IBDP Biology HL Cheat sheet - B3.2 Transport | IBDP Biology Cheatsheets

Capillaries: exchange surfaces

Capillaries are the main sites of exchange between blood and tissues.
Adaptations: very narrow diameter, highly branched networks, thin walls (one cell thick), and fenestrations in some capillaries where rapid exchange is needed.
Narrow lumen slows flow and keeps red blood cells close to the wall, reducing diffusion distance.
Large total surface area increases rate of exchange of oxygen, carbon dioxide, nutrients, and wastes.

This image shows how capillaries have very thin walls and tiny lumens, while arteries and veins are adapted for transport rather than exchange. It is useful for linking structure to function across all three vessel types. Source

Arteries and veins: structure and function

Arteries carry blood away from the heart at high pressure.
Artery walls contain thick layers of smooth muscle and elastic tissue to withstand pressure and maintain blood flow.
Arteries have a relatively narrow lumen compared with wall thickness.
Veins return blood to the heart at lower pressure.
Veins have thinner walls, less muscle and elastic tissue, and a wider lumen.
Valves in veins prevent backflow of blood.
Vein walls can be compressed by surrounding muscles, helping blood return to the heart.
In micrographs: arteries usually look rounder with thick walls; veins usually have larger lumens and thinner walls.

Pulse rates and coronary artery occlusion

Pulse rate gives an estimate of heart rate and can be measured at the radial or carotid pulse.
A pulse is caused by the surge of blood through arteries after ventricular contraction.
Coronary arteries supply the heart muscle with oxygen and glucose.
Occlusion of coronary arteries reduces blood supply to cardiac muscle and can cause coronary heart disease, angina, or myocardial infarction (heart attack).
Epidemiological data may show correlations between risk factors and disease, but correlation does not prove causation.

Tissue fluid and lymphatic drainage

Tissue fluid forms when plasma is forced out of capillaries by high hydrostatic pressure at the arteriole end.
Tissue fluid bathes cells and allows exchange of oxygen, nutrients, carbon dioxide, and wastes.
At the venule end, lower blood pressure allows much of the tissue fluid to re-enter capillaries.
Plasma contains more plasma proteins than tissue fluid because large proteins usually remain in the blood.
Excess tissue fluid drains into lymph ducts/vessels.
Lymph vessels have thin walls, gaps, and valves.
Lymph is eventually returned to the blood circulation.

Single and double circulation

In single circulation (for example, bony fish), blood passes through the heart once in each complete circuit: heart → gills → body → heart.
In double circulation (for example, mammals), blood passes through the heart twice in each circuit: heart → lungs → heart → body → heart.
Double circulation allows blood to be pumped to the body at higher pressure after leaving the lungs.
This supports faster delivery of oxygen and nutrients to tissues with high metabolic demand.

Mammalian heart: adaptations and blood flow

The heart is adapted to generate pressurized, one-way blood flow.
Cardiac muscle contracts continuously and has an excellent blood supply via coronary vessels.
The sinoatrial node (SAN) acts as the pacemaker, initiating the heartbeat.
Atria receive blood; ventricles pump blood out of the heart.
The left ventricle has a thicker wall than the right because it pumps blood to the whole body.
Atrioventricular valves prevent backflow from ventricles to atria.
Semilunar valves prevent backflow from arteries into ventricles.
The septum separates the left and right sides, preventing mixing of oxygenated and deoxygenated blood.
Unidirectional flow: vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium → left ventricle → aorta.

This diagram shows the cardiac cycle as alternating phases of atrial and ventricular activity. It is useful for understanding blood flow direction, valve action, and the timing of systole and diastole. Source

Cardiac cycle and blood pressure

The heartbeat starts at the sinoatrial node, causing atrial systole.
Blood then moves into the ventricles before ventricular systole begins.
During ventricular systole, atrioventricular valves close and semilunar valves open.
During diastole, the heart muscle relaxes and chambers refill with blood.
Systolic pressure = arterial pressure during ventricular contraction.
Diastolic pressure = arterial pressure during ventricular relaxation.
In data questions, be able to interpret graphs or tables showing changes in pressure over time.

