Home / Courses / Cell Biology / Unit 2

Cell Biology Unit 2: Membranes and Membrane Transport

This unit explains how membranes are built and how cells move ions and molecules across them, from simple diffusion to pumps and gradient-driven transporters.

Covers: Phospholipid bilayer and the fluid mosaic model, Membrane fluidity and lipid asymmetry, Membrane proteins, Diffusion and osmosis, Channels and carriers, Primary and secondary active transport, Electrochemical gradients and membrane potential

Study guide

Biological membranes are phospholipid bilayers. Each phospholipid has a polar head and two fatty-acid tails, so in water the tails face inward and the heads face out. The fluid mosaic model describes the bilayer as a two-dimensional fluid with proteins embedded in it. Lipids diffuse rapidly sideways but flip between leaflets only rarely, so the two leaflets can differ: phosphatidylserine is kept on the cytosolic side by ATP-driven flippases, and glycolipids face outward. Fluidity rises with shorter tails and more cis double bonds, and falls with long saturated tails. Cholesterol stiffens membranes at body temperature but keeps them from freezing solid in the cold.

Membrane proteins are integral (embedded, often crossing the bilayer with α-helices of about 20 hydrophobic residues, removed only with detergent) or peripheral (attached to lipid heads or other proteins, released by salt). Some are tethered by lipid anchors such as GPI. Proteins act as transporters, receptors, enzymes and anchors. Their lateral movement was shown by mouse–human cell fusion and by FRAP, in which a bleached spot regains fluorescence as unbleached molecules diffuse in; the plateau shows the mobile fraction. Many proteins are partly fenced in by the cytoskeleton.

Small nonpolar molecules (O₂, CO₂, steroids) and small uncharged polar molecules like water and ethanol cross the bilayer by simple diffusion, down their gradients. Ions and large polar molecules such as glucose cannot. Osmosis is the net flow of water toward the side with more solute; aquaporins speed it greatly. In a hypotonic solution animal cells swell and may lyse, while plant cells become turgid against their walls; in a hypertonic solution animal cells crenate and plant cells plasmolyze. Isotonic solutions cause no net water movement.

Transport proteins come in two types. Channels form aqueous pores that may be gated by voltage, ligands or stretch, and pass millions of ions per second down their electrochemical gradient. Carriers (transporters) bind solute and change shape, so they are slower and saturate, following v = Vmax[S]/(Km + [S]). Facilitated diffusion through GLUT1 is downhill and needs no energy. Active transport moves solutes uphill. Primary active transport uses ATP directly, as in the Na⁺/K⁺-ATPase (3 Na⁺ out, 2 K⁺ in per ATP), Ca²⁺ pumps and ABC transporters. Secondary active transport uses an ion gradient made by a pump: symporters move two solutes the same way (Na⁺–glucose) and antiporters move them opposite ways (Na⁺/Ca²⁺ exchanger).

For charged solutes, the driving force has two parts: the concentration gradient and the membrane potential. Together they form the electrochemical gradient, ΔG = RT ln([in]/[out]) + zFV. The Nernst equation, E = (61.5/z) log([out]/[in]) mV at 37°C, gives the potential at which an ion is at equilibrium. Typical values are E_K ≈ −90 mV and E_Na ≈ +65 mV. Resting cells are mostly permeable to K⁺ through leak channels, so the resting potential (about −70 mV) sits near E_K. When several ions are permeable, the potential is a permeability-weighted average (Goldman equation).

Key ideas

• Membranes are fluid bilayers whose leaflets differ in lipid composition.
• Short, unsaturated tails increase fluidity, and cholesterol buffers fluidity.
• Only small nonpolar or small uncharged molecules cross the bilayer freely.
• Channels are fast and gated; carriers are slower and saturate.
• Primary active transport uses ATP directly, while secondary active transport uses ion gradients built by pumps.
• Charged solutes move according to their electrochemical gradient, not concentration alone.
• The resting potential is near E_K because resting membranes are most permeable to K⁺.

Formulas and rules

Common mistakes

• Saying water moves toward lower solute concentration; it moves toward higher solute concentration.
• Ignoring the membrane potential when deciding which way an ion will flow.
• Forgetting to divide by z = 2 for Ca²⁺ in the Nernst equation.
• Calling Na⁺–glucose symport primary active transport; it uses the Na⁺ gradient, not ATP directly.
• Thinking cholesterol always increases fluidity; at body temperature it decreases it.

Before the test, make sure you can

• Explain the fluid mosaic model and how lipid composition affects fluidity.
• Classify membrane proteins as integral, peripheral or lipid-anchored.
• Predict which molecules cross a bilayer unaided.
• Predict water movement and cell shape in hypotonic, isotonic and hypertonic solutions.
• Compare channels, carriers, uniport, symport and antiport.
• Distinguish primary from secondary active transport with examples.
• Calculate equilibrium potentials with the Nernst equation and ΔG for transport.
• Explain why the resting potential is close to E_K.

Words to know

Fluid mosaic model

Membranes are fluid phospholipid bilayers with proteins embedded in or attached to them.

Amphipathic

Having both a hydrophilic part and a hydrophobic part, like a phospholipid.

Flippase

An ATP-driven enzyme that moves specific phospholipids to the cytosolic leaflet.

Integral protein

A membrane protein embedded in the bilayer that can be removed only with detergent.

Peripheral protein

A protein attached to the membrane surface by weak interactions, removable with salt.

FRAP

Fluorescence recovery after photobleaching; measures how fast and how much of a membrane component diffuses.

Osmosis

Net movement of water across a semipermeable membrane toward higher solute concentration.

Aquaporin

A channel protein that allows rapid passage of water but not ions.

Channel protein

A transport protein that forms a gated aqueous pore for fast downhill ion flow.

Carrier protein

A transporter that binds solute and changes shape to move it; saturable.

Facilitated diffusion

Protein-assisted downhill movement of a solute with no energy input.

Primary active transport

Uphill transport driven directly by ATP hydrolysis, as in the Na⁺/K⁺-ATPase.

Secondary active transport

Uphill transport driven by the downhill flow of another ion, usually Na⁺ or H⁺.

Symport

Co-transport of two solutes in the same direction.

Antiport

Co-transport of two solutes in opposite directions.

Electrochemical gradient

The combined driving force of an ion's concentration gradient and the membrane potential.

Nernst potential

The membrane potential at which an ion has no net flow across the membrane.

Resting membrane potential

The steady voltage across a resting cell membrane, about −70 mV in many animal cells.

Sample practice questions

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