CN: PAGE 22EQ: HOW DOES THE
STRUCTURE OF A MEMBRANE ENABLE IT TO CONTROL WHAT GOES IN &
OUT OF CELL
Plasma Membranes
EARLY FLUID MOSAIC MODEL
UPDATED MODEL of ANIMAL CELL PLASMA MEMBRANE
PHOSPHOLIPID BILAYER
HOW TO MAKE A PHOSPHOLIPID
GLYCEROL
+ phosphate group = “head”
+ 2 fatty acid “tails”
PHOSPHOLIPID
Fluidity of Membranes
Fluidity in Membranes
the more unsaturated tails the more fluid the membrane (cannot pack the tails as close together as straight saturated tails)
Fluidity in Membranes: Cholesterol
only in animal cell membraneswedged in between hydrophobic tails
Cholesterol in Membranes
Cholesterol’s Effect
@ 37ºC cholesterol makes membrane less fluid by restraining phospholipid movement
lowers temp required for membrane to solidify
Membrane Proteins
>50 membrane proteins 2 main categories:1. Integral Proteins
penetrate the hydrophobic inside of lipid bilayer
most are transmembrane proteins2. Peripheral Proteins
appendages loosely bound to either surface
Membrane Proteins
Membrane Proteins
on cytoplasmic side some proteins held in place by attachment to cytoskeleton
on ECF side some proteins attached to fibers in extracellular matrix
both give animal cells stronger framework
Major Functions of Membrane Proteins
1. TRANSPORT provides hydrophilic channel thru
hydrophobic interior of lipid bilayer some use passive some active transport
Transport Proteins
Major Functions of Membrane Proteins
2. ENZYMATIC ACTIVITYall enzymes are proteins so a membrane
protein could have all or part of its structure function as an enzyme
in some membranes several enzymes organized to carry out sequential steps in a metabolic pathway
Membrane Protein as Enzyme
Major Functions of Membrane Proteins
3. SIGNAL TRANSDUCTIONmembrane protein acts as receptor has
binding site with specific shape that exactly fits shape of the chemical messenger (signal molecule or ligand)
when signal enters receptor site usually the membrane protein changes shape (configuration) which relays message into cell, usually binding to a cytoplasmic protein
Signal Transduction
Major Functions of Membrane Proteins
4. CELL-CELL RECOGNITIONsome glycoproteins act as ID tags
recognized by membrane proteins of other cells which may bind to them
attachment short-lived
Cell-Cell Recognition
Major Functions of Membrane Proteins
5. INTERCELLULAR JOININGmembrane proteins of adjacent cells may
hook together in different types of cell jcts
tends to be long-lasting
Cell Junctios
Major Functions of Membrane Proteins
6. ANCHORINGcytoskeletal elements may be
noncovalently bound to membrane proteins: helps maintain cell shape & stabilizes location of membrane proteins
Cell Surface Proteins
medically important:1. some pathogens use them to
adhere/enter cell2. some medications designed to take
advantage of using them
Glycocalyx
glycoproteins + glycolipids usually ~15 sugar units
exterior surface of cell membranekey to cell-to-cell recognition
sorting cells in embryo Immune System
Plasma Membrane Asymmetry
like cell membrane exterior surface
Selective Permeability
plasma membrane example of emergent properties: each individual membrane protein, lipid, or carb together become a “supermolecule”
Selective Permeability
essential to cell’s existencesFluid Mosaic Model helps explain how
regulation occurs
24/7 steady stream on ions & small molecules in/out cell; each at their own rate
Selective Permeability
Selective Permeability
depends on:1. lipid bilayer2. specific transport protein built into
membrane
Selective Permeability
In general:small, nonpolar molecules get inions and polar molecules don’t get in
Transport Proteins
hydrophilic substances get thru hydrophobic lipid bilayer by going thru center of a transmembrane, transport protein
Channel Proteins
hydrophilic channel hydrophobic a.a. in portion of protein
that interfaces with lipid bilayerAquaporins: allow water molecules to
crosschannel open, allows up to 3 billion
water molecules/swater follows its concentration
gradient by osmosis
Aquaporins
Carrier Proteins
attach to their “passenger” change in shape so that passenger is shuttled thru membrane
very specific: 1 substance or small group of similar substances
Passive Transport
