Membrane Structure & Function

Plasma Membrane

Structure Boundary that

separates the living cell from its non-living environment

Selective permeability

Amphipathic~ hydrophobic & hydrophilic regions

Singer-Nicolson: fluid mosaic model

Plasma Membrane

Structure Phospholipid Yellow-

hydrophilic Blue-

hydrophobic

Singer and NicolsonSinger and Nicolson

In 19721972, Singer and Nicolson, Proposed that membrane proteinsmembrane proteins are dispersed and individually inserted into the phospholipid inserted into the phospholipid bilayer bilayer of the plasma membraneof the plasma membrane

Phospholipidbilayer

Hydrophilic region of protein

Hydrophobic region of protein

Fluid Mosaic ModelFluid Mosaic Model

A membrane is a fluid structurefluid structure with a “mosaic” of various proteins embedded in it when viewed from the top

PhospholipidsPhospholipids can move laterallylaterally a small amount and can “flex” their tails

Membrane proteinsMembrane proteins also move side to side or lateralllaterally making the membrane fluid

Freeze-fractureFreeze-fracture studies of the plasma membrane support the fluid mosaic model of membrane structure

A cell is frozen and fractured with a knife. The fracture plane often follows fracture plane often follows the hydrophobic interior of a membranethe hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layerstwo separated layers. The membrane proteins go wholly with one of the membrane proteins go wholly with one of the layerslayers.

The Fluidity of Membranes

Phospholipids in the plasma membrane can move within the bilayer two ways

Lateral movement(~107 times per second) Flip-flop

(~ once per month)

The type of hydrocarbon tailstype of hydrocarbon tails in phospholipids affects the fluidity of the plasma membrane

Fluid Viscous

Unsaturated hydrocarbontails with kinks

Saturated hydro-Carbon tails

The Fluidity of Membranes

The Fluidity of Membranes

The steroid cholesterolsteroid cholesterol Has different effects on membrane fluidity at different temperatures

Cholesterol

Membrane Proteins and Their Functions

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

Fibers of

extracellularmatrix (ECM)

Types of Membrane ProteinsIntegral proteinsIntegral proteins

Penetrate the hydrophobic core of the lipid bilayerAre often transmembrane proteinstransmembrane proteins, completely

spanning the membrane

EXTRACELLULARSIDE

Types of Membrane Proteins

Peripheral proteinsPeripheral proteinsAre appendages loosely bound to the

surface of the membrane

Membrane Structure Summary

Phospholipids~ membrane fluidity

Cholesterol~ membrane stabilization

“Mosaic” Structure~ Integral proteins~

transmembrane proteins Peripheral proteins~ surface

of membrane Membrane carbohydrates ~

cell to cell recognition; oligosaccharides (cell markers); glycolipids; glycoproteins

Six Major Functions of Membrane Proteins

Figure 7.9

Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane.

Enzymatic activity. A protein built into the membranemay be an enzyme with its active site exposed tosubstances in the adjacent solution. In some cases,several enzymes in a membrane are organized asa team that carries out sequential steps of ametabolic pathway.

Signal transduction. A membrane protein may havea binding site with a specific shape that fits the shapeof a chemical messenger, such as a hormone. Theexternal messenger (signal) may cause aconformational change in the protein (receptor) thatrelays the message to the inside of the cell.

(a)

(b)

(c)

ATP

Enzymes

Signal

Receptor

Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells.

Intercellular joining. Membrane proteins of adjacent cellsmay hook together in various kinds of junctions, such asgap junctions or tight junctions

Attachment to the cytoskeleton and extracellular matrix(ECM). Microfilaments or other elements of thecytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes

(d)

(e)

(f)

Glyco-protein

Six Major Functions of Membrane Proteins

The Role of Membrane Carbohydrates in Cell-Cell Recognition

•Cell-cell recognitionCell-cell recognition• IIs a cell’s ability to distinguish one type of neighboring cell

from another

•Membrane carbohydratesMembrane carbohydrates• Interact with the surface molecules of other cells,

facilitating cell-cell recognition

•Immunity• Recognition is a key to the organism’s immune system’s

ability to distinguish between self and invasion of foreign bodies

Synthesis and Sidedness of Membranes

•Membranes have distinct inside and distinct inside and outside facesoutside faces

•This affects the affects the movementmovement of proteins proteins synthesizedsynthesized in the endomembrane endomembrane system system (Golgi and (Golgi and ER)ER)

Passive Transport

• cell uses no energy• molecules move randomly• concentration is from high to low • three types

•SIMPLE DIFFUSION•FACILITATED DIFFUSION•OSMOSIS

http://programs.northlandcollege.edu/biology/Biology1111/animations/transport1.html

high

low

Weeee!!!

Simple DiffusionDiffusionDiffusion

Is the tendency for molecules of any substance to spread out evenlyspread out evenly into the available spaceMove from high to low concentrationhigh to low concentrationDownDown the concentration gradient

Effects of Osmosis on Water Balance

OsmosisOsmosisIs the movement of water across a semi-permeable membrane

It is affectedaffected by theby the concentration concentration gradient gradient ofof dissolved dissolved substances substances called thecalled the solution’ssolution’s tonicity tonicity

Water Balance of Cells Without Walls

Tonicity – Three StatesIs the ability of a solution to cause a

cell to gain or lose waterHas a great impact on cells without impact on cells without

wallswalls

Isotonic Solutions

If a solution is isotonicisotonicThe concentration of solutesconcentration of solutes is the samesame as

it is inside the cellThere will be NO NETNO NET movement of WATER

Hypertonic Solution

If a solution is hypertonichypertonicThe concentration of solutesconcentration of solutes is greatergreater than it

is inside the cellThe cell will lose water lose water (PLASMOLYSIS)(PLASMOLYSIS)

Hypotonic Solutions

If a solution is hypotonichypotonicThe concentration of solutesconcentration of solutes is lesless than it is

inside the cellThe cell will gain watergain water

Water Balance in Cells Without Walls

Animal cell. An animal cell fares best in an isotonic isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water.

Water Balance of Cells with Walls

Cell WallsCell WallsHelp maintain water balance

Turgor pressureTurgor pressureIs the pressure of water inside a plant cell pushing

outward against the cell membrane

If a plant cell is turgidturgidIt is in a hypotonichypotonic environmentIt is very firm, firm, a healthy state in most plantsa healthy state in most plants

If a plant cell is flaccidflaccidIt is in an isotonic or hypertonicisotonic or hypertonic environment

Water Balance in Cells with Walls

Plant cell. Plant cells are turgid (firmturgid (firm) and generally healthiest in a hypotonic environmenthypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell.

How Organisms Deal with Osmotic Pressure

Hypertonic Bacteria and plants have cell walls that prevent them

from over-expanding. In plants the pressure exerted on the cell wall is called tugor pressure.

Protists like paramecium has contractile vacuoles that collect water flowing in and pump it out to prevent them from over-expanding.

Salt water fish pump salt out of their specialized gills so they do not dehydrate.

Animal cells are bathed in blood. Kidneys keep the blood isotonic by remove excess salt and water. Paramecium: protist removing excess water video

How Will Water Move Across Semi-Permeable Membrane?

Solution A has 100 molecules of glucose per ml Solution B has 100 molecules of fructose per ml How will the water molecules move?How will the water molecules move?

There will be no net movement of waterno net movement of water since the concentration of solute in each solution is equal

How Will Water Move Across Semi-Permeable Membrane?

Solution A has 100 molecules of glucose per ml Solution B has 75 molecules of fructose per ml How will the water molecules move?How will the water molecules move?

There will be a net movement of water from Solution B to Solution A until both solutions have equal concentrations of solute

How Will Water Move Across Semi-Permeable Membrane?

Solution A has 100 molecules of glucose per ml Solution B has 100 molecules of NaCl per ml

How will the water molecules move?How will the water molecules move?

Each molecule of NaCl will dissociate to form a Na+ ion and a Cl- ion, making the final concentration of solutes 200 molecules per mil. Therefore, there will be a net movement of water from Solution A to Solution B until both solutions have equal concentrations of solute

32

Facilitated Diffusion & Proteins• Facilitated diffusion Facilitated diffusion is a type of PassivePassive Transport Aided by Proteins that

allow a specific substance to cross the membrane

Channel ProteinsChannel Proteins In some cases they act in

passive transport. Molecules diffuse randomly

through the opening, requiring no energy, moving from an area of high concentration to an area of low concentration.

Examples ion channel Glucose Amino acids

33

Facilitated Diffusion & ProteinsCarrier proteinsCarrier proteins

Undergo a subtle change in shape that translocates the solute-binding site across the membrane

A carrier proteincarrier protein alternates between two conformationsalternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either directioncan transport the solute in either direction, with the net movement being down the concentration gradientdown the concentration gradient of the solute.

