Membrane Structure and Membrane Structure and FunctionFunction

Membrane FunctionMembrane Function

Membranes organize the chemical Membranes organize the chemical activities of cells.activities of cells.

The outer plasma membrane The outer plasma membrane forms a boundary between a living cell and its forms a boundary between a living cell and its

surroundingssurroundings Exhibits selective permeabilityExhibits selective permeability

• Controls traffic of molecules in and outControls traffic of molecules in and out

Membrane FunctionMembrane Function

Internal membranes provide structural Internal membranes provide structural order for metabolismorder for metabolism

• Form the cell's organellesForm the cell's organelles• Compartmentalize chemical reactionsCompartmentalize chemical reactions

Fluid Mosaic Model of the PMFluid Mosaic Model of the PM

A membrane is a mosaicA membrane is a mosaic Proteins and other molecules are embedded Proteins and other molecules are embedded

in a framework of phospholipidsin a framework of phospholipids

A membrane is fluidA membrane is fluid Most protein and phospholipid molecules can Most protein and phospholipid molecules can

move laterallymove laterally

Membrane StructureMembrane Structure

Phospholipids are the major structural component of membranes.

Phospholipid

Membrane StructureMembrane Structure

All membranes are phospholipid bilayers All membranes are phospholipid bilayers with embedded proteins.with embedded proteins.

Label the:

Hydrophilic heads

Hydrophobic tails

Phospholipid Bilayer

Embedded in the bilayer are proteinsEmbedded in the bilayer are proteins Most of the membrane’s functions are Most of the membrane’s functions are

accomplished by the embedded accomplished by the embedded proteins. proteins. • Integral proteinsIntegral proteins span the membrane span the membrane• Peripheral proteinsPeripheral proteins are on one side or the other of are on one side or the other of

the membranethe membrane

Plasma Membrane ComponentsPlasma Membrane Components

GlycoproteinsGlycoproteins and and glycolipidsglycolipids are are proteins/lipids with short chain proteins/lipids with short chain carbohydrates attached on the carbohydrates attached on the extracellular side of the membrane.extracellular side of the membrane.

Fig. 5-1aFig. 5-1a

Cholesterol

Glycoprotein

Glycolipid

Carbohydrate ofglycoprotein

Phospholipid

Microfilamentsof cytoskeleton

Integrin

Types of Membrane ProteinsTypes of Membrane Proteins

1.1. Cell-cell recognition proteinsCell-cell recognition proteins2.2. IntegrinsIntegrins3.3. Intercellular junction proteinsIntercellular junction proteins4.4. Enzymes Enzymes 5.5. Signal transduction proteinsSignal transduction proteins

• Aka - Receptor proteins Aka - Receptor proteins

6.6. Transport proteinsTransport proteins– Passive and activePassive and active

Cell-cell recognition proteins Cell-cell recognition proteins - identify type - identify type of cell of cell andand identify a cell as “self” versus identify a cell as “self” versus foreignforeign Most are glycoproteins Most are glycoproteins

• Carbohydrate chains vary between species, Carbohydrate chains vary between species, individuals, and even between cell types in a given individuals, and even between cell types in a given individual.individual.

• Glycolipids also play a role in cell recognitionGlycolipids also play a role in cell recognition

IntegrinsIntegrins are a type of integral protein are a type of integral protein The cytoskeleton attaches to integrins on the The cytoskeleton attaches to integrins on the

cytoplasmic side of the membranecytoplasmic side of the membrane Integrins strengthen the membraneIntegrins strengthen the membrane

Intercellular junction proteins Intercellular junction proteins - help like - help like cells stick together to form tissuescells stick together to form tissues

Many membrane proteins are Many membrane proteins are enzymesenzymes This is especially important on the This is especially important on the

membranes of organelles.membranes of organelles.

Signal transduction (receptor) proteinsSignal transduction (receptor) proteins bind hormones and other substances on bind hormones and other substances on the outside of the cell.the outside of the cell. Binding triggers a change inside the cell.Binding triggers a change inside the cell.

