Cell membranes and transport: the fluid-mosaic model; diffusion and facilitated diffusion; osmosis and water potential; active transport; bulk transport by endocytosis and exocytosis; and the factors affecting the rate of movement.

What this dot point is asking

Eduqas wants you to describe the fluid-mosaic model, explain the five ways substances cross membranes (simple diffusion, facilitated diffusion, osmosis, active transport and bulk transport), define osmosis using water potential, and explain the factors that affect the rate of movement. This is a Core Concepts statement examined on every paper.

The fluid-mosaic model

The membrane is a phospholipid bilayer: the hydrophilic phosphate heads face the watery surroundings inside and outside the cell, and the hydrophobic fatty acid tails point inwards, away from water. This makes the membrane a partially permeable barrier; small non-polar molecules slip through, but ions and large polar molecules cannot.

Diffusion and facilitated diffusion

Simple diffusion is the net movement of particles from a region of higher to lower concentration, down the gradient, until evenly spread. Small, non-polar molecules (oxygen, carbon dioxide) cross the bilayer directly. It is passive (no ATP).

Facilitated diffusion moves ions and larger polar molecules (glucose, amino acids) down their gradient through channel proteins (water-filled pores, often gated) or carrier proteins (which change shape). It is still passive but needs the proteins because the solutes cannot cross the lipid bilayer.

Osmosis and water potential

A cell in a solution of higher water potential gains water; one in a solution of lower water potential loses water. An animal cell in pure water bursts (it has no wall); in a concentrated solution it shrinks (crenation). A plant cell in pure water becomes turgid (the cell wall resists, so it does not burst); in a concentrated solution the membrane pulls away from the wall (plasmolysis).

Active transport and bulk transport

Active transport moves substances against their concentration gradient using carrier proteins and ATP from respiration; the protein changes shape to carry the solute across. Bulk transport moves large amounts or large particles: endocytosis brings material in by engulfing it in a vesicle, and exocytosis releases material by fusing a vesicle with the membrane. Both require ATP.

The rate of movement rises with a steeper concentration (or water potential) gradient, a larger surface area, a higher temperature (faster-moving particles), and, for facilitated diffusion and active transport, more channel or carrier proteins (until they are saturated).

Examples in context

Example 1. Root hair cells and active transport. Root hairs take up mineral ions from soil even when the soil has a lower ion concentration than the cell, by active transport using carrier proteins and ATP. This is why respiratory poisons stop ion uptake but not simple diffusion.

Example 2. Cholesterol and temperature. Cholesterol sits between the phospholipids and stops the membrane becoming too fluid when warm or too rigid when cold, which is why it stabilises the membrane across a range of temperatures, a favourite Eduqas application of membrane structure.

Try this

Q1. Define osmosis in terms of water potential. [2 marks]

• Cue. The net movement of water molecules from a higher water potential to a lower water potential across a partially permeable membrane.

Q2. State two ways a cell can move a substance against its concentration gradient. [2 marks]

• Cue. Active transport (carrier proteins and ATP) and endocytosis (bulk transport in a vesicle, also using ATP).

Q3. Explain why an animal cell bursts in pure water but a plant cell does not. [3 marks]

• Cue. Water enters both by osmosis (down the water potential gradient); the plant cell wall resists the pressure so the cell becomes turgid, but the animal cell has no wall and bursts.