The Cell Membrane

• The cell membrane is selectively permeable to ions and organic molecules, regulating the movement of substances in and out of the cell
• Phospholipids form bilayers in water due to their amphipathic properties. They can move within the bilayer, making the membrane flexible or fluid
• The fluidity of the membrane allows for the formation of vesicles and for materials to be taken into the cell by endocytosis or released by exocytosis
• Endocytosis is when cells take in substances from outside the cell by engulfing them in a vesicle derived from cell membrane
• Exocytosis is when cells release materials from inside to the extracellular space
• A molecule is amphipathic when they contain both hydrophilic and hydrophobic parts
• Cholesterol is an amphipathic molecule containing a hydroxyl group, 4 linked hydrogen rings, and a hydrocarbon tail
• In low temperatures, the cell membrane has high viscosity (not as fluid) and is more densely packed. This makes it not as permeable and may cause it to break. Cholesterol prevents tight packing of fatty acid chains, maintaining membrane fluidity
• In high temperatures, the cell membrane has lower viscosity (more fluid), and is less densely packed. This makes it not hold shape and and causes it to become too permeable. Cholesterol restrains the movement of the fatty acid chains, making it less fluid and reducing permeability
• Integral membrane proteins are embedded in the phospholipid bilayer
• Peripheral proteins are indirectly, or loosely attached to the surface of the cell membrane
• Transport proteins move specific molecules and ions across the cell membrane, enabling specific hydrophilic solutes to cross the membrane without coming in contact with the hydrophobic interior of the lipid bilayer
• Channel proteins are a form of passive transport that form pores extending across bilayer. This allows specific solutes to move across the membrane down the concentration gradient (eg. Potassium Channel)
• Carrier proteins can be active or passive, binding to specific solutes and undergoing a series of shape changes to transfer the solute across the membrane (eg. Fructose Transporter)
• Pumps are a form of active transport that pumps certain solutes across the membrane using a source of metabolic energy. They move solutes against the concentration gradient. (eg. Proton Pump)
• Receptor proteins are embedded in the cellular membrane and bind to specific chemical signals from outside the cell. When it binds, the protein triggers a response by the cell. (eg. Peptide Hormone Receptor)
• Enzymes are proteins that catalyze chemical reactions. (eg. ATP Synthase)
• Recognition proteins are embedded in the cell membrane, allowing cells to identify each other and interact. They are often glycoproteins: proteins with oligosaccharide chains attached. (eg. Blood Type Proteins)
• Cellular membranes are semi-permeable, meaning only substances that are hydrophobic and have no charge can pass through (eg. lipids, gases)
• Large and charged systems are not able to pass through the membrane (eg. water)
• Cell membranes are selective, meaning that membrane proteins can regulate the passage of material
• Passive transport is the movement of materials down the concentration gradient. It does not require ATP.
• Simple diffusion is a type of passive transport which involves the movement of small and lipophilic molecules (eg. oxygen)
• Facilitated diffusion is a type of passive transport which involves the movement of large/charged molecules via membrane proteins (eg. aquaporin)
• Active transport is the movement of materials against the concentration gradient, requiring energy expenditure (eg. proton pumps)
• Osmosis is the net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration (until equilibrium is reached)
• Osmosis occurs because solutes cannot cross the cell membrane unaided, so free water will diffuse to equalize the two solutions. Osmosis always occurs from regions of low solute concentration to regions of high solute concentration.
• Hypertonic solutions (high osmolarity) have higher solute concentration than the cell. The water will leave the cell and the cell will shrivel (become flaccid)
• Hypotonic solutions (low osmolarity) have lower solute concentrations than the cell. The water will enter the cells, causing it to swell and potentially burst (become turgid)
• Isotonic solutions (equal osmolarity) have the same solute concentrations as the cell. There is no net water movement.
• Tissues or organs used in medical procedures are bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis.
• Membrane fluidity refers to the viscous flow of phospholipids
• At higher temperatures, the membrane has more fluidity. At lower temperatures, the membrane has less fluidity
• Longer fatty acid tails allow for more interactions between phospholipids, leading to less fluidity
• Unsaturated fatty acids have one or more double bonds in the fatty acid tails. It creates a "kink", pushing the adjacent phospholipids apart. This increased spacing increases fluidity.
• Because the membrane is fluid, cells can transport many molecules into (endo) or out (exo) of the cell at once.
• Membrane fluidity allows for spontaneous breaking/reforming of the bilayer, allowing larger materials to enter or leave the cell without having to cross the membrane. This requires ATP hydrolysis
• Endocytosis is when cell activity transports molecules into the cell by engulfing them into vesicles formed from the cell membrane. (eg. macrophages (WBC) engulfing pathogens when fighting infection)
• Endocytosis 1. Plasma membrane folds inwards, forming cavity that fills with extracellular fluid, dissolved molecules, food particles, pathogens and other substances
• Endocytosis 2. Membrane folds back on itself until ends of membrane meet. Traps fluid inside vesicle