• Called signal transductionCalled signal transduction• Example: The binding of insulin to insulin receptors Example: The binding of insulin to insulin receptors

causes the cell to put glucose transport proteins causes the cell to put glucose transport proteins into the membrane.into the membrane.

Fig. 5-1cFig. 5-1c

Messenger molecule

Activatedmolecule

Receptor

Transport ProteinsTransport Proteins

Passive Transport ProteinsPassive Transport Proteins allow water soluble substances (small polar allow water soluble substances (small polar

molecules and ions) to pass through the molecules and ions) to pass through the membrane without any energy costmembrane without any energy cost

Active Transport ProteinsActive Transport Proteins The cell expends energy to transport water The cell expends energy to transport water

soluble substances soluble substances againstagainst their their concentration gradientconcentration gradient

Fig. 5-1dFig. 5-1d

Transport of Substances Across Transport of Substances Across the Plasma Membrane (PM)the Plasma Membrane (PM)

1.1. Passive Transport Passive Transport (Simple) Diffusion (5.3)(Simple) Diffusion (5.3) Facilitated diffusion (5.6)Facilitated diffusion (5.6) Osmosis (5.4, 5.5)Osmosis (5.4, 5.5)

2.2. Active Transport (5.8)Active Transport (5.8)

3.3. Bulk Flow (5.9)Bulk Flow (5.9) EndocytosisEndocytosis ExocytosisExocytosis

Passive TransportPassive Transport

In In passive transportpassive transport substances cross substances cross the membrane by the membrane by diffusiondiffusion Diffusion - net movement of substances from Diffusion - net movement of substances from

an area of high concentration to low an area of high concentration to low concentrationconcentration• no energy requiredno energy required

Factors Affecting Diffusion RateFactors Affecting Diffusion Rate

Steepness of concentration gradientSteepness of concentration gradient Steeper gradient, faster diffusionSteeper gradient, faster diffusion

Molecular sizeMolecular size Smaller molecules, faster diffusionSmaller molecules, faster diffusion

TemperatureTemperature Higher temperature, faster diffusionHigher temperature, faster diffusion

Simple DiffusionSimple Diffusion

Nonpolar, hydrophobic moleculesNonpolar, hydrophobic molecules diffuse diffuse directly through the lipid bilayerdirectly through the lipid bilayer Simple diffusion does not require the use of Simple diffusion does not require the use of

transport proteins.transport proteins. Examples: OExamples: O22, CO, CO22, steroids, steroids

Polar, hydrophilic substancesPolar, hydrophilic substances cannotcannot pass pass directly through the lipid bilayer directly through the lipid bilayer Examples: water, ions, carbohydratesExamples: water, ions, carbohydrates

Simple DiffusionSimple Diffusion

small, nonpolar molecules(ex. O2, CO2)

Polar molecules(ex. Glucose, water)

ions(ex. H+, Na+, K+)

LIPID-SOLUBLELIPID-SOLUBLE WATER-SOLUBLEWATER-SOLUBLE

LIPID-SOLUBLELIPID-SOLUBLE

Facilitated DiffusionFacilitated Diffusion

In facilitated diffusion small polar In facilitated diffusion small polar molecules and ions diffuse through molecules and ions diffuse through passive transport proteinspassive transport proteins.. No energy neededNo energy needed

• Most passive transport proteins are solute Most passive transport proteins are solute specificspecific

• Example: glucose enter/leaves cells Example: glucose enter/leaves cells through facilitated diffusionthrough facilitated diffusion

Facilitated Diffusion

Passive transport protein

Lower concentration

Higher concentration of

OsmosisOsmosis

Osmosis – diffusion of water across a Osmosis – diffusion of water across a selectively permeable membraneselectively permeable membrane

Water moves from an area of _______ Water moves from an area of _______ water concentration to an area of _____ water concentration to an area of _____ water conc.water conc. Is energy required ?Is energy required ?

