Active transport
-moves substances against concentration gradient using cellular energy (ATP)
In animal cells, the most prominent active transport molecule is the sodium-potassium ion pump (Na+/K+ ATPase). The differential movement of positive ions across the membrane leaves the cell with a negative charge with respect to the ECM. This creates a voltage across the membrane. Only nerve and muscle cells have the ability to change the voltage.
Sodium and potassium ions leak across the membrane through channel proteins so the pump must operate continuously.
In other eukaryotic cells, voltage is generated by proton pumps. The H+ pumps puts hydrogen ions (protons) outside the cell to make the cell interior negative with respect to the outside. Chloroplast and mitochondria also rely on proton pumps; however, these pumps are driven by high-energy electrons.
Co-transport
A substance that has been pumped out of a cell can do work as it diffuses inward. Plants cells commonly "piggyback" sucrose, amino acids, and other nutrients when protons diffuse into cell through co-transport proteins. The piggybacked solutes are transported against their concentration gradient but no additional energy is required.
Bulk transport
The movement of macromolecules and large particles in and out of the cell requires the formation of transport vesicles and vacuoles. This disturbance in the membrane requires energy in the form of ATP.
When exporting large particles (EXOCYTOSIS), the vesicle and the cell membrane must fuse and the contents of the vesicle then spill out into the extracellular fluid (ECF).
In endocytosis, substances are brought into the cell. There are three categories of endocytosis: pinocytosis, in which a fluid droplet and all its contents are brought into the cell; phagocytosis, in which a large particle is engulfed by the cell to be digested when a lysosome fuses with the vacuole; and receptor-mediated endocytosis, in which specific substances (ligands) bind to receptor proteins, triggering the formation of a vesicle.
Reception of information
Membrane proteins on the surface may act as receptors for incoming messages, i.e. hormones and other chemicals. In many instances, the message, but not the molecule, enters the cell. A cascade of information begins that results in a change in cellular activity. Steroid hormones do actually enter the cell. They can penetrate the nucleus, act as a transcription factor, and cause a change in the expression of genes.
Cell Identity
Cell-cell recognition is crucial in development and in immunity responses.
Identity is established through the presence of key markers on the exterior surfaces of cells. These markers are branched chains of monosaccharides
(usually fewer than 15) covalently bound to lipids (glycolipids) or proteins (glycoproteins). These may vary among species, among individuals in the same species, and even on different cells within the same organism.
Glycoproteins on RBC surfaces determine the ABO blood groups.