Cell Membrane & Transport — A-Level Biology Revision

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What is Cell Membrane & Transport?

The cell membrane is a partially permeable barrier that controls the movement of substances into and out of the cell. The fluid mosaic model describes the membrane as a phospholipid bilayer with embedded proteins, cholesterol, glycoproteins, and glycolipids. Transport across the membrane can be passive (diffusion, facilitated diffusion, osmosis) or active (requiring energy).

Board notes: The fluid mosaic model of the cell membrane and the different mechanisms of transport are fundamental concepts covered by all A-Level Biology boards (AQA, Edexcel, OCR). The specific examples and experimental techniques, such as investigating the effect of temperature on membrane permeability, may differ.

Worked example

Common mistakes

• 1Confusing simple diffusion with facilitated diffusion. Simple diffusion is the movement of small, nonpolar molecules directly through the phospholipid bilayer, while facilitated diffusion requires a channel or carrier protein for polar molecules and ions.
• 2Not understanding the role of cholesterol. Cholesterol molecules are interspersed within the phospholipid bilayer, regulating the fluidity of the membrane. At high temperatures, it reduces fluidity, and at low temperatures, it prevents the membrane from becoming too rigid.
• 3Forgetting that active transport requires ATP. Active transport moves substances against their concentration gradient, from a region of lower concentration to a region of higher concentration, and this process requires energy from the hydrolysis of ATP.

Cell Membrane & Transport exam questions

Exam-style questions for Cell Membrane & Transport with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• What is osmosis?

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

• How does active transport work?

  Active transport uses carrier proteins that bind to specific molecules or ions. ATP provides the energy to change the shape of the protein, moving the substance across the membrane against its concentration gradient.