Membrane Structure and Function

Membranes Are Boundaries

• The plasma membrane is the boundary that separates the living cell from its surroundings

• The other membranes bound and define other compartments and organelles.

• Membranes exhibit selective permeability. They allow some substances to cross it more easily than others.

Concept 7.1: Cellular membranes are consist of lipids and proteins in a fluid mosaic arrangement.• Phospholipids are the primary lipids in most

membranes.

• amphipathic molecules

• The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

• Proposed by Singer and Nicholson in 1972.

• Proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water

Fluid Mosaic Model-Lipids

Hydrophilichead

WATER

Hydrophobictail

WATER

Phospholipids arranged in a bilayer = a unit membrane

Disambiguation

• Two layers of phospholipids = 1 lipid bilayer

• 1 bilayer = unit membrane or single membrane

• 2 bilayers (nucleus, ct, mt) = double membrane

• Hydrophobic parts hate water so they are on the inside-away from water

• Hydrophilic parts love water so they are on the outside-close to water

How do the proteins fit into the lipid bilayer?

They are not stuck to the opposite sides like bread on a sandwich. (at least not most of them)

EM studies reveal that many proteins are imbedded in the membrane (like chunks of peanuts in hot peanut brittle).

Biochemical studies revealed that some proteins were loosely attached and some tightly attached.

The Fluid Mosaic Model of Singer and Nicholson was developed to explain these results

Phospholipidbilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

Membrane proteins are integral (I) or peripheral (P)

I

P

Fluid Mosaic Model-Experimental Evidence

RESULTS

Membrane proteins

Mouse cellHuman cell

Hybrid cell

Mixed proteinsafter 1 hour

Prediction of the model: some proteins can move around in the membrane: “fluid” mosaic

Lateral movement is frequent but flip-flop is rare

Lateral movement(107 times per second)

Flip-flop( once per month)

-

Types of lipid movement in the fluid mosaicMany proteins can move or drift laterally as

well but they do not flip-flop

• As temperatures cool, membranes become less fluid as lipids solidify or “freeze”

• The temperature at which a membrane solidifies depends on the types of lipids

• Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids. Membrane composition influences membrane fluidity

• Membranes must be fluid to work properly; they have to be about salad oil consistency

Membrane fluidity and fatty acid saturation

Fluid

Unsaturated hydrocarbontails with kinks

Viscous

Saturated hydro-carbon tails

Cholesterol

Cholesterol within the animal cell membrane

The steroid cholesterol also helps to moderate changes in fluidity as temperatures fluctuate

Membrane Proteins and Their Functions

• A more complete overview of membrane structure

• A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

• Proteins determine most of the membrane’s specific functions

Fig. 7-7

Fibers ofextracellularmatrix (ECM)

Glyco-protein

Microfilamentsof cytoskeleton

Cholesterol

Peripheralproteins

Integralprotein

CYTOPLASMIC SIDEOF MEMBRANE

GlycolipidEXTRACELLULARSIDE OFMEMBRANE

Carbohydrate

• Peripheral proteins are bound to the surface of the membrane

• Integral proteins penetrate the hydrophobic core

• Integral proteins that span the membrane are called transmembrane proteins

The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids

N-terminus

C-terminus

HelixCYTOPLASMICSIDE

EXTRACELLULARSIDE

The hydrophobic parts are often in alpha helix formAs in this seven-alpha-helix protein

• What do these membrane proteins do?

• We can list six major functions of membrane proteins:– Transport

– Enzymatic activity

– Signal transduction

– Cell-cell recognition

– Intercellular joining

– Attachment to the cytoskeleton and extracellular matrix (ECM)

Some functions of membrane proteins

(a) Transport (b) Enzymatic activity (c) Signal transduction

ATP

Enzymes

Signal transduction

Signaling molecule

Receptor

Three more functions of membrane proteins

(d) Cell-cell recognition

Glyco-protein

(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

Importance of membrane proteins

•25-35% of genome devoted to membrane proteins

•But membrane proteins are hard to study

• In one early study of human membrane proteins:

1352 receptor proteins 817 transporters 533 enzymes 697 other types3309 unidentified

The Role of Membrane Carbohydrates in Cell-Cell Recognition (non-protein, non-lipid components)

• Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane

• Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)

• Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual

Concept 7.2: Membrane structure results in selective permeability

• A cell must exchange materials with its surroundings, a process that controlled by the plasma membrane

• Plasma membranes are selectively permeable, regulating the cell’s molecular traffic. They let some things through and they hold other things back.

