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Transcript
Cell Transport Moving materials in and out of the cell. Cell Membrane • “Fluid Mosaic Model” (fluid with a mosaic of proteins) • Phospholipid Bilayer- phosphate head, lipid tail • Amphipathic – a molecule that’s hydrophobic and hydrophilic. Selective Permeability • Semi-permeable – allows only some materials through** • Small nonpolar molecules, such as hydrocarbons, can dissolve in the lipid bilayer • Large Polar molecules, such as sugars, do not cross the membrane easily Freeze Fracture • Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer Maintaining Fluidity • The phospholipids constantly trade places from side to side. Every once in a while they will trade with a phospholip from the other layer. • High temperatures cause more movement, lower temperatures cause less. Observing Phospholipid Movement • When placed together, the phopholipids of a mouse and human cell ended up mixed due to all the movement. Cholesterol • Cholesterol – a steroid hormone found in membranes to help maintain fluidity. • It restricts excessive movement at high temps. It prevents sticking together at low temps. Membrane Proteins • Integral Proteins – go through the whole membrane • Peripheral Proteins – sit on the outside of the membrane (inside of the cell). Protein Roles • Cell membrane proteins do six main things 1. Transport materials in and out, 2. Act as Enzymes, 3. Pass signals. Protein Roles • 4. Identify other cells, 5. connect to other cells, 6. attach the cytoskeleton to the ECM fibers Carbohydrate Roles • Carbohydrates can bond to the lipids (glycolipids) or more often proteins (glycoproteins) in the membrane to aid in cell recognition, by protruding from the membrane. Passive Transport • Passive Transport- requires no energy Diffusion • Simple Diffusion- molecules move from an area of higher concentration to lower concentration (down their concentration gradient) • occurs until equilibrium is reached (molecules are evenly spread out) Ex. cologne Diffusion across a membrane Osmosis • Osmosis- diffusion of water molecules thru a selectively permeable membrane (high to low water concentration) • the force exerted by osmosis is called osmotic pressure Osmosis continued Cell in Solution • Hypertonic solution – more dissolved material in solution than in the cell (water flows out of the cell) ** hypertonic Cell in Solution • Hypotonic solution -more dissolved material in the cell than in the solution, causing (water flows into the cell) (increases osmotic pressure, cells may burst). hypotonic Isotonic • Isotonic- same amount of dissolved material in the solution and cell. (equilibrium is reached) Isotonic Osmoregulation (control of water balance) Why don’t cells burst? • cells in organisms don’t usually come into contact with pure water • plant cells have cell walls that keep the cell from expanding • some cells use pumps Facilitated Diffusion • Facilitated diffusion - diffusion with the assistance of integral transport proteins. (Still Passive transport) Channel Proteins • Channel Proteins – allow certain materials to flow in and out (down their concentration gradient.) • Aquaporin – channel protein that only lets water in. • Ion Channels – also called gated channels, open and close in response to a chemical or electrical stimulus Carrier Proteins • Carrier Proteins – change shape to escort certain material in and out of the cell. (down their concentration gradients.) Active Transport • Active transport - requires energy • Proteins act as pumps to move material from low to high concentration. (often called electrogenic pumps) Ex. Proton Pumps (Hydrogen Pumps) Pumps continued • Sodium-Potassium pump – pumps sodium out and potassium in. (low to high concentration) Electrochemical Gradient • Pumps create a build up of ions on either side of the membrane, which creates an electric charge. • The charge causes an electrochemical gradient, which moves the ions across the membrane like an electric current to do work. (ex. Nerve signals) Cotransport • Cotransport – electrochemical gradient used to move other molecules across the membrane Moving large objects • Endocytosis – a vesicle formed to take materials into the cell** • Exocytosis – a vesicle putting materials out of the cell 3 Types of Endocytosis • 1. Phagocytosis – engulfing solid particles • 2. Pinocytosis – engulfing liquid substances (extracellular fluid) 3 Types of Endocytosis • Receptor mediated endocytosis – particles called ligands bind to receptors, which signals the engulfing of the particles. You should now be able to: 1. Define the following terms: amphipathic molecules, aquaporins, diffusion 2. Explain how membrane fluidity is influenced by temperature and membrane composition 3. Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 4. Explain how transport proteins facilitate diffusion 5. Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps 6. Explain how large molecules are transported across a cell membrane Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings