Transport Across Membranes

Cell membranes control the movement of substances into and out of cells. The fluid mosaic model describes the plasma membrane as a dynamic structure composed of a phospholipid bilayer with embedded and peripheral proteins, cholesterol, and glycoproteins. Understanding membrane structure, the mechanisms of transport, the factors that affect membrane permeability, and the basics of cell signalling is essential for A-Level Biology.

Key Definition: The fluid mosaic model (proposed by Singer and Nicolson, 1972) describes the cell membrane as a phospholipid bilayer with a mosaic of proteins that can move laterally within it.

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The Fluid Mosaic Model

Component	Role
Phospholipid bilayer	Forms the basic barrier; hydrophobic interior prevents passage of water-soluble molecules and ions
Channel proteins	Provide hydrophilic pores for specific ions/polar molecules to pass through by facilitated diffusion
Carrier proteins	Change shape to transport specific molecules across the membrane (facilitated diffusion or active transport)
Cholesterol	Regulates membrane fluidity; prevents it becoming too rigid at low temperatures or too fluid at high temperatures; reduces permeability to small water-soluble molecules
Glycoproteins	Cell signalling, cell recognition, immune response (antigens on cell surface); act as receptors for hormones and neurotransmitters
Glycolipids	Cell-cell recognition, stability of membrane, and tissue compatibility
Peripheral proteins	Attached to the inner or outer surface of the membrane; involved in cell signalling and maintaining cell shape

The membrane is described as fluid because phospholipids and proteins can move laterally within the bilayer, and mosaic because of the varied, irregular pattern of proteins scattered throughout.

A diagram of the fluid mosaic model would show two rows of phospholipids with their hydrophilic heads (circles) facing outward towards the aqueous environment and their hydrophobic tails (two lines) facing inward. Embedded within the bilayer are large integral proteins spanning the whole membrane (transmembrane proteins), channel proteins with a central pore, and smaller peripheral proteins on the surface. Cholesterol molecules would be shown wedged between the phospholipid tails. Glycoproteins and glycolipids project from the outer surface with branching carbohydrate chains.

Exam Tip: When describing the fluid mosaic model, always mention both aspects: "fluid" (lateral movement of components) and "mosaic" (variety of proteins). Simply drawing a diagram without explaining these terms will lose marks on a "describe" question.

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Factors Affecting Membrane Permeability

The permeability of cell membranes can be affected by several factors. A common practical investigation uses beetroot cells (which contain a red pigment, betacyanin, in their vacuoles) to study these effects.

Temperature

• Below 0 °C — water in the membrane freezes, forming ice crystals that pierce the membrane, creating holes. When thawed, the membrane is severely damaged and highly permeable.
• 0–30 °C — the membrane is relatively stable and selectively permeable. Increasing temperature slightly increases permeability as phospholipids gain kinetic energy and move more.
• Above ~40 °C — the phospholipid bilayer becomes increasingly fluid; proteins begin to denature (tertiary structure is disrupted, active sites and channel shapes change). The membrane loses its selective permeability and becomes freely permeable. In the beetroot experiment, the solution turns increasingly red as betacyanin leaks out.

Solvents (Alcohol and Detergents)

• Organic solvents (e.g., ethanol, methanol) dissolve the phospholipids in the bilayer and disrupt hydrophobic interactions, increasing permeability. Higher concentrations of alcohol cause more damage.
• Detergents (e.g., washing-up liquid) are amphipathic molecules that insert themselves into the bilayer, disrupting its structure and creating pores or dissolving the membrane entirely.

Exam Tip: In the beetroot practical, the dependent variable is the intensity of red colour in the surrounding solution (measured using a colorimeter). The more permeable the membrane, the more betacyanin is released and the greater the absorbance reading. Control variables include beetroot disc size, volume of solution, and time of exposure.

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Passive Transport

Passive transport requires no metabolic energy (no ATP) and moves substances down their concentration gradient.

Simple 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.

• Small, non-polar molecules (O₂, CO₂, steroid hormones) and very small polar molecules pass directly through the phospholipid bilayer.
• Rate is affected by: concentration gradient, temperature, surface area, membrane thickness, and the nature of the diffusing molecule (size, polarity).
• Described mathematically by Fick's law:

Rate of diffusion ∝ (surface area × concentration difference) ÷ thickness of exchange surface

Worked Example 1 — Fick's Law:

Two exchange surfaces are compared:

• Surface A has a surface area of 50 cm², a concentration difference of 0.4 units, and a thickness of 0.2 cm.
• Surface B has a surface area of 100 cm², a concentration difference of 0.2 units, and a thickness of 0.1 cm.

Which surface has the greater rate of diffusion?

Solution: Using Fick's law (proportional relationship):

Surface A: Rate ∝ (50 × 0.4) ÷ 0.2 = 20 ÷ 0.2 = 100 arbitrary units

Surface B: Rate ∝ (100 × 0.2) ÷ 0.1 = 20 ÷ 0.1 = 200 arbitrary units

Surface B has the greater rate of diffusion, despite having a lower concentration difference, because it has double the surface area and half the thickness.

Facilitated Diffusion

• Movement of polar molecules and ions down their concentration gradient via channel proteins or carrier proteins.
• Channel proteins are specific (e.g., aquaporins for water, sodium ion channels for Na⁺). They provide a hydrophilic pore and are often gated (open/close in response to signals).
• Carrier proteins bind to the molecule, undergo a conformational change, and release it on the other side.
• Rate is limited by the number of available protein channels/carriers — exhibits a plateau at saturation (all channels/carriers occupied).

Osmosis

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