3The Nature of the Plasma Membrane The plasma membrane is a thin, fluid entity that manages to be very flexible and yet is stable enough to stay together despite being continually remade due to the constant movement of materials in and out of it.
4The Plasma MembraneIn animal cells, the plasma membrane has four principal components:1. A phospholipid bilayer.2. Molecules of cholesterol interspersed within the bilayer.
5The Plasma Membrane3. Proteins that are embedded in or that lie on the bilayer.4. Short carbohydrate chains on the cell surface, collectively called the glycocalyx, that function in cell adhesion and as binding sites on proteins.
6The Plasma Membrane glycocalyx phospholipids cholesterol proteins cell exteriorcytoskeletonintegralproteincell interiorperipheralproteinPhospholipidbilayer: a doublelayer ofphospholipidmolecules whosehydrophilic “heads”face outward, andwhose hydrophobic“tails” point inward,toward each other.Cholesterolmolecules that actas a patchingsubstance andthat help the cellmaintain anoptimal level offluidity.Proteins, whichare integral,meaning bound tothe hydrophobicinterior of themembrane, orperipheral,meaning notbound in this way.Glycocalyx: sugarchains that attachto proteins andphospholipids,serving as proteinbinding sites andas cell lubricationand adhesionmolecules.Figure 5.1
7The Phospholipid Bilayer Phospholipids are molecules composed of two fatty acid chains linked to a charged phosphate group.
8The Phospholipid Bilayer The fatty acid chains are hydrophobic, meaning they avoid water, while the phosphate group is hydrophilic, meaning it readily bonds with water.
9The Phospholipid Bilayer (a) Phospholipid molecule(b) Phospholipid bilayerwateryextracellularfluidpolarheadPP-hydrophilichydrophobicnonpolartailshydrophilicwaterycytosolHydrophobic moleculespass through freely.Hydrophilic moleculesdo not passthrough freely.Figure 5.2
10The Phospholipid Bilayer Such phospholipids arrange themselves into bilayers—two layers of phospholipids in which the fatty acid “tails” of each layer point inward (avoiding water), while the phosphate “heads” point outward (bonding with it).
11The Phospholipid Bilayer Phospholipids take on this configuration in the plasma membrane because a watery environment lies on either side of the membrane.
12The Phospholipid Bilayer In animal cells, the cholesterol molecules that are interspersed between phospholipid molecules in the plasma membrane perform two functions:They act as a patching material that helps keep some small molecules from moving through the membrane.They keep the membrane at an optimal level of fluidity.
13The Phospholipid Bilayer Some plasma membrane proteins are integral, meaning they are bound to the hydrophobic interior of the phospholipid bilayer.Others are peripheral, meaning they lie on either side of the membrane but are not bound to its hydrophobic interior.
14Membrane Protein Functions In animal cells, the cholesterol molecules that are interspersed between phospholipid molecules in the plasma membrane perform two functions:structural supportcell identification, by serving as external recognition proteins that interact with immune system cells
15Membrane Protein Functions communication, by serving as external receptors for signaling moleculestransport, by providing channels for the movement of compounds into and out of the cell
16The Plasma Membrane (a) Structural support (b) Recognition (c) Communication(d) Transportcell exteriorcell interiorMembrane proteinscan provide structuralsupport, often whenattached to parts ofthe cell’s scaffoldingor “cytoskeleton.”Protein fragmentsheld withinrecognition proteinscan serve to identifythe cell as “normal” or“infected” to immunesystem cells.Receptor proteins,protruding out fromthe plasma membrane,can be the point ofcontact for signalssent to the cell viatraveling molecules,such as hormones.Proteins can serveas channelsthrough whichmaterials can passin and out ofthe cell.Figure 5.3
17The Plasma MembraneThe plasma membrane today is described by a conceptualization called the fluid-mosaic model.It views the membrane as a fluid, phospholipid bilayer that has a mosaic of proteins either fixed within it or capable of moving laterally across it.
19Diffusion, Gradients, and Osmosis Diffusion is the movement of molecules or ions from a region of their higher concentration to a region of their lower concentration.
20Diffusion, Gradients, and Osmosis A concentration gradient defines the difference between the highest and lowest concentrations of a solute within a given medium.Through diffusion, compounds naturally move from higher to lower concentrations, meaning down their concentration gradients.
21Diffusion, Gradients, and Osmosis (a) Dye is dropped in.(b) Diffusion begins.(c) Dye is evenly distributed.watermoleculesdyemoleculesFigure 5.4
22Diffusion, Gradients, and Osmosis Energy must be expended to move compounds against their concentration gradients, meaning from a lower to a higher concentration.
23Diffusion, Gradients, and Osmosis A semipermeable membrane is one that allows some compounds to pass through freely while blocking the passage of others.
24Diffusion, Gradients, and Osmosis Osmosis is the net movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration.
25Diffusion, Gradients, and Osmosis Because the plasma membrane is a semipermeable membrane, osmosis operates in connection with it.Osmosis is a major force in living things; it is responsible for much of the movement of fluids into and out of cells.
