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Biol 205 Spring 2008 Week 3 Lecture 2

REading Assignment: Chapter 12 Membrane Transport pgs. 389-393 & Figure 12-7 on pg 394

The Lives of the Cell by Lewis Thomas

“ … It takes a membrane to make sense out of disorder in biology. You have to be able to catch energy and hold it, … releasing it in small shares……. To stay alive, you have to be able to hold out against equilibrium, maintain imbalance, bank against entropy and you can only transact this business with membranes in our kind of world…..”

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Maintaining an orderd structure: membranes, of course, define the cell itself and the subcellular compartments found in a eukaryotic cell

Identify each of these subcellular structures Answers on last page of lecture notes

10 = garbage disposal/recycler

7 = network of protein filaments

(Ignore 4 & 12)

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Many activities of living cells take place on or across membranes: • Defining and maintaining an ordered

compartment • Receiving and transducing environmental

signals: known as Signal transduction • Mediating Cell-cell interactions • Capturing & transforming energy • Storing potential energy in the form of

ion or solute asymmetries established and maintained by membrane associated proteins

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A preview of the plasma membrane

All biological membranes have a common general structure: a thin film of lipid and protein molecules held together by noncovalent interactions cell membranes are dynamic and fluid and most molecules can move about in the plane of the membrane

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LIPIDS: • large biological molecues that have little affinity

for water • are NOT polymers • consist mostly of hydrocarbons • include fats, steroids and phospholipids

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Phospholipids are the most abundant lipid in the membrane Phospholipids are amphipathic: • hydrophilic region and a hydrophobic region:

Polar head

(hydrophilic)

Nonpolar tail(hydrophobic)

Glycerol

Phosphate

Serine

Fatt

y a

cid

Fatt

y a

cid

Formula Schematic Space-filling Icon

CH2 CH2

CH2 CH2

CH2 CH2

CH2 CH2

CH2CH2

CH2CH2

CH2CH2

CH2 CH2

CH2CH2

CH2 CH2

CH2 CH2

CH2 CH2

CH2 CH2

CH2 CH2

CH2 CH2

CH2 CH2

CH3 CH3

C OC O

OO

CH2C CH2

H O

O–

O–

PO

CH2

NH3+

HC COOH

Figure 4.4a

Phospholipid

What happens when phospholipids are mixed with water?

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Would a bilayer form is there was no hydrophilic head on the molecule?

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Proteins are imbedded in membranes

Nature 438: 578 2005 Membranes are more mosaic than fluid

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Proteins are imbedded in membranes

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THE TRANSACTIONS OF THE CELL MEMBRANE Controls molecular traffic into and out of cell: which molecules and particles enter and exit the cell Transduces signals from outside to inside of the cell Mediates the physical interactions between animal cells Mediates many enzymatic reactions including those involving energy transformation in the chloroplast and mitochondria

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Features of the plasma membrane the thin film of lipid and protein molecules held together by noncovalent interactions membranes are dynamic and fluid and most molecules can move about in the plane of the membrane

Membrane fluidity is important for many reasons: • enables membrane proteins to diffuse rapidly

in the plane of the bilayer and to interact with one another

• provides a simple means of distributing lipids and proteins by diffusion from sites where they are added to the bilayer

• allows membranes to fuse with one another and mix their molecules -- important for many cellular events

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What factors might affect membrane fluidity?

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Membrane fluidity is determined by: • temperature • the nature of the hydrocarbon tails on

phospholipids • presence of steroid molecules such as

cholesterol (present in animals and absent in plants, yeast and bacteria)

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Vegetable Oil versus Butter ? What’s the difference?

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The kink resulting from the double bond is exaggerated for emphasis

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The fluidity of a membrane • __________ with increasing amounts of

saturated hydrocarbon tails: these can pack tightly together and are ____ fluid as a result

• _________with increasing amount of unsaturated hydrocarbons: kinks in the tail introduced by C=C _______ close packing of the hydrocarbon tails

The fluidity of a membrane • varies ____________ with the length of the

hydrocarbon tail • decreases with increasing amounts of

cholesterol: short, rigid molecule which modulates the fluidity of the bilayer

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Cholesterol • common component of animal cell membranes • cholesterol is a type of steroid • steroids are lipids consisting of four fused rings

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• cholesterol decreases membrane fluidity because its

rigid steroid ring system interferes with the motions of the hydrocarbon chains in the phospholipids

• but it also inhibits the close packing of the

hydrocarbon chains animations showing effect of cholesterol http://telstar.ote.cmu.edu/biology/MembranePage/index2.html

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Proteins can associate with the plasma membrane

Various ways in which membrane proteins can associate with the lipid bilayer. Most transmembrane proteins are thought to extend aacross the bilyaer as a (1) single alpha helix, (2) as multiples alphas helices, or (3) as a rolled-up beta sheet.

