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Biological Membranes Life at the Edge

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Biological Membranes Life at the Edge Biological membranes Life at the Edge The plasma membrane Is the boundary that separates the living cell from its nonliving surroundings About 8 nm thick Controls traffic into and out of the cell The plasma membrane exhibits selective permeability It allows some substances to cross it more easily than others Figure 7.1 Transport Across Membranes: Overcoming the Permeability Barrier •Overcoming the permeability barrier of cell membranes is crucial to proper functioning of the cell. •Specific molecules and ions need to be selectively moved into and out of the cell or organelle . •Membranes are selectively permeable. Definitions •Solution – mixture of dissolved molecules in a liquid •Solute – the substance that is dissolved •Solvent – the liquid Ion Concentrations •The maintenance of solutes on both sides of the membrane is critical to the cell –Helps to keep the cell from rupturing •Concentration of ions on either side varies widely –Na+ and Cl- are higher outside the cell –K+ is higher inside the cell –Must balance the number of positive and negative charges, both inside and outside cell •Ions and hydrophilic molecules cannot easily pass trough the hydrophobic membrane •Small and hydrophobic molecules can •Must know the list to the left Cells and Transport Processes Cells and cellular compartments - accumulate a variety of substances concentrations -very different from those of the surroundings substances that move across membranes - dissolved gases, ions, and small organic molecules; solutes Transport is central to cell function A central aspect of cell function - selective transport movement of ions or small organic molecules (metabolites) Cellular membranes are fluid mosaics of lipids and proteins Phospholipids Are the most abundant lipid in the plasma membrane Are amphipathic, containing both hydrophobic and hydrophilic regions For those who forgot… HYDROPHOBIC SUBSTANCE cannot be dissolved in water because they do not have affinity to water. Example is oil HYDROPHILIC SUBSTANCE can be dissolved in water because they have affinity to it. How are phospholipids and proteins arranged in the membranes of the cell? • The fluid mosaic model of membrane structure – States that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it – Or attached to a double layer of phospholipids Membrane Models: Scientific Inquiry Membranes have been chemically analyzed And found to be composed of proteins and lipids Scientists studying the plasma membrane Reasoned that it must be a phospholipid bilayer This bilayer of molecules exists as stable boundary between two aqueous compartments WATER Hydrophilic head Hydrophobic tail WATER The Davson-Danielli sandwich model of membrane structure Stated that the membrane was made up of a phospholipid bilayer sandwiched between two protein layers Was supported by electron microscope pictures of membranes However, there were 2 problems 1. generalization that all membranes of the cell are identical was challenged Plasma membrane is 7/8 nm thick and has three layered structure, and inner mitochondrial membrane is 6 nm thick and looks like a row of beads 2. placement of the proteins since membrane proteins are not very soluble in water Membrane proteins have hydrophobic and hydrophilic regions. If placed on the surface, hydrophobic parts would be in an aqueous environment… In 1972, Singer and Nicolson Proposed that membrane proteins are dispersed and individually inserted into the phospholipid bilayer Only their hydrophilic regions protrude far enough from the bilayer to be exposed to water According to this, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids Hydrophilic region of protein Phospholipid bilayer Hydrophobic region of protein The Fluidity of Membranes Membranes are not static sheets of molecules ! Held together by hydrophobic interactions which are weaker than covalent bonds Most of the lipids and some of the proteins can drift about laterally That is in the plane of the membrane Movement is rapid However, proteins are larger than lipids and they move slower The Fluidity of Membranes Membrane remains fluid as temperature decreases Phospholipids settle into closely packed arrangement and the membrane solidifies The solidification temperature depends on the types of lipids it is made of The membrane remains fluid at lower temperatures if it is rich in phospholipids with unsaturated hydrocarbon tails Those hydrocarbons have kinks in the tails where the double bonds are located so they cannot pack closely as saturated hydrocarbons The type of hydrocarbon tails in phospholipids Affects the fluidity of the plasma membrane Fluid Viscous