Transport into and out of cells

Substances move into and out of cells across the cell membranes by diffusion.

Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration. Learn that sentence — the words "net movement" and "higher to lower" are where the marks are.

Substances transported this way include oxygen and carbon dioxide in gas exchange, and the waste product urea, which diffuses from cells into the blood plasma for excretion by the kidney.

Three factors affect the rate of diffusion:

• the difference in concentrations — the concentration gradient. A steeper gradient means a faster rate. • the temperature. Warmer particles have more energy and move faster. • the surface area of the membrane. More surface means more places to cross at once.

Practise drawing this one, not just reading it. You are expected to recognise, draw and interpret diagrams that model diffusion, and the sketch is quick: a container split by a membrane, particles crowded on one side and sparse on the other, and an arrow showing the net movement down the gradient. Add a second, smaller arrow the other way if you want to show that particles cross both ways.

Reading someone else's diagram works the same way in reverse: find the membrane, compare how many particles are on each side, then state which way the net movement goes and why.

Size decides whether diffusion alone is enough. A single-celled organism has a relatively large surface area to volume ratio, which allows sufficient transport of molecules into and out of the cell to meet the needs of the organism.

As an organism gets bigger, its volume grows faster than its surface area, so the ratio falls. Every cell inside still needs supplying, but there is proportionally less surface to supply it through.

That is why multicellular organisms need exchange surfaces and a transport system: their surface area to volume ratio is too small for diffusion across the outside to supply everything inside. Surfaces and organ systems are specialised for exchanging materials, so that enough molecules can be transported into and out of cells for the organism's needs.

The effectiveness of an exchange surface is increased by:

• having a large surface area • a membrane that is thin, to provide a short diffusion path • in animals, having an efficient blood supply • in animals, for gaseous exchange, being ventilated.

Water moves across cell membranes by osmosis.

Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane. "Partially permeable" means the membrane lets water through but not the dissolved solute, so only the water can move to even things out.

Draw this one too. A diagram that models osmosis needs three things to earn its marks: the partially permeable membrane down the middle, water molecules on both sides with more of them on the dilute side, and solute particles drawn large enough to show that they cannot get through. The arrow goes from dilute to concentrated.

The maths here is percentages. You are expected to use simple compound measures such as rate of water uptake, to use percentages, and to calculate percentage gain and loss of mass of plant tissue — then plot, draw and interpret the graph.

Plotting and reading the graph. Concentration goes on the x-axis, because it is what you chose; percentage change in mass goes on the y-axis, because it is what you measured. Pick a scale that fills at least half the grid, mark each point with a small cross, and draw one smooth line of best fit rather than joining the dots.

Then interpret it. Points above the axis mean the tissue gained water, so that solution was more dilute than the cell contents; points below mean it lost water to a more concentrated solution. Where the line crosses zero there is no net movement, so the solution outside matches the concentration inside the cells — reading off that crossing point is a standard question, and it is an estimate from your line, not one of your measurements.

An application worth knowing: sports drinks. An isotonic drink is made at roughly the same concentration as body fluids, so there is no large net movement of water by osmosis either way — it rehydrates without upsetting the balance, which is what a runner wants during a race. A high energy drink is more concentrated, carrying more sugar for respiration; because it is more concentrated than body fluids it draws water into the gut by osmosis, so it fuels well but rehydrates more slowly. Which is the better choice depends on whether the athlete needs water or energy.

Active transport moves substances from a more dilute solution to a more concentrated solution — against a concentration gradient. This requires energy from respiration.

Two examples to know. Active transport allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil, and plants require those ions for healthy growth. It also allows sugar molecules to be absorbed from lower concentrations in the gut into the blood, which has a higher sugar concentration; those sugar molecules are then used for cell respiration.

You need to be able to describe how substances are transported into and out of cells by diffusion, osmosis and active transport, and to explain the differences between the three processes. The differences come down to three questions: what moves, which way, and does it cost energy?