IB Biology SL B3.2 Transport Notes

Capillaries are built for exchange, not high-pressure transport. They are narrow, highly branched, and close to body cells, so substances have a short path between blood and tissues. Their one-cell-thick endothelial walls reduce diffusion distance, and fenestrations in some capillaries allow rapid exchange and tissue fluid formation.

Link each structural feature to faster exchange rather than just naming parts.

Arteries and veins share basic wall components: endothelium, smooth muscle, elastic tissue, and collagen. Their proportions differ because pressure differs. Arteries have thicker walls and smaller lumens to withstand and maintain high pressure. Veins have thinner walls and wider lumens for low-pressure return.

Arteries receive blood at high pressure from ventricles. Thick walls and collagen prevent rupture, elastic fibres stretch during systole and recoil during diastole to even out pulse pressure and maintain flow, and smooth muscle in arteries and arterioles regulates blood distribution to tissues.

Pulse is the pressure wave created by ventricular contraction and felt in arteries such as the radial or carotid artery. Counting for a full minute is most accurate; shorter counts can be scaled, but errors are magnified when the count is multiplied.

Veins return blood at low pressure. Valves prevent backflow toward capillaries, thin flexible walls allow surrounding skeletal muscles to compress veins, and large lumens reduce friction so low-pressure blood can flow back to the heart.

The muscle pump matters because venous pressure is low.

Coronary arteries supply cardiac muscle with oxygen and nutrients. Atherosclerosis can form plaques beneath damaged coronary artery endothelium. If a plaque ruptures, thrombosis can occlude the artery, reducing oxygen delivery; cardiac muscle may die, causing myocardial infarction.

Water rises in xylem mainly because transpiration from leaf mesophyll creates tension in xylem water columns. Cohesion between water molecules transmits that tension from leaves toward roots, while adhesion to xylem walls helps maintain an unbroken transpiration stream.

Read the transpiration stream as a tension pathway from leaf to root.

Mature xylem vessels are dead, hollow tubes with absent or perforated end walls, so water can move with little resistance. Lignified walls resist collapse under negative pressure and waterproof the vessel. Pits allow lateral movement of water between vessels and surrounding tissues.

Each feature should be tied to transport efficiency or structural support.

A dicot stem plan diagram shows tissue distribution, not individual cells. Dicot stems have epidermis, cortex, pith, and vascular bundles in a ring. Each vascular bundle contains xylem, phloem, cambium, and supporting fibres. Plan diagrams should use outlines and tissue labels rather than cell detail.

A dicot root has epidermis with root hairs, cortex, endodermis, and central vascular tissue. The xylem forms a central cross with phloem between the arms. The Casparian strip in the endodermis blocks apoplast flow and forces water and ions through selective symplast entry before reaching xylem.

Use tissue position to separate a root plan from a stem plan.

Animal and plant transport answers should link structure to function. In animals, capillaries exchange, arteries maintain high-pressure flow, veins return low-pressure blood, pulse measures arterial pressure waves, and coronary occlusion blocks oxygen delivery to heart muscle. In plants, xylem transports water by transpiration tension and cohesion, while stem and root tissue plans show where xylem and phloem are arranged.

Use this for core questions on blood vessels, pulse, coronary occlusion, xylem, and stem/root tissue distribution.