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