26saltsolute(a) An aqueous solutiondivided by a semipermeablemembrane has a solute—in this case, salt—poured into its right chamber.solventsemipermeable membrane(b) As a result, thoughwater continues to flow inboth directions through themembrane, there is a netmovement of water towardthe side with the greaterconcentration of solutes in it.osmosis(c) Why does this occur?Water molecules that arebonded to the sodium (Na+)and chloride (Cl–) ions thatmake up salt are not free topass through the membraneto the left chamber of thecontainer.pure waterwater bound tosalt ionsFigure 5.5
27Osmotic ImbalancesOsmotic imbalances can cause cells either to dry out from losing too much water or, in the case of animal cells, to break from taking too much water in.Plant cells generally do not have this problem because their cell walls limit their uptake of water.
28Solute ConcentrationCells will gain or lose water relative to their surroundings in accordance with what the solute concentration is inside the cell as opposed to outside it.
29Solute ConcentrationA cell will lose water to a surrounding solution that is hypertonic—a solution that has a greater concentration of solutes in it than does the cell’s cytoplasm.A cell will gain water when the surrounding solution is hypotonic to the cytoplasmic fluid.
30(a) Hypertonic surroundings (b) Isotonic surroundings(c) Hypotonic surroundingsH2OAnimal cell:plasma membraneH2OH2OPlant cell:H2Oplasma membranecell wallH2OH2OwiltedturgidNet movement ofwater out of cellBalanced watermovementNet movement ofwater into cellFigure 5.6
31Solute ConcentrationWater flow is balanced between the cell and its surroundings when the surrounding fluid and the cytoplasmic fluid are isotonic to each other—when they have the same concentration of solutes.
32Plasma Membranes and Diffusion Animation 5.1: Plasma Membranes and Diffusion
34Moving Smaller Substances In and Out Some compounds are able to cross the plasma membrane strictly through diffusion; others require diffusion and special protein channels; still others require protein channels and the expenditure of cellular energy.
35ATP Passive transport Active transport simple diffusion facilitated diffusionATPMaterials move downtheir concentrationgradient through thephospholipid bilayer.The passage of materialsis aided both by aconcentration gradientand by a transportprotein.Molecules again movethrough a transportprotein, but now energymust be expended tomove them against theirconcentration gradient.Figure 5.7
36Transport Through the Plasma Membrane Active transport is any movement of molecules or ions across a cell membrane that requires the expenditure of energy.Passive transport is any movement of molecules or ions across a cell membrane that does not require the expenditure of energy.
37Types of Passive Transport There are two forms of passive transport: simple diffusion and facilitated diffusion.For either form of transport to bring about a net movement of materials into or out of a cell, a concentration gradient must exist.
38Types of Passive Transport A concentration gradient is all that is required for simple diffusion to operate.Facilitated diffusion, however, requires both a concentration gradient and a protein channel.
39Facilitated Diffusion In facilitated diffusion, transport proteins function as channels for larger hydrophilic substances—substances that, because of their size and electrical charge, cannot diffuse through the hydrophobic portion of the plasma membrane.
40Facilitated Diffusion glucosecell exteriorplasmamembranecell interior1. The transportprotein has abinding site forglucose thatis open to theoutside of the cell.2. Glucose bindsto the bindingsite.3. This bindingcauses the proteinto change shape,exposing glucoseto the inside ofthe cell.4. Glucose passesinto the cell andthe proteinreturns to itsoriginal shape.Figure 5.8
41Active TransportCells cannot rely solely on passive transport to move substances across the plasma membrane.A cell may need to maintain a greater concentration of a given substance on one side of its membrane.Yet, passive transport equalizes concentrations of substances on both sides of the plasma membrane.
42Active Transport To deal with such needs, cells use active transport. Chemical pumps move compounds across the plasma membrane against their concentration gradients.
43Active TransportOne example of such transport is the pumping of glucose into cells that line the small intestines.
45Moving Larger Substances In and Out Larger materials are brought into the cell through endocytosis and moved out through exocytosis.
46Exocytosis and Endocytosis Both mechanisms employ vesicles, the membrane-lined enclosures that alternately bud off from membranes or fuse with them.
47ExocytosisIn exocytosis, a transport vesicle moves from the interior of the cell to the plasma membrane and fuses with it, at which point the contents of the vesicle are released to the environment outside the cell.
49EndocytosisThere are two principal forms of endocytosis: pinocytosis and phagocytosis.
50EndocytosisPinocytosis is the movement of moderate-sized molecules into a cell by means of the creation of transport vesicles produced through an infolding or “invagination” of a portion of the plasma membrane.
51EndocytosisPhagocytosis is when certain cells use pseudopodia or “false feet” to surround and engulf whole cells, fragments of them, or other large organic materials.
52Formation of a pinocytosis vesicle. 12capturedmoleculesreceptorscoatedpitvesicle34In this form of pinocytosis, called clathrin-mediated endocytosis, cell-surfacereceptors bind to individual molecules of the substance to be taken into the cell andthen move laterally across the plasma membrane to a pit, coated on its undersidewith the protein clathrin, that will become a vesicle that moves into the cell.Formation of a pinocytosis vesicle.(b) Phagocytosisbacterium(or food particles)pseudopodiumvesicleIn phagocytosis, food particles—or perhaps whole organisms—are taken in by meansof “false feet” or pseudopodia that surround the material. Pseudopodia then fusetogether, forming a vesicle that moves into the cell’s interior with its catch enclosed.A human immune system cell called amacrophage (colored blue) usesphagocytosis to ingest an invading yeastcell.Figure 5.11
53EndocytosisIn pinocytosis, materials are brought into the cell inside vesicles that bud off from the plasma membrane.