Membrane Structure and Function
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Transcript of Membrane Structure and Function
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