BIOLOGICAL MEMBRANES:

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°Structure
°Functional characteristics
°Membrane-Bound proteins
°Resting membrane potential (anelectrical gradient that exists across the membranes of allliving cells and which results from the concentrations of ions inthe inside and outside of the cell and the presence in themembrane of particular membrane-bound proteins).


Now this information is going to be important in dealing with thetransmission of information through nerve impulses. It's alsogoing to be important in understanding how muscles function,hormones work, and is pretty basic information for a variety ofsubsequent topics.

Structure:

Now, as you have no doubt heard at some time in the past that themembrane is composed of lipids and proteins. About 50% of themass of the membrane is composed of lipid and about 50% of themass is protein. Most of the lipid in the membrane isphospholipid and the structure of these phospholipids is thatthey have a phosphate group which has a negative charge andattached to that are 2 long strings of carbon atoms strungtogether so you have somewhere in the neighborhood of 18 to 22 ormore carbons with OH groups attached to them, that's the lipidpart of the molecule. The reason that this structure is importantto understand is that the result of it is part of this moleculeis highly soluble in water and part of the molecule is highlysoluble in lipid. The hydrophilic part (the phosphate) likes thewater and is soluble in water. And the part that's soluble inlipid is not soluble in water and that's the part that is said tobe hydrophobic.

Most molecules are either soluble in lipid or they are soluble inwater but they are not soluble in both. If you take purifiedphospholipids like this, and you put them in a beaker with water,and shake it up you'll find that the molecules spontaneouslyarrange themselves in a structure which is called a phospholipidbilayer. If you can look at this layer of molecules from theside, you would see the phosphate groups lining along next to oneanother, the lipid parts lined up line next to one another andthen that's 1/2 of the bilayer, there would be more of thesemolecules arranged in a similar fashion. Think of this like agreat big huge pancake and we've cut the pancake in the side andthis is the side view of the pancake. The water is out here anddown here are these hydrophobic lipid molecules that are notsoluble in water. They have arranged themselves in order tocreate a continuous layer of lipid and this stuff is not going toarrange itself in a sheet like a pancake. It's going to arrangeitself in a little tiny sphere. Like a balloon with water in theinside of it and water on the outside and the walls of theballoon is analogous to this continuous layer of lipid. And thatis a cell membrane. That's what cell membranes look like. Or atleast that's what the lipid part of the cell membrane looks like.It's a lipid bilayer.

Unit membrane - A little balloon with water and cytoplasm on theinside, water and other electrolytes on the outside of the cellmembrane at this point is a bilayer. And it is found not onlysurrounding a cell where it forms the cell membrane or moretechnically the plasma membrane of a cell, but this same basickind of structure is found around the nucleus, forming thenuclear membrane. It's found around mitochondria forming themitochondrial membrane. And it is also found forming a structurecalled the Endoplasmic Reticulum (E.R.) which is a membranousstructure within the cell that has ribosomes on it and isinvolved with the function of production of proteins.

But this is our fundamental structure/building block of manydifferent important parts of the cell. Plasma membrane, nucleus,the mitochondria and etc. Now, if this was all that a plasmamembrane was composed of was just phospholipid, then what wewould find is some functional properties of one of these littlespheres (bilayer).

Functional Characteristics

1) Lipid soluble - highly permeable.
2) Large polar molecules (uncharged) that are not lipid solublehave low permeability.
3) Ions - low permeability.
4) Water - high permeability.

For anything to get across this membrane it must be able todissolve in lipid so in terms of functional characteristics, whatwe could see is that if things were lipid soluble they coulddiffuse through this membrane, they can dissolve in the membrane,they could move from the outside of the cell through the membraneinto the inside of the cell or vice versa. Just moving accordingto their own concentration gradient (difference inconcentration). If the concentration is higher on the inside thestuff would just leak out. If the concentration was higher on theoutside it would just diffuse in. It would be just like themembrane wasn't even there. So lipid soluble molecules will behighly permeable. Or the membrane is permeant. The membrane wouldbe permeable to lipid soluble molecules.

