Cell Membrane Transport Mechanisms PDF

Transport in and out of cells Cell Membrane Structure: Membrane Structure Cellular membranes have 4 components: 1. phospholipid bilayer 2. transmembrane proteins 3. interior protein network 4. cell surface markers 2 Phospholipids Phospholipid structure consists of -glycerol – a 3-carbon polyalco...

Transport in and out of cells Cell Membrane Structure: Membrane Structure Cellular membranes have 4 components: 1. phospholipid bilayer 2. transmembrane proteins 3. interior protein network 4. cell surface markers 2 Phospholipids Phospholipid structure consists of -glycerol – a 3-carbon polyalcohol acting as a backbone for the phospholipid -2 fatty acids attached to the glycerol -phosphate group attached to the glycerol Phospholipids The fatty acids are nonpolar chains of carbon and hydrogen. -Their nonpolar nature makes them hydrophobic (“water-fearing”). The phosphate group is polar and hydrophilic (“water-loving”). 4 Phospholipids The partially hydrophilic, partially hydrophobic phospholipid spontaneously forms a bilayer: -fatty acids are on the inside -phosphate groups are on both surfaces of the bilayer 5 Membrane Proteins Membrane protein is a protein molecule that is attached to, or associated with the membrane of a cell 6 Membrane Proteins Peripheral membrane proteins -anchored to a phospholipid in one layer of the membrane -possess nonpolar regions that are inserted in the lipid bilayer -are free to move throughout one layer of the bilayer 7 Membrane Proteins Integral membrane proteins -Integral proteins possess at least one transmembrane domain (span the lipid bilayer) -nonpolar regions of the protein are embedded in the interior of the bilayer -polar regions of the protein protrude from both sides of the bilayer 8 Passive Transport Passive transport is movement of molecules through the membrane in which -no energy is required -molecules move in response to a concentration gradient Diffusion is movement of molecules from high concentration to low concentration 9 Passive Transport In an aqueous solution -water is the solvent -dissolved substances are the solutes Osmosis is the movement of water from an area of high to low concentration of water -movement of water toward an area of high solute concentration 10 Passive Transport When 2 solutions have different osmotic concentrations -the hypertonic solution has a higher solute concentration -the hypotonic solution has a lower solute concentration Osmosis moves water through aquaporins (proteins embedded in the cell membrane that regulate the flow of water) toward the hypertonic solution. 11 Passive Transport Selective permeability: integral membrane proteins allow the cell to be selective about what passes through the membrane. Channel proteins have a polar interior allowing polar molecules to pass through. Carrier proteins bind to a specific molecule to facilitate its passage. 12 Passive Transport Channel proteins include: -ion channels allow the passage of ions (charged atoms or molecules) which are associated with water 13 Passive Transport -gated channels are opened or closed in response to a stimulus -the stimulus may be chemical or electrical 14 Passive Transport Facilitated diffusion is movement of a molecule from high to low concentration with the help of a carrier protein. Carrier proteins bind to the molecule that they transport across the membrane. -is specific -is passive -saturates when all carriers are occupied 15 Facilitated Diffusion No ATP involved. Binding alters the shape 16 Active Transport Active transport -requires energy – ATP is used directly or indirectly to fuel active transport -moves substances from low to high concentration -requires the use of carrier proteins 17 Active Transport Carrier proteins used in active transport include: -uniporters – move one molecule at a time -symporters – move two molecules in the same direction -antiporters – move two molecules in opposite directions 18 Active Transport Sodium-potassium (Na+-K+) pump -an active transport mechanism -uses an antiporter to move 3 Na+ out of the cell and 2 K+ into the cell -ATP energy is used to change the conformation of the carrier protein -the affinity of the carrier protein for either Na+ or K+ changes so the ions can be carried across the membrane 19 20 Bulk Transport Bulk transport of substances is accomplished by 1. endocytosis – movement of substances into the cell Endocytosis occurs when the plasma membrane envelops food particles and liquids 2. exocytosis – movement of materials out of the cell 21 1. phagocytosis – the cell takes in particulate matter 22 2. pinocytosis – the cell takes in only fluid 23