Membrane Structure and Membrane Structure and FunctionFunctionChapter 7

What is the Function of The What is the Function of The Plasma Membrane?Plasma Membrane?boundarymust be selectively permeableWhat, then is the structure that

allows the membrane to perform this function successfully?

PhospholipidsPhospholipidsRecall that phospholipids are

amphipathic (both hydrophilic and hydrophobic)

Artificial membranes showed phospholipids will form a layer in water

The BilayerThe BilayerWhy are membranes organized

into a bilayer?There are two “watery” areas

that interact with the membrane, outside of the cell and inside the cell

Science as a ProcessScience as a Process1935 Davson-Danielli

model “The Fat Sandwich”. The evidence:

Both protein and phospholipids were isolated from membranes

Thickness measured equals the bilayer

Phospholipids alone are not as attracted to water as the real membrane surfaces. Therefore coat with proteins!

Problems with the modelProblems with the model In what way are membrane proteins a

problem for this model?Hint: Membrane proteins are

amphipathicHydrophobic parts of the proteins are

in hydrophilic zones, resulting in an unstable structure

What other problem was there with this model?

The model suggests that all membranes are identical with regard to thickness. They are not!

Fluid Mosaic Model - 1972Fluid Mosaic Model - 1972In what way does

this model solve the problems?

Hydrophobic parts of proteins are embedded within the membrane

Thickness between different membranes is a function of the proteins

What evidence is there to support this model?

Fluid Mosaic Model - 1972Fluid Mosaic Model - 1972In what way does

this model solve the problems?

Hydrophobic parts of proteins are embedded within the membrane.

Thickness between different membranes is a function of the proteins

Freeze-fracture technique with electron

microscope

What evidence is there for What evidence is there for membrane fluidity?membrane fluidity?

Membrane FluidityMembrane FluidityWhy is it that membrane

phospholipids drift laterally, and rarely flip?

How is this fluidity How is this fluidity maintained?maintained?Kinks in unsaturated fatty acid

tails of phospholipidsCholesterol

How are proteins arranged to How are proteins arranged to contribute to membrane function?contribute to membrane function?Membrane proteins contribute to the

mosaic quality of the structureDifferent proteins convey different

properties to each membraneIntegral proteins are inserted within

the membranePeripheral proteins are attached to

membrane surfaceProteins attach to cytoskeleton or to

extracellular fibers to help give animal cells a stronger framework

Membrane CarbohydratesMembrane CarbohydratesFound only on the outside of the

membraneWhat is their function?Cell to cell recognitionSorting cells into tissues Immune defenseUsually oligosaccharides (15 or

less sugar units)glycolipids or glycoproteins

How do ions and other polar How do ions and other polar molecules pass into and out of molecules pass into and out of cells?cells?Transport proteinsProvide hydrophilic tunnel for

ionsThey are specific for the

substances they transport

What determines the direction What determines the direction of traffic across a membrane?of traffic across a membrane?DiffusionWhat causes diffusion? Why is it

spontaneous?Concentration gradient

represents potential energy Since the direction of movement

decreases the free energy of the system, it is spontaneous

Does the diffusion of more than Does the diffusion of more than one kind of particle work together one kind of particle work together or separately?or separately?

Do the particles stop moving once equilibrium

is reached?

Does the diffusion of more than Does the diffusion of more than one kind of particle work together one kind of particle work together or separately?or separately?

NO!

If a molecule can move freely If a molecule can move freely through the phospholipid bilayer, through the phospholipid bilayer, then what always controls the then what always controls the direction of its movement?direction of its movement?Concentration gradientRemember that the

concentration gradient represents potential energy

OsmosisOsmosisWhat is osmosis?The diffusion of water across a

semi-permeable membrane

Since water passes freely across the membrane, how can the cell control the direction of osmosis?

The cell can concentrate solutes that are not permeable to the phospholipid bilayer on one side of the membrane

Which way will water move? Water will follow the solutes What do the terms hypotonic,

hypertonic and isotonic mean?Hypotonic = lower solute concentrationHypertonic = greater solute

concentrationIsotonic = equal solute concentration

??

Which way will the water

move?

WHY?

Do Water Molecules Stop Do Water Molecules Stop Moving in Isotonic Moving in Isotonic Conditions?Conditions?NoThey continue to diffuse,

however there is no net movement

In general, which way does water move?

From hypotonic to hypertonic

Water Balance in CellsWater Balance in Cells

Facilitated DiffusionFacilitated DiffusionWhat is facilitated diffusion?Diffusion of solutes with the help

of transport proteinsIs this a passive or an active

process?passiveWhy do these solutes need a

protein to facilitate their diffusion?They are too polar to pass

through the lipid bilayer

Active TransportActive TransportWhat is active transport?Pumps molecules across the

membrane against their concentration gradients

Requires energy in the form of ATPUsed to help maintain ionic gradients

across membranesWhat do these ionic gradients

represent?Potential energy

Membrane PotentialMembrane PotentialMembrane potential is the

voltage across a membraneUsually around -70 mVHow is it maintained?An unequal distribution of anions

inside the cell to cations outside the cell

What two forces drive the What two forces drive the diffusion of ions?diffusion of ions?Concentration gradient of the ionEffect of membrane potential (charge)

on the ionThis is called the electrochemical

gradientIons diffuse down their electrochemical

gradientA transport protein that generates

voltage across a membrane is called an electrogenic pump

One example is the sodium-potassium pump

Proton pumps are the main electrogenic pumps of bacteria, fungi and plants.

Review Review of of passive passive and and active active transpotransportrt

Review Review of of passive passive and and active active transpotransportrt

Review Review of of passive passive and and active active transpotransportrt

What is co-transport ?What is co-transport ?

Transport of large Transport of large moleculesmoleculesEndocytosis –

◦Phagocytosis – endocytosis of large particulate substances

◦Pinocytosis – endocytosis of fluid and dissolved solutes

Exocytosis -

PhagocytosisPhagocytosis

PinocytosisPinocytosis