Diffusion, osmosis and active transport as ways substances move across membranes, the factors affecting the rate of diffusion, surface area to volume ratio, and the adaptations of exchange surfaces such as alveoli, villi and root hair cells.

What this topic is asking

OCR wants you to define diffusion, osmosis and active transport, explain the factors that affect the rate of diffusion, calculate and use surface area to volume ratio, and describe how exchange surfaces are adapted for efficient transport.

Diffusion, osmosis and active transport

• Diffusion is passive, meaning it needs no energy from the cell. Examples include oxygen and carbon dioxide moving across the membranes of the alveoli, and dissolved food and oxygen passing into cells. The rate of diffusion increases with a steeper concentration gradient, a higher temperature (particles move faster), and a larger surface area.
• Osmosis moves water specifically: water moves from a dilute solution (high water concentration) to a more concentrated solution (low water concentration) across a partially permeable membrane. This is how water enters root hair cells and how plant cells become firm (turgid).
• Active transport moves substances the "wrong" way, against the gradient from low to high concentration, so it needs energy from respiration. Examples are mineral ions moving from dilute soil water into root hair cells, and glucose being reabsorbed from the kidney tubules and the gut even when its concentration there is already low.

Surface area to volume ratio

You can calculate the ratio for a cube of side $s$: surface area $= 6s^2$, volume $= s^3$, so the ratio is $\dfrac{6s^2}{s^3} = \dfrac{6}{s}$. As $s$ increases, $\dfrac{6}{s}$ decreases, which is the mathematical reason the ratio falls with size. OCR may give you the dimensions of a model organism and ask you to compare ratios.

Exchange surfaces

Exchange surfaces are adapted to make diffusion fast by having a large surface area, a short diffusion distance and a steep concentration gradient. Key examples are the alveoli in the lungs (millions of tiny air sacs give a huge surface area, with thin walls one cell thick and a rich blood supply), the villi lining the small intestine (folds and finger-like projections increase the surface area for absorbing digested food, with a thin lining and a good blood supply), and root hair cells (each long projection increases the surface area for absorbing water and mineral ions from the soil). Ventilation (in lungs) and a flowing blood supply maintain the concentration gradient so diffusion keeps going.