Membrane Proteins and Their FunctionsA membrane contains different proteins, often grouped together, embedded in the fluid matrix of the lipid bilayerProteins determine most of the membranes specific functions

Peripheral proteins are bound to the surface of the membraneIntegral proteins penetrate the hydrophobic core (within the membrane)The Two Major Types of Membrane Proteins

Six major functions of membrane proteinsTransportEnzymatic activitySignal transductionCell-cell recognitionIntercellular joiningAttachment to the cytoskeleton and extracellular matrix (ECM)

Transport proteinAllow passage of hydrophilic substances across the membraneSome have a hydrophilic channel that certain molecules or ions can use as a tunnelChannel proteins called aquaporins facilitate the passage of water.A cell would die without aquaporins.Other T proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane.A transport protein is specific for the substance it moves.

Enzymatic activityHas active site exposed to substances in the adjacent solution.

In some cases many enzymes in a membrane are organized as a team that carries out series of steps of a metabolic pathway.

The Role of Membrane Carbohydrates in Cell-Cell RecognitionCells recognize each other by binding to surface molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane (outside the cell)Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)Basically a tag for identification.

Concept 7.2: Membrane structure results in selective permeabilityA cell must exchange materials with its surroundings, a process controlled by the plasma membranePlasma membranes are selectively permeable, regulating the cells molecular traffic

The Permeability of the Lipid BilayerHydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidlyPolar molecules, such as sugars, do not cross the membrane easily

Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investmentDiffusion is the tendency for molecules to spread out evenly into the available spaceAlthough each molecule moves randomly, diffusion of a population of molecules may be directionalAt dynamic equilibrium, as many molecules cross the membrane in one direction as in the other

Substances diffuse down their concentration gradient, the region along which the density of a chemical substance increases or decreasesNo work must be done to move substances down the concentration gradientThe diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell to make it happen

Animation: Membrane Selectivity Right-click slide / select Play

Animation: Diffusion Right-click slide / select Play

Effects of Osmosis on Water BalanceOsmosis is the diffusion of water across a selectively permeable membraneWater diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides

Figure 7.14Lower concentration of solute (sugar)Higher concentration of soluteSugar moleculeH2OSame concentration of soluteSelectively permeable membraneOsmosis

Water Balance of Cells Without WallsTonicity is the ability of a surrounding solution to cause a cell to gain or lose water (depends on both solute concentration and membrane permeability.Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membraneHypertonic solution: Solute concentration is greater than that inside the cell; cell loses waterHypotonic solution: Solute concentration is less than that inside the cell; cell gains water

Hypertonic or hypotonic environments create osmotic problems for organisms which do not have a cell wall.Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environmentsThe protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump

Video: Chlamydomonas

Video: Paramecium Vacuole

Figure 7.16Contractile vacuole50 m

Water Balance of Cells with WallsCell walls help maintain water balanceA plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm)If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt

In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis

Video: Plasmolysis

Video: Turgid Elodea

Animation: Osmosis Right-click slide / select Play

Facilitated Diffusion v/s Active TransportIn facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane (moving down the concentration gradient) requires no energy.

Active transport moves substances against their concentration gradientsActive transport requires energyActive transport is performed by carrier proteins embedded in the membranes

Active transport allows cells to maintain concentration gradients that differ from their surroundingsThe sodium-potassium pump is one type of active transport system

Animation: Active TransportRight-click slide / select Play

How Ion Pumps Maintain Membrane PotentialMembrane potential is the voltage difference across a membraneVoltage is created by differences in the distribution of positive and negative ions across a membrane

Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membraneA chemical force (the ions concentration gradient)An electrical force (the effect of the membrane potential on the ions movement)

An electrogenic pump is a transport protein that generates voltage across a membraneThe sodium-potassium pump is the major electrogenic pump of animal cellsThe main electrogenic pump of plants, fungi, and bacteria is a proton pumpElectrogenic pumps help store energy that can be used for cellular work

Figure 7.20CYTOPLASMATPEXTRACELLULAR FLUIDProton pumpHHHHHH

Cotransport: Coupled Transport by a Membrane ProteinCotransport occurs when active transport of a solute indirectly drives transport of other solutes Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell

Figure 7.21ATPHHHHHHHHProton pumpSucrose-H cotransporterSucroseSucroseDiffusion of H

Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosisSmall molecules and water enter or leave the cell through the lipid bilayer or via transport proteinsLarge molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesiclesBulk transport requires energy

ExocytosisIn exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contentsMany secretory cells use exocytosis to export their products

Animation: ExocytosisRight-click slide / select Play

EndocytosisIn endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membraneEndocytosis is a reversal of exocytosis, involving different proteinsThere are three types of endocytosisPhagocytosis (cellular eating)Pinocytosis (cellular drinking)Receptor-mediated endocytosis

Animation: Exocytosis and Endocytosis IntroductionRight-click slide / select Play

In phagocytosis a cell engulfs a particle in a vacuoleThe vacuole fuses with a lysosome to digest the particle

Animation: PhagocytosisRight-click slide / select Play

In pinocytosis, molecules are taken up when extracellular fluid is gulped into tiny vesicles

Animation: PinocytosisRight-click slide / select Play

In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formationA ligand is any molecule that binds specifically to a receptor site of another molecule

Animation: Receptor-Mediated EndocytosisRight-click slide / select Play

***************Figure 7.14 Osmosis.*****Figure 7.16 The contractile vacuole of Paramecium caudatum.******For the Cell Biology Video Water Movement through an Aquaporin, go to Animation and Video Files.

*For the Cell Biology Video Na+/K+ATPase Cycle, go to Animation and Video Files.*****Figure 7.20 A proton pump.**Figure 7.21 Cotransport: active transport driven by a concentration gradient.******For the Cell Biology Video Phagocytosis in Action, go to Animation and Video Files.

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