Revise Transport for IB Biology (B3.2)

Diffusion moves substances from where they are abundant to where they are scarce, and over microscopic distances it is remarkably fast. The trouble is that its effectiveness falls off sharply with distance. As an organism grows, its volume increases with the cube of its size while its exchange surface increases only with the square, so the surface-area-to-volume ratio drops. A large body therefore has too little surface, and too great an internal distance, for diffusion across the outer surface to supply the cells buried deep inside. The solution is a mass-transport system: a fluid (blood, or sap) is moved in bulk by a pump or a physical gradient, carrying materials the long distance quickly, so that diffusion only has to cover the last few micrometres between the transport fluid and the cells. This is why active animals have a circulatory system and tall plants have vascular tissue — both convert an impossible long-distance diffusion problem into an easy short-distance one.

The human heart is a muscular double pump with four chambers. The two upper chambers, the atria, are thin-walled and receive blood returning to the heart; the two lower chambers, the ventricles, are thick-walled and pump blood out under pressure. The right side handles deoxygenated blood: it receives blood from the body and pumps it to the lungs. The left side handles oxygenated blood: it receives blood from the lungs and pumps it around the whole body. Because the left ventricle must generate enough pressure to drive blood through the entire body, its muscular wall is noticeably thicker than that of the right ventricle, which only has to reach the nearby lungs. Valves between the atria and ventricles (atrioventricular valves) and at the exits into the arteries (semilunar valves) ensure blood flows in one direction only.

This arrangement is called a double circulation because blood passes through the heart twice for every complete circuit of the body. In the pulmonary circuit, the right ventricle sends deoxygenated blood to the lungs, where it is oxygenated and returns to the left atrium. In the systemic circuit, the left ventricle sends this oxygenated blood to the body's tissues, from which the now-deoxygenated blood returns to the right atrium. Passing through the heart between the two loops lets the blood be re-pressurised before the long systemic journey, making delivery to the tissues fast and efficient — an advantage for a warm-blooded, active organism.

One heartbeat, the cardiac cycle, has three brief phases. In atrial systole the atria contract and top up the ventricles with blood. In ventricular systole the ventricles contract; the rising pressure snaps the atrioventricular valves shut (preventing backflow into the atria) and forces the semilunar valves open, ejecting blood into the pulmonary artery and aorta. In diastole all chambers relax, the semilunar valves close, and the heart refills. Throughout, the valves guarantee one-way flow, and the coordinated timing keeps the two sides pumping in step.

Blood leaves the heart in arteries, passes through capillaries at the tissues, and returns in veins, and each vessel's wall is built for the pressure it must handle. Arteries carry blood away from the heart at high, pulsing pressure, so they have thick walls containing plenty of smooth muscle and elastic fibres and a relatively narrow lumen. The elastic tissue stretches as the ventricle pumps and recoils between beats, smoothing the flow; the muscle and the narrow lumen help maintain the pressure. Arteries do not normally need valves because the pressure only pushes blood one way.

Veins carry blood back to the heart at low pressure, so a thick muscular wall would be wasted; instead they have thin walls with little muscle or elastic tissue and a wide lumen that offers little resistance to the returning blood. Because the pressure is too low to guarantee forward flow on its own, veins contain valves along their length that snap shut to prevent blood flowing backward, while the squeezing of nearby skeletal muscles helps push the blood along. Capillaries, between the two, are where exchange actually happens, and their structure is discussed next.

Capillaries are the only vessels thin enough to exchange materials with the tissues, and every feature of their structure serves that role. Their walls are just a single layer of flattened endothelial cells, so the diffusion distance between the blood and the surrounding cells is as short as possible. Their lumen is so narrow that red blood cells pass through almost single-file, which slows the blood and presses the cells right up against the wall, giving oxygen the shortest possible path to diffuse out. Capillaries also branch into vast networks that penetrate the tissues, so their combined surface area for exchange is enormous and no cell is far from one. Finally, the walls are permeable — often with tiny gaps between cells — so that plasma can leak out to form tissue fluid, bathing the cells and carrying dissolved substances the final distance. Short diffusion distance, huge surface area and slow flow together make the capillary a near-ideal exchange surface.

Plants face the same long-distance problem as animals, and the xylem is their solution for water. Xylem vessels are formed from cells that die and lose their contents, leaving continuous, hollow tubes joined end to end. Their walls are reinforced with lignin, a strong waterproof material that stops the vessels collapsing inward under the tension of the water column and provides structural support to the plant. Crucially, xylem carries water in one direction only — upward, from the roots to the leaves.

The upward movement is explained by cohesion-tension theory. Water evaporates from the wet cell surfaces inside the leaf and diffuses out as vapour through the stomata — this is transpiration. Removing water from the top of the column lowers the pressure there, creating a tension (a negative pressure) that pulls on the water below. Because water molecules are polar, they hydrogen-bond to one another; this cohesion holds the water in a single, continuous, unbroken column so that a pull at the top is transmitted all the way down to the roots. Adhesion — the attraction of water molecules to the lignified xylem walls — helps hold the column up against gravity. The result is a continuous upward flow of water called the transpiration stream, driven ultimately by the sun's energy evaporating water at the leaf, and requiring no ATP from the plant.

Stomata are tiny pores, mostly on the underside of the leaf, whose opening is controlled by a pair of guard cells. They exist so carbon dioxide can diffuse in for photosynthesis, but every open stoma also lets water vapour escape — an unavoidable trade-off between feeding and drying out. The rate of transpiration therefore responds to conditions that affect either stomatal opening or the rate of evaporation and vapour removal. Higher light intensity tends to open the stomata and raises the rate; higher temperature speeds evaporation and raises the rate; higher wind speed sweeps away the humid layer of air sitting against the leaf, steepening the water-potential gradient and raising the rate. High humidity does the opposite: it reduces the water-potential gradient between the moist leaf interior and the surrounding air, so transpiration slows. A potometer, which measures water uptake by a cut shoot, is the standard tool for investigating how these factors change the rate.

While the xylem moves water, the phloem moves the sugars made in photosynthesis — mainly sucrose — in a process called translocation. Phloem tissue is alive, made of sieve tube elements joined end to end through perforated sieve plates, supported by neighbouring companion cells that supply the energy and machinery the sieve tubes lack. Translocation runs from a source to a sink. A source is any part that makes or releases sugar, typically a photosynthesising leaf; a sink is any part that uses or stores it, such as a root, a fruit or a growing shoot tip. At the source, sugar is actively loaded into the phloem; at the sink it is removed. Because sources and sinks are found both above and below the leaves, phloem transport can run either up or down the plant — unlike the strictly upward xylem — and a single organ can act as a source at one time of year and a sink at another.

Explain questions in B3.2 are marked analytically: each distinct, valid biological point is worth one mark, up to the number of marks available. Method marks (M) credit a correct step in the reasoning; answer marks (A) credit a correct linked conclusion; and error-carried-forward (ECF) means that provided your reasoning is written down, one weak step does not automatically cost you the marks that follow. Equivalent correct wording is always accepted. Study how each mark below attaches to a specific named idea, not to loose phrasing.

The logic of B3.2 — build a bulk-transport system, then let diffusion finish the job over a short distance across an adapted exchange surface — reappears throughout biology. The capillary's thin wall and huge surface area are the same design principles you meet in the alveoli of the lungs and the villi of the gut. The pressure-and-valve system of the heart and veins connects directly to gas exchange and to the control of blood flow. And the passive, cohesion-driven transpiration stream shows how a plant lifts water tens of metres using nothing but the sun's energy and the polarity of a water molecule. Master 'structure fits function' here and you have a template for every exchange and transport surface in the course.