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Biochemistry Unit 7: Membranes and Transport

Membranes separate the cell from its surroundings and control what enters and leaves. This unit explains bilayer structure, membrane proteins, the kinds of transport, and how to calculate the energy of moving solutes and ions.

Covers: Fluid mosaic model and bilayer properties, Membrane proteins, Simple and facilitated diffusion, Channels and selectivity, Primary active transport, Secondary active transport, Electrochemical gradients and membrane potential

Study guide

Biological membranes follow the fluid mosaic model: a lipid bilayer, about 5 nm thick, in which proteins float. Lipids diffuse quickly sideways within a leaflet, as FRAP experiments show, but rarely flip between leaflets without flippase enzymes, so the two leaflets differ in composition. Fluidity depends on lipid tails: cis-unsaturated and shorter chains increase fluidity, while saturated and longer chains decrease it. Cholesterol buffers fluidity, stiffening warm membranes and preventing cold ones from gelling. Small nonpolar molecules such as O₂ and CO₂ diffuse straight through; ions and large polar molecules need proteins.

Membrane proteins are integral or peripheral. Integral proteins cross the bilayer, often with α-helices of about 20 hydrophobic residues, or as β-barrels, and need detergents to extract. Peripheral proteins bind the surface through ionic and polar contacts and are released by salt or pH changes. Lipid-anchored proteins are tethered by covalent fatty acyl, prenyl or GPI anchors. Transport proteins come in two broad types: channels, which form pores that let ions or water flow quickly down their gradients, and transporters (carriers), which bind solutes and change shape by alternating access, showing saturation kinetics.

Passive transport moves solutes down their electrochemical gradient. Simple diffusion requires no protein and rises linearly with concentration. Facilitated diffusion uses a channel or carrier, such as GLUT1 for glucose or aquaporins for water. Ion channels can be highly selective: the K⁺ channel's selectivity filter uses backbone carbonyl oxygens that mimic K⁺'s water shell, so K⁺ passes about 1,000 times more easily than the smaller Na⁺. Many channels are gated by voltage, ligands or mechanical force. Carriers such as GLUT1 behave like enzymes: their rate follows v = Vmax[S]/(Kt + [S]), where Kt is the solute level giving half-maximal rate, and they can be competitively inhibited by similar molecules.

Active transport moves solutes uphill. Primary active transport uses ATP directly: P-type ATPases such as the Na⁺/K⁺-ATPase (3 Na⁺ out, 2 K⁺ in per ATP) and Ca²⁺-ATPases, plus ABC transporters. Because it moves net positive charge out, the Na⁺/K⁺ pump is electrogenic. Secondary active transport uses an ion gradient made by a primary pump. SGLT1 symports 2 Na⁺ with each glucose into intestinal cells, and the Na⁺/Ca²⁺ exchanger antiports Na⁺ in and Ca²⁺ out. Drugs such as ouabain and digoxin inhibit the Na⁺/K⁺ pump, raising cell Na⁺ and, in the heart, Ca²⁺.

The free energy of transport is ΔG = RT ln(C_in/C_out) + zFΔψ for moving a solute into the cell, where Δψ is the inside potential relative to outside. For uncharged solutes only the concentration term applies; a tenfold gradient is worth about 5.9 kJ/mol at 37 °C. For ions, the membrane potential matters: with the inside at −70 mV, cations are pulled in. The Nernst equation, E = (RT/zF) ln([out]/[in]), gives the potential at which an ion is at equilibrium. The resting potential lies near the K⁺ Nernst potential because resting membranes are most permeable to K⁺.

Key ideas

• Membranes are fluid, asymmetric bilayers with embedded and attached proteins.
• Small nonpolar molecules diffuse freely; ions and polar molecules need proteins.
• Channels let solutes flow quickly down gradients; carriers use alternating access and saturate.
• Primary active transport uses ATP; secondary active transport uses ion gradients.
• The Na⁺/K⁺-ATPase moves 3 Na⁺ out and 2 K⁺ in per ATP and is electrogenic.
• Ion movement depends on both concentration gradient and membrane potential.
• The resting potential is near E(K⁺) because K⁺ permeability dominates at rest.

Formulas and rules

Common mistakes

• Forgetting the electrical term zFΔψ when calculating ΔG for ions.
• Getting the sign of Δψ wrong; for inward movement use the inside potential relative to outside.
• Calling secondary active transport 'passive' because it does not use ATP directly.
• Mixing up the Na⁺/K⁺ pump stoichiometry (it is 3 Na⁺ out, 2 K⁺ in).
• Assuming smaller ions always pass channels more easily, ignoring dehydration energy.

Before the test, make sure you can

• Describe the fluid mosaic model and factors that affect membrane fluidity.
• Classify membrane proteins as integral, peripheral or lipid-anchored.
• Distinguish simple diffusion, facilitated diffusion, and primary and secondary active transport.
• Explain how the Na⁺/K⁺-ATPase works and why it is electrogenic.
• Calculate ΔG for moving uncharged solutes and ions across membranes.
• Calculate Nernst potentials and explain the resting membrane potential.
• Explain channel selectivity and carrier saturation kinetics.
• Predict effects of pump inhibitors such as ouabain and digoxin.

Words to know

Fluid mosaic model

A model of membranes as fluid lipid bilayers with mobile embedded proteins.

Flippase

An enzyme that moves lipids between membrane leaflets to keep asymmetry.

Integral protein

A protein embedded in or spanning the bilayer, removable only with detergent.

Peripheral protein

A protein bound loosely to the membrane surface, removable by salt or pH changes.

Channel

A pore protein allowing rapid flow of specific ions or water down their gradient.

Transporter (carrier)

A protein that binds a solute and changes shape to move it, showing saturation.

Facilitated diffusion

Protein-mediated movement down a gradient with no energy input, such as GLUT1.

Primary active transport

Uphill transport powered directly by ATP hydrolysis.

Secondary active transport

Uphill transport powered by an ion gradient, such as SGLT1.

Symport

Movement of two solutes in the same direction by one transporter.

Antiport

Movement of two solutes in opposite directions by one transporter.

Na⁺/K⁺-ATPase

A P-type pump exporting 3 Na⁺ and importing 2 K⁺ per ATP.

Electrogenic

Producing net charge movement across a membrane.

Nernst potential

The membrane voltage at which a given ion is at equilibrium.

Resting membrane potential

The steady voltage across a resting cell membrane, about −70 mV in neurons.

Selectivity filter

The narrow part of a channel that determines which ion can pass.

Aquaporin

A channel protein that conducts water rapidly while excluding protons.

FRAP

Fluorescence recovery after photobleaching, used to measure lateral diffusion.

Sample practice questions

30 more questions like these, from level 1 to 5, are in the practice set.