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Membrane Structure and Function

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Plasma Membrane Plasma Membrane OverviewOverview

Structure= Structure= Fluid mosaicFluid mosaic of lipids and proteins- Lipid bilayerbilayer

- Contains embedded- Contains embedded proteins proteinsFunction= the boundaryboundary that

separates the living cell from its nonliving surroundings

- Selectively Permeable- Selectively Permeable (chooses what may cross the membrane)

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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

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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)

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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

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PhospholipidsPhospholipids

- Are the most abundantmost abundant lipid in the plasma membrane

- Are amphipathicamphipathic, containing both hydrophilic (head) and hydrophobic regions (tails)

1. 1. Head-Head- composed of phosphate group attached to one carbon of glycerol is hydrophilichydrophilic

2. Tails- two fatty acid tailstails are hydrophobichydrophobic

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Hydrophilichead

HydrophobicHydrophobic tail tail

WATER

WATER

Phospholipid Bilayer

Steroid Cholesterol

The steroid cholesterolsteroid cholesterol moderates membrane fluidity (different effects at different temperatures)

Cholesterol

http://telstar.ote.cmu.edu/biology/MembranePage/index2.html

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Membrane Carbohydrates- Interact with the surface

molecules of other cells, facilitating cell-cell recognition

Cell-cell recognition- Cell-cell recognition- IIs a cell’s ability to distinguish one type of neighboring cell from another

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Membrane ProteinsMembrane Proteins

In 19721972, Singer and Nicolson, Proposed that membrane membrane proteinsproteins are dispersed and individually inserted into the inserted into the phospholipid bilayer phospholipid bilayer of the of the plasma membraneplasma membrane

Phospholipidbilayer

Hydrophilic region of protein

Hydrophobic region of protein

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Types of Membrane Proteins

Integral proteinsIntegral proteinsPenetrate the hydrophobic core of

the lipid bilayerAre often transmembrane proteinstransmembrane proteins,

completely spanning the membrane

EXTRACELLULARSIDE

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Types of Membrane Proteins

Peripheral proteinsPeripheral proteinsAre appendages loosely bound

to the surface of the membrane

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Functions of Membrane Proteins

Figure 7.9

1.Transport2.Enzymatic activity3.Signal transduction4.Cell to cell recognition5. Intercellular joining6.Attachment to the

cytoskeleton and extracellular matrix

(ECM).

ATP

Enzymes

Signal

Receptor

Glyco-protein

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Fibers of

extracellularmatrix (ECM)

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Synthesis and Sidedness of Membranes

Membrane proteins and lipidsMembrane proteins and lipids are made in the ER ER andand Golgi Golgi apparatusapparatus

ER

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Membrane Permeability

Membrane structurestructure results in selective permeabilityselective permeability

A cell must exchange cell must exchange materials with its materials with its surroundingssurroundings, a process controlled by the plasma membrane

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Permeability of the Lipid Bilayer

Polar moleculesPolar molecules- Do NOT cross

the membrane rapidlyrapidly

- Often assisted - Often assisted by transport by transport proteinsproteins

Hydrophobic moleculesHydrophobic moleculesAre lipid solublelipid soluble and can pass through the membrane rapidlyrapidly

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Passive Transport

Passive transportPassive transport is diffusion of a substance across a membrane with no energyno energy investment

COCO22, H, H22O, and OO, and O22 easily diffuse across plasma membranes

Diffusion of water is known as OsmosisOsmosis

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Simple DiffusionDiffusionDiffusion

Is the tendency for molecules of any substance to spread out evenlyspread out evenly into the available space

Move from high to low concentrationhigh to low concentration

DownDown the concentration gradient

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Osmosis

Osmosis- Is the movement of water across a semi-permeable membrane

- Is affected by the concentration gradient of dissolved substances called the solution’s tonicity

Tonicity- Is the ability of a solution to cause a cell to gain or lose water- Has a great impact on cells without impact on cells without

wallswalls

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Three States of Tonicity

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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)

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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

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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

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Water Balance in Cells Without Walls

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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.

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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 How will the water molecules move?move?

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

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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 How will the water molecules move?move?

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

ANSWER:

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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 ml. Therefore, there will be a net movement of water from Solution A to Solution B until both solutions have equal concentrations of solute

ANSWER:

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Facilitated Diffusion

Facilitated diffusionFacilitated diffusionIs a type of PassivePassive Transport Aided by

ProteinsProteins

In facilitated diffusion

Transport proteinsTransport proteins speed the movement of molecules across the plasma membrane

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Facilitated Diffusion & ProteinsChannel proteinsChannel proteins

Provide corridors that allow a specific molecule or ion to cross the membraneEXTRACELLULAR

FLUID

Channel proteinSolute

CYTOPLASM

A channel protein (purple) has a channel through which water molecules or a specific solute can pass.

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Facilitated Diffusion & Proteins

Carrier proteinsCarrier proteinsUndergo 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.

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Active Transport

Active transportUses energyUses energy to move solutes againstagainst their concentration gradients

Requires energy, usually in the form of ATPATP

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The sodium-potassium pumpsodium-potassium pumpIs one type of active transport system

Active Transport

PP i

EXTRACELLULARFLUID

Na+ binding stimulatesphosphorylation by ATP.2

Na+

Cytoplasmic Na+ binds tothe sodium-potassium pump.

1

K+ is released and Na+

sites are receptive again; the cycle repeats.

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

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

6 Loss of the phosphaterestores the protein’s original conformation.

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CYTOPLASM

[Na+] low[K+] high

Na+

Na+

Na+

Na+

Na+

PATP

Na+

Na+

Na+

P

ADP

K+

K+

K+

K+K+

K+

[Na+] high[K+] low

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Passive vs. Active TransportPassive transport. Active transport.

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

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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

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Proton Pump

EXTRACELLULARFLUID

+

H+

H+

H+

H+

H+

H+Proton pump

ATP

CYTOPLASM

+

+

+

+

–

–

–

–

–

+

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Example of CotransportCotransport-Occurs when active transportactive transport of a specific

solute indirectly drives the active transport of another solute (driven by a concentration gradient)driven by a concentration gradient)

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Bulk Transport

Exocytosis- Exocytosis- transport vesiclestransport vesicles migrate to the plasma membrane, fuse with it, and release their contents

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Bulk TransportEndocytosis- ndocytosis- tthe cell takes in

macromolecules by forming new forming new vesicles from the plasma vesicles from the plasma membranemembrane

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In 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.

Three Types of Endocytosis

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.

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0.25 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 coa 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.

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