MEMBRANE STRUCTURE & FUNCTION Campbell and Reece Chapter 7

MEMBRANE STRUCTURE

& FUNCTION

Campbell and ReeceChapter 7

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

Viscous Membrane Fluid Membrane

Animal Membranes

Cholesterol reduces membrane fluidity at moderate temps by reducing phospholipid movement; at low temps it hinders solidification by disrupting the regular packing of phospholipids

Evolution of Differences in Membrane Lipid Composition

Species-specific membranes are evolutionary adaptations made to maintain appropriate fluidity to accommodate specific environmental conditions

Examples

fish that live in lakes that freeze over in cold months have higher proportion of unsaturated hydrocarbon tails membranes stay fluid in colder temps

Examples

certain prokaryotes can change the composition of membranes depending on the temperature at which they are growing

Chryseobacterium greenlandensis

Examples: Plant Adaptations

% of unsaturated phospholipids increases as temps decrease which keeps membranes from solidifying

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

Oxygen

higher concentration in air inhaled in alveolar sacs diffuses into capillaries in alveoli thru circulatory system diffuses from capillaries in tissues where there is a higher concentration individual cells where concentration lower than capillaries mitochondria where concentration lower still

Osmosis

Water Balance

Tonicity: ability of a surrounding solution to cause a cell to gain or lose water

Depends on: concentration of solutes that cannot

cross membrane relative to the concentration of all solutes in cell

Isotonic Solutions

concentration of solutes same inside as outside cell

Hypotonic & Hypertonic Solutionsw/out a Cell Wall

Osmoregulation

control of solute concentrations & water balance

http://www.stolaf.edu/people/giannini/movies/paramecium/para%20cont.mov

Water Balance with Cell Walls

includes cells of plants, fungi, prokaryotes, & some protists

walls inelastic so cells in hypotonic solutions so wall can expand very little before it exerts backpressure on cell = turgor pressure which opposes further water intake

plants w/out wood require cells to be turgid for mechanical support

With a Cell Wall

Turgid vs. Flaccid

Cell Walls in Hypertonic Solution

plasma membrane pulls away from cell wall (called plasmolysis) plant wilts dies if does not receive water

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

Cells use the sucrose-H+ cotransporters to store sugars made in photosynthesis in veins of leaves

Plant will distribute sugar to other parts of plant as needed

another example

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)