Membrane Structure (Cell Membranes)

MEMBRANE STRUCTURE CELL MEMBRANES Cell (plasma) membranes enclose the contents of the cell, separa>ng intracellular components from the extracellular environment. This allows for the precise control of internal condi>ons (i.e. homeostasis). Cell membranes have two key proper>es that promote this homeosta>c regula>on: • They are semi-permeable, in that some material cannot cross the membrane without assistance • They are selective, in that membrane scan regulate the passage of certain material according to need PHOSPHOLIPID BILAYER Membranes consist of a phospholipid bilayer. Each phospholipid consists of a polar phosphate head and two non-polar faOy acid tails. The phosphate head is hydrophilic (water-loving), while the faOy acid tails are hydrophobic (water-ha>ng). This makes the phospholipid amphipathic (both hydrophilic and hydrophobic). Phospholipids will spontaneously arrange into a bilayer, with the hydrophilic phosphate heads facing out towards the surrounding aqueous solu>ons (i.e. cytosolic and extracellular fluids), while the hydrophobic faOy acids face inwards to avoid exposure to the polar fluids. The bilayer is therefore held together by the weak hydrophobic associaCons between the faOy acid tails, allowing for membrane fluidity and flexibility (it can easily break and reform). Hydrophilic head AOracted to H2O Hydrophobic tail Repelled by H2O Inters>>al Fluid (Extracellular) Cytosolic Fluid (Intracellular) MEMBRANE PROTEINS Phospholipid bilayers are embedded with proteins, which may be permanently or temporarily aOached: • Integral proteins are transmembrane (span the bilayer) and permanently attached to the membrane • Peripheral proteins associate with one side of a membrane and are temporarily attached to the bilayer Integral proteins cannot readily be dissociated from the membrane without disrup>ng the bilayer (such as through the use of detergents). Examples of integral proteins include ion channels, carrier proteins and protein pumps. Peripheral proteins can easily be dissociated from the membrane by using a polar solvent. Examples include receptor complexes associated with signal transduc>on pathways (such as G proteins). PROTEIN FUNCTIONS Membrane proteins serve a variety of key func>ons: • Junctions: They can connect cells together to form tissues (tight junctions) • Enzymes: Immobilising enzymes on membranes localises specific reactions • Transport: Allows passage of material across the bilayer (channel proteins) • Recognition: May function as markers for cell identification (e.g. antigens) • Adhesion: Act as attachment points for cytoskeleton or extracellular matrix • Transduction: Functions as receptors for signalling pathways (glycoproteins) Hint: JET RAT GLYCOSYLATION Phospholipids and membrane proteins can have carbohydrate chains aOached via the process of glycosylaCon. Glycosyla>on of phospholipids result in glycolipids, whereas glycosyla>on of membrane proteins produce glycoproteins. The carbohydrate chains are located on the extracellular side of the membrane and play important roles in cell adhesion and cell recogni>on. • Adhesion: Surface carbohydrates can serve as attachment points for cells (glycoproteins act as sperm binding sites) • Recognition: Surface carbohydrates can also act as a point of recognition between cells (ABO antigens are glycolipids) Glycoproteins and glycolipids also play a role in maintaining the structural integrity of the extracellular matrix. The carbohydrate chains can link extracellular molecules together, helping to make the matrix a cohesive network that provides external structure. The glycocalyx is a sugar coat found in ova that mediates sperm binding FLUID-MOSAIC MODEL The fluid-mosaic model of membrane structure describes two of the key quali>es of a plasma membrane: • • Fluid: Phospholipids can move position, making membranes amorphous (able to change size or shape) Mosaic: The bilayer is embedded with proteins and carbohydrates, resulting in a mosaic of components The fluid-mosaic model was proposed by Singer and Nicolson in 1972 and is the currently accepted model. integral protein glycoprotein cholesterol Phospholipid phosphate fa1y acids peripheral protein