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MCAT
Course: MCAT > Unit 9
Lesson 8: Nucleic acids, lipids, and carbohydrates- Nucleic acids, lipids, and carbohydrates questions
- Nucleic acid structure 1
- Antiparallel structure of DNA strands
- Saponification - Base promoted ester hydrolysis
- Lipids - Structure in cell membranes
- Lipids as cofactors and signaling molecules
- Carbohydrates - Naming and classification
- Fischer projections
- Carbohydrates - Epimers, common names
- Carbohydrates - Cyclic structures and anomers
- Carbohydrate - Glycoside formation hydrolysis
- Keto-enol tautomerization (by Sal)
- Disaccharides and polysaccharides
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Lipids - Structure in cell membranes
Created by Ryan Scott Patton.
Want to join the conversation?
- Is anyone able to link the video he is referencing in the first 10 seconds of this video, when he says "in a previous video I introduced a class of biological molecule called lipids"?(8 votes)
- So what exactly is the definition of lipids? (Not only give examples but also generalize the structure of them)(5 votes)
- Lipids are a group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E, and K), monoglycerides, diglycerides, triglycerides, phospholipids, and others.(4 votes)
- If the ester bond of waxes can be broken down via hydrolysis, then why are waxes so good at protecting against water? I know that he said the nonpolar tail is the reason but wouldn't the hydrolysis reaction break down the wax also?(4 votes)
- This is why your body has enzymes. Likely the activation energy is too high and so needs to be lowered by enzymes to have any chance at occuring(3 votes)
- Atyou draw sphingosine with 21 carbons. Is sphingosine a class of molecules, like triglycerides, that can have varying number of carbons or does sphingosine refer to a specific molecule? I have always been taught that sphingosine is a molecule that has 18 carbons 6:50(4 votes)
- Good spot!
It is actually drawn with 20 carbons, but you are correct this is a mistake.
Sphingosine has 18 carbons — the saturated tail should be (CH₂)₁₂CH₃ rather than (CH₂)₁₄CH₃.
I've posted a "Mistake" notification.(2 votes)
- : Isn't triacylglycerol the same thing as triglyceride? Or, Why the pop-up announcement? What is triglycerol, then? 0:51(1 vote)
- It is the same thing. Although "triglycerol" is not a scientific word. Triacylglycerol would be the appropriate word in his example.(2 votes)
- Should the polar part be at the other end, where the R group(s) are hanging and the "head" area is polar due to the deprotonated O-?(1 vote)
- When he says polar head, he refers to the whole phosphodiester linkage. The nonpolar tail results from the long hydrocarbon R groups.(1 vote)
- do you know what about a lipids structure makes it good at what it does(1 vote)
- Is it a combination of van der Waals forces including London dispersion and dipole-dipole interactions that hold the non-polar tails of phospholipids together?(1 vote)
- London dispersion forces are the only intermolecular forces that occur between non polar molecules, which include the hydrocarbon tails of lipids. :)(1 vote)
- Molecular structure of a cell(1 vote)
- At, shouldn't the oxygen of the OR be protonated before the tetrahedral intermediate collapses (since alkoxides are a bad leaving group)? 2:42(1 vote)
Video transcript
- [Voiceover] Alright so in
a previous video I introduced a class of biological
molecules called lipids. I broke lipids down into
two main categories. We have hydrolyzable lipids, hydrolyzable, and then not hydrolyzable. Essentially what this
means and we've kind of talked about this is these
lipids can be broken down into smaller sections, and these Lipids is non-hydrolyzable. Lipids cannot be broken
down into smaller sections and in this we fit probably
the most abundant lipid, triglycerol and if you remember triglycerol has the main
function in our bodies at least of energy storage. So we break down triglycerol
because it's a hydrolyzable lipid to get energy. I want to continue talking
about these hydrolyzable lipids but the other ones that
I think are important instead of kind of having
a predominant energy storage function, they
have a structural function in our cells and kind of in the biological role,
so a structural function. So the first one I wanna
talk about are phospholipids. Phospholipids are hydrolyzable lipids, that contain a phosphorus atom. So this phosphorus atom
usually comes in a form of a phosphodiester bond and I have no expectation of you to know what a phosphodiester bond
is because I don't think we talked about it so we
start with phosphoric acid, which is H three, P O four,
that's phosphoric acid and it has a, kind of has
a structure like this. We've got phosphorus
double bonded to oxygen and then bonded three
other times to O H group. So we have four oxygens in total and three hydrogens and a phosphorus atom. That might look a little bit goofy if you're paying real close attention because this phosphorus
is bonded five times but remember that phosphorus
is a third row element unlike carbon, so it can
bond more than four times. It has, it's capable of
sharing more than eight valence electrons. So we've got phosphoric acid right here and if we take these two side O H groups and replace them with O R groups, so let me kind of redraw
the base structure here. Still have it double bonded to an oxygen but if we replace those sides with O R groups, say for example in a dehydration reaction this becomes a phosphodiester. And it's an -ester because this phosphorus is double bonded to an oxygen and an O R group and
