welcome in this video we're going to be looking at the structure and function of lipids so lipids are a category of molecules that contain fats and oils as well as membrane lipids so let's get right into it when we look at the structure of lipids one of the interesting things about them is they're not really characterized by specific functional groups a number of the structures of lipids have a pretty diverse look to them and as a result we can't necessarily just look at a molecule and be able to put them into this category of lipids the one thing that they all share in common is that they're non-polar which means they're insoluble in water as we've mentioned in the past water is a polar molecule and will dissolve other polar or charged molecules like dissolves like but when we put lipids that are nonpolar into water that is polar we get phase separation usually so we don't have mixing we get oil and water oil would be one of the lipids that we're going to be talking about in this chapter all right because of the the diverse set of structural features that these lipids contain their functions tend to have an equally diverse variety of things that they do so we know that fats and oils can be used for energy storage we do store fat in our adipose tissue as lipid droplets we also know that the lipids are present in membranes so if you've heard of the lipid bilayer before lipid bilayer obviously would be containing lipids we have a few fat soluble vitamins so the fat soluble vitamins would be fitting into this lipid category so those are a d e and k vitamin a vitamin d vitamin e vitamin k all of the other vitamins are water-soluble vitamins and would not be fitting to this category we have some of the hormones that are lipids obviously we have a huge diversity of structure for our hormones some of those are proteins some of those are other small molecules that are hydrophilic and then we have some that are our lipids body all right so one of the things that we want to talk about are fatty acids fatty acids themselves are lipids but in addition to that they also can be used as a part of other lipids and look at them as part of fats and oils as well as parts of the lipids that we find in membranes so fatty acids are just long chain carboxylic acids so we've looked at the carboxyl group before the carbonyl connected to a hydroxyl group in this particular case we do have a carboxylic acid but we have a long chain of carbons that are connected to that carboxylic acid and that's called a fatty acid if you remember when we were talking about carboxylic acids we said that the smaller ones the ones with fewer numbers of carbons are soluble in water so kind of one to three or four are soluble in water and then as we begin adding more carbons it gets less soluble in water so these longer chain fatty acids are not soluble in water and therefore we term them fatty acids because they're dissolvable or soluble in nonpolar solvents in addition they also usually contain an even number of carbons and that's just based on how they're being produced in the organisms that are producing them whether that be plants or animals or fungi when we generally when we begin synthesizing fatty acids we're going to be connecting two carbon fragments together and then ultimately the total number of carbons would be an even number so if you look at this particular structure what you'll find is that there are 14 carbons in that particular molecule and the molecule kind of up above is shown in its kind of condensed structural formula and then down below kind of even more condensed where we're taking the all these cht groups and we're kind of condensing them down and telling just how many of those we have which obviously helps us a lot to be able to draw the structure without making a lot of work for ourselves many of these fatty acids in addition to having an iupac systematic name also have common names so this particular one is called myristic acid because of the number of carbons that it has and and the fact that it only has carbon-carbon single bonds um again the the common name myristic acid unfortunately unless you've memorized how many carbons mursic acid has the the word meristic doesn't really tell us how many carbons that particular common names but it's less helpful in terms of what does the structure look like all right so as we've mentioned back in our chapter when we were talking about alkenes we can have alkanes and alkenes saturated and unsaturated again saturated would be only carbon-carbon single bonds and unsaturated be one or more carbon-carbon double bonds and the saturated and unsaturated terms also apply to fatty acids so if you look at the fatty acid on the top you'll see that it just contains carbon-carbon single bonds and that would be called a saturated fatty acid and when we think about fats and oils later we're going to have higher concentrations of these saturated fatty acids that are components of fats and oils in contrast the molecule on the bottom has a carbon-carbon double bond and that carbon-carbon double bond makes that fatty acid an unsaturated fatty acid and you can see from the geometry around the carbon-carbon double bond that we have hydrogens coming off on the same side these carbons are coming off on the same side if we draw a line through here that gives us a cis and trans isomer this particular isomer is cis so that's a cis fatty acid most of the fatty acids that are produced in plants and animals are formed in cis but we also have been able to synthesize or convert cis fatty acids to trans fatty acids in labs some of the physical properties of these saturate these fatty acids that we find is that the saturated fatty acids tend to have higher melting points because they form more london dispersion force interaction again remember most of the interactions that we find in just carbons