Unsaturated hydrocarbon Saturated hydro- tails with kinks Carbon tails (b) Membrane fluidity The Fluidity of Membranes Phospholipids in the plasma membrane Can move within the bilayer Lateral movement Flip-flop (~107 times per second) (~ once per month) (a) Movement of phospholipids Lateral movement Within the same membrane surface Fast process Flip-flop Or transverse diffusion From one membrane surface to another Slow process The Fluidity of Membranes The membranes must be fluid to work properly Fluid as salad oil When solid it changes its permeability and enzymatic proteins in the membrane become inactive Solutes Cross Membranes Simple Diffusion, Facilitated Diffusion, and Active Transport •Three quite different mechanisms are involved in moving solutes across membranes •A few molecules cross membranes by simple diffusion, the direct unaided movement dictated by differences in concentration of the solute on the two sides of the membrane •However, most solutes cannot cross the membrane this way The Role of Membrane Carbohydrates in Cell-Cell Recognition Cell-cell recognition Is a cell’s ability to distinguish one type of neighboring cell from another Important for organisms functioning Basis for the rejection of foreign cells by immune system The way cells recognize other cells is by binding to surface molecules Usually carbohydrates Membrane carbohydrates Interact with the surface molecules of other cells, facilitating cell-cell recognition Usually short Some are covalently bonded to lipids forming molecules called glycolipids Most of them are bonded to proteins forming glycoproteins Synthesis and Sidedness of Membranes Membranes have distinct inside and outside faces This affects the movement of proteins synthesized in the endomembrane system Membrane proteins and lipids 1 •Synthesis of membrane proteins and Transmembrane glycoproteins lipids in the ER. Carbohydrates are ER added to the proteins making them Secretory protein glycoproteins Glycolipid •Inside Golgi they undergo Golgi 2 carbohydrate modifications becoming apparatus glycolipids Vesicle •Proteins are transported in vesicles to the plasma membrane •The vesicles fuse with the membrane 3 releasing secretory proteins form the Plasma membrane: cell Cytoplasmic face 4 Extracellular face Transmembrane Secreted glycoprotein protein Membrane glycolipid Membrane structure results in selective permeability A cell must exchange materials with its surroundings, a process controlled by the plasma membrane A steady traffic of small molecules and ions moves across the membrane in both directions Sugars, amino acids and other nutrients enter the cell while waste products leave the cell The cell takes in oxygen for cellular respiration and expels CO2 It also regulates concentration of inorganic ions The Permeability of the Lipid Bilayer Hydrophobic molecules Are lipid soluble and can pass through the membrane rapidly Examples are oxygen, hydrocarbons and CO2 Polar molecules Do not cross the membrane rapidly Examples are glucose and other sugars, water Charged atom or molecule and its surrounding shell of water penetrate the membrane even more difficult Transport Proteins Transport proteins Allow passage of hydrophilic substances across the membrane Some of them act as channel proteins where they have hydrophilic channel that certain molecules use as a tunnel Others act as carrier proteins which hold onto their passengers and change shape in a way that shuttles them across the membrane In both cases the transport protein is specific for the substance it translocates Active transport In other cases, transport proteins move solutes against the concentration gradient; this is called active transport. Active transport requires energy such as that released by the hydrolysis of ATP or by the simultaneous transport of another solute down an energy gradient. Concentration gradient or Electrochemical Potential The movement of a molecule that has no net charge is determined by its concentration gradient Simple or facilitated diffusion involve exergonic movement “down” the concentration gradient (negative ΔG) Active transport involves endergonic movement “up” the concentration gradient (positive ΔG) The electrochemical potential The movement of an ion is determined by its electrochemical potential the combined effect of its concentration gradient and the charge gradient across the membrane The active transport of ions across a membrane creates a charge gradient or membrane potential (Vm) Active transport of ions Most cells have an excess of negatively charged solutes inside the cell This charge difference favors the inward movement of cations such as Na+ and outward movement of anions such as Cl– In all organisms, active transport of ions across the plasma
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