IB DP Biology: Transport: blood vessels, heart, xylem and phloem (DP Biology) – Revision Notes

These revision notes condense IB Diploma Programme Biology section B3.2 (Transport) from the IB Biology guide, first assessment 2025. Statements B3.2.1 to B3.2.10 are SL and HL. Statements B3.2.11 to B3.2.18 are additional higher level, so they are HL only.

For full explanations and worked examples, read the study guide, then test yourself with the practice questions. Wider theme: Theme B revision notes. Hub: IB DP Biology. Checklist: printable checklist. To locate weak areas, try the free 10-minute diagnostics.

Key definitions

Term	Meaning
Capillary	Narrow, thin-walled vessel where exchange between blood and tissues happens
Pulse	Wave of wall stretch and recoil in an artery after each ventricular contraction
Occlusion	Blockage of a vessel
Atherosclerosis	Plaque build-up beneath the lining of an artery
Transpiration	Loss of water vapour from a plant, mainly through stomata
Cohesion	Attraction between water molecules through hydrogen bonds
Tension	Negative pressure potential in xylem
Translocation	Movement of sap in phloem from sources to sinks
Tissue fluid (HL only)	Fluid filtered from plasma that bathes cells
Systolic and diastolic pressure	Peak and lowest arterial pressure in each cycle

Vessels at a glance (B3.2.1 to B3.2.5)

Vessel	Wall	Lumen	Key adaptation
Artery	Thick; elastic fibres and smooth muscle	Narrow	Stretch and recoil keep pressure up and smooth the flow
Vein	Thin, flexible	Wide	Valves stop backflow; muscle compression moves blood
Capillary	One cell thick	About one red cell wide	Large total surface area, short diffusion distance

Fenestrations are pores through capillary endothelium, in sites needing rapid exchange (kidney glomerulus, intestinal villi, endocrine glands).

Method: telling artery from vein in a micrograph

1. Work out magnification: scale bar length in the image ÷ real length.
2. Measure lumen diameter and wall thickness; divide by the magnification.
3. Calculate wall ÷ lumen. A high ratio means artery; a low ratio means vein.
4. Check the muscle and elastic layers if they are visible.

Pulse (B3.2.4)

• Use fingertips on the carotid (neck) or radial (wrist) artery; not the thumb.
• Beats per minute = beats counted × (60 ÷ seconds counted).
• Repeat and average. Compare with a digital monitor, which avoids counting error but can misread if the sensor moves.

Coronary occlusion (B3.2.6)

Step	Event
1	Damage to the artery lining; plaque builds up
2	Lumen narrows; plaque may rupture and a clot forms
3	Less oxygen reaches cardiac muscle
4	Angina if partial; heart attack if blocked completely

Risk rises with smoking, high blood pressure, high blood cholesterol, a diet high in saturated fat, inactivity and inherited factors.

Data skills: compare rates per 100 000, not raw counts. Correlation coefficients run from −1 to +1 and show strength, not cause. A weak correlation weakens a hypothesis; a strong one does not prove it.

Water in plants (B3.2.7 to B3.2.10)

Transpiration pull in steps

1. Water evaporates from mesophyll cell walls into air spaces.
2. It leaves by stomata.
3. Water is drawn out of xylem through leaf cell walls by capillary action.
4. Tension (negative pressure potential) develops in the xylem.
5. Cohesion keeps the column continuous; the tension pulls the column up.

Xylem vessel adaptations: no cell contents and missing or incomplete end walls (free flow); lignified walls (resist collapse under tension); pits (water enters and leaves sideways).

Plan diagrams: outlines only, no cells, label lines ruled, title and magnification given.

Region	Stem	Root
Vascular tissue	Bundles in a ring; xylem inner, phloem outer	Central; xylem often star-shaped, phloem between arms
Cortex	Between epidermis and bundles	Wide, surrounding the vascular tissue
Epidermis	Outer protective layer	Outer layer with root hair cells

Tissue fluid and lymph (B3.2.11 to B3.2.13, HL only)

• Arteriolar end: high hydrostatic pressure pushes plasma out of the capillary, forming tissue fluid.
• Venular end: lower pressure lets much fluid drain back.
• Tissue fluid has less protein than plasma and no red blood cells.
• The rest drains into lymph vessels (thin walls with gaps, valves) and returns to the blood near the heart.

Worked reminder. Net force = hydrostatic pressure − protein pull. If the arteriole end is 3.8 kPa and the protein pull is 3.1 kPa, the net is +0.7 kPa outwards.

