxygenated blood.

Common Mistakes

• Answering "vena cava" — the venae cavae carry deoxygenated blood, but they are veins carrying blood to the right atrium, not from the heart.
• Answering "aorta" — the aorta leaves the heart but carries oxygenated blood to the systemic circulation.
• Writing "pulmonary vein" — the pulmonary vein carries oxygenated blood from the lungs to the heart.

Things to Be Careful About

The mark scheme accepts the plural "pulmonary arteries" because there are two of them (one to each lung), but "pulmonary artery" is the standard singular answer. Do not add qualifiers such as "left" or "right" unless you are certain, since the mark scheme does not require it.

Background Concept

Phagocytosis is the engulfment and intracellular digestion of solid particles such as bacteria, cell debris and dust. The cells that carry it out are called phagocytes. The two principal phagocytes of the human body are:

• Macrophages — large, long-lived phagocytes derived from monocytes. They are found in tissues (e.g. alveolar macrophages in the lungs, Kupffer cells in the liver, microglia in the CNS) and are highly effective at engulfing cell debris, dead cells and pathogens.
• Neutrophils (also called polymorphonuclear leucocytes, PMNs) — short-lived, highly mobile phagocytes that are usually the first responders to a site of infection. They circulate in the blood and migrate into infected tissue in large numbers.

Both recognise non-self material, engulf it to form a phagosome, fuse the phagosome with a lysosome, and digest the contents with lysozyme and other hydrolytic enzymes.

Understanding the Question

The stem gives three clues:

1. The cell ingests material — i.e. it carries out phagocytosis.
2. It digests what it has ingested — i.e. it has lysosomal enzymes.
3. The material includes cell debris and bacteria — a non-specific, innate function.
4. The location is the lungs.

A phagocyte in the lungs fits all four criteria. The cell that does this most efficiently in the alveoli is the alveolar macrophage, but the question asks for the general cell type, not the named tissue-specific form.

Approach

The functional description ("ingests and digests") is the textbook definition of a phagocyte. The location (lungs) simply specifies the site — the same cell type is found in many other tissues.

Step-by-Step Reasoning

• "Ingests" + "digests" + "cell debris and bacteria" = phagocytosis.
• A cell that phagocytoses is a phagocyte.
• The most accurate name for the phagocyte that is resident in the lung tissue is the alveolar macrophage (a tissue-resident macrophage), but the general name phagocyte or macrophage is also credited.
• Neutrophils are also credited; they migrate into the lungs from the blood during infection to phagocytose bacteria.

Key Takeaways

• A phagocyte is any cell that engulfs and digests solid particles using lysosomal enzymes.
• In the lungs, the resident phagocyte is the alveolar macrophage; during infection, large numbers of neutrophils are also recruited.
• Phagocytosis is a non-specific (innate) defence — it does not require prior exposure to the pathogen.

Common Mistakes

• Writing "white blood cell" or "leucocyte" unqualified — these are too broad; the mark scheme rejects them. Many white blood cells (e.g. lymphocytes) are not phagocytic.
• Writing "PMN" alone — this is a rejected abbreviation in the mark scheme.
• Writing "ciliated cell" — cilia do not phagocytose; they move mucus.
• Writing "goblet cell" — goblet cells secrete mucus; they do not ingest bacteria.

Things to Be Careful About

The mark scheme accepts phagocyte, macrophage or neutrophil but rejects "leucocyte" or "white blood cell" on their own because not all leucocytes are phagocytic. "PMN" as an isolated abbreviation is also rejected.

Background Concept

B-lymphocytes (B cells) are the lymphocytes responsible for the humoral immune response. When a B cell encounters its specific antigen (and receives co-stimulation from a helper T cell), it proliferates and differentiates into two main cell types:

• Plasma cells (also called effector B cells) — large, antibody-secreting cells. A single plasma cell can secrete thousands of antibody molecules per second.
• Memory B cells — long-lived cells that mount a rapid secondary response on re-exposure to the same antigen.

Antibodies (immunoglobulins) are Y-shaped proteins that bind specifically to the antigen that triggered their production. They neutralise pathogens, opsonise them for phagocytosis, activate complement, and agglutinate antigens.

Understanding the Question

The stem asks for "the cell that secretes antibodies". The clue is purely functional: which cell type manufactures and releases antibodies into the blood and tissue fluid?

Approach

Recall the differentiation pathway of the B-lymphocyte: B cell → plasma cell (secretes antibodies) + memory B cell (long-lived). The cell doing the secretion is the plasma cell.

Step-by-Step Reasoning

• Lymphocytes come in two main classes: B-lymphocytes and T-lymphocytes.
• T-lymphocytes do not secrete antibodies; they act directly on infected cells (cytotoxic T cells) or co-ordinate other immune cells (helper T cells).
• B-lymphocytes, once activated, differentiate into plasma cells, which are the antibody factories of the immune system.
• Therefore the cell that secretes antibodies is the plasma cell (a differentiated B-lymphocyte).

Key Takeaways

• B-lymphocytes → activated B cells → plasma cells (antibody secretion) + memory B cells (long-term immunity).
• Plasma cells are the only cells of the immune system that secrete antibodies in significant quantities.
• Antibody secretion is a feature of the humoral (antibody-mediated) immune response, not the cell-mediated response.

Common Mistakes

• Writing "lymphocyte" alone — the mark scheme rejects this because T-lymphocytes also exist and do not secrete antibodies.
• Writing "T cell" or "T-lymphocyte" — T cells do not secrete antibodies; they are involved in cell-mediated immunity.
• Writing "phagocyte" or "macrophage" — these ingest and digest; they do not secrete antibodies.
• Writing "effector cell" unqualified — too vague; the mark scheme rejects this.

