Cell Membrane - Functions (Enumerated)

1. Selective barrier / permeability control - The lipid bilayer is impermeable to water-soluble substances (ions, glucose, urea) but freely permeable to fat-soluble substances (O₂, CO₂, alcohol), thereby regulating what enters and exits the cell.
2. Transport of substances - Integral membrane proteins form channels and carrier systems that mediate diffusion, facilitated diffusion, and active transport of ions, nutrients, and waste products across the membrane.
3. Cell signaling and receptor function - Membrane proteins act as receptors for hormones and neurotransmitters, initiating intracellular signal cascades (e.g., G-protein-coupled pathways, enzyme-linked receptors).
4. Enzymatic activity - Peripheral proteins attached to the membrane surface function as enzymes catalyzing chemical reactions.
5. Cell recognition and adhesion - Membrane carbohydrates form the glycocalyx, which serves as a recognition layer for cell-to-cell interactions and adhesion sites for extracellular proteins.
6. Endocytosis and exocytosis - The membrane mediates ingestion of large particles (phagocytosis) and fluid/solutes (pinocytosis), as well as secretion of substances out of the cell.
7. Maintenance of electrochemical gradients - Via the Na⁺-K⁺ ATPase pump embedded in the membrane, a negative intracellular electrical voltage is maintained, which is fundamental to nerve and muscle function.