and hydrogens are london dispersion force interactions we can't form hydrogen bonding interactions between uh carbons and hydrogens or if a hydrogen is covalently attached to a carbon to form a hydrogen bond we have to have a hydrogen covalently attached to a nitrogen oxygen or fluorine a very electronegative atom and so the bulk of these fatty acid molecules contain carbons covalently bonded to other carbons and hydrogens we don't have any polar bonds there and we don't have the ability to hydrogen bond so we really only have london dispersion force interactions and so you can see in the structure on the bottom of that saturated those those four saturated fatty acids what we can see is that because they have a fairly consistent geometry they can begin stacking together and forming longer range dispersion forces or london dispersion force interactions and as a result of those longer range or more linear dispersion force interactions we get a stronger attraction between one fatty acid and another and as a result of that they have high melting points so usually at room temperature these saturated fatty acids tend to be solids and we would have to bring the temperature up to actually convert them into a liquid in contrast the unsaturated fatty acids as a result of the carbon-carbon double bond and again most of them are in cis there is a little bit of a kink in its structure and it kind of prevents any kind of stronger london dispersed force interactions because of that kink they don't really stack together well they disrupt london dispersion force interactions and as a result they have lower melting points in fact those compounds that contain higher amounts of these unsaturated fatty acids tend to be liquids at room temperature which would suggest that the temperature of the room or 25 degrees celsius is enough to melt something that would have normally been solid and keep that in the form of a liquid the phase of the liquid again compounds that have mostly or higher concentrations of these carbon carbon double bonds or are unsaturated will tend to be liquids at room temperature so if we think about how these relate to fats and oils fats would generally have more higher concentration of the saturated fatty acids because they are typically solid at room temperature in contrast the oils would tend to have a higher concentration of the unsaturated fatty acids and therefore would be liquids at room temperature just to note so that c-o-h we are using now to abbreviate the carboxyl group which again is the carbonyl that's covalently a bonded to a hydroxyl group so now that we have an understanding of what fatty acids are we can switch over and begin talking about what is the structure of fat and oil so fats and oils are called in organic chemistry triglycerides or triacylglycerols those use those words are used interchangeably they are composed of fatty acids but those fatty acids have been linked to a molecule called glycerol if you look at the structure on the left you'll see glycerol it has three carbons so that would be propane it's an alcohol but it actually has three alcohol groups two connected to it uh one on carbon one one carbon two and one on carbon three so that would be one two three propane and then because we have three hydroxyl groups we have to use the word try to represent that we have three trial one two three propane trial so it has an iupac name the more common name that we use is called glycerol and if we connect fatty acids to that glycerol molecule we connect them through east ester linkages so if you look at the linkage here we have a carbonyl and an oxygen we have r groups on either side that is an ester linkage and these blue molecules would be the fatty acids so there's one fatty acid here the second one here the third one here and they're being connected into these carbons through ester linkages so the black atoms here came from the glycerol the blue atoms came from the fatty acids and this molecule is called a triglyceride or triacylglycerol so triacylglycerol means three acyl groups so these are called acyl groups once we've removed the hydrogen so there's one two three that's triacyl and connected to glycerol so that's the um that's the derivation of that word trace the glycerol in the clinic if you go and you have your blood tested and you have a lipid panel the word that is used is triglycerides so that would be the amount of this that is in your blood so if we look at in more detail these triglycerides typical triglycerides contain two or three different fatty acids so if we look at the structure down below there we actually have a saturated fatty acid shown in red and that's connected to carbon one and then on carbon two the blue atoms you can see that it has two carbon-carbon double bonds in it so this would be called a polyunsaturated fatty acid and then the bottom one has just one carbon-carbon double bond that would be called a monounsaturated fatty acid so just a single carbon-carbon double bond so in this particular case you can connect different types of fatty acids to each of the hydroxyl groups of glycerol and that produces what's called a mixed triglyceride so we don't have a triglyceride that's composed of the same exact type of fatty acid we have a mixture of fatty acids connected to glycerol when we look at the composition of fats and oils again the major defining feature between fats and oils is fats are salts and oils or liquids but generally the structure is the same they're containing this glycerol connected to three fatty acids the difference then is the mixture of triglycerides and they differ in the amount of unsaturation so oils would have triglycerides that contain fatty acids or more more fatty acids with carbon-carbon double bonds or or unsaturated fatty acids in contrast the fats would have triglycerides that contain higher concentrations