Cell Biology: Plasma Membrane Structure

Study Notes

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.

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.

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 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.

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.

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.

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 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.