Osmosis and membrane transport

The cell surface membrane controls the movement of substances using a range of mechanisms suited to different sizes, polarities, and concentration gradients.

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

• Includes simple diffusion and facilitated diffusion
• Moves substances down their concentration gradient without using ATP

Active transport

• Pumps substances against their concentration gradient, using ATP as the immediate energy source
• Works through specific carrier proteins

Endocytosis and exocytosis

• Move bulk amounts of material into and out of cells using membrane vesicles
• Both require ATP

Channel and carrier proteins are central to membrane transport.

Channel proteins

• Form water-filled pores that allow facilitated diffusion of specific ions and polar molecules
• Many are gated, opening and closing to control which substances pass through

Carrier proteins

• Bind to specific substances and change shape to move them across the membrane
• Function in both facilitated diffusion (down the gradient) and active transport (against the gradient, using ATP)

Understanding which mechanism applies to which substance is a key examiner focus, alongside precise terminology such as “water potential gradient” and “ATP hydrolysis”.

Recognising that some processes need ATP while others rely on passive movement is essential for distinguishing the four main transport types.