is diffusion of substance across membrane w/no nrg investment
Diffusion
In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated. (it will move down its concentration gradient)
No work required: spontaneous because particles have KE and are in constant motion ex: O2 & CO2
Osmosis
Isotonic Solutions
concentration of solutes same inside as outside cell
Hypotonic & Hypertonic Solutionsw/out a Cell Wall
http://www.stolaf.edu/people/giannini/movies/paramecium/para%20cont.mov
Facilitated Diffusion
channel or carrier proteins that allow hydrophilic substances to cross membranes moving down their concentration gradients
if transport ions called ion channelsmany are Gated Ion Channels
open/close mechanism works in response to stimuli (electrical, specific ligand)
Facilitated Diffusion
http://programs.northlandcollege.edu/biology/Biology1111/animations/passive3.swf
Gated Ion Channels
Glucose Transporters
Cystinuria
example of disorder due to absence of carrier protein for cysteine & other a.a. in kidney cells
normally a.a. reabsorbed in kidneys using carrier proteins
in this disorder the a.a. accumulate kidney stones
Active Transport
moves substances against their concentration gradient
requires energyallows cell to maintain concentration
gradients
Na+/K+/ATPase Pump
Na+/K+/ATPase Pump
http://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/animations/ion_pump/ionpump.html
How Ion Pumps Maintain Membrane Potential
all cells have voltages across the plasma membrane
(-) because cytoplasmic side (-) relative to ECF side
overall inside/outside cell neutral but just inside (-) & just outside (+)
Gradients across thePlasma Membrane
a difference in charge across membrane is called:
membrane potentialrange is -50 to -200 mV
Membrane Potential
like any battery has potential energy cell uses it to control movement of all
charged particles across plasma membrane
inside (-) compared to outside so passive movement of cations into cell & anions out of cell favored
Ions Move Down Electrochemical Gradient
2 forces drive diffusion:1. chemical gradient
concentration gradient2. electrical gradient
cations move into cell, anions out
Example: Absorption in Small Intestine
Electrogenic Pumps
transport protein that generates voltage across a membrane
major one in animal cells is Na+/K+/ATPase pump
major one in plants, fungi, & bacteria is a proton pumpactively transports protons (H+) out of
cells increases + charge outside and
increases – charge inside cell
Proton Pumps
Electrogenic Pumps
by generating voltage across a membrane potential energy is increasedcan be used for cellular workused in mitochondria to make ATPused in cotransport
cotransport
a substance that has been pumped against its concentration gradient holds potential energy
that energy can be used to do work as it moves back across the membrane down its concentration gradient
2nd protein (not the pump) called a cotransporter can couple the downhill diffusion this substance with a 2nd substance moving up its own concentration gradient
cotransporters
Bulk Transport Across the Membrane
used by large macromolecules or large volumes of smaller molecules
1. Exocytosis2. Endocytosis
Exocytosis
transport vesicles from Golgi move along microtubules to plasma membrane
membrane of vesicle comes in contact with plasma membrane
proteins in membranes rearrange lipids in vesicle membrane & plasma membrane so that they fuse
contents released into ECF
Exocytosis
Exocytosis
Endocytosis
cell takes in substances vesicle made with membrane from cell membrane
uses different membrane proteins than in exocytosis but looks like reverse of exocytosis
3 types:1. phagocytosis2. pinocytosis3. receptor-mediated endocytosis
Phagocytosis
“cell-eating”wraps pseudopods around substance
creating a membranous sac = food vacuole lysosome to be digested
Pinocytosis
“cell-drinking”cell takes “gulps” of ECF for solutesnonspecific
Receptor-Mediated Endocytosis
allows cells to take in specifically what it needs
specific ligands bind to specific membrane proteins
receptor proteins with ligands in place cluster together into “coated pits” (on cytoplasmic side)
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