of saturated fatty acids or maybe even in some case longer fatty acids so in addition to the degree of unsaturation the number of carbons in the fatty acid also defines whether we have something that is solid or liquid longer chain saturated fatty acids means even greater london dispersion force interactions and as a result they tend to be solids at room temperature and maintain their solid form unlike oils all right so again each one of these fatty acids has a common name so the red saturated fatty acid is called palmitic acid and that's the the main one that we can synthesize in our bodies in our cells is palmitic acids got 16 carbons the one in the middle in blue is linoleic acid you can see it has two carbon carbon double bonds and then the one in green that has just a single carbon carbon double bond is called oleic acid so now that we have an idea of some of the lipids that are functioning as fats and oils and we have kind of our understanding of the fatty acids that are the base of those molecules of for lipids we want to switch over and begin talking about the other major lipid that we find in our cells and that is the membrane lipids so membrane lipids we know are components of the cell membrane they are they form a lipid bilayer and you can see i'm kind of shown down the bottom right lipid bilayer in books are generally shown with this circle and two squiggly lines that coming out of the circle and usually there's two layers of those so you can see there's a layer on top with this kind of sea of heads head groups from these lipids with the little tails kind of sticking down and then this other layer down here with the head groups kind of positioned down and the little squiggly line is positioned up and so if we think of these little squiggly lines as tails and little circles as heads then we kind of have a tail to tail connection when we look at this one layer connected to the other layer and this bilayer type construction is what's forming the lipid membrane as well as membranes of other organelles inside of eukarya organisms these circular head groups are hydrophilic which means they're water loving the water is actually interacting outside the cell so all this is aqueous solution interacting with these water soluble and hydrophilic head groups and then inside the cell would be the intracellular environment of the cytoplasm which would also be aqueous and the water would be forming interactions with these head groups inside the cell and also hydrophilic the tails of these the two squiggly lines that are connected to the heads are all hydrophobic water fearing and you can see that if we have this lipid bilayer that goes all the way around into a sphere the only connections that these tails make are between those tails and all the tails that are hydrophobic around it and we don't really have a lot of interaction or connection with water because those are being shielded by these hydrophilic head groups again the bilayer is tail to tail so the tail in one layer is connected to the tail and the other if we look at the composition of the membrane and we ask the question what what are these lipids actually look like beyond just a circle into squiggly lines well if we if we dig into the molecular structure what we find is that there are different types of lipids it's not just circle and squiggly lines but many of them kind of adhere to that same generic shape there are three different types phospholipids glycolipids and sphingolipids that have a kind of a similar type structure with a circle and two squiggly lines coming out of them and we're going to dig into that what is the circle what are the squiggly lines and then the last one in at least in animals we have a molecule called cholesterol that is also found in the membranes i think many times cholesterol gets a bad rap i think a lot of times people associate weight associate cholesterol with heart disease but that's only in cases where we have high cholesterol cholesterol is a normal component of the membranes remember it provides rigidity and strength to the membranes animal membranes don't have a cell wall like plant membranes do and to some extent cholesterol provides some strength in the membrane to make up for the lack of a cell wall all animals have a cholesterol or cholesterol-like molecule or i'm sorry all organisms have a cholesterol or cholesterol-like organ or molecule animals molecule is called cholesterol if we look at the analogous molecules in plants and fungi it's ergosterol and stigma sterol and they have kind of similar structures and so all of the animal or all the plants all the fungi all the organisms that we find have these um cholesterol-like molecules at least the higher-order eukarya prokaryotes we don't really see that too much all right so we're going to look at each one of these lipids and compare and contrast one lipid structure to another the first lipid we're going to look at is phosphoglycerides the other word for that is called glycerophospho those used those words are used interchangeably in this particular case we're still based on glycerol and you can see from the name glyceride is telling us we have glycerol glycerophosphates again glycerol so we have our molecule of glycerol right here and to that glycerol we're connecting two fatty acids and those two fatty acids are connected again by ester linkages but instead of having a third fatty acid down here to make a triglyceride or trisoglycerol we're connecting a phosphate group and that's where the phospho portion comes from phosphoglycerate or glycerophospholipid we have this phosphate group connected to that one and that one is connected to the glycerol by what's called a phosphoester so it kind of looks like a carbon-based ester where we have a carbonyl oxygen and r groups on either side here we have a phosphorus double bonded oxygen with oxygen and carbon and that produces what's called a phosphoester this is different than triglycerides because we know that phosphate is not only polar but it's also charged you can see the two negative charges once we've taken a phosphate which is po43 minus and we've connected it to a carbon and formed a covalent bond we have two negatives that are still left on that phosphate group which means that we have not a molecule that is nonpolar we have a molecule that is mostly nonpolar but we have a phosphate group that is both charged and polar and hydrophilic this molecule is the circle in squiggly lines so the two squiggly lines that we've drawn in these lipid bilayers is these fatty acid chains that are coming off of the glycerol and the circle group the hydrophilic group would be in this case the phosphate and remember that the geometry around these carbons is all tetrahedral they're not 90 degrees in addition there's free rotation so this molecule can rotate over here and produce kind of the circle and the squiggly lines are kind of embedded in that lipid bilayer if we think of that in as a lipid membrane a membrane lipid this particular one is called phosphatidic acid because it only has the phosphate group we also have other glycerol phospholipids that have the ability to produce even more structural diversity but there are still phosphoglycerides and that occurs when we are able to connect additional groups other molecules to the phosphate in our glycerol phospholipid or phosphoglyceride the same fatty acids are still attached the glycerol's still here the phosphate is still here but if we attach additional molecules like one of these two onto the end we produce even greater diversity in addition we can also produce different types of charges so you can see that once we've connected that particular molecule to it which is called ethanol amine so that particular ethanol mean is right here and if we connect our ethanol amine to our phosphate we get a neutral charged head group or circle and in this particular case we have a negative one here we have a positive one here and those balance out and give us a net zero charge but it's still polar right we have polar bonds between the phosphorus and oxygens we still have polar bonds between the nitrogen and hydrogens so this group is still polar it's just no longer charged but again it gives us some greater diversity in the type of the hydrophilic group that might be part of that membrane that's just one of these head groups that can be attached we can also attach other molecules like choline so this molecule instead of this ethanol amine which is here if we remove that we can take this and put it on here we can also produce a different type of glycerol fossil lipid each one of these has a unique name so once we attach this ethanol amine to here this would be called phosphatidyl ethanol amine they also have more common names than that called this one specific is called cephalin and has unique functions in the membrane all right these are called amino alcohols or amino acids so the amino alcohols here can be directly attached i'm not showing an example of an amino acid but all those can produce a greater diversity in phosphoglycerides the second type of memory label that we're going to look at is are called the spingolipids and these are lipids that are made using not glycerol but a molecule called sphingosine so here's our sphingosine molecule it's unique in that the three carbons are here kind of like glycerol but the other um thing that creates a difference between this molecule and glycerol is this long chain of hydrocarbons and so you can see i've taken this molecule and i've created a sphingolipid out of it and what you'll see is that this sphingosine molecule is shown in red so here i'm showing it as a condensed structural formula here i'm showing it as to some extent some line angle formula and maybe some condensed over here but what you'll notice is that there are 12 of these ch2 groups in here and those are all kind of shown over here but what that does is in sphingosine it actually has kind of a built-in hydrocarbon chain so there's no requirement to put a fatty acid or connect that to this base structure because it's already present in the molecule sphingosine the other thing that's unique about spingosine is that it has a nitrogen in carbon 2 and that nitrogen is then therefore able to connect to a fatty acid and that linkage is called an amide so a carbonyl connected to a nitrogen with r groups on either side gives us an amide linkage and that gives us the two squiggly lines that we see in the membrane lipid and then in addition to that we still have phosphate groups on many of their spingolipids as well as in some cases head groups like amino alcohols or saccharides or amino acids so again a variety of things could be placed here to give a lot of diversity to this particular molecule this particular sphingolipid is called spingomyelin it's part of the myelin sheath of nerve cells and it's there to insulate the nerve depolarization of the membrane as it carries the nerve impulse down the axons and neurons of those types of cells so certainly has a unique function in that way one of the ways that it's able to do that is because this built-in hydrocarbon chain into the base molecule is not able to be hydrolyzed like we could with an ester linkage or in this case an amide linkage ester linkages and phosphoglycerides amide linkages here and in the sphingolipid in addition to sphingolipids that have like a phosphate and an alcohol amine or amino acid we also have glycosphingolipids which contain uh carbohydrate in some form monosaccharide or disaccharide or oligosaccharide so you can see in this particular case we still have our spingosine so we still have this hydrocarbon that's connected to the base molecule we'll see how we still have