3.3.4 Mass transport

Mass transport is how organisms move things in bulk, oxygen, sugars, water, when diffusion alone is too slow. AQA examines two parallel systems: the cardiovascular system in animals, with haemoglobin as the oxygen carrier and a four-chambered pump driving the loop; and xylem and phloem in plants, both of which move fluid without a pump at all.

Haemoglobin loads and unloads oxygen reversibly along a sigmoidal dissociation curve.

Haemoglobin is a globular protein with a quaternary structure of four polypeptide subunits. Each subunit holds a prosthetic haem group with one Fe²⁺ ion at its core. Each Fe²⁺ ion reversibly binds one oxygen molecule, so each haemoglobin carries up to four. Oxygen is loaded where the partial pressure is high, in the lungs, and unloaded where it is low, at respiring tissues.

1. The first oxygen molecule binds to one haem group on the haemoglobin molecule.
2. The binding causes a change in the tertiary (or quaternary) structure of the haemoglobin molecule.
3. The structural change uncovers a second binding site that is more accessible to the next oxygen molecule.
4. The next oxygen binds more easily, and the third and fourth bind progressively more easily still; this is the cooperative effect.

Write tertiary structure change (or quaternary structure change) paired with second binding site uncovered. The label positive cooperativity on its own, without the mechanism chain, scores zero. The chain is what the mark scheme credits.

Write binding site or haem group for the oxygen-binding location on haemoglobin. Don't write active site; haemoglobin is not an enzyme, and AQA rejects the term on any non-enzyme protein.

The Bohr effect releases more oxygen where metabolism is highest; fetal haemoglobin's curve sits to the left.

Respiring cells produce carbon dioxide. The CO₂ dissolves in blood plasma to form carbonic acid, which dissociates and releases hydrogen ions. The lowered pH alters haemoglobin's tertiary structure, and its affinity for oxygen falls.

Write haemoglobin has lower affinity; name the molecule and include the comparative lower (or higher). Affinity belongs to haemoglobin, not to the organism. Pair the affinity statement with more oxygen released; the comparative is required there too.

The mammalian heart is a double pump whose valves open and close by pressure differences.

The mammalian heart is a double pump in a closed double circulatory system. The right side receives deoxygenated blood from the vena cava and pumps it via the pulmonary artery to the lungs. The left side receives oxygenated blood from the pulmonary veins and pumps it via the aorta to the body. The left ventricle wall is thicker than the right because it must generate the higher pressure that drives blood through the systemic circuit.

Write bicuspid valve or tricuspid valve; name left or right. AV valve without specifying the side is rejected on any question that distinguishes the two.

1. A valve opens when upstream pressure exceeds downstream pressure; for example, the atrioventricular valves open in diastole because atrial pressure has risen above ventricular pressure.
2. A valve closes when downstream pressure exceeds upstream pressure; for example, the atrioventricular valves close in ventricular systole because rising ventricular pressure exceeds atrial pressure.

Valve answers need a pressure difference in both directions: opening (upstream above downstream) and closing (the reverse). Timing words like during ventricular systole describe when, not why. They earn no mark on their own.

The heart is myogenic; it initiates its own contraction at the sinoatrial node. A ring of non-conducting tissue at the base of the atria stops the wave passing directly into the ventricles, so the AVN delay is the only route through. The delay lets atrial contraction finish before the ventricles begin, and contraction starts at the apex and travels upward.

Arteries, capillaries, and veins are structured for their pressure regime; atherosclerosis narrows arteries.

The three blood-vessel types are each adapted to a different pressure regime. Arteries take high-pressure pulsatile flow from the heart. Capillaries are the exchange vessels. Veins return blood at low pressure.

Write endothelium (or endothelial) for the inner lining of any blood vessel. Epithelial is rejected. For the aorta wall, write elastic recoil; muscles pump, muscles maintain pressure, and vasoconstriction in the aorta are all rejected. Vasoconstriction is an arteriole behaviour, not an aorta one.

On conclude questions about aneurysm or atherosclerosis trends, name the outcome the stem specifies. Describing a trend in aorta-wall data without writing aneurysm (or whichever outcome the stem names) scores zero.

Tissue fluid forms by hydrostatic pressure and returns by osmosis.

Blood plasma carries dissolved nutrients, oxygen, and waste products, plus large plasma proteins that cannot cross the capillary wall. Tissue fluid is the fluid that forms in the tissue spaces around capillaries when water and small solutes are forced out of the blood.

