What are the molecules in the plasma membrane that provide basic membrane structure, cell identity, and membrane fluidity?
These molecules are called phospholipids (2 fatty acid "tails" with a phosphate group "head"). They tend to form phospholipid bilayers because of the fact that the fatty acid "tails" are hydrophobic (repel / do not mix with water) while the phosphate groups are hydrophilic (attracted to / mix with water) due to its charge.
The plasma membranes of cells consist of a phospholipid bilayer (the hydrophilic "heads" face the water-containing interiors and exteriors of each cell, while the hydrophobic "tails" face each other).
Additionally, there may be proteins embedded in the plasma membrane (functioning as channel proteins, protein pumps, receptors, etc.) that help with the cell function.
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