chapter 7 membrane structure and function. fibers of extracellular matrix (ecm) glycoprotein...
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Chapter 7Chapter 7Membrane Structure and Function
Fibers ofextracellularmatrix (ECM)
Glycoprotein
Carbohydrate
Microfilamentsof cytoskeleton
Cholesterol
Integralprotein
Peripheralproteins
CYTOPLASMIC SIDEOF MEMBRANE
EXTRACELLULARSIDE OFMEMBRANE
Glycolipid
Overview: Life at the EdgeOverview: Life at the Edge
• The plasma membrane is the boundary that separates the living cell from its nonliving surroundings.
Fibers ofextracellularmatrix (ECM)
Glycoprotein
Carbohydrate
Microfilamentsof cytoskeleton
Cholesterol
Integralprotein
Peripheralproteins
CYTOPLASMIC SIDEOF MEMBRANE
EXTRACELLULARSIDE OFMEMBRANE
Glycolipid
Hydrophilichead
Hydrophobictail
WATER
WATER
Phospholipids, the most abundant lipid in membranes, are “amphipathic” molecules, containing hydrophobic and hydrophilic regions.
Hydrophilic regionof protein
Hydrophobic region of protein
Phospholipidbilayer
The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it.
The Fluidity of MembranesThe Fluidity of Membranes
Phospholipids in the plasma membrane can move within the bilayer.
Most of the lipids, and some proteins, drift laterally.
Rarely does a molecule flip-flop transversely across the membrane.
Lateral movement(~107 times per second)
Flip-flop(~ once per month)
Membranes must be fluid to work properly.Membranes rich in unsaturated fatty acids
are more fluid that those rich in saturated fatty acids.
The Fluidity of Membranes The Fluidity of Membranes (continued)(continued)
ViscousFluid
Unsaturated hydrocarbontails with kinks
Saturated hydrocarbon tailswithout kinks
As temperatures cool, membranes switch from a fluid state to a solid state.
The temperature at which a membrane solidifies depends on the types of lipids.
The Fluidity of Membranes The Fluidity of Membranes (continued)(continued)
Liquid Crystal Crystal
NormalTemperature
LowTemperature
At lower temperatures, membranes undergo a transition to a crystalline state in which fatty acid tails are fully extended, packing is highly ordered, and van der Waals interactions between adjacent chains are maximal.
The steroid cholesterol has different effects on membrane fluidity at different temperatures.
At warm temperatures, cholesterol restrains movement of phospholipids.
At cool temperatures, cholesterol maintains fluidity by preventing tight packing.
The Fluidity of Membranes The Fluidity of Membranes (continued)(continued)
A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer.
Proteins in the membrane can drift within the bilayer.Proteins are larger than lipids and move more slowly.
The Fluidity of Membranes The Fluidity of Membranes (continued)(continued)
To investigate whether membrane proteins move, researchers fused a mouse cell and a human cell.
Membrane proteins
Mixed proteinsafter 1 hour
Hybrid cellHuman cell
Mouse cell
EXTRACELLULARSIDEN-terminus
C-terminusCYTOPLASMICSIDE
Helix
The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha
helices.
Membrane Proteins and Their Membrane Proteins and Their FunctionsFunctions
1. Transport2. Enzymatic activity3. Signal transduction4. Cell-cell recognition5. Intercellular joining6. Attachment to the cytoskeleton and
extracellular matrix
Six major functions of membrane proteins:
Glyco-protein
Cell-cell recognition Intercellular joining Attachment to thecytoskeleton and extra-cellular matrix (ECM)
EnzymesSignal
ReceptorATP
Transport Enzymatic activity Signal transduction
The Role of Membrane The Role of Membrane CarbohydratesCarbohydrates
• Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane.
Membrane carbohydrates may be bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins).
Plasma Membranes Are Selective Plasma Membranes Are Selective PermeablePermeable
A cell must exchange materials with its surroundings, a process controlled by the selectively permeable plasma membrane.
Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane easily.
Polar molecules and large macromolecules do not cross the membrane easily.
Channel proteins called “aquaporins” facilitate the passage of water through the membrane.
The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others.
Transport ProteinsTransport Proteins
Transport proteins allow passage of specific substances across the membrane.
Channel proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel.
• Carrier proteins bind to molecules and change shape to shuttle them across the membrane.
Channel Protein Carrier Protein
Passive Transport Requires No Passive Transport Requires No EnergyEnergy
Diffusion is the tendency for molecules to spread out evenly into the available space.
Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction.
At dynamic equilibrium, as many molecules cross one way as cross in the other direction.
Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another.
No work must be done to move substances down the concentration gradient.
Passive Transport Uses No EnergyPassive Transport Uses No Energy
Net diffusion Net diffusion Equilibrium
Net diffusion Net diffusion Equilibrium
Effects of Osmosis on Water Effects of Osmosis on Water BalanceBalance
Osmosis is the diffusion of water across a selectively permeable membrane.
The direction of osmosis is determined only by a difference in total solute concentration.
Lowerconcentrationof solute (sugar)
Higherconcentrationof sugar
Same concentrationof sugar
Selectivelypermeable mem-brane: sugar mole-cules cannot passthrough pores, butwater molecules can
H2O
Osmosis
Water Balance of Cells Water Balance of Cells WithoutWithout WallsWalls
Tonicity: the ability of a solution to cause a cell to gain or lose water
Isotonic solution: solute concentration is the same as that inside the cell; no net water movement across the plasma membrane
Hypertonic solution: solute concentration is greater than that inside the cell; cell loses water
Hypotonic solution: solute concentration is less than that inside the cell; cell gains water
Filling vacuole50 µm
50 µmContracting vacuole
The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts
as a pump.
Water Balance of Cells Water Balance of Cells With With WallsWalls
Cell walls help maintain water balance.A 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.
Animalcell
Lysed
H2O H2O H2O
Normal
Hypotonic solution Isotonic solution Hypertonic solution
H2O
Shriveled
H2OH2OH2OH2OPlantcell
Turgid (normal) Flaccid Plasmolyzed
Facilitated Diffusion Is Aided by Facilitated Diffusion Is Aided by ProteinsProteins
In facilitated diffusion, transport proteins speed movement of molecules across the plasma membrane.
Channel Proteins Carrier Proteins
Active Transport Uses EnergyActive Transport Uses Energy
Active transport uses energy, usually in the form of ATP, to move substances against their concentration gradient.
Diffusion Facilitated diffusion
Passive transport
ATP
Active transport
Cotransport: Coupled Transport by a Cotransport: Coupled Transport by a Membrane ProteinMembrane Protein
Cotransport occurs when active transport of a solute indirectly drives transport of another solute.
H+
ATP
Proton pump
Sucrose-H+
cotransporter
Diffusionof H+
Sucrose
H+
H+
H+
H+
H+H+
+
+
+
+
+
+
–
–
–
–
–
–
Bulk Transport: Exocytosis and Bulk Transport: Exocytosis and EndocytosisEndocytosis
Large macromolecules cross the membrane via vesicles.
In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents.
In endocytosis, the cells take in molecules by forming vesicles from the plasma membrane.
Exocytosis Endocytosis