m embrane s tructure and f unction chapter 7 p. 124-130
TRANSCRIPT
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
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