the nitrogen and that can still form an amide linkage to this fatty acid and again once we condense it down it's a little bit harder to see kind of the zigzagginess of this but you have to kind of look at how many ch2 groups are at we'll have 12 okay there's going to be a long chain here same thing here we have 12 carbons here so there's a long hydrocarbon group kind of in here to form the two squiggly lines that we see in the membrane lipid on the other end rather than having a phosphate and any other additional group that might be attached to phosphate we have carbohydrates and so in this particular case we can form glycosidic bonds bonds between a carbohydrate and the spingo lipid so if we have um just one monosaccharide attached to the sphingosine we get a cerebral side if we have multiple monosaccharides so if we have a disaccharide so we have another one right here next to this connected into this hydroxyl group to make like a one four linkage that we talked about in carbohydrates or maybe we have like three or four five or an oligosaccharide then we get what are called gangliosides so cerebrosides have a single monosaccharide and gangliosides would have more than one monosaccharide so disaccharide trisaccharide multiples that are all connected together this particular one that we're looking at is called galacto cerebroside so it has galactose and if you remember from galactose the hydroxyl group on carbon two was down and then up on three and then up on four unlike glucose which would be down on four so this is galactose which gives us galacto-cerebrocytes cerebroside again so riversides are those that contain one monosaccharide so just a single monosaccharide connected into the sphingosine molecule so it is a cerebral side and this particular one galactoside is actually another component of the myelin sheath in addition to spingomyelin we also have uh this particular molecule which is called galactoside the final category of membrane lipids that we find in the membranes of eukarya are based on the molecule steroid so steroids are category lipids that all share the same structure of four cycloalkanes and they have the connections in a very distinct pattern so you can see there's three six-membered rings and one five-membered ring and they're all fused together so you can see these two carbons fuse these two these two carbons fuse these two rings and then these two carbons view these two rings and all the steroid-based molecules will have this very distinct ring structure the three six-membered rings attached in this way and a five-membered ring attached in this way this is called the steroid skeleton or steroid nucleus has this distinct psychoalkane ring system where we've fused rings together to form uh the steroid nucleus so now that we have an idea of steroids we want to kind of shift back a little bit and look at cholesterol so cholesterol is a membrane lipid it is providing strength and rigidity to the membrane so we will push cholesterol into our membranes to give those membranes strength it is a membrane lipid and in addition to that it also produces a membrane that is less fluid so we can actually control the fluidity of the membrane based on how much of the cholesterol sits in the membrane so you can see i've kind of drawn the cholesterol like this where we have kind of just the one small hydrophilic group that would just be this hydroxyl group over here that's the only thing that is hydrophilic on the whole molecule the rest of this is just hydrocarbons which are just carbon carbon bonds carbon hydrogen bonds we know there's none of those are polar or charged they can't form hydrogen bonds so really just london dispersion force interactions and they don't form favorable interactions with water the only thing that does is this hydroxyl group so we just have this really small hydrophilic group on the end of our steroid nucleus you can see the stereo nucleus there six members six members six member and five member rings and in addition to that we kind of have a shorter hydrocarbon tail here and that's kind of this shorter hydrocarbon tail that's kind of sticking off the end of this five-membered ring and this whole thing in terms of its length is similar to the lengths of the circle and two squiggly lines and as a result fits into the membrane uh fairly easily um interacts with the the squiggly lines the fatty acids that are surrounding it from the glycerophos lipids and aspingolipids all right in addition to providing strength to the membrane cholesterol is also used to make um a variety of other steroid-based molecules in our body so those include the steroid-based hormones the bile salts and vital acids as well as vitamin d so vitamin d has a structure that has a steroid kind of cycloalkane structure with fused rings as well of course we also know that high blood plasma concentrations of cholesterol are correlated with atherosclerosis and heart disease i'm it seems like the literature suggests that most of that is due to the standard american diet although there is there are some genetic diseases as well that cause that cholesterol to be high but again cholesterol generally is an important molecule so again if we if we need the steroid-based hormones if we need the bile salts and vital acids if we need vitamin d then we certainly need cholesterol if we need strength and rigidity from membrane we need cholesterol so kind of a love-hate relationship with cholesterol we certainly need it but we can't have too much of it circularly right now circulating around in our blood all right so let's look at some of the molecules that cholesterol is used to make in our body so the first ones we're going to look at are the steroid-based hormones and there's two categories of hormones that we'll look at the adrenocorticoid hormones you can see those are shown in blue aldosterone and