Membranes and membrane transport

Biology · B Form and function. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Biology topics

The idea

Every membrane is built on a phospholipid bilayer. The hydrophilic heads face the water on each side and the hydrophobic tails face inwards. This hydrophobic core stops ions and polar molecules, but small non-polar molecules such as oxygen and carbon dioxide diffuse straight through.

Proteins give the membrane its other abilities. Channel proteins let chosen particles diffuse through, which is facilitated diffusion. Pump proteins use energy from ATP to move particles against the concentration gradient, which is active transport.

The fluid mosaic model describes the result: phospholipids and proteins that move sideways, with proteins scattered among the lipids.

Rules to remember

Simple diffusion, facilitated diffusion and osmosis are passive: down the concentration gradient, no ATP.
Active transport: against the concentration gradient, needs ATP and pump proteins.
Osmosis = net movement of water from a lower solute concentration to a higher solute concentration across a partially permeable membrane.
Aquaporins are channel proteins that greatly increase the rate at which water crosses.
Integral proteins are embedded in the bilayer; peripheral proteins are attached to its surface.
Glycoproteins and glycolipids have their carbohydrate chains on the outside of the cell.
percentage change in mass = (final mass − starting mass) ÷ starting mass × 100

Common mistake

Describing osmosis as water moving from low concentration to high without saying which concentration. Always say solute concentration: water moves towards the more concentrated solution.

Worked example

A piece of carrot of mass 5.0 g is left in a salt solution. After an hour its mass is 5.6 g. Find the percentage change in mass and explain it.

Change in mass = 5.6 − 5.0 = +0.6 g.
Percentage change = 0.6 ÷ 5.0 × 100 = +12%.
The tissue gained mass, so water entered the cells by osmosis.
Water moves towards the higher solute concentration, so the cells had a higher solute concentration than the solution.

Answer: +12%; the solution was less concentrated than the cell contents (hypotonic), so water entered by osmosis.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

B2.1 Membranes and Membrane Transport

Keep going

← Proteins Organelles and compartmentalization →Take a Biology test