Cell Membrane Structure and Function

Water Distribution in the Body

• Water makes up about 60% of the lean body mass of the human body
• Water is distributed in two large compartments: Intracellular Fluid (ICF) and Extracellular Fluid (ECF)
• ICF constitutes two-thirds of total body water and provides a specialized environment for the cell
• ECF contains about one-third of total body water and is distributed between the plasma and interstitial compartments

Ionic Compositions of Intracellular & Extracellular Fluids

• Not mentioned in the provided text

Properties of the Cell Membranes

• Membranes consist of an asymmetric lipid bilayer with distinct inner and outer surfaces
• Membranes are dynamic, highly fluid structures consisting of a lipid bilayer and associated proteins
• Plasma membrane exchanges material with the extracellular environment by exocytosis and endocytosis due to its flexibility
• Gap junctions are special areas of membrane structure that allow adjacent cells to communicate by exchanging material
• Plasma membrane has selective permeabilities generated through the action of specific transporters and ion channels
• Plasma membrane plays key roles in cell–cell interactions and in transmembrane signaling

Structure of the Cell Membrane

• The lipid bilayer is impermeable to most water-soluble molecules due to their insolubility in the hydrophobic core
• Gases such as oxygen, CO2, and nitrogen readily diffuse through the hydrophobic regions of the membrane
• Permeability coefficients of small molecules in a lipid bilayer correlate with their solubilities in nonpolar solvents
• Molecules that move rapidly through a given membrane have a high permeability coefficient

Plasma Membrane Components

• Membranes consist of lipid, carbohydrate, and protein parts
• Major lipids in mammalian membranes are Phospholipids, Glycosphingolipids, and Sterols
• Cholesterol is the most common sterol in animal cell membranes
• Lipid bilayer has hydrophilic and hydrophobic parts, making it amphipathic

Membrane Proteins

• Integral proteins interact extensively with phospholipids and span the bilayer as a bundle of α-helical transmembrane segments
• Examples of integral proteins include transporter molecules, ion channels, various receptors, and G proteins
• Glycophorin A is a simple membrane protein that spans the membrane only once
• Peripheral proteins do not interact directly with the hydrophobic cores of the phospholipids in the bilayer
• Peripheral proteins bind to the hydrophilic regions of specific integral proteins and head groups of phospholipids

Functions of Membrane Proteins

• Integral proteins provide specific points of attachment between cells or between a cell and proteins of the extracellular matrix (surface adhesion)
• Examples of integral proteins with surface adhesion functions include Integrins, Cadherins, and Selectins
• Transmembrane proteins serve as channels for water-soluble molecules or have receptor functions
• Detergents solubilize (release) integral and transmembrane proteins

Membrane Markers

• Not mentioned in the provided text

Membrane Composition

• Various cellular membranes have different lipid and protein compositions
• Myelin has the highest membrane lipid content
• Mitochondrial inner membrane has the highest membrane protein content due to its high selectively permeable nature and the presence of the electron transport chain

Membrane-Related Diseases

• Abnormalities of membranes can result in diseases or pathologic states

Specialized Features of Plasma Membranes

• Lipid Rafts: Specialized areas of the exoplasmic (outer) leaflet of the lipid bilayer enriched in cholesterol, sphingolipids, and certain proteins, involved in signal transduction
• Caveolae: Flask, or tube-shaped invaginations in the plasma membrane into the cytosol, possibly derived from lipid rafts
• Other specialized features include Tight Junctions, Gap Junctions, Desmosomes, Adherens junctions, and Microvilli