it's a -diester because it happens twice, so
this is a phosphodiester. This is the form that phosphorus
is gonna come into play for our lipids. So if we start with triglycerol, which is kind of that
basic hydrolyzable lipid and I guess I better go ahead
and draw out triglycerol here. So this is triglycerol and
if we replace this lower fatty acid chain, remember
all of these are fatty acids, if we replace that with a phosphodiester it's gonna look like this. So this would be one of the O R groups. This whole rest of the molecule
would be the other O R group and I guess in our bodies not to confuse you too much but phosphodiesters had a pH in our body of around 7.4, this hydrogen right here is actually usually deprotonated so usually carries a negative
charge on this oxygen so we wouldn't have an H right here instead we'd have a negative charge. So this right here is a phospholipid and one of the cool
things about phospholipids is unlike triglycerols now we have a specifically polar section,
so this right here is polar and then we have a non-polar
section to this molecule. So this is all still non-polar and the purple part principally
because of this kind of being negative charge and just this the polar nature of this phosphodiester but that allows it to
play a pretty cool role in cell membranes. You've probably heard of
a phospholipid bi-layer and what happens is you
have a phosphodiester head, so kind of a phosphodiester head here and then you have two kind of
fatty acid tails coming off and this is polar, this
purple part is polar and the orange part would be non-polar and you get a whole bunch
of these and they kind of line up together, the
polar heads kind of line up and the non-polar tails kind of line up. And those non-polar
tails become attracted to another set of non-polar tails, and another polar head right here, and this forms a two layer
membrane to our cells. So this is kind of the
thing that separates the inside contents of our
cell from the outside contents and this whole middle section right here is not attracted to water
because of this kind of non-polar lipid characteristic and
it keeps water over here and water over here but
separates these two fluid compartments so that's
a pretty neat function, a structural function of the phospholipid and I don't think you can
really talk about phospholipids without stopping at least
briefly to mention sphingolipids, they're another hydrolyzable lipids. So sphingolipid, and you need to talk about sphingolipids because they're another
lipid that incorporate this phosphodiester unit, but instead of having it kind of on a base of a triglycerol, it's
gonna be on the base of the amino alcohol sphingosine. So let me kind of draw in sphingosine. And so sphingosine is an amino alcohol, it's got these O H groups,
but it's also got a lot of lipid characteristics
because it's really high molecular weight alcohol. This is a lot of carbon here, 15 carbons on this kind of tail, it
would extend out like that if I kind of drew them all out but if we replace this O H
group with a phosphodiester just like we did with our triglycerol so kind of replace it. It also develops a structural
function within cell membranes but mostly within nerve
cells and I'll show you why. So we've got a nerve cell here with a long axon and nerves are surrounded
by the insulation of myelin, so we've got myelin right here. The properties of myelin,
which allowed to insulate this nerve axon right here
are really predominantly due to the sphingolipid
concentration in myelin. So sphingolipids kind of have
a similar structure function within cell membranes but
mostly within their cells and then I guess the
last hydrolyzable lipid that I really wanna talk about are waxes, so waxes. Again, I'll kind of do this briefly but waxes are also esters and they're made of a high molecular weight alcohol. So again a long carbon
chain alcohol that might look like this with an R group but it'd be really long, so it'd kind of look like this and then they've got a fatty acid so -- A fatty acid, again really
high molecular weight. If you form an ester out
of these two molecules you end up with a product
that looks a little something like this, so a long chain, double bonded to an oxygen
with an O R group right here. This would be the ester and so you got an ester right here but it's kind of a
unique ester in the sense that it's got two really
long carbon chains making it two kind of non-polar sections here. These two non-polar
sections make the molecules very hydrophobic, waxes
are very hydrophobic and so we see waxes in
nature often forming a real barrier against
water and this happens in leaves, we put waxes on our
cars to protect the surface of our cars from rain and
from the kind of the humidity but waxes are another type
of hydrolyzable lipid. And their hydrolyzable
again, I just kind of wanna beat this point
home because I've got this ester group right here,
carbon double bonded to oxygen with an O R group that can be hydrolyzed. You can do an ester hydrolysis reaction. So that's with waxes, same
thing with sphingolipids they've got this, sorry let me make sure you know that that's an oxygen, they've got this ester group right here that you can break down. You can do the same thing if
I go up with phospholipids. Again we have this O, this kind of this ester bond right
here that we can hydrolyze and we can do it with these bonds as well, just like we could with triglycerols. So all of these, again are
lipids that can be broken down into smaller units through
hydrolysis reactions.