cortisol they again have a very distinct cholesterol nucleus or cholesterol skeleton steroid skeleton structure you can see that the shorter hydrocarbon chain that was on the five-membered ring has been cut off and these molecules are certainly more hydrophilic we got carbonyl groups that we know are polar we have hydroxyl groups that are polar the hydroxyl group that was on the end has been converted to a ketone so those are still polar so you can see that there's more polarity here than there was in cholesterol and again in these particular hormones they're used to regulate water balance as well as calcium levels and probably like starting to look at some of these in a and b the other category of hormones are the sex-based hormones estradiol which is an estrogen and testosterone which is an androgen and you can see that those have been modified from cholesterol there's not this short hydrocarbon tail on these five carbon ring structures estradiol still has the hydrox group and you can see from the die all kind of word there's two hydroxy groups in estradiol testosterone you can see the o-n-e ending is indicative of the ketone on the other end rather than the alcohol group that was coming from the cholesterol and you can see that some of the names are in kind of keeping with that prefix ending aldosterone has a carbonyl several of them so there's one here and one here there's an aldehyde here cortisol has alcohol groups you can see there's two alcohol groups connected to cortisol so some of them have been using some of the organic iupac suffix names to indicate a certain functional group that's present within those molecules in addition to the steroid-based hormones we also have bile salts and bile acids that are also derived from cholesterol so cholesterol is used in the liver to produce these bile salts and vital acids and you can see both of them still have that short little hydrocarbon chain on the five-membered ring but also have been kind of converted into other things you can see there's a carboxylic acid here and over here there's an amide linkage and another carboxylic acid that has donated a proton so it's not an acid anymore it's now a negatively charged anion which hooks up with the sodium and so it's basic so it's it's been neutralized the acid has been neutralized and we have assault so that's where the bile salt is coming from the bile acid here hasn't been neutralized it still has that h plus there that's able to be donated it's a proton donor and so this particular bile acid and this particular bile salt are fairly common ones but they're not the only bile salts and vital acids that we see in our body once they're made in the liver they're stored in the gallbladder and i'm sure you'll start looking through those organs in anatomy and physiology so you can see where those are located and those are connected up to the gi tract and they can be deposited or released into the gi tract and their function is emulsification so in addition to the water soluble molecules that we consume in our diet we also consume some amount of fat and oil in our diet and these bile salts and bile acids are there to emulsify which essentially means that they're able to be somewhat hydrophobic and somewhat hydrophilic at the same time so you can see that the steroid-based ring has lots of hydrocarbons on it that would be the hydrophobic portion but you'll notice that the bile acid that i'm showing here has several hydroxyl groups which are all hydrophilic and the carboxylic acid which is hydrophilic because they all contain very polar molecules or polar functional groups as well as functional groups that are able to form hydrogen bonds with water bile salt in a similar way we still have these hydroxy groups that can form hydrogen bonds with water we have some polarity in the amide we have some polarity in the salt that is formed when it's neutralized by a base and in addition to that again we still have the hydrocarbon stereonucleus which is hydrophobic so this allows us to form interactions with the fats and oils but also form interactions with the water that is in the gi tract as well as the enzymes that are there to begin breaking down the fats in our diet so these emulsifiers make suspensions of two non-dissolvable or undissolvable liquids so in this particular case if we consume aqueous and fat and we always do if we're eating cellular-based foods we're going to have that separate out but we need to have access to those fats to begin to digest those with our enzymes we need to emulsify an agent and that is bile salts and bile acids and again in the whole the whole point is to allow enzymes to gain access to the fats so we can begin digesting those and and getting energy from them all right the last thing we want to talk about in this particular chapter now that we've talked about the the structural components that are lipids and we've shifted over and again looking at the function of those is either as energy storage as fats and oils or as components of membranes that whose function is really to separate one aqueous solution the extracellular environment from another aqueous solution the intracellular environment we want to talk briefly about membrane transport how do we move molecules across the membrane when we know we have very hydrophobic hydrophilic head groups but we have this hydrophobic tail-to-tail structure inside of the membrane how do we get hydrophilic things or hydrophobic things kind of through the membrane from outside the cell to inside the cell or vice versa inside the cell to outside the cell so if we have molecules that are fairly hydrophobic we can do passive diffusion so passive diffusion is shown here on the left and passive diffusion really is just molecules moving from an area of high concentration to an area of low concentration and they're driving toward an equilibrium