Substances that must be transported

1. Every cell in a large organism needs a steady supply of raw materials and a steady removal of the waste its reactions produce.
2. The substances a transport system moves in an animal are oxygen, carbon dioxide, water, dissolved food molecules, mineral ions and urea.
3. Each one is carried between an exchange surface, where it enters or leaves the body, and the cells that use or produce it.
4. Some are carried in solution in the liquid part of the blood, and oxygen is carried bound to a protein inside a specialised cell.
5. The direction each substance travels depends on where it enters the body and where it is needed, so oxygen and food travel inwards while carbon dioxide and urea travel outwards.
6. The quantity of each substance an organism moves depends on what that organism requires, so an active mammal shifts far more oxygen and glucose per kilogram than a slow-moving one.

Oxygen and carbon dioxide

1. Oxygen is needed by every respiring cell because aerobic respiration releases far more energy from glucose than anaerobic respiration does.
2. Oxygen enters the body across the lungs, diffusing from the air in the air sacs into the blood in the capillaries wrapped around them.
3. It is then carried to the respiring tissues, where it diffuses out of the blood and into the cells.
4. Carbon dioxide is the waste product of aerobic respiration and travels in the opposite direction, from the tissues to the lungs.
5. Carbon dioxide has to be removed because it dissolves in water to form a weak acid, and a build-up would lower the pH of the blood and damage enzymes.
6. Most of it is carried dissolved in the liquid part of the blood as hydrogencarbonate ions, and it diffuses out into the air in the lungs and is breathed out.
7. A plant transports these two gases very little, because the air spaces inside its leaves and stem reach almost every cell directly.

Water

1. Water is the solvent in which every other transported substance dissolves, so nothing else can be carried without it.
2. Cytoplasm is mostly water, and the chemical reactions of a cell take place between substances dissolved in it.
3. In a mammal water is absorbed from the small intestine and the large intestine and carried in the blood to every tissue.
4. Water also spreads heat around the body, which is how a warm-blooded animal keeps a steady internal temperature.
5. In a plant water is absorbed by the roots, carried up to the leaves and used as a raw material for photosynthesis.
6. Water inside plant cells also presses outwards on the cell walls, and that pressure is what holds a soft young stem upright.

Dissolved food molecules

1. Large food molecules cannot be transported, so digestion breaks them into small soluble molecules that dissolve and travel in the blood.
2. Starch is digested to glucose, which every cell needs as the fuel for respiration.
3. Proteins are digested to amino acids, which cells reassemble into the particular proteins they need for growth and repair.
4. Lipids are digested to fatty acids and glycerol, which are used to build membranes and are stored as an energy reserve.
5. These products are absorbed across the wall of the small intestine and carried away in the blood, glucose and amino acids straight into the capillaries of the villi.
6. A plant makes its own glucose in the leaves and moves it as sucrose, together with amino acids, to wherever it is used or stored.

Mineral ions

1. Mineral ions are needed in small quantities but are essential, because without them particular molecules cannot be built at all.
2. In a mammal, iron ions are needed to make haemoglobin, so a shortage lowers the amount of oxygen the blood can carry.
3. Calcium ions and phosphate ions are needed to build bone and teeth, and calcium ions are also needed for muscles to contract and blood to clot.
4. Sodium ions and potassium ions carry the electrical signals along nerve cells.
5. Mineral ions are absorbed in the intestine, dissolve in the liquid part of the blood and are carried to wherever they are needed.
6. A plant takes its mineral ions from the soil solution, and nitrate ions are needed to turn glucose into amino acids and so into protein.
7. A plant short of nitrate ions grows poorly and stays small, and a plant short of magnesium ions cannot make chlorophyll and its leaves turn yellow.

Urea

1. Amino acids cannot be stored, so any the body does not need are broken down in the liver.
2. The nitrogen-containing part is removed and converted first to ammonia, which is highly toxic, and then to the far less toxic urea.
3. Urea dissolves in the liquid part of the blood and is carried from the liver to the kidneys.
4. The kidneys filter it out of the blood and it leaves the body dissolved in urine.
5. Urea must be transported continuously because a rising concentration in the blood would poison the tissues.
6. A plant makes no urea, because it takes in only the nitrogen it needs as nitrate ions rather than eating protein it must break down.

Matching supply to demand

1. The quantity of each substance an organism has to transport depends on how fast its cells are respiring.
2. A resting mammal respires slowly, so oxygen and glucose are delivered at a modest rate and carbon dioxide is removed at a modest rate.
3. During hard exercise the muscles respire many times faster, so all three rates rise together.
4. The body meets that demand by breathing faster and more deeply and by making the heart beat faster and more strongly.
5. A sprinting animal reaches a point where oxygen cannot be delivered fast enough, and the muscles then respire anaerobically for a short time.
6. That limit on delivery is why an all-out sprint can be held for only a short burst, whatever the animal.

Substances transported in plants

1. A plant transports water and mineral ions upwards from the roots to the leaves.
2. It transports dissolved sugars and amino acids from the leaves to the roots, the growing tips, the flowers and the fruits.
3. The two routes are separate tissues running side by side, and each carries its load in one characteristic direction.
4. Gases are the exception, because oxygen and carbon dioxide move through the air spaces between the cells rather than through a transport tissue.