First, there is the exchange of fluid molecules through a semipermeable membrane (osmosis), the pore size of which is sufficient to allow small particles such as ions or water molecules to pass through unimpeded. Osmosis occurs, for example, in plasmolysis but also in the light reaction of photosynthesis and in the respiratory chain during ATP synthesis.
Transmembrane transport proteins (sometimes called permeases) transport substances either passively with the concentration gradient or actively against the concentration gradient under direct or indirect energy consumption. While active transport requires energy in the form of e.g. ATP, no energy consumption is necessary for the passive transport process. According to the type of membrane transport, a distinction is made between several membrane transport proteins.
Carrier proteins transport molecules actively or passively across the membrane by selective transport. An example is Na+-K+ pump (nervous system) and glucose transporter (sugar uptake into the cell). Another form of selective transport is the passive transport of molecules via tunnel proteins/channels through the membrane, where the channel is adapted to the molecule to be transported. Voltage-gated sodium or potassium channels of the nerve cell are an example.
Cell transport can also be mediated by receptor proteins. In this case, the information transfer between the outside and the inside of the cell takes place via ligands that bind to the receptor protein on the outside of the cell. The information is transferred to the inside of the cell by a reaction of the inner part as for example in second-messenger in the hormone system or in the uptake of neurotransmitters from the receptors of the postsynaptic membrane.
Transport proteins like structural proteins maintain the cell in its particular form as in the anchoring of the cytoskeleton in the membrane. Important structural proteins are keratin and myosin.
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Furthermore, there are communication proteins that serve the cell-cell interaction. As a rule, these are glycoproteins that serve tissue formation and immune defense.