IB Biology · Theme B: Form and function · SL and HL
B3.2 Transport
A one-page summary of B3.2 Transport, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
What adaptations facilitate transport of fluids in animals and plants?
What are the differences and similarities between transport in animals and plants?
What B3.2 covers
Moving blood, moving sap
- B3.2.1The capillary: exchange at its narrowest
- B3.2.2Same three layers, different proportions
- B3.2.3Arteries: built to survive the surge
- B3.2.4The pulse is a pressure wave, not blood itself
- B3.2.5Veins: built to return blood uphill
- B3.2.6When a coronary artery closes
- B3.2.6 · NOSA strong correlation still isn't proof
- B3.2.7Cohesion-tension: pulling water without a pump
- B3.2.8The xylem vessel: dead tubes built to not break
- B3.2.9A dicot stem, ring by ring
- B3.2.10A dicot root: the star at the centre
Fluid balance, circulation and the heartbeat
- B3.2.11 · HLTissue fluid: leaking out, soaking back in
- B3.2.12 · HLTissue fluid: the middleman for every cell
- B3.2.13 · HLWhere the leftover 10% goes
- B3.2.14 · HLA fish: one circuit, one pressure drop
- B3.2.14 · HLA mammal: two circuits, re-pressurised
- B3.2.15 · HLThe heart's own walls, unequal by design
- B3.2.15 · HLValves and a pacemaker: flow that only goes one way
- B3.2.16 · HLSystole: atria first, then ventricles
- B3.2.16 · HLDiastole, and reading the pressure trace
- B3.2.17 · HLRoot pressure: a backup, not the main pump
- B3.2.18 · HLSieve tube elements: stripped down for flow
- B3.2.18 · HLCompanion cells power the pressure-flow
B3.2 Transport: summary
Blood vessels
- Capillaries: thin, branched, some fenestrated.
- Arteries: thick + elastic; veins: thin + valves.
Pulse & the coronary problem
- Pulse = pressure wave, not blood flow.
- Atherosclerosis → MI; correlation ≠ causation.
Xylem & plant tissues
- Cohesion-tension pulls water; no pump needed.
- Stem: xylem in, ring. Root: xylem star, centre.
Tissue fluid & lymph HL
- Filtered out (arterial end), reabsorbed (venous end).
- Excess drains via lymph back to the blood.
Circulation & the heart HL
- Single (fish) vs double (mammal) circulation.
- Valves + SA node pacemaker; systole then diastole.
Root pressure & phloem HL
- Root pressure: weak backup, ion pumping.
- Pressure-flow: sieve tubes + companion cells.
Key terms
- Tunica media
- Middle vessel-wall layer of smooth muscle and elastic fibres; its thickness differs sharply between artery and vein.
- Pulse
- The pressure wave from ventricular contraction, travelling through artery walls faster than the blood itself.
- Atherosclerosis
- Build-up of cholesterol-rich plaques in an artery wall, narrowing the lumen.
- Cohesion-tension theory
- Evaporation at the leaf generates tension; cohesion between water molecules transmits it down the xylem.
- Casparian strip
- Suberin band in the root endodermis that blocks the apoplast, forcing water through a cell membrane.
- Double circulation HL
- Separate pulmonary and systemic circuits, letting blood be re-pressurised after the lungs.
- Pressure-flow hypothesis HL
- Active loading of sucrose at a source raises pressure, driving bulk flow of phloem sap to a sink.
Sample exam questions
Three of the 66 multiple-choice questions for B3.2. Try each one before opening the answer.
Question 1. The human heart is described as a double pump. This means:
- Blood is pumped twice through the heart on each circuit
- The right side pumps blood to the lungs (pulmonary circuit) and the left side pumps blood to the body (systemic circuit)
- The heart pumps both blood and lymph simultaneously
- The heart has two chambers that pump alternately
Show the answer
Answer: B. Double circulation: right side → pulmonary (low pressure, gas exchange); left side → systemic (high pressure, delivery to body). Keeps oxygenated and deoxygenated blood separate.
Question 2. The left ventricle wall is significantly thicker than the right ventricle wall because:
- The left ventricle contains more blood than the right ventricle
- The left ventricle beats more frequently than the right ventricle
- The left ventricle pumps oxygenated blood, which is denser than deoxygenated blood
- The left ventricle must generate higher pressure to pump blood through the entire systemic circulation, while the right ventricle only pumps to the nearby lungs
Show the answer
Answer: D. Systemic circuit has higher resistance, requiring ~120 mmHg systolic pressure. Pulmonary circuit is low-resistance, ~25 mmHg. Thicker wall enables higher pressure generation.
Question 3. How is root pressure generated, and when does it make the greatest relative contribution to water transport in plants?
- Active transport of mineral ions from soil into the root xylem lowers the water potential there, drawing water in by osmosis and building hydrostatic pressure; it contributes most at night when stomata are closed
- Transpiration pulls water up through the xylem by cohesion-tension; it contributes most during the day when stomata are open
- Root cells actively pump water directly into the xylem using ATP; it contributes most during periods of high transpiration
- Osmosis alone, without any active transport, generates root pressure equally throughout the day and night
Show the answer
Answer: A. Root cells actively transport mineral ions from the soil into the root xylem using ATP. This lowers the water potential inside the xylem, so water moves in from the root cortex by osmosis. The resulting build-up of water in the enclosed xylem generates a positive hydrostatic pressure (root pressure) that pushes xylem sap upward. This is a minor contributor compared with transpiration pull (cohesion-tension), but becomes the dominant mechanism at night when stomata are closed and transpiration is minimal — evidenced by guttation, where water droplets are forced out of leaf margins.
Linking questions
Questions that connect B3.2 to other parts of the course, the kind that come up in Paper 2.
- Oxygen is transported from the lungs (B3.1) to respiring tissues by haemoglobin in red blood cells. Explain how the structure of haemoglobin (B1.2) enables cooperative binding of oxygen, and how this produces the sigmoidal oxygen dissociation curve. (see B1.2, B3.1)
- The cohesion of water molecules is essential for xylem transport. How do the hydrogen-bonding properties of water (A1.1) enable the transmission of tension from the leaf to the root? (see A1.1)
- Phloem transports sucrose to developing fruits, which are strong sinks. How does the respiration of sucrose in the fruit (C1.2) maintain the fruit's status as a sink and sustain the pressure gradient driving phloem flow? (see C1.2)
- Water moves from xylem into phloem sieve tubes by osmosis. How does the concept of water potential (D2.3) explain this movement, and what role do solute concentrations in source and sink tissues play? (see D2.3)
Practise B3.2
Study notes, every question and full markschemes for B3.2 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.