Large polar molecules have charged components on them, like aminoacids and proteins, or even uncharged large molecules like sugarmolecules. They would not be able to get across this membrane. Itis very very low permeability to even medium size molecules andparticularly low permeability to charged molecules like aminoacids and proteins. So large, medium, polar molecules that arenot lipid soluble, the membrane would have a very lowpermeability.

And finally ions which are in most cases fairly small, arecomposed of 4 or 5 atoms, ions which are very small and have acharge on them so it would be almost impossible for them to getthrough the membrane. The membrane would have a very lowpermeability to ions.

But the membrane would have a fairly high permeability to water.Apparently the water can sort of squeeze between the lipidmolecules, this kind of membrane does have fairly highpermeability.

Now there is another kind of lipid that is also found in theplasma membrane of cells and this other lipid appears to bearranged here sort of in spaces between the phospholipids andthat lipid is called cholesterol. Again that may also contributeto the fact that there is this continuous lipid bilayer. But as Itold you actually plasma membranes are not composed of a purephospholipid bilayer. The actual plasma membranes that are foundsurrounding cells are only about 50% of the mass will be composedof phospholipids and the other 50% is composed of proteins. Theseproteins, the actual location of these proteins was the subjectof fairly intensive research 20 years ago and in 1972 a pair ofscientists (Singer & Nicolson) proposed a hypothesis whichhas been shown to be true which they called the Fluid MosaicModel of the membrane which says that these proteins that arefound associated with membranes, the membrane-bound proteins, arein fact not attached to the outside of a membrane but they areactually floating in the membrane. And what we know is that theseproteins are long, strings of amino acids and many of those aminoacids have charged places on them, but some amino acids are fatsoluble amino acids and so typically you would represent aprotein by the long string like this and then in some cases thesemembrane-bound proteins go all the way through the membrane. Theygo from, if this is the outside of the cell and this is theinside of the cell down here at the bottom, it would go all theway through. And they have in critical locations around theiredges of this 3 dimensional structure of this protein are goingto be amino acids that are fat soluble. And they will have a highaffinity (they will want to attach to the phospholipids that areadjacent to them) so you end up with what are called annularlipids which are the lipids that form a ring, that's what annularmeans a ring. You have a ring of lipids that are attached not bycovalent bonds but by the low sort of weak molecular forces thefat soluble amino acids are kind of attaching these lipids tothem so that the protein sits in the membrane in a very specificorientation which is determined by the primary structure by thesequence of amino acids in a way that thing folds up. So thisprotein has a very specific shape and a very specific design sothat it sits in the membrane in a very specific way and some ofthem go all the way through the membrane, some of them are onlyexposed on one side of the membrane and both of the possibilitiesdo exist, in other words there are some of these proteins thatare only exposed on the outside of the cell and they aresurrounded by their annular lipids that hold them there.

Now the reason why Singer & Nicolson called this the FluidMosaic Model is that there are 2 ideas here, one of them is thatthese proteins are kind of, if you think of a person whosesitting in an inner-tube floating in the river, well theinner-tube is like your annular lipids and your legs are danglingdown in the river, that's like being down in the membrane and thetop of your body is sticking out up in the air and you're kind offloating around in the river, that's the idea of the way in whichthese proteins are floating around in the membrane. That's thefluid part of the idea is that they are floating in the membraneand they are maybe even mobile. They can move around in themembrane just like the person floating on the surface of theinner-tube. But it's also referred to as a Mosaic because if youwere able to look at that membrane from outside what you wouldsee is something like a whole bunch of people floating on a lakein inner-tubes. You'd see a bunch of water but there'd be allthese little people with their inner-tubes floating around andthat would look like a Mosaic effect with proteins sticking outof the cell all around the whole entire surface of the cell andthe presence of these proteins in the membrane has a veryimportant effect on the functional properties of the membranebecause these membrane-bound proteins have 3 basically differentfunctions.

We use some special terms (above) when referring to thesedifferent functions.

