B3.2 TransportIB Biology SL: Revision notes

Capillaries are adapted for exchange between blood and tissues: branching and narrow diameters give a large surface area; walls one cell thick give a short diffusion distance; fenestrations (pores) occur where exchange must be particularly rapid, e.g. villi and kidney glomeruli.

Arteries carry blood away from the heart at high pressure. Thick layers of smooth muscle and elastic fibres let them withstand pressure; elastic fibres stretch at each beat and recoil to maintain pressure between beats. Their wall is thick relative to a narrow lumen and they look round in section.

Veins return blood at low pressure. They have valves to stop backflow and thin, flexible walls that are compressed by skeletal muscle contraction. In micrographs they have a thin wall relative to a wide lumen and often a collapsed outline.

Heart rate can be measured by feeling the carotid (neck) or radial (wrist) pulse with two fingertips, counting for 15 or 30 s and multiplying up to beats per minute. Digital monitors (pulse oximeters, smartwatches) are quicker and avoid counting errors but may be affected by movement or poor contact.

Occlusion of the coronary arteries by fatty plaque (atheroma) narrows the lumen; a ruptured plaque can trigger a clot. Heart muscle receives too little oxygen, causing angina or, if blocked completely, a heart attack (myocardial infarction). Risk factors: high saturated fat / LDL, smoking, high blood pressure, obesity, inactivity, genetics.

A correlation coefficient (r, from −1 to +1) measures the strength of a correlation. Low values may be evidence against a hypothesis, but even a strong correlation (such as saturated fat and CHD) does not prove causation.

Transpiration from leaf cell walls draws water out of xylem through cell walls by capillary action, generating tension (negative pressure potential). This tension pulls water up the xylem, and cohesion between water molecules keeps the column continuous.

Xylem vessel adaptations: no cell contents and incomplete or absent end walls for unimpeded flow; lignified walls to withstand tension without collapsing; pits for water to enter and leave.

Stem (transverse section): outer epidermis, cortex, a ring of separate vascular bundles near the edge, each with phloem towards the outside and xylem towards the centre, and central pith. The ring gives support against bending.

Root: epidermis (with root hairs), a wide cortex, and a central vascular cylinder in which xylem forms a star or X shape with phloem between its arms. Central vascular tissue resists pulling forces.