B3.2 Transport

Once an organism grows beyond a few cells, diffusion alone cannot move materials fast enough between its exchange surfaces and its interior. The solution is a transport system: a network that carries substances in bulk across the body. In animals this is the circulatory system driven by the heart; in plants it is the vascular tissue that lifts water from root to leaf and distributes sugars. B3.2 asks you to connect the structure of vessels and pumps to the job each does, and to explain the elegant physics — tension, pressure and adhesion — that moves fluids without a plant ever needing a heart.

Blood vessels

Mammals have a closed circulation with three vessel types, each built for its role:

• Arteries carry blood away from the heart at high pressure. They have thick walls with much elastic tissue and smooth muscle and a narrow lumen; the elastic walls stretch and recoil to smooth the pulses of pressure.
• Capillaries are the site of exchange. They have walls just one cell thick and a very narrow lumen, giving a short diffusion distance and large surface area for substances to pass between blood and tissue.
• Veins return blood to the heart at low pressure. They have thinner walls, a wide lumen and valves that prevent backflow; nearby muscle movement helps squeeze blood along.

Always match the feature to its function: thick elastic walls for high pressure, thin walls for exchange, valves for low-pressure return.

The heart and the circulation

Mammals have a double circulation: the right side of the heart pumps deoxygenated blood to the lungs (pulmonary circuit) and the left side pumps oxygenated blood to the body (systemic circuit). This keeps the two streams separate and maintains a high systemic pressure. The left ventricle has a thicker, more muscular wall than the right because it must pump blood around the whole body.

The heartbeat is myogenic — it originates within the heart muscle itself. The sinoatrial node (SAN) in the right atrium acts as the pacemaker, setting the rhythm by sending out waves of electrical excitation that make the atria contract; the signal then passes via the atrioventricular node (AVN) to the ventricles, causing them to contract slightly later. During the cardiac cycle, valves between the atria and ventricles, and at the artery exits, open and close to ensure one-way flow. Heart rate is adjusted by nerves and by the hormone adrenaline.

Transport of water in plants

Water travels up the plant through dead, hollow xylem vessels in the transpiration stream. The driving force is evaporation: water evaporates from the moist cell walls of the leaf mesophyll and diffuses out through the stomata, a process called transpiration. This loss lowers the water potential in the leaf and pulls water up to replace it.

The pull works because of the properties of water from A1.1. Cohesion — hydrogen bonding between water molecules — holds the water in an unbroken column under tension, and adhesion to the xylem walls helps support it. This is the cohesion–tension theory. The rate of transpiration rises with higher temperature, more light, lower humidity and more wind, because each steepens the gradient or speeds evaporation. Roots take up water by osmosis and minerals by active transport.

Transport of sugars: translocation

Sugars made in photosynthesis are transported through living phloem tissue in a process called translocation. Phloem moves dissolved organic compounds — chiefly sucrose — from sources (where sugar is made or stored, such as leaves) to sinks (where it is used or stored, such as roots, fruits and growing tips).

Unlike the one-way transpiration stream, translocation can go up or down the plant depending on where the sources and sinks are. Sucrose is actively loaded into the phloem at the source, lowering its water potential so water follows by osmosis; this raises the pressure and drives the contents along to the sink, where sucrose is removed. Because loading requires active transport and therefore ATP, translocation is an energy-requiring process — a key contrast with the passive transpiration stream.