Now what do we mean by recognition the first of these functions?There are a number of different situations where cells need to beable to detect the presence of a particular molecule in thesolution outside of the cell. A good example of that is in thecase of a hormone where the hormone may be a large proteinmolecule which cannot get inside the cell but the hormone needsto tell the cell to do something and so when that is the casethen the cell has a membrane-protein, one of the ones that'sexposed just on the outside which has exactly the properstereospecific configuration so that it can combine with thathormone that it needs to detect. Proteins can catalyze chemicalreactions, they function as an enzyme and they are able to causea particular chemical reaction to occur because they will combinewith the substrate in a lock and key fashion. That lock and keyfashion, that shape, the charge/arrangement that allows a proteinto combine with a substrate molecule is the same thing that thisrecognition protein is doing, it has the proper shape, and chargedistribution, and when that molecule is floating around in thesolution surrounding the cell by random molecular movement it'sgoing to bump into this recognition protein and that's going totrigger some subsequent sequence of events that makes the cellaware of the fact that the hormone is out there. The name givento this membrane-bound protein whose function is recognition is areceptor protein.

The second function is Catalysis. That is, these are enzymes.That's the name we give to a protein whose function catalysis.And you know what enzymes are, they cause particular chemicalreactions to happen at higher rates than they would without thechemical reaction. Cells have billions of enzymes floating aroundin their cytoplasm, catalyzing all kinds of chemical reactions,they have enzymes in their mitochondria that carry on electrontransport and the Krebs Cycle so you learned about lots ofdifferent kinds of proteins and lots of different enzymes butthese Membrane-bound enzymes are a very small specialized sub-setof enzymes which are actually attached to the cell membrane.Sometimes they are attached and exposed on the outside where theyactually catalyze chemical reactions in the fluid outside of thecell. Sometimes they are exposed only on the inside where theycatalyze specific chemical reactions in cytoplasm of the cell butinstead of floating around in the cytoplasm the way mostcytoplasmic enzymes do, these enzymes are actually floating inthe membrane. One example of such a MBP enzyme is when a hormonecomes along, combines with the receptor, the receptor turns on anenzyme and the enzyme catalyzes the chemical reaction inside ofthe cell that makes the cell aware of the presence of the hormoneon the outside.

Now the third major function of membrane-bound proteins isCarrier Mediated Transport and the most important word in this isTRANSPORT - that is moving things across the membrane. If themembrane were composed just of lipid ions, sugar molecules, aminoacids would not be able to get into or out of the cell. But weknow that those things have to be able to get into and out ofcells and the reason that they are able to do that is because ofmembrane-bound proteins that have this function of transportallowing things that could not get through the membrane bythemselves to get through the membrane. There are many thingsthat are fat soluble things that can get through the membranewithout being helped by a membrane-bound protein but the onesthat we're talking about here are the ones that have to be helpedby the proteins. So Carrier Mediated Transport means movementacross the membrane which is made possible by a carrier and thatcarrier is the membrane-bound protein. The membrane-protein isthe carrier and it somehow manages to allow the amino acid or thesugar molecule or the ion or whatever it is to get through thatmembrane.

There are 2 different sub-sets of Carrier Mediated Transport. Oneis called Facilitated diffusion and the second sub-set is ActiveTransport.

Facilitated diffusion is a sub-set of diffusion. Diffusion is themovement of a substance from an area of high concentration to anarea of low concentration. In other words pure random molecularmovement will tend to cause something to move from where it findsitself in a high concentration to where it finds itself in a lowconcentration. And that would be simple diffusion - just movingby random molecular movement. But in this case we're talkingabout something that could not get through the membrane if itweren't for the particular membrane-bound proteins that are thereto help stuff get through. Facilitated is a word that just means"aided or assisted". So there are membrane-boundproteins whose function is to help things go through the membranein the direction that they would normally diffuse. And we refereto those proteins whose funciton is facilitated diffusion as"channels". We cab think of a channel as a proteinhaving a shape like a doughnut with a water-filled center throughwhich only specific molecules are allowed to pass.

