Transport across membranes and osmosisEdexcel International A Level Biology: Revision notes

Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is passive, needing no ATP. Small non-polar molecules such as oxygen cross the phospholipid bilayer directly.

Facilitated diffusion is passive transport through membrane proteins. Channel proteins form water-filled pores for specific ions; carrier proteins bind a specific molecule and change shape to move it across. Both move substances down a concentration gradient and are needed for charged or polar substances that cannot cross the hydrophobic core.

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.

Water potential (ψ, in kPa) is a measure of the tendency of water molecules to move: pure water at standard conditions is 0 kPa, and solutions have negative values. The more solute in a solution, the more water molecules are attracted to it, so the fewer free water molecules there are and the lower (more negative) the water potential.

Water always moves down the water potential gradient, from the less negative to the more negative. At equilibrium water molecules still cross in both directions but there is no net movement.

Active transport is the movement of molecules or ions across a membrane against their concentration gradient, using ATP and carrier proteins. The carrier binds the substance, ATP is hydrolysed, the protein changes shape and releases the substance on the other side.

Cells use active transport when they need to accumulate a substance, for example root hair cells absorbing mineral ions. Rate depends on ATP supply, so it stops if respiration is inhibited.

Large molecules and particles cannot pass through channel or carrier proteins, so cells use vesicles; both processes need ATP.

• Endocytosis: the cell surface membrane folds inwards around the material and pinches off to form a vesicle inside the cell, for example a phagocyte taking in a bacterium.
• Exocytosis: a vesicle moves to the cell surface membrane and fuses with it, releasing the contents outside, for example secretion of an enzyme such as amylase.

Cut identical cylinders of tissue such as potato, blot and weigh them, and place each in a different concentration of sucrose solution for a set time. Blot and reweigh. Calculate the percentage change in mass and plot it against concentration.

Where the line crosses zero there is no net water movement, so the solution has the same water potential as the tissue. Read this concentration and use a table or calibration curve to find the water potential in kPa.

Control temperature, volume, time and tissue source; blot consistently; repeat and calculate means. Use percentage change because starting masses differ.