Plasma membrane structure (fluid mosaic model)

Every cell is surrounded by a plasma membrane. It is not a rigid wall but a thin, flexible boundary that controls what moves between the cell and its surroundings.

The membrane is far too thin to see in detail under a light microscope, so its arrangement is described by the fluid mosaic model. This model explains how the ways substances cross the membrane, diffusion, osmosis, facilitated diffusion and active transport, actually work, which is the core of this chapter.

Parts and functions

How structure suits function

Each component of the fluid mosaic model contributes to the membrane's job of controlling exchange.

• Phospholipid bilayer, the hydrophobic tails inside form a barrier to water-soluble substances, while small, uncharged molecules such as oxygen and carbon dioxide pass straight through, making the membrane partially permeable.
• Channel and carrier proteins, they give specific ions and larger molecules such as glucose a route across the bilayer they could not otherwise take, so the cell controls exactly which substances enter and leave.
• Fluid, mobile layer, the phospholipids and proteins can move sideways, so the membrane is flexible enough to change shape, letting a white blood cell engulf bacteria and letting vesicles fuse with it.
• Carbohydrate markers on glycoproteins and glycolipids, they act as receptors and recognition sites, so cells and hormones can identify one another.
• Cholesterol between the phospholipids, it keeps the membrane stable and at the right fluidity, so it neither leaks nor sets too rigid.

Related processes

The structure of the plasma membrane explains every way substances cross it. Small, uncharged molecules cross the phospholipid bilayer directly by diffusion, moving down a concentration gradient, and water crosses by osmosis.

Ions and larger water-soluble molecules such as glucose cannot cross the bilayer, so they move through channel or carrier proteins by facilitated diffusion, still down the gradient and without using energy.

When a cell needs to move a substance against its concentration gradient, carrier proteins use ATP to pump it across by active transport, as root cells do to take up mineral ions. The membrane's flexibility also lets large particles be taken in whole: the membrane folds around them to form a vesicle in endocytosis, which includes the phagocytosis of bacteria by white blood cells.

These processes are studied in the chapter on the movement of substances across the plasma membrane.

Common labelling errors

Worked question

Question (in the style of Paper 2 Section A): Diagram N shows the plasma membrane. Layer G is a double row of molecules, each with a rounded head and two tails; structure H spans the layer and has a channel through the middle.

(a) Name layer G and structure H. [2 marks] (b) Oxygen crosses the membrane through G, but glucose crosses through H. Explain this difference. [3 marks] (c) A root cell takes up potassium ions from soil that is more dilute than its cytoplasm.

Name the process and state why it needs H and energy. [2 marks]

Model answer: (a) G is the phospholipid bilayer; H is a channel (or carrier) protein. (b) Oxygen is a small, uncharged molecule, so it can diffuse directly through the phospholipid bilayer; glucose is larger and water-soluble, so it cannot cross the bilayer and must pass through a protein by facilitated diffusion.

(c) The process is active transport; the ions move against the concentration gradient, so a carrier protein and ATP (energy) are needed to pump them in.

Marking note: in part (b) the marks are for oxygen being small/uncharged so it diffuses through the bilayer, and glucose being larger so it needs a protein.

Source:SRC-DSKP-EN

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