Circulation and the heart (B3.2.14 to B3.2.16, HL only)

Feature	Bony fish	Mammal
Circulation	Single	Double
Passes through heart per circuit	Once	Twice
Sequence	Heart, gills, body, heart	Right side, lungs, left side, body, right side
Pressure after gas exchange organ	Low	Restored by the left ventricle

Heart structures: cardiac muscle (coordinated contraction, many mitochondria); pacemaker (sinoatrial node); atria (thin, receive and fill ventricles); ventricles (thick, left thicker); atrioventricular valves (stop backflow into atria); semilunar valves (stop backflow from arteries); septum (separates two sides); coronary vessels (feed the muscle).

Left-side cycle

1. Pacemaker fires; atrium contracts.
2. Ventricular pressure exceeds atrial: atrioventricular valve shuts.
3. Ventricular pressure exceeds aortic: semilunar valve opens; ejection.
4. Ventricle relaxes: aortic pressure exceeds ventricular, semilunar valve shuts.
5. Ventricular pressure below atrial: atrioventricular valve opens; filling.

Heart rate = 60 ÷ cycle time. Pulse pressure = systolic − diastolic.

Root pressure and phloem (B3.2.17 and B3.2.18, HL only)

• Root pressure: active transport of mineral ions into xylem, then osmosis, gives positive pressure. It helps when transpiration is low (humid air, spring before leaf opening).
• Sieve tube elements: sieve plates, little cytoplasm, few organelles, no nucleus.
• Companion cells: many mitochondria for active loading; plasmodesmata link them to sieve tubes.
• Flow: loading at a source draws water in and raises pressure; unloading at a sink lowers it; sap flows down the pressure difference.

Calculations to know

Skill	Formula	Reminder
Magnification	image size ÷ actual size	Convert to the same unit first
Actual size	image size ÷ magnification	Give units, usually µm
Pulse rate	beats × 60 ÷ seconds	Average several counts
Percentage change	change ÷ original × 100	Use the starting value as the divisor
Rate of disease	cases ÷ population × 100 000	Allows fair comparison

Worked reminder. A vessel measures 36 mm in an image at ×120. Actual width = 36 ÷ 120 = 0.30 mm = 300 µm. A resting pulse of 70 beats per minute rises to 112 after exercise: change = 42, and 42 ÷ 70 × 100 = 60% increase.

Must-know distinctions

Compare	Point
Transpiration pull and root pressure	Pull is negative pressure potential from above; root pressure is positive pressure from below
Xylem and phloem	Dead and hollow versus living; water and minerals versus sugars
Tissue fluid and lymph	Tissue fluid surrounds cells; lymph is the excess inside lymph vessels
Artery and vein	Compare wall thickness relative to lumen, and valves

Quick self-test

1. A micrograph has a 30 mm scale bar for 60 µm. A capillary is 4 mm wide in the image. Find the actual width.
2. A pulse gives 19 beats in 15 s. Calculate beats per minute.
3. A town has 84 new coronary cases among 60 000 people. Give the rate per 100 000.
4. A root has water potential −0.4 MPa and a leaf −1.9 MPa. Which way does water move, and what is the difference?
5. State why xylem walls are lignified.
6. Hydrostatic pressure at a capillary is 3.8 kPa and the protein pull is 3.1 kPa. State the net movement.
7. Calculate heart rate for a cycle of 0.9 s, to the nearest whole number.
8. A reading is 140/90 mmHg. Calculate the pulse pressure.
9. State two adaptations of companion cells.
10. State one reason a mammal’s double circulation delivers blood faster than a fish’s single circulation.

Answers

1. Magnification = 30 000 ÷ 60 = ×500; width = 4 mm ÷ 500 = 8 µm.
2. 19 × 4 = 76 beats per minute.
3. 84 ÷ 60 000 × 100 000 = 140 per 100 000.
4. Towards the lower (more negative) value, so root to leaf; difference 1.5 MPa.
5. To resist collapse under tension.
6. 3.8 − 3.1 = +0.7 kPa, so net movement is out of the capillary.
7. 60 ÷ 0.9 = 67 beats per minute.
8. 140 − 90 = 50 mmHg.
9. Many mitochondria; plasmodesmata linking to sieve tube elements.
10. The heart re-pressurises blood after the lungs.

Where marks are usually lost

• Describing arteries as thick-walled without mentioning elastic recoil and what it does.
• Stating that a capillary wall is thin, but not linking this to a short diffusion distance.
• Saying valves “push” blood. They only prevent backflow.
• Forgetting units in micrograph and pulse calculations.
• Counting cases rather than comparing rates per head of population.
• Writing “correlation shows causation”.
• Saying transpiration “pushes” water up. It pulls, by tension, and cohesion keeps the column unbroken.
• Mixing up valve closures with the pressure that causes them in cardiac cycle questions.
• Forgetting that mature xylem vessels are dead and phloem sieve tubes are living.

Official syllabus

This resource follows the IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization.