Cell Membrane Overview

Cell Membrane Overview Cell Membrane Introduction: Cell membranes are semi-permeable (some things can enter while others cannot) Phospholipid: building block of the cell membrane and have 3 components: phosphate head group, glycerol backbone (holds fatty acid tails to the phosphate head), and two fatty acid tails The molecule is amphipathic (one side that is hydrophobic and one side that is hydrophilic) Typically small, nonpolar molecules can pass through the plasma membrane (AKA “passive diffusion”) - This include gases (O2, CO2) while water and ethanol take much longer (they’re small but polar) - Benzene is large but also nonpolar, so this also passes through slowly - Large and polar molecules (i.e. glucose) and charged molecules (ions and amino acids) will not pass through passive diffusion Phospholipid Structure: 2 fatty acids (basically a carboxylic acid with a long carbon chain) One has a double bonded kink (cis double bond) in the structure while the other is completely saturated Glycerol has a 3 carbon structure with three OH groups coming off each carbon Phosphate group can bind to other structures through one of its OH groups Cell Membrane Overview & Fluid Mosaic Model: 3 separate components of the cell membrane: 1. Phospholipids 2. Cholesterol: a steroid that inserts between phospholipids, maintaining the fluidity of the cell membrane (important for changes of temperature-keeps phospholipids from coming too close together in low temperatures and at high temperatures it raises its melting point) 3. Proteins: integral proteins (transmembrane proteins are specifically ones that act as pathways for ions and molecules and span the entire membrane) and peripheral proteins - huge player in the functions of cell membranes (act as receptors and help transport molecules in and out of the cell) Carbohydrates also bind to either the lipids or proteins in the membrane and play a very important role in cell-cell communication (allows one cell to recognize another cell); they are typically located on the outer membrane (either on glycoproteins or glycolipids) Fluid Mosaic Model: a lot of different pieces come together to form the cell membrane and is considered fluid because all the proteins, lipids, etc. can move around the cell membrane in basically any direction Membrane Proteins: Lipid Bound Protein: very rare type of protein and is found inside the lipid bilayer (literally has no basic function - can’t interact with the inside or outside of the cell) 2 Types of Integral Proteins: 1. Channel Protein: allows for a specific ion to pass/flow through using passive diffusion (down its concentration gradient) 2. Carrier Protein: carries the specific ion/molecule to the other side of the membrane (can sometimes use energy and pump against the concentration gradient Glycoprotein: sugar + protein and is important in signaling (allows a cell to recognize another cell) Cell Membrane Fluidity: Membrane Fluidity: three things can affect this, temperature, cholesterol, and unsaturated/saturated fatty acids At low temperatures, phospholipids will begin clustering together closely (crystalized state), which lowers the fluidity of the cell membrane Cholesterol allows the phospholipids from packing together too tightly (increasing the fluidity) At high temperatures, phospholipids have more energy and more motion (distance between one another), which increases the fluidity of the cell membrane Cholesterol allows the phospholipids to pull themselves closer together cause they want to attach themselves to the cholesterol (lowering the fluidity at high temperatures) Cholesterol acts as a buffer, keeping the membrane from not getting too fluid but also not losing too much fluidity Saturated fatty acids decrease the fluidity of the membrane (allows the phospholipids to stack onto one another) while unsaturated fatty acids increase the fluidity of the membrane (the phospholipids can’t stack onto one another as much) Membrane Dynamics: Uncatalyzed Movement: no need for a catalyst Transbilayer Diffusion (“flip flop”): where a phospholipid switches from outer leaflet to the inner leaflet (or vice versa) and does NOT need a catalyst (very slow and doesn’t happen very often) Lateral Diffusion: where a phospholipid moves around within its leaflet and occurs pretty fast (does not need a catalyst either) Catalyzed Movement: needs a catalyst in order to occur (typically a protein) Flippase: an transmembrane protein that uses ATP in order to switch a phospholipid from the outer leaflet to the inner leaflet (fast) Floppase: a transmembrane protein that uses ATP in order to switch a phospholipid from the inner leaflet to the outer leaflet (fast) Scramblase: a transmembrane protein that takes a phospholipid from the inner to outer leaflet and another phospholipid from the outer to the inner leaflet (does not require ATP) (fast) Quick Note: glucose enters via a carrier protein (facilitated diffusion)