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How can a protein be imbedded in the hydrophobic interior of a membrane?

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Functions of integral membrane proteins: • Transport proteins: control molecular traffic into and out of

cell • Enzymes: photosynthesis and respiration • Transduce signals from outside to inside of the cell: • transduction of polypeptide hormone signals • transduction of nerve impulses between cells • many other processes • Mediate the physical interactions between animal cells (cell

adhesion): form intercellular junctions • Attach to the cytoskeleton

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MEMBRANES: So WHAT?

Woe to that child which when kissed on the forehead tastes salty. He is bewitched and soon must die. This adage, from northern European folklore, is an early reference to the common genetic disease recognized today as cystic fibrosis. As the saying implies, the disorder once routinely killed children in infancy and is often identifiable by excessive salt in sweat.. (Scientific American Dec. 1995)

cystic fibrosis: most common severe recessive monogenic disorder affecting people of European descent Info about cystic fibrosis http://www.nlm.nih.gov/medlineplus/cysticfibrosis.html http://ghr.nlm.nih.gov/condition=cysticfibrosis

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The “cystic fibrosis” gene codes for a transmembrane protein involved in chloride transport (note gene is named for its mutant phenotype and not for the protein that it specifies)

CFTR= cystic fibrosis transmembrane conductance regulator CFTR= a chloride channel PREVIEW: This gene is 270 kb long but the mRNA is only 6129 bp long

These figures illustrate the domain structure of the protein

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To understand the significance of this defect we need to first consider membrane permeability

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The plasma membrane lies at the interface of the controlled interior and fluctuating exterior of a cell Membrane controls two distinct and crucial activities: • Selective Permeability: select among different

molecular species, slowing down the permeation of some while allowing others to pass unimpeded

• Active Transport: brings about the transport of

material, either inward (accumulation) or outward (excretion, secretion) against a concentration gradient

Permeate: to diffuse through or penetrate something

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What factors control whether an atom or molecule can cross the plasma membrane? Lipid composition of the membrane: a high % of cholesterol makes a membrane less

permeable to small molecules a high % of unsaturated hydrocarbon chains

increases permeability Characteristics of the molecule itself SUCH AS? Presence of transporter proteins

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The relative permeability of a synthetic lipid bilayer to different classes of molecules. The smaller the molecule and, more importantly, the less strongly it associates with water, the more rapidly the molecule diffuses across the bilayer

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Permeability coefficients for the passage of various molecules through synthetic lipid bilayers What is the rate at which a given molecule will cross a protein-less lipid bilayer? • the smaller the molecule the more readily it will cross the membrane • more hydrophobic, the more readily it will cross the membrane

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Nice animations on diffusion and transport http://www.wiley.com/legacy/college/boyer/0470003790/animations/membrane_transport/membrane_transport.swf

Small nonpolar molecules O2: the covalent bond between the two atoms is nonpolar because the electrons are shared equally It is a small molecule and therefore it will readily dissolve in a lipid bilayer and diffuse across it CO2 also readily diffuses Small uncharged polar molecules if small enough, uncharged polar molecules such as water and ethanol will diffuse rapidly across a bilayer glycerol diffuses less rapidly Large uncharged polar molecules glucose: hardly at all Charged molecules - large or small- lipid bilayers are highly impermeable to any charged molecule, no matter how small Charge and degree of hydration of such molecules prevents them from entering the hydrocarbon phase of the bilayer

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MEMBRANE TRANSPORT PROTEINS The passage of large, polar or charged molecules requires special membrane proteins Two major classes of membrane transport proteins: Carrier proteins and channel proteins • Both form continuous protein pathways across

the lipid bilayer • All membrane transport proteins are relatively

specific with respect to what molecules they transport

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Channel Proteins Forms a hydrophilic water-filled pores that extends across the lipid bilayer Specific for inorganic ions of the appropriate size and charge allowing them to cross the membrane Transport through channel proteins occurs at a much faster rate than transport through carrier proteins (discussed below)

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Channel Proteins Pores can be open or closed Allosteric Regulation of a cation channel:

Top and lateral representations of a cation channel • The channel is made of four identical blue subunits. • The orange triangles are small molecules (called

cyclic nucleotides) that can bind to the subunits of the channel.

