MEMBRANE STRUCTURE AND FUNCTIONChapter 7

p. 124-130

THE PLASMA MEMBRANE Controls the flow of nutrients in & out of

cellSelectively Permeable: allows some

substances to cross more easily than others

PERMEABILITY OF LIPID BILAYER Many small molecules pass easily across

membrane in both directions i.e.: O2, CO2, H2O

However, cell excludes others from passing i.e.: Na+, K+, etc..

Hydrophobic molecules pass easily; hydrophilic do not

Proteins help hydrophilics across Transport Proteins: span membrane &

allow ions & polar molecules to pass by: 1) Creating hydrophilic channels 2) Physically moving molecules across

MEMBRANE STRUCTURE Composed of:

Phospholipids Proteins Carbohydrates**lipids & proteins are amphipathic (both hydrophobic and

–philic) Fluid Mosaic Model: membrane is fluid

structure w/proteins embedded in and attached to phospholipids

FLUIDITY OF MEMBRANES Phospholipids & some proteins move

laterallyPhospholipids (PL’s) move rapidly If temp ↓ PL’s pack closely & membrane

solidifiesUnsat. tails maintain fluidity (kinks in tails

keep PL’s apart)Sat. tails ↑ solidificationCholesterol restricts movement & prevents

tight packing of PL’s If solid, permeability changes &

proteins become inactive

MEMBRANE PROTEINS Proteins determine most of the membranes

functions 1) Integral Proteins: penetrate hydrophobic

core Transmembrane Protein: all the way through Protein’s hydrophobic area is α helices of non-polar

amino acids 2) Peripheral Proteins: loosely bound to

integral proteins on surface

FUNCTIONS OF MEMBRANE PROTEINS 1) Transport: as hydrophilic channels

OR as pumps (w/help of ATP) 2) Enzymatic Activity: active site is

exposed to solution 3) Signal Transduction: binding site for

chemical messengers 4) Intercellular Joining: connecting

cells 5) Cell-Cell Recognition: glycoproteins

serve as ID tags 6) Attachment to Cytoskeleton:

maintains cell shape

MEMBRANE CARBOHYDRATES

Branched oligosaccharides (small fragments), mostly bonded to proteins (glycoproteins) that aid in cell recognition

Cell recognition is important for: Organizing cells into tissues Embryo development Rejecting foreign cells Blood typing

CELLULAR TRANSPORTChapter 7

p. 130-140

PASSIVE TRANSPORT Diffusion: tendency of

molecules to spread out into available space Substances move from ↑

conc. to ↓ conc. “Concentration Gradient” Is passive; requires NO

energy bcs ↑ entropy Not affected by other

substances Dynamic equilibrium:

when molecules move equally in both directions

OSMOSIS IS PASSIVE TRANSPORT OF H2O

Solute concentrations determine direction of osmosis1) Hypertonic:

solution w/↑ solute conc.

2) Hypotonic: solution w/↓ solute conc.

3) Isotonic: solution w/equal solute conc.

CELL SURVIVAL DEPENDS ON H2O BALANCE Cells w/out walls must

regulate H2O balance in different surroundings In hypertonic soln, H2O

moves out of cell In hypotonic soln, H2O

moves into cell Most cells in isotonic

solns; H2O moves in & out equally

Osmoregulation: control of H2O balance

OSMOREGULATION OF PARAMECIUM

Open contractile vacuole

Lets H2O in

Closed contractile vacuole

Squeezes excess H2O out

OSMOREGULATION IN CELLS WITH WALLS

Plants, prokaryotes, fungi, & protists have cell walls In Hypotonic solns: cell is turgid (firm)

Healthy state for plant cells In Isotonic solns: cell is flaccid (limp)

Plant will wilt In Hypertonic solns: cell plasmolyzes

(membrane pulls away from wall) Lethal to plants

TRANSPORT PROTEINS FACILITATE DIFFUSION Facilitated Diffusion: diffusion of H2O, polar

molecules, & ions w/ help of transport proteins Is PASSIVE (no energy) Specific to molecule transporting May be inhibited by “imposters” 1) Aquaporins: water channel proteins 2) Ion Channels: ion channel proteins (i.e Na+)

May be gated , requiring a stimulus to open/close 3) Carrier Proteins: proteins subtly change shape

ACTIVE TRANSPORT PUMPS SOLUTES AGAINST GRADIENTSActive Transport:

requires energy (ATP) to pump solutes against conc. gradientHelps maintain

internal condition of cell

i.e. Na+/K+ Pump

SOME ION PUMPS GENERATE VOLTAGE Voltage: electrical potential energy

Cytoplasm more (-) than surroundings Membrane Potential: difference in voltage

across cell membrane -50 to -200 millivolts Favors passive transport of (+) ions into cell and (-)

ions out of cell

Electrochemical Gradient: diffusion of ions is affected by: 1) chemical factor (conc.

gradient) 2) electrical factor

(membrane potential) Electrogenic Pump:

generates voltage across a membrane Animals: Na+/K+ Pump Plants, Bacteria, etc.:

Proton Pump (H+ ions)

COTRANSPORT Cotransport: energy

from 1 ATP-powered pump indirectly drives active transport of another solute After being pumped out,

solute will diffuse back in i.e.: in plants, Proton

Pump drives active transport of amino acids, sugars, etc.

EXOCYTOSIS & ENDOCYTOSIS TRANSPORT LARGE MOLECULES Proteins,

polysaccharides, & other large molecules use vesicles to move in & out of cell

Exocytosis: secretion of macromolecules by fusion of vesicle w/plasma membrane Transport vesicle from Golgi

fuses w/membrane & expels contents out of cell

i.e.: insulin into blood; carbs out of plant cells

Endocytosis: intake of macromolecules & matter using vesiclesPlasma membrane indents, pinches in, &

forms intracellular vesicle1) Phagocytosis: “cell-eating”; intake of

solid matter Food is digested when vesicle fuses w/lysosome

2) Pinocytosis: “cell-drinking”; intake of extracellular fluid Non-specific

3) Receptor-Mediated: proteins w/specific receptor sites move large quantities of specific substances into cell i.e.: cholesterol into cell to make membrane