1. At the arterial end of the capillary, blood hydrostatic pressure exceeds the hydrostatic pressure of the surrounding tissue spaces; water and small solutes are forced out through the capillary wall.
2. Tissue fluid lacks the plasma proteins, which remain in the capillary; the water potential of the blood inside the capillary therefore falls relative to the tissue fluid outside.
3. At the venous end, hydrostatic pressure has fallen as the blood slowed and lost fluid; the osmotic effect of the plasma proteins now dominates, and water moves back into the capillary by osmosis.
4. The surplus tissue fluid, together with any small plasma proteins that leaked out, drains into lymphatic capillaries and returns to the systemic circulation via the thoracic duct.

The subject of osmosis is water; write water returns by osmosis. Tissue fluid returns by osmosis is rejected. The precision the mark scheme requires is that water moves, not the fluid as a whole.

Water moves from soil to leaf through the xylem by the cohesion-tension mechanism.

Water enters the plant at root hair cells by osmosis; the root hair sap has a lower water potential than the surrounding soil water. Water then crosses the root cortex by the apoplast pathway (through cell walls) or the symplast pathway (through cytoplasm via plasmodesmata). At the endodermis, the Casparian strip blocks the apoplast and forces all water through the symplast at that ring.

1. The xylem holds a continuous column of water from root to leaf; the vessels are dead, lignified, open-ended tubes that do not collapse under tension.
2. Water molecules are held together by hydrogen bonds (cohesion) and adhere to the lignified vessel walls; these forces keep the column continuous even when it is under stress.
3. Transpiration evaporates water from the surfaces of mesophyll cells inside the leaf, and the water vapour diffuses out through the stomata; the resulting low water potential at the top of the column generates a tension that pulls the column upward.

Pitfall — Cohesion-tension is a three-step compound mark

All three steps must appear in order; cohesion alone caps the answer at one mark.

On the canonical cohesion-tension question, fewer than 5% of students achieve all three marks. The continuous column and the cohesion step are the points most students reach. The tension step, transpiration evaporates water from the mesophyll, lowering the water potential at the top of the column and generating the tension that pulls the column upward, is the consistently dropped one. Write all three points in order; partial answers cap at one mark.

Name a specific environmental factor: light intensity, temperature, humidity, or wind. The sun and it gets warmer are rejected. Photosynthesis rate as the cause of transpiration is rejected; it is correlated, not causal. Stomata open to allow water loss is also rejected; stomata open for CO₂ uptake, and water loss is a consequence.

Phloem translocates sucrose from source to sink by mass flow.

Phloem moves sucrose and other assimilates from sources, primarily photosynthetically active leaves, to sinks, primarily roots, fruits, and growing shoot tips. The functional units are sieve tube elements (living cells with perforated end walls, called sieve plates, and almost no organelles) and companion cells (dense with mitochondria and connected to the sieve tubes via plasmodesmata).

1. Companion cells use ATP to actively pump hydrogen ions out; H⁺ re-enters via co-transporters that simultaneously carry sucrose into the companion cell; sucrose then passes via plasmodesmata into the sieve tube, lowering the water potential inside the sieve tube at the source end.
2. Water moves into the sieve tube from the adjacent xylem by osmosis, raising the hydrostatic pressure inside the sieve tube at the source end.
3. At the sink, sucrose is removed (converted to starch or used in respiration); water potential rises and water leaves the sieve tube by osmosis, so hydrostatic pressure falls at that end; sucrose solution flows passively from high pressure to low pressure by mass flow.

Write sucrose; sugar and carbohydrate are insufficient. Write mass flow and hydrostatic pressure gradient. Diffusion along the phloem is rejected; mass flow is bulk flow down a pressure gradient, not diffusion down a concentration gradient.

Pitfall — The phloem tracer credit chain is fixed

On tracer questions, the four-link chain must be articulated; describing the data is not enough.

Radioactive carbon dioxide is fixed in photosynthesis, incorporated into sucrose, transported in the phloem, and the movement requires living cells. Heat-killed phloem stops translocation, which is why metabolic poisons block the process. Ringing the bark removes the phloem but leaves the xylem intact, so movement stops only on one side of the ring. Describing the tracer data without applying this chain scores zero.

Key terms

• haemoglobin
• pressure
• valve
• tissue fluid
• osmosis
• water potential
• aorta
• elastic recoil
• xylem
• cohesion
• transpiration
• phloem