Chapter 7Chapter 7Membrane Structure and Membrane Structure and FunctionFunction

– The plasma membrane is located at the boundary of every cell The plasma membrane is located at the boundary of every cell

– It functions as a selective barrierIt functions as a selective barrier

– It allows the passage of nutrients, oxygen and wasteIt allows the passage of nutrients, oxygen and waste

The membrane is made up of phospholipids The membrane is made up of phospholipids Phospholipids are Phospholipids are amphipathicamphipathic

– i.e.. It has both the i.e.. It has both the hydrophobic and hydrophilichydrophobic and hydrophilic regions regions

Fluid mosaic: Fluid mosaic: a term used to describe the structure of plasma a term used to describe the structure of plasma membrane with freedom of movement of molecules within the membrane with freedom of movement of molecules within the

membranemembrane

Cholesterol molecules:Cholesterol molecules: strengthens the fluid mosaic and makes strengthens the fluid mosaic and makes it more stableit more stable

Proteins:Proteins: determine which particles can pass across the membranedetermine which particles can pass across the membrane – Some protein serves as enzymeSome protein serves as enzyme

– Some serve as markers involved in disease fighting Some serve as markers involved in disease fighting

Traffic across the membraneTraffic across the membrane Plasma membrane serves as a communication mediaPlasma membrane serves as a communication media between a cell between a cell

and itand it’’s environments environment In addition to regulation of gas exchange, it also regulates the In addition to regulation of gas exchange, it also regulates the

concentration of inorganic ions – concentration of inorganic ions – NaNa++, K, K++, Ca, Ca++ and Cl and Cl-

It allows the passage of certain molecules and prohibits others – selective permeability

Hydrophobic core impedes transportation of ions and polar molecules

Hydrophobic as hydrocarbons – CO2 and O2 can dissolve and pass with ease

Hydrophilic substances avoid contact with the lipid bi-layer by passing through the transport protein (selective)

Passive transport Diffusion: the net movement of particles from area of high

concentration to area of low concentration– As a result of random movement of particles

Passive transport Diffusion: the net movement of particles from

area of high concentration to area of low concentration– As a result of random movement of particle

Diffusion depends on concentration gradientsDiffusion depends on concentration gradients– Concentration gradientConcentration gradient is the difference in the is the difference in the

concentration of substances across a spaceconcentration of substances across a space

Dynamic EquilibriumDynamic Equilibrium:: a continuous movement a continuous movement but no overall change but no overall change

Plasma Membrane (7)Plasma Membrane (7)

Intracellular versus extracellularIntracellular versus extracellular Membrane potentialMembrane potential Glycolipids and glycoproteinsGlycolipids and glycoproteins Fluid-mosaic modelFluid-mosaic model

Membrane Lipids (7)Membrane Lipids (7)

Phospholipids form a lipid Phospholipids form a lipid bilayerbilayer– HydrophilicHydrophilic (water-loving) (water-loving) polar headspolar heads– HydrophobicHydrophobic (water-fearing) nonpolar heads (water-fearing) nonpolar heads

Cholesterol: Determines fluid nature of Cholesterol: Determines fluid nature of membranemembrane

Membrane Proteins Membrane Proteins (7) in(7) in

Integral or intrinsicIntegral or intrinsic– Extend from one Extend from one

surface to the othersurface to the other Peripheral or Peripheral or

extrinsicextrinsic– Attached to either Attached to either

the inner or outer the inner or outer surfaces of the lipid surfaces of the lipid bilayerbilayer

Movement through the Plasma Movement through the Plasma MembraneMembrane

Passive TransportPassive Transport– DiffusionDiffusion– OsmosisOsmosis– FiltrationFiltration

Mediated transport mechanismsMediated transport mechanisms– Facilitated diffusionFacilitated diffusion– Active transportActive transport– Secondary active transportSecondary active transport

DiffusionDiffusion Movement of solutes from an area Movement of solutes from an area

of higher concentration to lower of higher concentration to lower concentration in solutionconcentration in solution– Concentration or density gradientConcentration or density gradient

Difference between two pointsDifference between two points

– Viscosity:- Viscosity:- the state of being thick, the state of being thick, sticky, and semifluid in consistency, sticky, and semifluid in consistency, due to internal friction.due to internal friction.Oil is more viscous than waterOil is more viscous than water

DiffusionDiffusion

OsmosisOsmosis DiffusionDiffusion of water (solvent) across of water (solvent) across

a selectively permeable membranea selectively permeable membrane ImportantImportant because large volume because large volume

changes caused by water changes caused by water movement disrupt normal cell movement disrupt normal cell functionfunction

Cell shrinkage or swellingCell shrinkage or swelling– Isotonic:Isotonic: cell cell neither shrinks nor neither shrinks nor

swellsswells– Hypertonic:Hypertonic: cell shrinks cell shrinks (crenation)(crenation)– Hypotonic:Hypotonic: cell swells cell swells (lysis)(lysis)

OsmosisOsmosis

OsmosisOsmosis

FiltrationFiltration

Works like a Works like a sievesieve Depends on pressure differenceDepends on pressure difference on on

either side of partitioneither side of partition Moves from side of Moves from side of greater pressure greater pressure

to lowerto lower– Example: In kidneys in urine formationExample: In kidneys in urine formation

Mediated Transport Mediated Transport MechanismsMechanisms

Facilitated diffusionFacilitated diffusion– Higher to lower Higher to lower

concentration without concentration without metabolic energymetabolic energy

Active transportActive transport– Requires ATPRequires ATP– Moves particles Moves particles

against concentration against concentration gradientgradient

Secondary active Secondary active transporttransport– Ions or molecules Ions or molecules

move in same move in same (symport) or different (symport) or different direction (antiport)direction (antiport)

Secondary Active TransportSecondary Active Transport

EndocytosisEndocytosis

Internalization Internalization of of substances by substances by formation of a formation of a vesiclevesicle

TypesTypes– PhagocytosisPhagocytosis– PinocytosisPinocytosis– Receptor-Receptor-

mediated mediated endocytosisendocytosis

Pinocytosis and Pinocytosis and Receptor-Mediated Receptor-Mediated

EndocytosisEndocytosis

ExocytosisExocytosis

Accumulated vesicle Accumulated vesicle secretions secretions expelledexpelled from cellfrom cell

ExamplesExamples– Secretion of digestive Secretion of digestive

enzymes by pancreasenzymes by pancreas– Secretion of mucus by Secretion of mucus by

salivary glandssalivary glands– Secretion of milk by Secretion of milk by

mammary glandsmammary glands

The EndThe End