Transmembrane Potential refers to the difference in electrical potential (voltage) across the membrane of a lining cell. There are a number of factors which contribute to this difference:
- Sodium ions are found in a high concentration outside cells.
- Potassium ions are found in a high concentration inside cells.
2. The cell membrane is selectively permeable.
- K+ channels are more permeable than Na+ channels.
- Proteins cannot pass through channels (too large).
3. As a result of #s 1 and 2, the cytoplasm tends to be negatively charged. The extracellular area is positively charged. The resistance of the lipid bilayer to a free flow of substances across the membrane helps to maintain this potential difference across the membrane. In a neuron this difference measures -70 millivolts.
Electrochemical gradient refers to the sum of the electrical and chemical forces acting across the cell membrane.
- The chemical gradient pushes Na+ into the cell.
- The electrical gradient pulls Na+ into the cell.
2. For K+:
- The chemical gradient pushes K+ out of cell.
- The electrical gradient retards the movement of K+ out of the cell.
- K+ is attracted by the intracellular protein (-).
- K+ is repulsed by the accumulation of positive charges extracellularly.
Resting Potential As a result of all of these conditions, a resting cell tends to show a fairly stable transmembrane membrane potential across the membrane. This is called the resting potential. Each cell type has a different resting potential:
Cell Type Resting Potential
Adipose cell -40mv
Thyroid cell -50mv
Skeletal muscle cell -85mv
Cardiac muscle cell -90mv