There is another group of membrane-bound proteins that arehelping things get through membranes but they have the ability tomove things against the concentration gradient. That is in theopposite direction to the one in which they would move bydiffusion. These membrane-bound proteins can make things movefrom an area of low concentration to an area of highconcentration, against the concentration gradient. And thisfunction is referred to as "active transport" and wecall those proteins "pumps". In order for a pump tomove something against the gradient it has to use energy. In mostcases these pumps consume ATP. Just like an electrical pump useselectricity. These are little proteins sitting in the membraneand they use ATP and the grab something in the area of lowconcentration and they move it to an area of high concentration.Transporters are usually the type of protein that go all the waythrough the membrane. If they are channels they may be allowingthings to diffuse in from the outside or if they may be allowingthings to diffuse from the inside out. For a channel, thedirection of movement is not going to be determined by theprotein, it's going to be determined by the direction ofconcentration gradient. But the channels are very specific, thesize, or the charge distribution or something about the channelis very specific so you have sodium channels that only allowsodium ions or we have potassium channels that only allowpotassium through, don't let sodium through even though they areboth fairly similar sized molecules. Pumps on the other hand havea direction. They will move something from one side of themembrane to the other. They might be pumping something from theoutside in or they might be pumping something from the inside outbut pumps are also very specific, they use energy and they'regoing to move a specific kind of molecule in one direction only.So in that case the directionality is determined by the membrane.

Channels and pumps are both very specific, they only work withparticular substrates. Thus, they have to be able to recognizethe they substrates are going to work on. In other words, for asodium channel to be specific, it has to know "this is asodium I'm going to let through here." For a pump to pump aparticular substance in one direction it has to recognize thatparticular substance as being that one substance which it isgoing pump in or out of the cell. And, in addition to that, apump is going to take ATP and turn it into ADP, so in a sense apump recognizes the substrate and it catalyzes the chemicalreaction splitting ATP, but it's function is to be movingsomething from one location to another; whereas a receptor, theonly thing the receptor does is it just combines with theparticular molecule that it recognizes and anything else thathappens is going to be a function of one of these othermembrane-bound proteins.

A very important characteristic of channels is that they caneither be in the "off" (or closed) state or"on" state. So there are different things that willcause some channels that are normally closed to open up and allowwhatever it is they are specific for to diffuse through. Andsimilarly, membrane-bound enzymes like the one I talked aboutearlier with the hormone, can sit there in the "off"status until the hormone comes along and combines with a receptorand then the receptor says, "hey I've got a hormone outhere" and then the enzyme turns on and starts catalyzingit's reaction. But pumps generally are always on, they willfunction and they will pump whatever they are designed to pump ifthey can find any of it.

A receptor is there to recognize the presence of a particularmolecule in the solution surrounding the cell, to detect thepresence of it. It's like a doorbell on your house, and whensomebody comes up and they want to tell you that they are there,they'll push the doorbell. Well the doorbell is like thereceptor, and the bell that rings inside your house is maybe morelike the enzyme that turns on inside the house and lets you knowthat they are there.

Resting Membrane Potential:

There is an electrical gradient across the membrane of all cellsand the next thing that I want to talk about is what causes thatResting Membrane Potential to exist. Why is there a RestingMembrane Potential?

To explain the resting membrane potential requires 4 facts.

1) There is a difference in the concentration of potassiumbetween the inside of the cell and the outside of the cell. Thatis to say that there is a potassium gradient. Specifically, thepotassium concentration inside the cell is a lot higher than thepotassium concentration outside.

2) The membrane is permeable to potassium. What that means isthat the potassium can get through the membrane and the reasonfor that there are many potassium channels in the membrane.

3) The majority of the negative charges inside the cell areproteins so that for every potassium inside the cell there is anegative charge on a protein (not totally true, but nearly true).So the negative charge on the protein inside the cell nearlyelectrically balances the positive charge on the potassium.

4) The protein cannot get out. The membrane is not permeable toprotein because it's too big. We said the lipid bilayer is notgoing to be permeable to large molecules, proteins are largemolecules. So proteins can't get out of cells.