Transport in plants: transpiration stream

Water moves from roots to leaves in the xylem during transpiration.
Transpiration from moist cell walls in leaves lowers water potential and pulls water out of the xylem.
This creates tension (negative pressure) in xylem.
Cohesion between water molecules keeps an unbroken column of water.
Water also moves through cell walls by capillary action.
The transpiration stream is a passive process driven mainly by water loss from leaves.

Xylem: adaptations for water transport

Xylem vessels transport water and mineral ions.
Vessel elements are dead at maturity and have no cytoplasm, reducing resistance to flow.
End walls are absent or incomplete, allowing continuous flow.
Walls are lignified, preventing collapse under tension.
Pits allow sideways movement of water between vessels and surrounding tissues.
Xylem is specialized for one-way transport upward.

Root pressure and phloem translocation

Root pressure is positive pressure generated when mineral ions are actively transported into xylem.
Water enters xylem by osmosis, pushing sap upward when transpiration is low.
Root pressure is especially important in high humidity or early spring before leaves open.
Phloem transports sucrose and other organic solutes in sap.
Sieve tube elements have reduced cytoplasm, few organelles, and no nucleus, reducing resistance to flow.
Sieve plates allow movement between adjacent sieve tube elements.
Companion cells contain many mitochondria and are linked by plasmodesmata to sieve tubes.
Companion cells use ATP for loading solutes at sources and unloading at sinks.

This image shows the contrasting roles of xylem and phloem in plant transport. It is useful for remembering that xylem carries water/minerals and phloem carries products of photosynthesis. Source

Dicot stem and root transverse sections

In a dicot stem transverse section, identify epidermis, cortex, vascular bundles, xylem, and phloem.
In stems, vascular bundles are arranged in a ring near the outside.
In a dicot root transverse section, identify epidermis, cortex, xylem, and phloem.
In roots, xylem is more central, with phloem between the arms of xylem.
In exam diagrams, always link position to function: xylem = water transport/support, phloem = translocation, cortex = storage/transport pathway, epidermis = outer protection.

This diagram helps identify the arrangement of tissues in a dicot stem cross section. It is especially useful for plan-diagram questions where you must place vascular tissues in the correct relative positions. Source

This image shows the tissue arrangement in a dicot root, including the central xylem and surrounding phloem. It is ideal for practicing root tissue identification from diagrams or micrographs. Source

HL only: tissue fluid, lymph, heart, and plant transport

Tissue fluid forms by pressure filtration of plasma from capillaries.
Exchange between tissue fluid and cells supplies materials and removes wastes.
Excess tissue fluid enters lymph ducts, which return lymph to the blood.
Mammals have double circulation, unlike the single circulation of bony fish.
The heart’s structure supports high-pressure circulation and one-way flow.
The cardiac cycle includes atrial systole, ventricular systole, and diastole.
Root pressure can move water when transpiration pull is weak.
Phloem translocation depends on specialized sieve tube elements and companion cells.

Checklist: can you do this?

Distinguish arteries, veins, and capillaries from structure alone, including in micrographs.
Trace blood flow through the mammalian heart and relate structures to one-way transport.
Interpret systolic and diastolic blood pressure data and the sequence of the cardiac cycle.
Explain transpiration pull, xylem adaptations, root pressure, and phloem translocation.
Draw and annotate plan diagrams of dicot stem and dicot root transverse sections.

High-yield comparisons

Arteries vs veins: arteries = high pressure, thick muscular/elastic walls, no valves near heart; veins = low pressure, thinner walls, valves present.
Capillaries vs other vessels: capillaries are specialized for exchange, not bulk transport.
Xylem vs phloem: xylem transports water/minerals upward; phloem transports organic solutes from sources to sinks.
Single vs double circulation: double circulation allows higher pressure to body tissues.
Transpiration pull vs root pressure: transpiration pull = main mechanism, negative pressure; root pressure = backup/support, positive pressure.

Written by:

Dr Shubhi Khandelwal

Qualified Dentist and Expert Science Educator

Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.