Membrane: Structure And Function

Membrane Structure And FunctionChapter title Membrane Structure and Function. The ability of the mobile ph wholeness to discriminate in its chemical exchanges with the environment is fundamental to life, and it is the plasma membrane that makes this selectivity possible.MembraneThe membranes that are found at heart cells (plus the plasma membrane touch cells) live of phospholipids (and other lipids plus membrane proteins) arrayed by hydrophobic exclusion into two-dimensional liquids known as known as lipid bilayersPhospholipidsPhospholipids are amphipathic molecules center that they have both a hydrophobic and a hydrophilic endLipid bilayerPhospholipids can exist as bilayers in aqueous settlementsThe hydrophobic sight of the phospholipid is shielded in middle of these bilayersThe hydrophilic portion is exposed on both sides to waterLipid bilayers are held together mainly by hydrophobic interactions (including hydrophobic exclusion)Fluid mosaic modelThe plasma membrane contai ns proteins, sugars, and other lipids in addition to the phospholipidsThe model that describes the arrangement of these mettles in and about lipid bilayers is called the fluid mosaic modelBasically, membrane proteins are suspended within a two-dimensional fluid that in turn is made up mostly of phospholipidscholesterinCholesterol, a kind of steroid, is an amphipathic lipid that is found in lipid bilayers that serves as a temperature-stability bufferAt higher temperatures cholesterol serves to impede phospholipid fluidityAt lower temperatures cholesterol interferes with solidification of membranes (e.g., cholesterol functions similarly, in the latter case, to the effect of unsaturated fatty acids on lipid-bilayer fluidity)Cholesterol is found particularly in puppet cell membranesMembrane proteinsProteins are typically associated with cell membranesIntegral membrane proteins are typically hydrophobic where they interact with the hydrophobic portion of the membrane or hydrophilic whe re they interact with the hydrophilic portion of the membrane and overlyingFunctions of membrane proteinsFunctions of membrane proteins includeTransport of substances crosswise membranesEnzymatic activitycell dialogueCell-to-cell joiningAttachment to the cytoskeleton and extracellular matrixSelective permeabilityLipid bilayers exhibit selective permeabilityIn general, intact lipid bilayers are pervious toaquaphobic molecules (including many gasses)Small, not-ionized moleculesSimultaneously, lipid bilyaers are NOT permeable toLarger, polar molecules (e.g., sugars)Ions, regardless of sizeThus, lipid bilayers are selectively permeable barriers that allow the launch of small or hydrophobic molecules while blocking the entry of larger polar or even small charged substancesTransport crosswise membranes impetus crosswise membranes is important, for instance as a means of removing wastes from a cell or bringing food into a cellCategories of substance enjoy crossways membranes includeP assive impartFacilitated diffusionActive transport (including cotransport)Endocytosis, phagocytosis, and exocytosis, also considered below, technically are not mechanisms of expungement of substances crosswise lipid bilayers (though these do act as apparent motions of substances into and out of cells to be movement across the euakaryotic cell membrane, a substance must actually pass through an endomembrane lipid bilayer) honour that in considering transport across membranes we will once again confront the concept of movement away from or towards equilibrium, i.e., endergonic and exergonic processesThere are three basic types of movement across membranes simple diffusion, passive transport, and active transportSimple diffusionSimple diffusion is the movement of substances across lipid bilayers without the aid of membrane proteinsThis image (below) shows how substances move through membranes, regardless of net direction and concentration gradientsThis image (below) shows how subst ances net move through membranes in the direction of their concentrations gradients (i.e., with their concentration gradients)-note that regardless of how net movement is accomplished, all simple diffusion across membranes occurs in the manner illustrated above, i.e., it is a process that is driven by the random movement of moleculesThis figure (below) indicates the kinds of molecules that are fit of moving across membranes via simple diffusionPassive transportPassive transport is the term used to describe the diffusion (as well as what is termed facilitated diffusion, below) of substances across lipid bilayersPassive transport is a consequence of movement through the lipid bilayer (whether by diffusion through the membrane or with movement across facilitated by an integral membrane protein) a concentration gradient thereby contrasting with active transportDown the concentration gradientDiffusion is a random process that tends to result in the net movement of substances from areas of high concentration to areas of low concentrationThis includes movement from one side of a permeable lipid bilayer to the other from the higher concentration side to the lower concentration side (i.e., passive transport)Movement from high to low concentration areas is described as going down its concentration gradient.The direction of movement of substances across lipid bilayers by passive transport is controlled by concentration gradientsOsmosisMovement of water across selectively permeable membranes down the water concentration gradient is called osmosisNote that this is movement toward equilibrium (exergonic process)Tonicity ( isosmotic, hypertonic, hypotonic)Picture a membrane separating two solutions, one side with a higher solute concentration than the otherThe side with the higher solute concentration is tell to be hypertonicThe side with the lower solute concentration is said to be hypotonic(I keep track of the difference by recalling that a hypodermic syringe is so named because the tip of the needle is placed beneath the dermis, i.e., infra the skin a hypotonic solution has a solute concentration that is beneath, i.e., lower than that of the reference solution)If both sides have the