The plasma membrane is selectively permeable due to the proteins embedded within it. Specifically, transmembrane proteins exist, one portion extracellularly and the other intracellular and provide for channels that may shield substances from the lipid bilateral. When bound to other substances, the molecular structure of proteins may be changed and thereby influencing cellular physiology and plasma membrane permeability.







PROTEINS





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Proteins are the most versatile molecules in the human body. Proteins are crucial to sustaining life. There is likely no major physiological reaction in the body that does not involve at least one protein.





Proteins are organic molecules made up of chains of amino acids.


Amino acids are molecules composed of carbon, oxygen, hydrogen, and nitrogen.


There are 20 different amino acids involved in protein synthesis.

Proteins are polymers. Polymers are molecules composed of a chain of same or similar molecules.


The same or similar molecule of a polymer is called a monomer. The monomer of a protein is an amino acid.


​Proteins are also known as peptides. A peptide is a molecule made up of two or more amino acids.




Proteins may have complex structure. The molecule, based on hydrogen and covalent bonds, and chemical binding of different peptide chains present with four levels of structure: primary, secondary, tertiary, and quaternary.







Primary Structure

The sequence of the amino acid monomers is the primary structure of a protein. As previously stated there are 20 amino acids associated with protein synthesis. This means there is a nearly unlimited variation of potential sequences.

Secondary Structure

Intramolecular hydrogen bonds may call the protein to coil or form a helix. This coiling is known as the protein’s secondary structure.

Tertiary Structure

Covalent interactions between side chain molecules cause the protein to fold. This is known as the tertiary structure.

Quatenary Structure

Quatenary structure is formed when same or different peptide chains are chemically bound.



TRANSMEMBRANE PROTEIN FUNCTIONS







AQUAPORINS







Plasma membranes are embedded with hundreds specialized transmembrane proteins known as aquaporins. Aquaporins are channels specialized for the transport of water. As previously stated, it is the proteins within the membrane that make it selectively permeable. You probably recall that lipids are hydrophobic, that means water does not mix with them. Given the membrane lipid layer, water trying to enter or leave the cell would get trapped in the lipid layer. The aquaporins shield the water from the lipid layer and therefore water freely enters and leaves the cell.