they're trying to equilibrate the concentration on both sides so we have molecules on the left that are moving toward the right we have molecules on the right that are moving toward the left but because we have a higher concentration over here than over here we have generally more moving this direction in this direction until the concentration equals out and then we have molecules moving both directions at the same rate and as a result we kind of have equilibrated the concentration of these molecules that are shown as red spheres in both the outside and inside this is called passive diffusion and it requires molecules that are somewhat hydrophobic to be able to move through the hydrophobic lipid tail groups of the membrane in contrast if we have molecules that are hydrophilic or charged so if we have ions that need to move through the membrane or for example glucose that is being moved from the blood into the inside of cells those require proteins to facilitate their passage through the membrane so this is called facilitated diffusion and in this particular case we have to have some kind of protein this is kind of a cutaway of the protein so this would kind of form a this tube and so you can see the inner area of the tube so this has been cut away kind of a tube or a cylinder and so it's these molecules can or ions can pass through the inside of the cylinder um and that protects those molecules from seeing the hydrophobic groups of the fatty acid tails of these lipids in the bilayer facilitated diffusion also works by moving molecules from an area of high concentration to an area of low concentration so in this case it's also driven by equilibrium and so usually in these channels these molecules or ions will move in both directions if there's higher concentration on one side or the other we have more of these moving toward this direction and then once we have equal amounts on both sides then we have some moving in both directions and maintaining that equilibrium again moving at a consistent rate both into the cell and out of the cell and so really not changing the concentration once we've reached equilibrium all right both in the case of passive diffusion and facilitated diffusion this would be molecules moving from an area of high concentration to an area of low concentration all right so an example of this would be after you eat a meal and your blood glucose concentration is high we have proteins in the cell membrane called glutes or gluts these are glucose transporters so short for that is glut or glute in these particular cases glucose would be high in concentration in the blood or capillaries that are neighboring the cells that are needing the glucose and because the concentration of the blood is high we're going to just passively move through the membrane and increase the concentration on the other side of the membrane and then the other other thing about this is once the concentration of glucose outside in the blood is similar to the glucose concentration inside of the cells there's not going to be a continuous move through we kind of have an equilibrium set up and we're kind of maintaining that concentration of glucose both inside and outside of the of the cells all right the final type of membrane transport is called active transport so active transport is different in then either passive or facilitated diffusion in that active transport is moving molecules or ions from an area of low concentration to an area of even higher concentration all right so we're in this case moving against equilibrium equilibrium would would suggest that these molecules or ions need to go back this direction to equilibrate the concentrations on both sides but in this particular case we're moving even what small amount is still left on this side of the membrane toward the other to build up an even greater concentration on the other side so again we're moving from an area of low to high that's unique and this is why it's called active transport we are still requiring some kind of protein channel to facilitate the movement of these ions or molecules across the membrane but because we're not using equilibrium to drive the direction of the reaction and we're actually wanting to drive a higher concentration on one side or the other we actually have to have some input of energy to do that in most cases it's atp so we're going to hydrolyze atp hydrolyze means using water to break bonds we're going to break off one of the phosphates on the triphosphate of atp and we're going to be left with the diphosphorylated form adp and a free inorganic phosphate so that's what pi is inorganic phosphate which would just be po4 3 minus all right so that's just an easier way to write that than writing out p or 4 3 minus all right it's active transport because we're actually using energy to drive these molecules toward a higher concentration on one side versus the other moving against the the normal direction that equilibrium would want to push those molecules and we certainly have cases in our cells where we actually need to push more more concentration from one side to the other and this is where active transport is really helping us do that but again it comes at an energetic cost so moving these molecules across the membrane obviously has to be worth it in terms of the energy that is being used to do it all right that wraps up our discussion on lipids so just in brief review we've talked about the the major characteristics of lipids we looked at fatty acids which are a component of several of the lipids that we see both fat and oils as well as membrane lipids we looked at fats and oils in their structure triglycerides trisoglycerols and then we shifted over and looked at several different types of membrane lipids and how they build up the lipid membrane or bilayer and then finally we looked at transport across the membrane