Things to Be Careful About

The mark scheme accepts "plasma cell", "B-lymphocyte" (as the cell of origin) or "effector B cell". The safest single-word answer is plasma cell because the stem says "secretes" — that is the effector form, not the naive B-lymphocyte that is waiting to be activated. Do not just write "B cell" without qualification if the question specifically asks about the cell doing the secreting, because naive B cells do not secrete antibodies until they have differentiated.

Background Concept

The gas exchange system (trachea, bronchi, bronchioles) is lined by a ciliated pseudostratified columnar epithelium with two important specialist cells:

• Goblet cells — modified epithelial cells shaped like a wine goblet, with the apical (lumen-facing) cytoplasm packed with mucinogen granules. They secrete mucus, a sticky glycoprotein-rich fluid that traps dust, microbes and other inhaled particles.
• Ciliated cells — epithelial cells bearing many motile cilia on their apical surface. The cilia beat in a synchronised, wave-like motion that moves the sheet of mucus (and any trapped debris) upwards towards the pharynx, where it is swallowed. This is the mucociliary escalator.

Mucus secretion and ciliary beating together keep the lower airways clean and free of infection.

Understanding the Question

The stem gives two diagnostic clues:

1. The cell is epithelial.
2. It secretes mucus.
3. The location is the trachea.

A mucus-secreting epithelial cell in the trachea is, by name, a goblet cell. (Submucosal mucous glands in the trachea also secrete mucus, but they are not epithelial cells — they are compound tubuloacinar glands, so they are not what the stem asks for.)

Approach

Match the function (secretes mucus) to the cell type that performs it in the tracheal epithelium. The cell with mucinogen granules in its apical cytoplasm is the goblet cell.

Step-by-Step Reasoning

• The trachea is lined by ciliated pseudostratified columnar epithelium.
• Within this epithelium, the specialised mucus-secreting cells are goblet cells.
• The ciliated cells move the mucus, but do not secrete it.
• Submucosal glands also secrete mucus, but the stem specifies "the epithelial cell", so the answer is goblet cell.

Key Takeaways

• Goblet cells are the mucus-secreting cells of the respiratory epithelium.
• They are part of the mucociliary escalator, working with ciliated cells to clear inhaled debris.
• Mucus traps particles; cilia move the mucus; the cell that makes the mucus is the goblet cell.

Common Mistakes

• Writing "mucous gland" — these are also mucus-secreting structures in the tracheal wall, but the stem asks for an epithelial cell, not a gland in the submucosa.
• Writing "ciliated cell" — these move the mucus but do not secrete it.
• Writing "epithelial cell" alone — too vague; the mark scheme requires the specific cell type.
• Confusing goblet cells with brush border cells of the intestine — both are apical-specialised epithelial cells but with different functions.

Things to Be Careful About

The question asks for the cell, not the gland. The mark scheme wants "goblet cell". Do not write "mucus-secreting cell" generically — the mark scheme needs the specific name.

Background Concept

The trachea is held open by incomplete rings of hyaline cartilage in its wall. These C-shaped rings:

• are rigid (the matrix is dominated by type II collagen and proteoglycans, with chondrocytes embedded in lacunae);
• keep the tracheal lumen patent (open) during inhalation, when the air pressure inside falls and the wall would otherwise collapse inwards;
• are open at the back (posteriorly), where the trachealis muscle (smooth muscle) bridges the gap and allows the diameter of the trachea to be adjusted slightly during coughing and forced exhalation.

The cartilage is technically described as being in the form of C-shaped rings (or "plates" further down the airway), but the tissue itself is cartilage — and that is what the mark scheme credits.

Understanding the Question

The stem gives two diagnostic clues:

1. The tissue prevents collapse of the trachea.
2. This prevention is needed specifically during inhalation (when the intraluminal pressure is lowest).

A rigid supporting tissue that resists collapse under negative pressure is, by definition, cartilage.

Approach

Think of the mechanical function required: a tissue that resists compressive forces and holds a tube open. The supporting tissue in the tracheal wall is hyaline cartilage.

Step-by-Step Reasoning

• The trachea must remain open to allow air to flow in and out of the lungs.
• During inhalation, the pressure inside the trachea falls below atmospheric pressure; without support, the soft tissue wall would be sucked inwards and the airway would collapse.
• Rigid C-shaped rings of hyaline cartilage in the tracheal wall prevent this collapse.
• Therefore the tissue is cartilage.

Key Takeaways

• Hyaline cartilage gives the trachea (and bronchi) its rigidity.
• It prevents collapse of the airway during the negative pressure of inhalation.
• The cartilage is in C-shaped rings, but the tissue itself is what the mark scheme asks for.

Common Mistakes

• Writing "bone" — bone is found in the ribs and vertebrae around the thorax, but the trachea itself is supported by cartilage, not bone.
• Writing "smooth muscle" — the trachealis smooth muscle at the back of the trachea adjusts airway diameter, but it does not prevent collapse; in fact, if it contracted maximally, it would narrow the lumen.
• Writing "elastic fibres" — these allow the lungs and airway to recoil, but they are too extensible to keep the trachea open against negative pressure.
• Writing "ciliated epithelium" — this is the lining of the trachea, not a supporting tissue.
• Writing "C-shaped rings" or "plates" alone — the mark scheme will ignore the descriptor but still needs the tissue name "cartilage". A candidate who writes only "rings" without naming the tissue would not be credited.

Things to Be Careful About

The mark scheme says "ignore plates / rings" — meaning that if you write "cartilage rings" or "cartilage plates", you will still get the mark, but the required word is cartilage. Do not omit the tissue name in favour of the shape.