Introduction
This topic covers the physiology of the cell: how substances cross the plasma membrane, how the resting and action membrane potentials arise, how vesicles carry material in and out of the cell, and how a cell receives an external signal and converts it into a response. It builds on the structure of the membrane and the organelles described in the earlier topics.
Transport across the plasma membrane
The lipid bilayer lets small non-polar molecules such as \mathrm{O}_{2} and \mathrm{CO}_{2} pass freely, but it is poorly permeable to ions and to large or polar molecules. Everything else crosses by carrier proteins, channels or vesicles.
Passive transport
- Simple diffusion moves a substance down its concentration gradient without a carrier and without energy, as with \mathrm{O}_{2}, \mathrm{CO}_{2} and steroid hormones.
- Facilitated diffusion also moves a substance down its gradient, but through a carrier or channel. It is saturable, as with glucose entering cells through GLUT transporters.
- Osmosis is the movement of water across a semipermeable membrane, toward the side with higher solute concentration.
Active transport
Active transport moves a substance against its gradient and needs energy. Primary active transport uses ATP directly. The \mathrm{Na}^{+}/\mathrm{K}^{+}-ATPase pumps 3 \mathrm{Na}^{+} out and 2 \mathrm{K}^{+} in per cycle, which maintains the gradients on which all other transport depends. Secondary active transport uses the gradient of one ion, usually \mathrm{Na}^{+}, created by a primary pump. A symporter moves both substances in the same direction, as with glucose and \mathrm{Na}^{+} in the intestine and kidney. An antiporter moves them in opposite directions, as with the \mathrm{Na}^{+}/\mathrm{Ca}^{2+} exchanger.
Vesicular transport
Endocytosis brings material into the cell. Phagocytosis takes in large particles, as done by macrophages and neutrophils. Pinocytosis takes in fluid and dissolved solutes. Receptor-mediated endocytosis uses clathrin-coated pits to concentrate specific ligands, as in the uptake of LDL cholesterol. Exocytosis releases secretory products and adds membrane to the surface. It can be constitutive or regulated, and regulated exocytosis is triggered by a rise in cytosolic \mathrm{Ca}^{2+}.