Transport across Cell Membranes

The cell-surface membrane

The membrane controls what enters and leaves the cell. Its structure is described by the fluid mosaic model:

• A phospholipid bilayer — hydrophilic phosphate heads face outwards (to water); hydrophobic tails face inwards. This makes the membrane a barrier to water-soluble substances.
• Proteins are scattered through it (a "mosaic") and can move (fluid): channel and carrier proteins for transport, plus receptor and enzyme proteins.
• Cholesterol sits between phospholipids, controlling fluidity and stability.
• Glycoproteins and glycolipids act in cell recognition and as receptors.

Passive transport (no energy needed)

Simple diffusion

Net movement of small, non-polar molecules (O₂, CO₂) down a concentration gradient, directly through the bilayer. Rate depends on the gradient, temperature, surface area and membrane thickness.

Facilitated diffusion

Movement of larger or charged/polar molecules (glucose, ions) down a gradient through proteins:

• Channel proteins — form water-filled pores for specific ions.
• Carrier proteins — change shape to move a specific molecule across.

Osmosis

The diffusion of water across a partially permeable membrane, from a higher (less negative) water potential to a lower (more negative) water potential. Pure water has a water potential of 0; adding solute makes it more negative.

Active transport (needs energy)

Movement of substances against the concentration gradient using carrier proteins and ATP (energy from respiration). Example: absorbing mineral ions into root cells. Co-transport (e.g. glucose absorption in the ileum) couples the movement of one substance (sodium ions) to bring another (glucose) against its gradient.

Factors affecting transport rate

• Diffusion/facilitated diffusion: steeper gradient, higher temperature, larger surface area and thinner membrane → faster.
• Facilitated diffusion and active transport also depend on the number of transport proteins.

Worked example

Why can't glucose cross the membrane by simple diffusion, and how does it enter cells?

• Glucose is large and polar, so it can't pass through the hydrophobic bilayer. It enters by facilitated diffusion through a specific carrier protein (or co-transport with sodium in the ileum). ✓

Common mistakes

• Saying osmosis moves "solute" — it moves water, from higher to lower water potential.
• Forgetting active transport needs ATP and carrier proteins (not channels).
• Confusing channel proteins (ions, pores) with carrier proteins (change shape).

Exam tips

• Describe the membrane using the fluid mosaic model and each component's role.
• Distinguish simple vs facilitated diffusion vs active transport (energy? proteins? gradient direction?).
• Use water potential language for osmosis (more negative = lower).

Key facts to remember

• Fluid mosaic model: phospholipid bilayer + proteins (channel/carrier), cholesterol, glyco-proteins/lipids.
• Passive: simple diffusion, facilitated diffusion (channel/carrier), osmosis (water, high→low water potential) — no energy.
• Active transport uses carrier proteins + ATP against the gradient; co-transport couples two substances (e.g. Na⁺ and glucose).