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CFTR= cystic fibrosis transmembrane conductance regulator CFTR= a chloride channel http://www.genet.sickkids.on.ca/cftr/CftrDomainPage.html?domainName=MSD9

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All channel proteins allow the solute to cross the membane passively: down the concentration gradient This is known as passive transport or facilitated diffusion

NOTE: the elegant secondary and tertiary structure

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Diffusion: • net drift of molecules down a concentration

gradient due to random thermal movement • net random drift of molecules in the direction

of lower concentration Fun web site on diffusion and osmosis: http://physioweb.med.uvm.edu/diffusion/tocpage.htm

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A conformational change in the transporter mediates the passive movement of a solute.

Carrier Proteins (aka Transporters): have a specific binding pocket for the solute to be transported; only solute molecules that fit into the binding pocket can be transported – conferring a level of specificity similar to that of the substrate-active site interaction seen with enzymes The transporter shown can exist in two conformational states: in state A, the binding sites for solute are exposed on the outside of the lipid bilayer; in state B, the same sites are exposed on the other side of the bilayer. The transition between the two states can occur randomly. It is completely reversible and does not depend on whether the solute binding site is occupied. Therefore, if the solute concentration is higher on the outside of the bilayer, more solute binds to the transporter in the A conformation than in the B conformation, and there is a net transport of solute down its concentration gradient (or, if the solute is an ion, down its electrochemical gradient).

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Transport animations http://www.wiley.com/legacy/college/boyer/0470003790/animations/membrane_transport/membrane_transport.swf

passive transport or facilitated diffusion: channel or carrier-mediated diffusion down

the a concentration gradient active transport: carrier mediated active pumping of certain solutes across

membrane against their concentration gradient active transport is tightly coupled to a source of metabolic energy

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Three ways of driving active transport. The actively transported molecule is shown in yellow, and the energy source is shown in red. Coupled transport uses potential energy in the form of one solute gradient (see next page) to drive the movement of another solute against its concentration gradient

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Ion Transport across cell membranes is of central importance in biology: • Cells maintain an internal ion concentration that is

different from that of the fluid around them • These ion gradients are critical to a number of

cellular processes Comparison of Ion Concentrations Inside and Outside of a Typical Mammalian Cell Component Intracellular

Concentration (mM) Extracellular Concentration (mM)

Cations: Na+

5-15

145

K+ 140 5 Mg++ 0.5* 1-2 Ca++ 10-7* 1-2 H+ 7 X10-5

[10-7.2M = pH 7.2] 4 X 10-5 [10-7.4 or pH 7.4]

Anions Cl- Fixed Anions**

5-15 high

110 0

* concentrations of free ions in the cell; the total cellular concentration is much higher ** negatively charged small and large organic molecules that are trapped inside the cell unable to cross the plasma membrane

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OPTIONAL STUFF HOW TO LET A BIG ION THROUGH WITHOUT LETTING A SMALLER ION OF THE SAME CHARGE THROUGH K+ ions are attracted to the selectivity filter of the Potassium Channel by the electrostatic potential set up by amino acids with a negative charge. Focused around the Channel's entrance, the negatively charged amino acids also repels anions, therefore increasing the concentration of cations (K+) at the pore's entrance.

Around the channel water molecules surround the cations, keeping them stable. The selectivity filter is not big enough to accommodate an ion and its water molecules. Therefore, before any ion can enter the selectivity filter it must lose all of its water molecules. However, to enter the selectivity filter the ion must remain stable. Oxygen atoms in the backbone act as surrogate water for K+ ions, keeping them stable. The smaller Na+ ion is not big enough for the oxygen atoms to act as surrogate water and stabilize it. Therefore, only K+ ions can enter the selectivity filter and remain stable.

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