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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key diagram
The cell membrane is made of a phospholipid bilayer with proteins embedded in it. It is partially permeable, controlling which substances enter and leave the cell by diffusion, osmosis and active transport.
The cell membrane diagram is tested at both Core and Extended levels, though Extended students must understand the fluid mosaic model in more detail. Every transport question relies on knowing that the membrane is partially permeable.
Drawing the phospholipid bilayer
Draw two parallel rows of phospholipid molecules. Each phospholipid has a circular hydrophilic head (water-attracting) facing outward and two wavy lines for the hydrophobic tails (water-repelling) facing inward. The two rows are arranged tail-to-tail, forming the bilayer.
This arrangement means the interior of the membrane is hydrophobic, preventing most water-soluble molecules from passing through freely.
Proteins in the membrane
Draw large irregular shapes spanning the bilayer for channel proteins and carrier proteins. Channel proteins provide a pore for specific ions or small molecules to pass through. Carrier proteins change shape to move substances across: these are involved in active transport.
Some proteins sit only on one surface. The overall arrangement of phospholipids and proteins is called the fluid mosaic model because the components can move within the layer, like tiles in a mosaic.
Partially permeable nature
The membrane allows small, non-polar molecules (such as oxygen and carbon dioxide) to pass through the phospholipid bilayer by diffusion. Water passes through by osmosis, partly via channel proteins called aquaporins. Larger molecules and ions require carrier proteins or channel proteins. Active transport uses carrier proteins and energy from respiration to move substances against the concentration gradient.
Label checklist
Check these labels are present and correctly positioned before finishing your drawing.
✓Phospholipid bilayer
✓Hydrophilic head
✓Hydrophobic tails
✓Channel protein
✓Carrier protein
✓Glycoprotein (Extended)
Try a diagram question (3 marks)
The diagram shows the structure of a cell membrane. Explain why the membrane is described as partially permeable. [3]
Show the answer and marking guidance
The hydrophobic interior of the phospholipid bilayer prevents large or charged molecules from passing through.
Small non-polar molecules such as oxygen can diffuse through the bilayer.
Specific proteins provide channels or carriers for other substances, so only selected molecules can cross.
Original Cambridge-style practice written for this site.
Common diagram mistakes
Check these against your own drawing before moving on.
Drawing the phospholipids all facing the same direction instead of tail-to-tail.
Writing "semi-permeable" instead of "partially permeable": the syllabus uses "partially permeable".
Forgetting to show proteins embedded in the bilayer.
Confusing channel proteins (passive) with carrier proteins (active transport).