same solute concentration, they are said to be isotonicAnimal cells and tonicityNormally animal cells are bathed in an isotonic solutionPlacement of an animal cell in a hypertonic solution causes the cell to shrink (i.e., water is lost from the cell by osmosis)Placement of an animal cell in a hypotonic solution causes it to take on water then burst (lyse, i.e., die) (water is gained by the cell, lost from the environment bathing the cell, both by osmosis)TurgidityNormally a coiffure cell exists in a hypotonic environmentThe hypotonicity causes the plant cytoplasm to expandHowever the plant cell does not lyse and this is due to the presence of its cell jettyThis conditions is known as turgidity (i.e., the pressing of the plant plasma membrane up against its cell wall )Plant cells prefer to display turgidityPlasmolysisA plant or bacterial cell placed in a hypertonic environment will show a shrinkage of its cytoplasmThis shrinkage is called plasmolysisAt the very least plasmolysis will inhibit growthOften plasmolysis will lead to cell deathThis is the principle upon which foods are preserved in extremely osmotic solutions (e.g., salt or sugar) such solutions impede most microbial growthFlaccidityPlant cells bathed in isotonic solutions will fail to display turgidityInstead they display flaccidityAt a whole-organismal level, flaccidity is otherwise known as wiltingTransport proteinsSubstances (e.g., sugars) that are not permeable through lipid bilayers may electrostatic cross via membrane-spanning transport proteinsFacilitated diffusionFacilitated diffusion is the movement of a substance across a membrane via the avocation of a transport protein, where net movement can only occur with the concentration gradient, is called facilitated diffusionTh e key thing to keep in mind is that facilitated diffusion, in contrast to other mechanisms of transport-protein-mediated membrane crossing, does not require any input of energy beyond that necessary to place the protein in the membrane in the premier place (i.e., facilitated diffusion is an exergonic process)Passive versus active transportTwo general categories of transport across membranes existThose that dont require an input of energy (passive transport, simple diffusion, facilitated diffusion)Those that do require an input of energy (active transport)Passive TransportActive TransportConcentration gradientWith (Down)Against (Up)Without Integral ProteinYes (Simple Diffusion)NoWith Integral ProteinYesYesExamplesSmall or Hydrophobic Substances, Osmosis(by simple diffusion) or Not-Small or Charged Substances (by facilitated diffusion)Cotransport, Proton Pump, Sodium-Potassium PumpActive transportActive transport is the movement of substances across membranes against their concentrat ion gradientsMoving things against their concentration gradients requires an phthisis of energy (i.e., it is an endergonic process)This energy can be in the form of ATP (e.g., sodium-potassium core)This energy can also be in the form of electrochemical gradients (i.e., cotransport)Note that the movement of substances by active transport is in a direction that is away from equilibriumSodium-potassium pumpOne means by which cells actively transport substances across membranes is via the sodium-potassium pumpThe sodium-potassium pump is important especially in animal cells, and is the means by which the sodium-potassium electrochemical gradient is established by these cellsProton pumpThe sodium-potassium pump is the means by which animal cells generate membrane potentialsIn bacteria, plants, and fungi, proton pumps play the same roleThe proton pump is simply ATP-driven active transport in which the substance pumped across the membrane is a hydrogen ionCotransportMuch of the active tr ansport accomplished by a cell isnt directly powered by ATPInstead, much active transport is powered by membrane potentials (i.e., electrochemical gradients)Such electrochemical-gradient-driven active transport is called cotransportIn cotransport, one substance, such as a sugar, is driven up its concentration gradient while a second substance, e.g., sodium ions or protons, are allowed to fall down their electrochemical gradient the energy gained from the latter is employed to power the former (i.e., energy coupling)EndocytosisEndocytosis is a general category of mechanisms that move substances from outside of the cell to inside of the cell, but neither across a membrane (technically) nor into the cytoplasm (again, technically speaking)Instead, substances are moved from outside of the cell and into the lumens of endomembrane system membersTo enter the cytoplasm an endocytosed substance must still be moved across the membrane of the endomembrane system, e.g., following their digestion (typically hydrolysis) to smaller moleculesExamples include phagocytosis, pinocytosis, and receptor-mediated endocytosisPhagocytosisPhagocytosis is the engulfing of extracellular particles is achieved by wrapping pseudopodia around the particles, thus internalizing the particles into vacuolesAmoebas employ phagocytosis to eat almost protozoa obtain their food by engulfing, i.e., via some form of endocytosisThe advantage of endocytosis as a mechanism of food gathering has to do with minimizing the volume within which digestive enzymes must work in order to digest food, i.e., the engulfed food particleCells in our own bodies, called phagocytes and macrophages employ phagocytosis to engulf (and then destroy) debris rootless around our bodies as well as to engulf and destroy invading bacteriaPinocytosisPinocytosis is the engulfing of liquid surrounding a cellThis is how developing ova obtain nutrients from their surrounding nurse cells (ova are very large cells so have surface-to-volu me problems-pinocytosis solves the problem of nutrient acquisition by allowing nutrients to be obtained across many internal membranes earlier than being limited to crossing the plasma membrane)Receptor-mediated endocytosisReceptor-mediated endocytosis involves the binding of extracellular substances to membrane-associated receptors, which in turn induces the formation of a vesiclesExocytosisExocytosis is more or less the mechanistic antonym of endocytosis