Biophysics of Cells and Membranes: A Beginner's Guide

Biophysics is the application of physics to living systems, and one of its most important subjects is the cell membrane: the ultra-thin barrier, only about 8 nanometres thick, that separates the inside of every cell from the outside world. Far from being a passive wall, it is a selective gatekeeper that controls exactly which ions and molecules may pass.

This matters because every nerve impulse, every heartbeat and every muscle contraction is an electrical event created by ions moving through membranes. Understanding membrane physics is the foundation of neuroscience, cardiology and much of modern drug design.

Key Concepts

• Phospholipid bilayer: two layers of lipid molecules with water-loving heads facing outward and water-fearing tails facing inward, forming the membrane's fabric.
• Selective permeability: small uncharged molecules slip through, while ions need dedicated passages.
• Ion channels: protein pores that let specific ions, such as Na⁺ or K⁺, flow down their gradients.
• Sodium-potassium pump: a protein that burns ATP energy to push 3 Na⁺ ions out and pull 2 K⁺ ions in, maintaining the gradients.
• Membrane potential: the voltage across the membrane, typically about −70 mV inside relative to outside at rest.
• Diffusion: the random thermal motion that drives particles from high to low concentration.
• Osmosis: the diffusion of water across a semi-permeable membrane toward the saltier side.
• Capacitance: the membrane stores charge like a capacitor, about 1 µF/cm².

Formulas

• Nernst equation: E = (RT/zF) × ln(Cout/Cin), where E is the equilibrium voltage in volts (V), R the gas constant, T the temperature in kelvin (K), z the ion's charge, F Faraday's constant, and C the concentrations.
• At body temperature (37°C): this simplifies to E ≈ 61.5 mV × log10(Cout/Cin) for a singly charged ion.

The equation assumes only one ion type can cross and that it has reached equilibrium, with no pump working against it. Real cells balance several ions plus the constantly running sodium-potassium pump, so the resting −70 mV sits between the individual Nernst values.

Worked Example

Potassium has Cin = 140 mM inside the cell and Cout = 5 mM outside. What is its Nernst potential at 37°C?

1. Concentration ratio = 5 ÷ 140 = 0.0357.
2. log10(0.0357) = −1.447.
3. E = 61.5 mV × (−1.447) = −89.0 mV.

So potassium alone would drive the membrane to −89 mV. Sodium's Nernst potential is about +60 mV, and the real resting potential of −70 mV is the compromise set by how leaky the membrane is to each ion.

Common Misconceptions

• "The membrane is a solid wall." It is a fluid, dynamic sheet: lipids and proteins drift laterally, and the membrane constantly bends, buds and fuses during cell life.
• "Ions just diffuse through it." The oily interior blocks charged ions almost completely; they cross only through specialised channels, carriers or pumps.
• "Resting potential means nothing is moving." Quite the opposite: ions leak continuously and the sodium-potassium pump works non-stop, burning a large share of the brain's energy, to hold the voltage steady.

Real-World Applications

• ECG and EEG: heart and brain monitors read the summed electrical activity of membranes from the skin and scalp.
• Anaesthetics: many anaesthetic drugs work by blocking sodium channels in nerve membranes, silencing pain signals.
• Antibiotics: several antibiotics punch holes in bacterial membranes while sparing human ones.
• Dialysis: artificial membranes filter waste from blood when kidneys fail, using the same diffusion physics.

The cell membrane is where physics meets life most directly: voltages, currents and diffusion, the same concepts as in electronics, running the machinery of every living cell.