Filtration

smaller molecules are forced through porous membranes

hydrostatic pressure important in the body

molecules leaving blood capillaries

Active Transport

high

low

This is gonna be

hard

work!!

•Cell uses energy

•Actively move molecules to where they are needed

•Concentration is from low to high

•Three typesPROTEIN PUMPSPROTEIN PUMPSEXOCYTOSISEXOCYTOSISENDOCYTOSISENDOCYTOSIS

Protein Pumps Some transport proteinstransport proteins

actively use energy from ATP in the cell to drag molecules from area of low concentration to areas of high concentration (working directly against diffusion)

An example of this is the sodium/potassium pump.sodium/potassium pump. Here the energy of a phosphate (shown in red) is used to exchange sodium atoms for potassium atoms.

Sodium Potassium Pumps

Protein changes shape to move

molecules: this requires energy!

The sodium-potassium pump sodium-potassium pump is one type of active transport system

Active Transport

PP i

EXTRACELLULARFLUID

Na+ binding stimulatesphosphorylation by ATP.

Na+

Cytoplasmic Na+ binds tothe sodium-potassium pump.

1

K+ is released and Na+

sites are receptive again; the cycle repeats.

Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside.

Extracellular K+ binds to the protein, triggering release of the Phosphate group.

6

Loss of the phosphaterestores the protein’s original conformation.

CYTOPLASM

[Na+] low[K+] high

Na+

Na+

Na+

Na+

Na+

P ATP

Na+

Na+

Na+

P

ADP

K+

K+

K+

K+ K+

K+

[Na+] high[K+] low

Comparison of Passive & Active TransportPassive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane.

Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.

Diffusion. Hydrophobicmolecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer.

Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins,either channel or carrier proteins.

ATP

Maintenance of Membrane Potential by Ion Pumps

Membrane potentialMembrane potentialIs the voltage difference across a membrane

An electrochemical gradientelectrochemical gradientIs caused by the concentration electrical gradient of ions across a

membrane

An electrogenic pumpelectrogenic pumpIs a transport protein that generates the voltage across a membrane

Permeability of the Lipid Bilayer

•Hydrophobic moleculesHydrophobic molecules

•Are lipid solublelipid soluble and can pass through the membrane rapidlyrapidly

• Polar moleculesPolar molecules

•Do NOT cross the membrane rapidlyrapidly

• Transport proteinsTransport proteins

•Allow passage of hydrophilic hydrophilic substancessubstances across the membrane

Co-transportCo-transportCo-transport

Occurs when active transportactive transport of a specific solute indirectly drives the active transport of another solute

Involves transport by a membrane protein membrane protein

Driven by a concentration gradientconcentration gradient

Example of Co-transport

Co-transport: active transport driven by a driven by a concentration gradientconcentration gradient

Bulk Transport

In ExocytosisExocytosisTransport vesiclesTransport vesicles migrate to the plasma

membrane, fuse with it, and release their contents

In EndocytosisEndocytosisThe cell takes in macromolecules by forming forming

new vesicles from the plasma membranenew vesicles from the plasma membrane

Taking large molecules into a cell Uses energy Cell membrane in-folds pinocytosis – substance

is mostly water phagocytosis –

substance is a solid receptor-mediated

endocytosis – requires the substance to bind to a membrane-bound receptor

Endocytosis

EndocytosisIn phagocytosisphagocytosis, a cellengulfs a particle by engulfs a particle by Wrapping pseudopodiaWrapping pseudopodia around it and packaging it within a membrane-enclosed sac large enough to be classified as a vacuole vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes.

PHAGOCYTOSIS

In pinocytosispinocytosis, the cell ““gulps” droplets of gulps” droplets of extracellular fluidextracellular fluid into tinyvesicles. It is not the fluiditself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosisis nonspecific in the substances it transports.

Endocytosis

460.25 µm

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor

Ligand

Coat protein

Coatedpit

Coatedvesicle

A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs).

Plasmamembrane

Coatprotein

Receptor-mediated endocytosis enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules (green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle.

Exocytosis

•Forces large molecules out of cell

• Membrane surrounding the material fuses with cell membrane• Cell changes shape – requires energy• Hormones or wastes released from cell

Transcytosis

endocytosis followed by exocytosis

transports a substance rapidly through a cell

HIV crossing a cell layer