Chapter 7 Life on the edge surround Membrane Proteins 6 Functions Plasma membrane is the boundary that separates the living cell from its Cellular Permeability Certain things cross easily other can t Transport Enzymatic Activity Signal transduction Cell Cell Recognition Intercellular Joining Attachment to the cytoskeleton and extracellular matrix Concept 7 2 Cell must exchange with its surrounding Plasmas membrane regulates molecular traffic Control input glucose needed Control output urea waste not needed Permeability of the lipid bilayer Hydrophobic molecule can move through membrane easily Gases such as O2 and CO2 can cross as well as lipids Polar molecules sugars can t cross easily cross because of hydrophobic layer of membrane Transport proteins use as a tunnel the membrane Allow passage of hydrophilic substances across membrane Channel proteins Hydrophilic channel that certain molecules or ions can Bypasses the hydrophobic region Aquaporin s Facilitate the passage of water Carrier Proteins bind to molecules change shape to shuttle them across Transport protein is specific to one substance Glucose only not fructose Concept 7 3 Passive transport is diffusion with no energy investment Diffusion Molecules spreading evenly into the available space Although each molecule is random diffusion can move in one At dynamic equilibrium rate of ions entering rate of ions leaving direction cell Substances diffuse down the concentration gradient Movement is unaffected by the concentration of other substances No work must be done to move substances down the concentration gradient Diffusion is passive transport Delta G Spontaneous and exergonic Simple diffusion is the movement of gases and lipids through the Facilitated diffusion For larger molecules helps from proteins to pass phospholipid layer membrane No energy Facilitated Diffusion Passive Transport Speed the passive movement of molecules across the plasma membrane Transport proteins Channel Proteins Aquaporin Ion Channel Carrier Proteins Some diseases are caused by the malfunctions of specific transport systems for example the kidney disease Cystinuria Cysteine not transported back into kidney cells lacks carrier protein Remains in urine and crystalizes Concept 7 4 Active Transport Move against diffusion needs energy Active energy Active transport uses ATP not spontaneous Delta G Preformed by carrier proteins Sodium Potassium Pump Terminal phosphate to carrier protein How ion pumps maintain membrane potential Voltage difference is created by differences in the distribution of positive and negative ions across a membrane Electrogenic pump transport protein that generates voltage across membrane and stores energy that can be tapped for cellular work Sodium potassium pump Proton pump used by fungi plants and bacteria Co Transport Active transport of a solute indirectly transports another Exocytosis contents Transport vesicles migrate to the membrane fuse with it and release their Moves out large number of molecules Many secretory cells use exocytosis to export their products Hormone secreting cells enzymes from cells of the digestive tract Cell takes in macromolecules by forming vesicles from the plasma Endocytosis membrane 3 Types Phagocytosis Cellular Eating Pinocytosis Cellular Drinking Receptor Mediated endocytosis Phagocytosis cell engulfs a particle in a vacuole Vacuole fuses with lysosomes to digest the particle Filled with digestive enzyme used to destroy the particle Pinocytosis Cell drinking Receptor Mediated endocytosis binding of ligands to receptors triggers A ligand is any molecule that binds specifically to a receptor site Ex Human cells use receptor mediated endocytosis to take in vesicle formation cholesterol Hypercholesterolemia inherited disease where LDL receptors are defective cholesterol is not taken into the cells leads to atherosclerosis Osmosis Diffusion of H2O across a selectively permeable membrane Water moves from high to low solute concentration till its equal 2 solutions of Na Cl one is 5M the other is 2 5M if a water permeable membrane separates the two solutions water will move from the 5M solution to the 2 5M solution till equilibrium is reached Water Balance of Cells Tonicity Ability of a surrounding solution to cause a cell to gain or lose water cells The membrane has an effect on Tonicity Isotonic Solution Solute outside the cell is same as in the cell Hypertonic Solution Cell loses water Cell shrivels Hypotonic Solution Cell gains water Cell expands Hypertonic Solutions and Hypotonic Solutions create osmotic problems for Osmotic Regulation Control of solute and water concentrations Protist Paramecium Hypertonic compared to its pond environment has a contractile vacuole that acts as a pump Cell wall helps maintain water balance Plant cell in hypotonic solutions swell till the walls are turgid in a hypertonic solution becomes flaccid this can lead to an effect called plasmolysis Concept 7 1 Cellular membranes are fluid mosaics of lipids and proteins Phospholipids are the most abundant lipid in the plasma membrane Amphipathic Molecules are phospholipid containing a hydrophobic region Fluid mosaic Model Membrane is a fluid structure with a mosaic of various and hydrophilic region proteins and lipids Membrane Models 1915 Membranes from red blood cells erythrocytes were analyzed and composed of phospholipids and proteins 1925 Garter and Grendel reasoned that the membrane is composed of a phospholipid bilayer 1935 Hugher Davison and James Derrick proposed a sandwich model in which the phospholipid bilayer lies between two globular proteins 1972 S J Singer and G Nicolson proposed that the membrane is a mosaic of proteins dispersed within a lipid bilayer Freeze Fracture studies of the plasma membrane supported the fluid Splice the membrane along the middle of the phospholipid bilayer mosaic model Fluidity of Membranes Phospholipids in the plasma membrane can move within the bilayer such as across or even flip with one on the opposite side of the bilayer As temperatures cool membranes move from fluid to solid the lipids determine the temperature that solidifies them Membranes rich in unsaturated fats acids are more fluid Cell needs to fluid to function Cholesterol has effect on membrane fluidity at different temps Warm Vicious Cold Fluid Membrane Proteins 1 Peripheral Bound to surface 2 Integral proteins penetrate hydrophobic core Trans membrane proteins Span membrane Six Major Functions of Membrane proteins 1
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