Now those 4 facts explain the existence of a Resting MembranePotential which is a slight excess of negative charge on theinside of the cell. You can measure the voltage (the electricalpotential) across this cell with a little vice called amicro-electrode and you hook it up to a volt meter and youmeasure the electrical difference across the cell, it's going tobe different in different species and in different cells but onaverage, it's going to be about 70 millivolts negative on theinside and so the convention we say that the potential is minus70 millivolts (0.070 of a volt). Why is the cell slightlynegative on the inside compared to the outside? Well, we canthink about it this way, the potassium wants to diffuse out ofthe cell. There's a higher potassium concentration inside thecell than outside so there's a chemical gradient that favorspotassium leaving the cell and the membrane is permeable topotassium so that the potassium can go. And so what happens is afew potassium ions exit the cell, they will actually move out ofthe cell. But their balancing proteins cannot follow them becausethe membrane is not permeable to protein and so these littlelonely negative charged proteins get left behind and there's avery slight separation of charge and the inside of the cellbecomes negatively charged. And that causes the Resting MembranePotential.

Why don't all of the potassium ions inside the they all leak outof the cell? The negative charge holds them in. After the firstfew potassium ions leave, and the inside of the cell is negative,think about what happens to this little potassium ion that sortof sits here like this, part way through the channel thinkingabout leaving. That potassium ion now has 2 opposing forcesacting on it. One force is tending to make that potassium ionleave and that force is the difference in potassiumconcentration. But what force is balancing that? The negativecharge. This is a positively charged ion that's trying to leavethe cell and the inside of the cell is now negative. We know thatopposite charges attract so the negative charge that was createdby the first few potassiums leaving now acts to keep all the restof the potassiums from leaving. And so there are 2 forces thatare acting in opposition to eachother, and they tend to balanceeach other.

INTERESTING COMPLEXITIES:

There is another positively charged ion that's very important andthat is sodium. And the concentration of sodium on the outside ismuch higher than the concentration of sodium on the inside. Whichway is sodium going to want to move? Into the cell. What forcesare making sodium want to move into the cell? The concentrationgradient for one, what else? The negative charge. That sodium ionis sitting out there, thinking "what a neat place to be, Imean there's no sodiums in there and it's negative in there"so there are 2 forces acting on sodium making it tend to go in.Consequently, there is a very slow leak of sodium into the cellwithout a channel. The sodium kind of wedges its way betweenthose lipid molecules and there's such a powerful attraction thatit manages to squeak through that lipid bilayer because of theforces acting on it. The negative charge and the concentrationgradient. So a few sodiums leak into the cell.

Now every time one of those sodiums leaks into the cell, thatlets another potassium out. Over the course of days, this cellwould come to have the same potassium concentration on the insideas on the outside and it wouldn't have a Resting MembranePotential, it'd be a dead cell. So the cell has to do somethingto counteract that inescapable leak of sodium in and potassiumout. So what the cell does is it has a pump that pumps sodium andpotassium at the same time. It kicks sodium out and it pullspotassium back in. And this is called the SODIUM POTASSIUMEXCHANGE PUMP. It must be there in order to keep the potassiumconcentration higher on the inside and to keep the sodiumconcentration lower on the inside. That's what it's there for,it's there to make up for this inescapable slow leak of sodiuminto the cell. This pump does not have to be working for there toa Resting Membrane Potential. In fact you can poison the sodiumpotassium exchange pump with a chemical called Ouabain(pronounced "wah-bane"). And the cell will continue tohave a resting potential for hours. Is the membrane potentialconstant for hours and hours? No. It's going to move from minus70 to 0. Why? The slow leak of sodium into the cell inside isgoing to let potassium go out. Consequently, the concentration ofpotassium is going to decline and driving force for diffusion ofpotassium will decline and the electrical gradient will decline.

So, in summary, the membrane potential exists because there is ahigher concentration of potassium inside the cell and thepotassium can leave through the potassium channels and thenegatively charged proteins cannot. This produces an electricalpotential across the membrane called the membrane potential. Thesodium pump is important for creating the potassium concentrationgradient but it is only indirectly responsible for the membranepotential. Since the potassium pump is actually moving chargedparticles, it was originally hypothesized that the pump directlycreated the membrane potential, but the experiment with Ouabainproved that the membrane potential would continue to exist, evenif the pump stopped working. This was a very important experimentfor understanding the real basis of the membrane potential.