Cell Biology: Plasma Membrane and Proteins

Plasma Membrane and Membrane Proteins

• Plasma membrane separates the interior of the cell from its environment
• It acts as a biological membrane, protecting the cell
• The membrane is a lipid bilayer, with phospholipids and cholesterol as key components, maintaining fluidity at varying temperatures
• The fluid mosaic model describes the membrane's structure, proposed by Singer and Nicolson in 1972
• The model shows embedded proteins like integral, peripheral, and lipid-anchored proteins
• Integral proteins span the membrane, having hydrophilic and hydrophobic domains interacting with internal and external molecules

Proteins of Plasma Membrane

• Integral proteins span the membrane and have hydrophobic membrane-spanning regions, which interact with internal and external molecules; examples include ion channels, proton pumps, and G protein-coupled receptors
• Lipid-anchored proteins are linked to the membrane via covalent bonds with lipids; they don't directly interact with the membrane, instead embedding into the lipid bilayer
• Peripheral proteins bind to integral membrane proteins or lipid bilayer regions; they have temporary interactions with the membrane

Carbohydrates in Plasma Membrane

• Plasma membranes contain carbohydrates (predominantly glycoproteins, but also glycolipids)
• Carbohydrates play roles in cell-cell recognition
• Recognition happens on the cell surface, where carbohydrates recognize host cells
• Viruses can use these receptors to cause infection

Membrane Fluidity and Dynamics

• Material can be incorporated into or removed from the membrane via exocytosis and endocytosis, respectively
• Exocytosis involves fusion of intracellular vesicles with the membrane
• Endocytosis involves the membrane forming blebs around extracellular material that then pinch off into vesicles

Functions of Plasma Membrane

• Controls substance movement in and out of cells; selectively permeable to ions and certain molecules
• Provides a physical barrier between the external environment and inner cell organelles, including communication and signaling between cells
• Anchors the cytoskeleton, contributing to the cell's shape and maintaining cell potential