Water travels in/out of the cell through Water travels in/out of the cell through aquaporinsaquaporins

Osmosis TermsOsmosis TermsConsider two solutions separatedConsider two solutions separated

by a plasma membrane.by a plasma membrane. HypertonicHypertonic

solution with a relatively high concentration of solutesolution with a relatively high concentration of solute HypotonicHypotonic

solution with a relatively low concentration of solutesolution with a relatively low concentration of solute IsotonicIsotonic

solutions with the same solute concentrationsolutions with the same solute concentration

Watermolecule

Selectivelypermeablemembrane

Solutemolecule

H2O

Lowerconcentration

of solute

Higherconcentration

of solute

Equalconcentration

of solute

Solute molecule withcluster of water molecules

Net flow of water

Osmosis and Animal CellsOsmosis and Animal Cells

Osmosis and Plant CellsOsmosis and Plant Cells

OsmosisOsmosis

When a Cell is Placed in a Hypotonic When a Cell is Placed in a Hypotonic SolutionSolution Water concentration is _________ the cell.Water concentration is _________ the cell. Water flows ___________ the cell.Water flows ___________ the cell.

OsmosisOsmosis

When a Cell is Placed in a Hypertonic When a Cell is Placed in a Hypertonic SolutionSolution Water concentration is _________ the cell.Water concentration is _________ the cell. Water flows ___________ the cell.Water flows ___________ the cell.

Isotonic solution Hypotonic solution Hypertonic solution

H2O H2O

(1) Normal (2) Lysed

H2O

H2O H2O H2O

Animalcell

Plantcell

(4) Flaccid (5) Turgid (6) Shriveled (plasmolyzed)

(3) Shriveled

Plasmamembrane

H2O

H2O

See page 83

Osmosis SummaryOsmosis Summary

When a cell is placed in a Hypotonic solutionWhen a cell is placed in a Hypotonic solution:: Cell gains water through osmosisCell gains water through osmosis Animal cell lyses; plant cell becomes turgid (firm)Animal cell lyses; plant cell becomes turgid (firm)

When a cell is placed a Hypertonic solutionWhen a cell is placed a Hypertonic solution:: Cell loses water through osmosisCell loses water through osmosis Animal cell shrivels; plant cell plasmolyzesAnimal cell shrivels; plant cell plasmolyzes

Active TransportActive Transport

Active transport proteins move substances Active transport proteins move substances across the PM across the PM againstagainst their concentration their concentration gradient.gradient. Requires energy (ATP)Requires energy (ATP) Active transport proteins are highly selectiveActive transport proteins are highly selective Active transport is needed for proper Active transport is needed for proper

functioning of nerves and musclesfunctioning of nerves and muscles

Active Transport of “X”Active Transport of “X”

– Active transport proteins span the Active transport proteins span the plasma membraneplasma membrane

– They have openings for “X” on only one They have openings for “X” on only one side of the membraneside of the membrane

– ““X” enters the channel and binds to X” enters the channel and binds to functional groups inside the transport functional groups inside the transport protein.protein.

– Cytoplasmic ATP binds to the transport Cytoplasmic ATP binds to the transport proteinprotein

Active Transport of “X”Active Transport of “X”– A phosphate group is transferred from A phosphate group is transferred from

ATP to the transport protein ATP to the transport protein – protein is energized by the added –P.protein is energized by the added –P.

– The energized transport protein changes The energized transport protein changes shape and releases “X” on the other side shape and releases “X” on the other side of the cell.of the cell.

– The phosphate group is released from The phosphate group is released from the transport protein and it resumes its the transport protein and it resumes its original shape.original shape.

– Process repeats.Process repeats.