The Permeability of the Lipid Bilayer

• Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly

• Polar molecules, such as sugars or amino acids, do not cross the membrane easily

Cartoon version: small non-polar molecules can move through a membrane-water is an exception

Transport Proteins

• Transport proteins allow passage of hydrophilic substances across the membrane

• Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel

• Special type of channel proteins called aquaporins facilitate the passage of water: water moves freely across the membrane (aka water channels).

• Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane

• Some transport proteins open and shut rapidly in response to signals:these are called gates

• A transport protein is specific for the substance it moves

Review: three classes transport proteins

• Channel proteins

• Carrier proteins

• Gates

Preview for 7.3-7.5: How do substances cross membranes?

What are the mechanisms-given the general involvement of proteins and the differing nature of substances involved?

• Diffusion

• Facilitated Diffusion

• Active Transport

• Endo- and Exocytosis

Concept 7.3: Passive transport is movement of a substance across a membrane with no energy investment

• 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-from high to low concentration

• At dynamic equilibrium, as many molecules cross one way as cross in the other direction

Molecules of dye

Passive Transport involves diffusion across a membrane

Moves solute from high to low concentration

Moves from high to low concentrationMembrane (cross section)

WATER

Net diffusion Net diffusion

(a) Diffusion of one solute

Equilibrium

Net diffusion stops at equilibrium

• 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

• The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen (only requires a concentration gradient)

• Substances cross membrane

• Move down concentration gradient: no energy input

• Rely on transport proteins

Facilitated diffusion

Concept 7.4: Active transport uses energy to move solutes against their gradients

• Facilitated diffusion is still “passive” because the solute moves down its concentration gradient

• Some transport proteins, however, can move solutes against their concentration gradients

The Need for Energy in Active Transport

• Active transport moves substances against their concentration gradient

• Active transport requires energy, usually in the form of ATP-the cell’s energy currency

• Active transport is performed by specific proteins embedded in the membranes

Active transport allows cells to maintain concentration gradients that differ from their surroundings

•The sodium-potassium pump is one type of active transport system-probably the best understood

EXTRACELLULAR

FLUID [Na+] high [K+] low

Na+

Na+

Na+ [Na+] low[K+] high CYTOPLASM

Cytoplasmic Na+ binds tothe sodium-potassium pump. 1

Na+ binding stimulatesphosphorylation by ATP.

Na+

Na+

Na+

ATP P

ADP

2

Phosphorylation causesthe protein to change itsshape. Na+ is expelled tothe outside.

Na+

P

Na+ Na+

3

K+ binds on theextracellular side andtriggers release of thephosphate group.

P P

K+

K+

4

Loss of the phosphaterestores the protein’s originalshape.

K+

K+

5

K+ is released, and thecycle repeats.

K+

K+

6

Summary and Comparison-Diffusion,Passive Transport, Active Transport

Passive transport

Diffusion Facilitated diffusion

Active transport

ATP

EXTRACELLULARFLUID

H+

H+

H+

H+

Proton pump

+

+

+

H+

H+

+

+

H+

–

–

–

–

ATP

CYTOPLASM

–

The proton pump illustrates how electrogenic pumps work

Active transport can establish electric potential

Cotransport or Coupled Transport

• Cotransport occurs when active transport of a solute indirectly drives transport of another solute

• Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell

• Symport system-both substances move in the same direction relative to the membrane

• In an antiport system, substances move in opposite directions

Proton pump

–

–

–

–

–

–

+

+

+

+

+

+

ATP

H+

H+

H+

H+

H+

H+

H+

H+

Diffusionof H+

Sucrose-H+

cotransporter

Sucrose

Sucrose

Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

• Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins

• Large molecules, such as polysaccharides and proteins, cross the membrane by bulk transport using vesicles

• Bulk transport requires energy

Exocytosis

• In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents

• Many secretory cells use exocytosis to export their products

• Acinar cells of the pancreas are examples-they secrete digestive enzymes into the intestine

Endocytosis

• In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane

• Endocytosis is a reversal of exocytosis, involving different proteins

• There are three types of endocytosis:

– Phagocytosis (“cellular eating”)

– Pinocytosis (“cellular drinking”)

– Receptor-mediated endocytosis or RME

CYTOPLASM EXTRACELLULARFLUID

Pseudopodium

“Food” orother particle

Foodvacuole Food vacuole

Bacterium

An amoeba engulfing a bacteriumvia phagocytosis (TEM)

Pseudopodiumof amoeba

1 µm

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

Often used as a defense

Plasmamembrane

Vesicle

0.5 µm

Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM)

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

Non-specific-all dissolved substances enter

• In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation

• A ligand is any molecule that binds specifically to a receptor site of another molecule

• Some viruses enter in this way

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor Coat protein

Coatedpit

Ligand

Coatprotein

Plasmamembrane

0.25 µm

Coatedvesicle

A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs)