Introduction
Most cells carry a negative surface charge. This charge keeps cells apart, affects how they interact with each other and with drugs, and explains the way they move in an electric field.
Origin and importance of the surface charge
- Ionized surface groups: the main source. Carboxyl groups of sialic acid in glycoproteins and glycolipids, and the phosphate groups of phospholipids, are negatively charged at physiological pH.
- Adsorbed ions: ions from the surrounding medium can bind to the surface and change its net charge.
- Importance: repulsion between red blood cells stops them sticking together in the circulation. The charge also affects cell adhesion, cell recognition and how drugs and ions bind to the membrane.
The electric double layer
A charged surface attracts ions of the opposite charge, called counter-ions, from the medium. Together, the surface charge and these counter-ions form an electric double layer.
- Stern layer: counter-ions held close to the surface. The potential falls steeply and in a straight line here.
- Diffuse layer: counter-ions spread out further away, balancing electrical attraction against thermal movement. The potential falls gradually with distance.
The potential at the plane where the moving liquid slips past the particle is the electrokinetic (zeta) potential. It is the part of the surface potential that can be measured in practice. Raising the ionic strength compresses the double layer, so the potential falls off over a shorter distance.