Cell Biology: Plasma Membrane Structure

Plasma Membrane Structure

• The plasma membrane is a fluid mosaic model
• Composed of phospholipids, proteins, and carbohydrates
• Phospholipids form a bilayer with hydrophilic heads and hydrophobic tails
• Proteins are embedded within the bilayer, some spanning the entire membrane (transmembrane), others are on the surface.
• Carbohydrates are attached to proteins or lipids, primarily on the extracellular side, forming the glycocalyx.
• Cholesterol is a component of animal cell membranes, contributing to membrane fluidity.

Membrane Lipids

• Phospholipids are the most abundant lipids in the membrane.
• Phospholipids include phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and sphingomyelin
• Glycolipids are found almost exclusively on the exterior, non-cytoplasmic face of the plasma membrane.
• Glycolipids are comprised of a carbohydrate portion and a lipid portion
• Cholesterol is another major lipid component in animal cell plasma membranes.

Membrane Proteins

• Integral membrane proteins are embedded in the phospholipid bilayer and may span the entire membrane.
• Peripheral membrane proteins are associated with the surface of the membrane.
• There are several types of membrane proteins that serve diverse purposes in cellular processes.
• Transmembrane proteins have a significant role in transport.
• Proteins associated with the bilayer may consist of the beta-barrel, alpha-helix, and pleated-sheet structures.

Membrane Fluidity

• Membrane fluidity is influenced by lipid composition and temperature.
• Unsaturated hydrocarbon tails with kinks reduce interactions and increase fluidity, while saturated tails are more rigid.
• Cholesterol moderates the effect of temperature on membrane fluidity.

Membrane Transport

• Passive transport occurs down the concentration gradient (from higher to lower concentration). This requires no energy from the cell.
• Simple diffusion occurs without any membrane protein assistance.
• Facilitated diffusion uses channel or carrier proteins to transport molecules across the membrane.
• Active transport moves molecules against the concentration gradient, requiring energy (often from ATP).
• Passive transport methods include simple diffusion and facilitated diffusion.
• Active transport methods include uniporters, symporters, and antiporters.

Membrane Functions

• Transport: Movement of substances into and out of the cell (passive and active).
• Enzymatic activity: Carry out metabolic reactions.
• Signal transduction: Receive and transmit signals.
• Cell-cell recognition: Allows cells to identify each other.
• Intercellular joining: Cells join at points of contact.
• Attachment to the cytoskeleton and extracellular matrix (ECM): Provides structural support and anchors cells during processes.

Membrane Specializations

• The glycocalyx, on the exterior, has significant roles including cell-cell recognition, protection, and cell adhesion.
• Lipid rafts are localized regions of the membrane due to the composition of cholesterol and glycosphingolipids. These likely play a critical role in signaling and transport.
• Membrane compartmentalization and transport are very important factors in maintaining the function of the cell.

Endocytosis and Exocytosis

• Endocytosis is the process of taking in large molecules and particles.
• Phagocytosis takes in solid particles, while pinocytosis takes in fluids.
• Exocytosis is the opposite process, releasing large molecules or particles from the cell.