Chapter 7Membrane Structure and Function

Plasma MembraneFlexible boundary separating living cell from

nonliving surroundingsSelectively permeable = choosy about what

enters and exitsControls traffic into and out of cell

Structure

Phospholipid BilayerHydrophilic heads are facing watery

environment (extracellular fluid and cytoplasm)Hydrophobic tails are inside bilayer

Fluid Mosaic ModelProteins embedded in bilayer of phospholipid

moleculesFreeze Fracture evidence supports

Freeze, cut, see that proteins are embedded

FluidityMembranes are held together by weak

hydrophobic interactionsAllows movement of phospholipidsCan move laterally (side to side) or flip (rare)

Need to be fluid to workUnsaturated tails prevent tight packingCholesterol restrains phospholipid movement

in warmer temps., and prevents close packing at lower temps.

Prot

eins

Integral proteinsTransmembrane (span entire membrane)

Peripheral proteinsNot embedded, just attatched to either ECM or

cytoskeleton

8

Six Major Functions of Membrane Proteins

Figure 7.9

Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane.

Enzymatic activity. A protein built into the membranemay be an enzyme with its active site exposed tosubstances in the adjacent solution. In some cases,several enzymes in a membrane are organized asa team that carries out sequential steps of ametabolic pathway.

Signal transduction. A membrane protein may havea binding site with a specific shape that fits the shapeof a chemical messenger, such as a hormone. Theexternal messenger (signal) may cause aconformational change in the protein (receptor) thatrelays the message to the inside of the cell.

(a)

(b)

(c)

ATP

Enzymes

Signal

Receptor

9

Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells.

Intercellular joining. Membrane proteins of adjacent cellsmay hook together in various kinds of junctions, such asgap junctions or tight junctions

Attachment to the cytoskeleton and extracellular matrix(ECM). Microfilaments or other elements of thecytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes

(d)

(e)

(f)

Glyco-protein

Six Major Functions of Membrane Proteins

Synthesis of MembranesMade in ERTransport vesicle

to GolgiModified and

packaged in Golgi

Secretory vesicle to cell membrane

Cell-to-Cell RecognitionHelps cell recognize if a cell is alike or

differentImportant for immune responseCarbohydrates act as markers on cell

membraneOligosaccharides (<15 monomers)Can be found attached to proteins or lipids

Glycoprotein = protein + oligosaccharide Glycolipid = lipid + oligosaccharide

Moving through MembranesNonpolar (hydrophobic) molecules

Cross easilyHydrocarbons, Gases

Polar (hydrophilic) moleculesSmall can pass (water, ethanol)Large cannot (glucose)Ions have a hard time (Na+, H+)Can use transport proteins

Passive TransportNo energy required

- ∆GMoves down concentration gradient

Difference in solute concentration Net movement: High Low ConcentrationThrough Membrane = DiffusionThrough Transport Protein = Facilitated

DiffusionWater through Membrane = Osmosis

OsmosisIsotonic Solution = no net movement, equal

solute concentration on both sides of membrane, equilibrium, water moves equally in and out.

Hypotonic Solution = solution has LESS solute concentration than inside the cell, net movement into cell, cell swells, animal cells can burst (lyse).

Hypertonic Solution = solution has MORE solute concentration than inside the cell, net movement out of cell, cell shrinks.

15

How Will Water Move Across Semi-Permeable Membrane?

Solution A has 100 molecules of glucose per ml

Solution B has 100 molecules of fructose per ml

How will the water molecules move?

There will be no net movement of water since the concentration of solute in each solution is equal

16

How Will Water Move Across Semi-Permeable Membrane?

Solution A has 100 molecules of glucose per ml

Solution B has 75 molecules of fructose per ml

How will the water molecules move?

There will be a net movement of water from Solution B to Solution A until both solutions have equal concentrations of solute

17

How Will Water Move Across Semi-Permeable Membrane?

Solution A has 100 molecules of glucose per ml

Solution B has 100 molecules of NaCl per ml How will the water molecules

move?

Each molecule of NaCl will dissociate to form a Na+ ion and a Cl- ion, making the final concentration of solutes 200 molecules per mil. Therefore, there will be a net movement of water from Solution A to Solution B until both solutions have equal concentrations of solute

Cell (compared to beaker) hypertonic or hypotonic

Beaker (compared to cell) hypertonic or hypotonicWhich way does the water flow? in or out of cell

.05 M .03 M

Do you understand Osmosis…

Differences in Animals + Plants

AdaptationsContractile Vacuole in parameciumSalt pumps in bony marine fish

Full

Empty

Facilitated DiffusionSpecific to soluteMax rate occurs at saturation of soluteCan be inhibited by molecules that resemble

solute

Channel Proteins – no shape changeCarrier Proteins – shape changeGated Channels – open only in response to

stimuli

Channel

Carrier

Gated

EXTRACELLULARFLUID

Channel protein SoluteCYTOPLASM

Active TransportEnergy requiringTransport protein pumps a molecule

AGAINST the concentration gradientNet movement: Low High+ ∆GImportant to maintain ion gradients (Na, K,

Cl, Ca)

Sodium-Potassium PumpNa+ binding sites towards cytoplasmK + binding sites towards exteriorATP becomes ADP and phosphorylates

proteinCauses shape change from Na + receptive to

K + receptiveSolutes travel across membrane3 Na + out of cell for every 2 K + into cellKeeps inside of cell negative compared to

outsideMajor pump in animal cells (esp. neurons!)

Proton PumpMajor pump in plants, bacteria, and fungiFound in Mitochondria and Chloroplasts

Cotransport1 ATP driven pump transports 1 solute and

indirectly drives the transport of other solutes against their concentration gradient

Large MoleculesEndocytosis = importing large

macromolecules by forming vesicles from plasma membranePhagocytosis: endocytosis of solid particles

Ex: Ameoba’s pseudopodiaPinocytosis: endocytosis of fluid dropletsReceptor Mediated Endcytosis: a ligand

(molecule that bonds to a receptor) binds to initiate endocytosis Cholesterol

In blood, cholesterol is bound to lipid and protein complexes called low-density lipoproteins (LDL). The LDLs bind to LDL receptors on cell membrane to initiate endocytosis of cholesterol. Defective LDL receptors mean build up of cholesterol in blood.

Large MoleculesExocytosis = exporting macromolecules

from a cell by fusion of vesicles with the cell membraneVesicles from ER or GolgiEx: Insulin from pancreatic cells