Transport in Cells
Revision notes written from the specification by AP Guru tutors.
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. It is a passive process — it requires no energy from the cell.
Examples you should know: oxygen and carbon dioxide moving in gas exchange, and urea diffusing from cells into the blood plasma for excretion by the kidney.
The rate of diffusion is increased by:
- a steeper concentration gradient,
- a higher temperature,
- a larger surface area of the membrane.
Osmosis
Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane. The membrane lets water molecules through but not the larger solute molecules.
A precise definition earns marks. Saying "water moves from high to low concentration" is ambiguous — be clear you mean water moving from a dilute solution (high water potential) to a concentrated solution (low water potential).
Required practical: osmosis
The standard investigation uses potato cylinders in sugar solutions of different concentration.
- Cut potato cylinders of equal size and blot them dry.
- Measure and record the mass of each.
- Place each in a different concentration of sugar solution for a set time.
- Remove, blot dry, and re-measure the mass.
- Calculate the percentage change in mass for each concentration.
Percentage change is used rather than raw change because the cylinders are not all of exactly equal starting mass, so it allows fair comparison.
Interpreting results: a gain in mass means water entered the cells, so the solution was more dilute than the cell contents. A loss means water left the cells. The concentration at which there is no change in mass is equal to the concentration inside the potato cells.
Active transport
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 learn:
- Root hair cells absorb mineral ions from very dilute soil solutions, allowing plants to take up the ions they need for healthy growth.
- The small intestine absorbs glucose from the gut into the blood, even when the glucose concentration in the gut is lower than in the blood.
Exchange surfaces
A single-celled organism has a large surface area to volume ratio, so diffusion across its surface is sufficient. As organisms get larger, the ratio falls and diffusion alone becomes too slow.
Effective exchange surfaces are adapted by having:
- a large surface area,
- a thin membrane, giving a short diffusion path,
- in animals, an efficient blood supply to maintain the concentration gradient,
- in animals for gas exchange, ventilation to move the medium across the surface.
You should be able to calculate and compare surface area to volume ratios for simple shapes such as cubes — a frequent source of marks.