Fig. 5-8-1Fig. 5-8-1

Transportprotein

Solute

Solute binding1

Fig. 5-8-2Fig. 5-8-2

Transportprotein

Solute

Solute binding1 Phosphorylation2

Fig. 5-8-3Fig. 5-8-3

Transportprotein

Solute

Solute binding1 Phosphorylation2 Transport3

Proteinchanges shape

Fig. 5-8-4Fig. 5-8-4

Transportprotein

Solute

Solute binding1 Phosphorylation2 Transport3

Proteinchanges shape

Protein reversion4

Phosphatedetaches

Active TransportActive Transporttell the story…tell the story…

ATPATP P

ADP

Bulk FlowBulk Flow

Vesicles are used to transport large Vesicles are used to transport large particles across the PM.particles across the PM. Requires energyRequires energy

Types:Types: ExocytosisExocytosis EndocytosisEndocytosis

• Phagocytosis, pinocytosis, receptor-mediatedPhagocytosis, pinocytosis, receptor-mediated

Vesicle

Fluid outside cell

Protein

Cytoplasm

ExocytosisExocytosis

Bulk FlowBulk Flow

ExocytosisExocytosis Cytoplasmic vesicle merges with the PM Cytoplasmic vesicle merges with the PM

and releases its contentsand releases its contents Example:Example:

• Golgi body vesicles merge with the PM an Golgi body vesicles merge with the PM an release their contentsrelease their contents

• How nerve cells release neurotransmittorsHow nerve cells release neurotransmittors

Vesicle forming

EndocytosisEndocytosis

Endocytosis can occur in three waysEndocytosis can occur in three ways• Phagocytosis ("cell eating")Phagocytosis ("cell eating")• Pinocytosis ("cell drinking")Pinocytosis ("cell drinking")• Receptor-mediated endocytosisReceptor-mediated endocytosis

EndocytosisEndocytosis

EndocytosisEndocytosis PM sinks inward, pinches off and forms a PM sinks inward, pinches off and forms a

vesiclevesicle Vesicle often merges with Golgi for Vesicle often merges with Golgi for

processing and sorting of its contentsprocessing and sorting of its contents

Endocytosis - termsEndocytosis - terms

Phagocytosis – cell eating – cell eating Membrane sinks in and captures Membrane sinks in and captures solid solid

particlesparticles for transport into the cell for transport into the cell Examples:Examples:

• Solid particles often include: bacteria, cell Solid particles often include: bacteria, cell debris, or fooddebris, or food

Pinocytosis – cell drinkingPinocytosis – cell drinking Cell brings in a liquidCell brings in a liquid

Endocytosis - Endocytosis - commentscomments

Phagocytosis and pinocytosis are not Phagocytosis and pinocytosis are not selectiveselective Membrane sinks inward and captures Membrane sinks inward and captures

whatever particles/fluid present.whatever particles/fluid present. Vesicle forms and merges with the Golgi Vesicle forms and merges with the Golgi

body…body…

Receptor Mediated EndocytosisReceptor Mediated Endocytosis

Receptor Mediated Endocytosis is a is a highly specific form of endocytosis.highly specific form of endocytosis.

Receptor proteins on the outside of the cell Receptor proteins on the outside of the cell bind specific substances and bring them into bind specific substances and bring them into the cell by endocytosisthe cell by endocytosis

Receptor Mediated EndocytosisReceptor Mediated Endocytosis

1.1. Receptor proteins on PM bind specific Receptor proteins on PM bind specific substances substances (vitamins, hormones..)(vitamins, hormones..)

2.2. Membrane sinks in and forms a pitMembrane sinks in and forms a pit– Called a coated pitCalled a coated pit

3.3. Pit pinches closed to form a vesicle around Pit pinches closed to form a vesicle around bound substancesbound substances

• Cytoskeleton aids in pulling in the membrane and Cytoskeleton aids in pulling in the membrane and vesicle formationvesicle formation

Fig. 5-9cFig. 5-9c

Coatedvesicle

Coatedpit

Specificmolecule

Receptor-mediated endocytosis

Coat proteinReceptor

Coatedpit

Material boundto receptor proteins

Plasma membrane

Fig. 5-9Fig. 5-9 Phagocytosis

EXTRACELLULARFLUID

Pseudopodium

CYTOPLASM

Foodvacuole

“Food” orother particle

Pinocytosis

Plasmamembrane

Vesicle

Coatedvesicle

Coatedpit

Specificmolecule

Receptor-mediated endocytosis

Coat proteinReceptor

Coatedpit

Material boundto receptor proteins

Plasma membrane

Foodbeingingested