RAVEN & JOHNSON CHAPTER 5 CAMPBELL CHAPTER 8

Membrane Structure & Function

Membrane StructureLipids (phospholipids) & proteins

They are amphipathic – both hydrophobic & hydrophilic

Fluid-mosaic model(Singer & Nicolson, 1972)

FluidityHeld by weak hydrophobic interactions

Movement is lateral

FluidityMembrane must be fluid to work properly

Cholesterol is wedged into phospholipid tails – restrains movement in warm temp (less fluid), but also lowers solidifying temp (more fluid)

Fluidity

MosaicEmbedded proteins (over 50 types) determine specific function of the membrane

Integral proteinsTransmembraneHydrophobic (-helical

amino acids) & hydrophilic regions

MosaicPeripheral proteins

On the surface of membrane

Protein Functions

Cell RecognitionCrucialDetermined by surface molecules (carbohydrates) on plasma membrane

Cell RecognitionOligosaccharides (polysaccharide fewer than 15 units) vary for different types of cells

Used as markers to distinguish

Example – A, B, AB, O

Molecular MovementHydrophobic molecules move across easilyHydrocarbons, CO2, O2

Hydrophilic have trouble moving through hydrophobic core (ions, polar molecules, water, glucose, sugars)Need transport proteins (facilitated diffusion)

Molecular MovementProteins are specific to molecule it transports

Function: move molecules across the membrane that would not be able to do so otherwise

Two types:

Molecular Movement

Molecular Movement

1) Channel Proteins Water or small ions If water, called aquaporins If needs electrical or chemical

stimulus to open & close, called gated channel

Molecular Movement2) Other carrier proteins

Some transport proteins change shape to transport molecules

Molecular MovementReview:Review: Passive transport (diffusion)Active transportConcentration gradientEquilibriumOsmosisHypertonic, Hypotonic, Isotonic ( & what happens to the cell)

Molecular Movement

Molecular Movement

Molecular MovementAnimal cells need isotonic environment

If not, cells must adapt for osmoregulation (water balance)Ex – Paramecium’s contractile vacuole

Active TransportAgainst concentration gradient – requires energy (ATP)

Ability to have cell with concentrations very different from surroundingsEx) Animal cell has much K+ and little Na+ compared to surroundings

Active TransportAccomplished by membrane proteinsATP transfers P to transport protein, protein changes shape, molecule moves in or out

Ex) Sodium-Potassium pumpOne ATP pumps three Na+ out and two K+ in

Active Transport

CotransportCoupling of the “downhill” diffusion of one substance to the “uphill” transport of another against its own concentration gradient

Exocytosis & Endocytosis

For macromoleculesExocytosis – cell secretes macromolecules by fusion of vesicles with plasma membrane

Exocytosis & Endocytosis

Endocytosis – cells takes in macromolecules by forming new vesicles

Three types: Phagocytosis (particles) Pinocytosis (any liquid) Receptor-mediated endocytosis

(specific)

Exocytosis & Endocytosis

Exocytosis & Endocytosis

Receptor-mediated endocytosisProteins embedded in membrane –

specific receptor sites (area where they are – coated pits)

Extracellular substances (ligands) bind to sites – triggers vesicle formation

Process enables cell to acquire bulk quantities of specific substances (cholesterol)

ReviewReview Website

(http://www.wiley.com/college/pratt/0471393878/student/animations/membrane_transport/index.html)