Transport Across Membranes

This lesson covers the mechanisms by which substances move across cell membranes, including passive processes (diffusion, facilitated diffusion, osmosis) and active processes (active transport, co-transport). Understanding these processes is essential for many areas of A-Level Biology, from nutrient absorption in the gut to nerve impulse transmission. This material corresponds to AQA specification section 3.2.3.

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Diffusion

Key Definition: Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is a passive process — no metabolic energy (ATP) is required.

Principles

• Diffusion occurs because molecules are in constant random motion (Brownian motion). There is no directional force; the net movement arises statistically because more molecules move out of a concentrated region than into it.
• Diffusion continues until dynamic equilibrium is reached — molecules still move in both directions, but there is no net movement.
• Only small, non-polar molecules (e.g., O₂, CO₂, N₂) and some very small polar molecules (e.g., water, ethanol) can diffuse directly through the phospholipid bilayer.

Factors Affecting the Rate of Diffusion

The rate of diffusion is described by Fick's law:

Rate of diffusion ∝ (surface area × concentration difference) / thickness of membrane (diffusion distance)

Factor	Effect on Rate
Concentration gradient	Steeper gradient → faster diffusion
Surface area	Greater surface area → faster diffusion (e.g., microvilli in the small intestine)
Diffusion distance (thickness)	Shorter distance → faster diffusion (e.g., thin alveolar walls)
Temperature	Higher temperature → faster diffusion (molecules have more kinetic energy)
Size of molecule	Smaller molecules diffuse faster
Polarity	Non-polar molecules cross lipid bilayers more easily

Exam Tip: When asked to explain how a biological surface is adapted for rapid diffusion, always link back to Fick's law. For example, the alveoli of the lungs have a large total surface area (~70 m²), thin walls (one cell thick, ~0.5 µm), a rich blood supply maintaining the concentration gradient, and ventilation continuously replacing air — all features that maximise the rate of gas exchange.

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Facilitated Diffusion

Key Definition: Facilitated diffusion is the passive movement of molecules or ions across a membrane through specific transport proteins (channel proteins or carrier proteins), down a concentration gradient. No ATP is required.

Channel Proteins

• Provide a hydrophilic pore through the membrane for ions or small polar molecules.
• Are specific — each channel typically allows only one type of ion or molecule through.
• Many are gated: they can open or close in response to specific stimuli (e.g., voltage-gated Na⁺ channels in neurones open in response to depolarisation; ligand-gated channels open when a specific molecule binds).
• When open, ions flow through very rapidly — up to 10⁸ ions per second per channel.

Carrier Proteins

• Bind to a specific molecule on one side of the membrane.
• Undergo a conformational change (change in shape) that moves the molecule to the other side of the membrane and releases it.
• Are slower than channel proteins because each transport event requires a shape change.
• Examples: glucose transporter (GLUT) proteins in cell membranes facilitate the uptake of glucose into cells down its concentration gradient.

Key Differences from Simple Diffusion

• Facilitated diffusion requires specific proteins and is therefore saturable — the rate reaches a maximum (V_max) when all transport proteins are occupied. Simple diffusion is not saturable.
• Facilitated diffusion shows specificity — only certain molecules are transported.
• Both are passive processes driven by the concentration gradient.

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Osmosis

Key Definition: Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential (Ψ) to a region of lower (more negative) water potential.

Water Potential (Ψ)

• Water potential (Ψ, measured in kilopascals, kPa) is a measure of the tendency of water molecules to move from one place to another. It is the 'free energy' of water per unit volume.
• Pure water has a water potential of 0 kPa. Dissolving solutes lowers the water potential, making it negative.
• Water always moves from a higher (less negative) water potential to a lower (more negative) water potential.

For plant cells:

Ψ (cell) = Ψ_s + Ψ_p

Where:

• Ψ_s = solute potential (always negative or zero) — the effect of dissolved solutes lowering water potential.
• Ψ_p = pressure potential (usually positive in plant cells due to turgor pressure, can be zero or negative in xylem).

Osmosis in Animal Cells

Animal cells have no cell wall, so they are sensitive to changes in osmotic conditions: