Biology 1A - Lecture 5: Membrane structure and function




Uploaded by UCBerkeley on 05.09.2012

Transcript:
>>INSTRUCTOR: Good morning everybody I hope you had a nice Labor Day weekend, I didn't
I had to prepare your future lectures. It's my fault I was late.That's done now. So one
thing I wanted to make you aware of. Some of your colleagues, started a study group
in Hildebrandt. I do not know these students an I do not know what they're going to discuss
so I cannot officially endorse the content. So go at your own peril. So last week we start
stopped with the summary of the cell and I showed you a little movie and as I know the
tablet PC was inadequate to show you the movie so I wanted to show you the movie again as
a summary on my Mac.I'm not allow to make any advertisement for anything, but this one
should, should, let's see if it holds up, should be able to show the movie in its flawless
entirety.We'll see.So the first movie will be on the animal cells that you have seen
already and then, one of the movie is going, I'll switch to a second movie, and the second
movie is about the plant cells and it will highlight the three cellular structure that
plant cells have, that animals cells do not have.That's the whole purpose of this.And
again, we're going to do special effects, so again you're going to hear the song by
Hans Zimmer and James Newton Howard from the movie The Dark Knight to show you the drama
of what's happening in your cells. Okay.Let's see.[VIDEO]>>INSTRUCTOR: Okay. That was it.It
worked.[Applause]So I hope that woke you up.So, are there any questions to any of the little
thingies that were shown there?Did you recognize all of the structures that were there? I promise
you will not get a movie in your exam an you have to find out what it all is.Okay.So that
was the cell.And so today we are going to continue with membranes.So this is what we're
going to discuss, we're going to discuss the fluid mosaic model, diffusing active transport
and then endo- and exocytosis, we have talked about previously, so concerning the struck
which haveÊa membrane, we talked about this before, membranes are made out of phospholipids
and these phospholipids form a bilayer, here, where you have the hydrophobic part that's
the fatty acids pointing inwards and they align with each other and you have the hydrophilic
part so that's the phosphate group and in the case of a [Indiscernible] it's the [Indiscernible]
group, they case the aqueous sides of this. That's not the only component in a membrane,
you have proteins in the membrane, and they mediate what's happening on one side with
what's happening on the other side. So there are two types of proteins that you can find
within a membrane, so there is this thing that completely spans the whole membrane and
this is called a transmembrane protein.Because it goes from to both sides. And then we have
proteins that are stuck to one side of the protein, not of the lipid layer. And these
are called: Peripheral proteins.So we talked about that the shape of the protein is dictated
by the amino acids sequence.And so also, if you think about these proteins inserted in
this particular bilayer, the amino acids, the amino acid side chains will dictate how
this protein is oriented here. So for example you can see here on the side of this protein
here, you expect to have hydrophobic amino acids, because then the hydrophobic side chains
will point toward the lipid bilayer. But here, when it comesÊ when it's close here to the
water phase that's when you have hydrophilic regions so you will have polar amino acids
sitting particularly on this side to make sure that the protein is properly inserted
into the membrane.Okay.Now, the membranes, are considered fluid mosaic, that was a name
that was coined a few decades ago, and a very elegant experiment to demonstrate this is
this one here, so there's a mouse cell that has a certain protein in its plasma membrane
here.And here's a human cell that has a different protein in its cell.And when you take these
two cells and you fuse them, you do not expect that one side has a just protein A and the
other side has protein B, no in one hour all of these proteins are mixed within it so that
nicely demonstrate that had the membrane is not a static component it's actually a liquid.
And the mosaic that's the proteins they can flow through the membrane.And that's why they
diffuse out and distribute themselves all over the membrane.Unless they're held in place
by certain molecules. But overall you should consider the membrane not solid wall, but
a liquid where things can move.And so that's what's shown here. It's fluid that allows
protein mosaic to move laterally through out the membrane.So, with this model people have
measured the speed of the fluidity of the membranes.So here you just have the phospholipids,
and so if these phospholipids move across the membrane that's lateral movement here
that's often that's on the order of 10Êmillion times per second.So this happens very often
that the phospholipids move through out they're lipid bilayer. Now we have two layers here,
we have a bilayer so there's also the flipflopping here where the lipid can move from one bilayer
and flops to the other side of the bilayer but that is very rare. So that only occurs
once per month.Okay. So these very rare compared to this. So this hardly ever happens. And
therefore once the composition of the phospholipid layers is determined, it remains essentially
in tact, there is very little change.Excuse me.Now, the membranes, they can contain a
number of phospholipids, and particularly the fades that are attached to the glycerol,
remember lipid is a glycerol in case of two fatty acids they can vary and depending on
what youÊ what fatty acids you have, you change the fluidity of the membrane.For example
here: You have these unsaturated fatty acids, and if you have this, then the membrane is
much more fluid.And this is important for example in polar fish.Because the temperature
is so low that membrane like this, which has only saturated hydrocarbon tails that will
solidify and that means the fish wouldn't survive, but if they have unsaturated in cold
water in the arctic and unarctic and then they have a lot of unsaturated fats so the
membrane remains liquid.So if you have saturated hydrocarbon and then it is more tightly packed
so this can be considered viscus.And then of course you can also have other components
in the membrane. For example: Here what is this? Oh, it's says here cholesterol and as
you can see cholesterol again as a steroid has the 4 rings and it has only one group,
the hydroxyl group, and this will orient into the membrane like this and will disturb in
the space and this one is also considered fluid.So, under cold temperatures, membranes
that contain cholesterol also remain fluid.But actually under normal temperature, they don't
become superfluid because the cholesterol causes a hindrance for this lipids to move.So,
cholesterol in that sense is quite special because at normal temperatures they hinder
the movement of the phospholipids.Okay.Oops.So. If you think of the membrane, you're not only
have a liquid that allows movement to occur, it also blocks movement across the membrane.And
therefore, that's why cells have internal organelles because they create reaction spaces,
where they can contain certain types of molecules.And that's the problem with eukaryotes they don't
haveÊ they do not organelles they don't have an internal membrane structure so the whole,
let's say the E.Êcoli has only one reaction space and all of the metabolites and the compounds
that this cell needs and produces and utilizes will all diffuse through out this entire compound
where as in the eukaryotic cells because you these membrane enclosed reaction spaces you
can specialize your reaction. So that not everything in your cell will end up in this
space. And so that's the main function of membranes and of course the plasma membrane
is very important because it defines the inside from the outside ofÊthe cell.So that not
everything inside of the cell is going to leech out into the environment.Now, there
are molecules that can pass the membrane, unhindered and these are hydrophobic molecules,
so as you can image hydrophobic molecules they will have no problem to pass through
the lipid bilayer so they will pass.Polar molecules for example, sugars or charge molecules,
amino acids, they will not be able to pass because they're not efficiency hydrophobic
to pass the membrane, so they will not pass.And so in order for those molecules to pass, you
require transport proteins. So that's the key now of these proteins in these membranes,
because they mediate the reaction space, they mediate what goes into the cell and what goes
out of the cell and this allows the cell, or the organelle to regulate what goes in
and what goes out.All right.Please get your iClickers out.According to the fluid mosaic
model which of the following statements is true for phospholipids? So here you have the
5 statements and only one of them is true. So please go ahead.Now. okay, 10 more seconds.3,
2, 1.We're already over time.All right.So, 98% of you got it right.And that was answer
C.Excellent.So, let's go through it and see what's correct.They have hydrophilic tails
in the interior of the membrane, no it has to be hydrophobic.They are free to depart.
No, because they have a hydrophobic tail so they have to stick. They cannot go quietly
into the aqueous medium. Yes they do move laterally. This is all hydrophilic actions.
They occur in the membrane proteins restricted to the surface, no they can also be [Indiscernible]
membrane proteins. So you got this very right so 98% of my right.My colleagues tell me your
questions are too simple, I say no, my I'm a good teacher. Are my questions too simple
or am I good teacher?>>STUDENT: Good teacher.>>INSTRUCTOR: So arguments hold to remember that when you
get the evaluations at the end of the class, okay.Okay. So, now, we move to diffusion.
So diffusion is the tendency of molecules to spread evenly in space.So, if you have
a concentration of molecules for example: A dye. It will diffuse, it will fill up the
open space, so at the end you have an equilibrium and the concentration of the molecule is equal
in the entire space.So, in this case here we also have a membrane, but the membrane
is permeable to this particular dye, and therefore it is not a hindrance and therefore the molecules
will diffuse into equilibrium.If you have two different molecules on the different size
of the membrane, if the membrane is permeable to both molecules, again, they will distribute
both into equilibrium.For this process to occur, you do not require external energy.Okay.This
will just happen.And they're thermodynamics roots why this will happen and we get to this
next week. But this will happen. So there's always movement from molecules from a high
concentration down the gradient to a low concentration.And no energy input is required. So this is passive
transport. Now, with any molecule, this is also true for the molecule of water.And the
diffusion of water is called: Osmosis. So here we have a U glass container, we have
a membrane. And there we have two types of molecules, we have these larger molecules,
here in green, of which we have a low concentration, sugars in this case, of which we have a low
concentration here. And the high concentration here.So this is the sugar concentration here.And
then because of the concentration water is of course the opposite. We have a higher concentration
of water here than we have here.Yes? Because there's more water in here. More water molecules
in here than here. So the membrane here, is now a selective permeable membrane. And if
the membrane only allows water molecules to pass, but not the big color molecules here,
then it's always called semipermeable membrane.So in this environment, the green sugar molecules
they cannot pass through the membrane, but water can.And so water will move from its
high concentration to its low concentration, high concentration to its low concentration
and it will do so until the concentration is equal.So the water concentration is equal
and at that stage the concentration which is the amount of molecules per volume of solution,
the sugar concentration will also be equal.And that's how much the water will move.This is
in your book and this is not completely correct.Why not?Because this does not describe this really.Anybody?
Yes, please.>>STUDENT: [Indiscernible]>>INSTRUCTOR: That's correct they move in equilibrium but
that mean it is net concentration remain it is same in each component.Yes?>>STUDENT: [Indiscernible]>>INSTRUCTOR:
Yes. Free more water molecules, yes.Yes?>>STUDENT: [Indiscernible]>>INSTRUCTOR: Yes.Perfect.
So gravity kicks in.If you would do this experiment in space, this would work like this...And
the concentrations would be exactly equal.And therefore, the water concentration and the
sugar concentration would be exactly equal, but here, as you can see the water level is
higher, than this water level and therefore it is exerts gravity, it puts a phosphorous
on the solution, and because of gravity, the phosphorous the counter acting pushing force
the water concentration is slightly less and the concentration of the sugar will be slightly
higher, but for the concept of diffusion and osmosis this doesn't water I hope you understand
the concept and I hope these values of these concentrations are equal, but not correct
if you're on earth in this room, the gravity will counter act it a little bit.Okay.So,
the water balance of the cell, is also called: Tonicity.So tonicity, is the ability of the
surrounding solution to cause a cell to gain or to lose water.So, for tonicity to occur,
one solution is never enough, you need to consider two spaces.Yeah? So if I give you
a solution and ask you what is the tonicity of the solution, you will not be able to know
this because you always need the reference solution to this.So for example here, we have
a concentration, a situation where the concentration on the inside is the same as the concentration
on the outside, and if you have this, then water molecules move in and out, but the net
influx and efflux is equal and therefore there is no net influx of water into the cell.And
such solution where the concentration is the same outside that you add to the cell, as
the inside is called: Isotonic.So, you also have the case here, where you add a solution
where the inside isÊ what is it, smaller? Has a lower concentration than the outside.So,
water will diffuse through down a concentration gradient so it will move out to the larger
concentration, so water will move out and the cell shrink. So when it's a higher concentration
on the outside, it's called a hypertonic solution.You can also have the opposite case here, you
have the case where the inside concentration is larger than the outside concentration.And
in such a case water will move into the cell, there will be net movement of water into the
cell, as shown here, and so this is called: A hypotonic solution.So, hypotonic solutions
areÊ can be to Celts quite dang louse because as you can see here with erythrocytes in your
blood. If you put an erythrocyte in a solution that is hypotonic the water will move and
the cell will burst and it causes hemolysis, so you do not want that to happen. So when
you go to a hospital and they give you a is a lean solution, it's not for the salt in
there it's so they don't give you a hypotonic solution so your blood cells do not burst,
they need to make sure that you have at least an isotonic solution so your blood cells remain
in tact.So, the same problem of dealing with water in an out flux, is also in plant cells.So
in the plant cells you have the same situation. If you put plant cells in an isotonic solution
the in equals the out the net influx is the same and this cell is called [Indiscernible].
But you don't really need to know that. If you have a hypertonic solution, and you nearly
always have a hypertonic solution in plant cells because the concentration of the soil
is so high in plant cells, the plant cell will introduce water but it will not burst
because it has the cell wall.So because it has the cell wall, it will not burst.So the
cell wall is strong enough to overcome the pressure of the water being added to the cell.If
it has a high solute concentration and that means the cell is [Indiscernible] but you
can also have the opposite case where you have a hypertonic solution that is more concentrated
than a plant cell and there will be a net efflux of water and the cell will shrink but
the cell wall will not shrink it's everything within the plasma membrane that will shrink
so the plasma membrane will peel off the cell wall and it will shrink and that's called
plasmolysis.And the cells is plasmolyzed and you can watch this under the microscope. And
I think I have a movie here of this if it works on the PC again.So here you see your
plant cell, and you put it in a hypertonic solution and you see how the tissue, and you
see how the plasma membrane streaks off the cell wall and the cell curls up, so the outer
boarder is the plasma membrane. The cell wall remains in tact.Yeah?Now, for water to move
in or out it doesn't really matter what the solute is.Okay. So what counts here is the
number of molecules.What doesn't count is what type of molecules they are. So for osmolarity
for water to diffuse in and out it matters whatever molecules are in the cell.Okay.So
we're going to go another iClicker question but for this one you will not get points because
this is a question in your homework. As you know homework is due for this lecture in two
days.So you can basically work already on your homework and the reason why I mentioned
this here because it turns out that 20% of you who have done that homework already had
the most problems with this question.And so I put it here.So you guys can work it out
already.So, to prevent hemolysis so the bursting of the cell, an animal cell must be placed
in an eyes tonic solution and isotonic solution is 0.09% sodium chloride or 5% glucose.So
which of the following solutions, or solution can be considered hypertonic?So, here you
the three solutions and again you don't get points but you should discuss it with your
neighbors and I'm going to count to see how you guys did on this.Okay. 15 more seconds.Okay.3,
2, 1.So here's what you guys voted. A or D. And that's exactly the problem of this question.So
let's go through it.So, what you want is at least an eyes tonic solution or a higher concentration,
so if you have 6.97% glucose that's higher than 5%, so this is correct.Distilled water
is a hypotonic solution.So, this is certainly not correct.Now, 5% glucose end .09% chloride
is also correct and the reason is it's its and not or.Okay.So when you think of theÊ
when you think of the solution, the type of molecule doesn't count, what counts is the
number of molecules, so in this scenario, you have 5% glucose and 0.95 sodium chloride
so you have more than each one of those. So pay attention to this when you go your homework
and because it's A and C it is actually answer D. that's correct. And this one is wrong.Is
this clear?Good.Okay.Moving on.So, what we discussed so far is diffusion, diffusion is
a passive transport and we have molecules that can cross a membrane and we have other
molecules that cannot cross a membrane, so for the molecules that cannot cross a membrane,
you have proteins that facilitate the transport of membranes.And so, there are two types of
proteins that can do that, you have these here the channel proteins they literally have
a channel where the molecule can go through the protein and therefore can cross the membrane,
and you have carrier molecules that change the confirmation so the molecule can run down.
As you note in both cases, there's a high concentration outside and there's a small
concentration inside.And so in both cases, you still have diffusion, so even though they're
protein mediated you still have diffusion, high concentration, goes down a gradient to
low concentration, and therefore no energy is required.So both of these cases are called:
Facilitated diffusion.So, one example are the aquaporin this is is channel proteins
for water and water will run down according to the concentration gradient of water. So
osmosis will occur. So what type of amino acids do you think line the channel of this?Anybody?Yes?>>STUDENT:
[Indiscernible]>>INSTRUCTOR: The polar. Yes.So the polar amino acids will line the channel
here so, that the water can go down its diffusion gradient. Correct.The carrier proteins because
of their confirmation of changes are very nice systems to regulate flow, this one you
cannot really regulate because it's a matter of concentration of the twoÊ of the substance,
here you can regulate it. For example you can have a gaited ion channel so if you have
a certain stimulus the confirmation can change and the gate will be opened and the molecules
can flow and if you're stimulus molecule is gone the gate can close, so these are really
gates. This one is just an open hole basically. But in both cases because of solute goes down
a concentration gradient you do not need any energy.So in some cases, you go need energy.Yes,
please?>>STUDENT: [Indiscernible]>>INSTRUCTOR: Yeah.>>STUDENT: [Indiscernible]>>INSTRUCTOR:
So, the question is whether the moleculesÊ why the molecules or can the molecules not
move through the gate because there's a higher concentration outside? Is that correct? >>STUDENT:
[Inaudible]>>INSTRUCTOR: Yeah, so in this particular case here, in the channel proteins
they would, and they could, but not in the gated proteins and if you for example see
here in this pictogram you see that the gate is actually shut. So despite the higher concentration
the molecules will not be able to move through.But it depends on theÊ on the carrier proteins.
Some carrier proteins they're sensitive to concentrations and they open up if you have
a higher concentration, but most carrier proteins are sensitive to another stimulus. For example,
a hormone.Hormone comes along then the gate will open, and then the molecules will go
through by along the gradient, along the concentration gradient, unless that hormone comes along
the gate will be shut and the concentration gradient won't matter.Yeah?>>STUDENT: [Indiscernible]>>INSTRUCTOR:
Okay.>>STUDENT: [Indiscernible]>>INSTRUCTOR: Yes. So in this case, it depends again on
the protein there are many of these, if theÊ you can also have the opposite case, the stimulus
molecule, if a stimulus molecule can also shut down a gate.And so it would for example
change the confirmation of the carrier protein.By the stimulus molecule can also open the gate.
But the point here is that there's an external signal that changes the confirmation that
leads to either opening or closing and then the molecules will move through but only based
on concentration differential here.So the gate, the carrier protein gate is not actively
involved in pushing the molecules through it just opens the gate.So, there are situations in the cell where you
want to move molecule against a concentration gradient so for example here, you see these
molecules here, they are high concentration inside of the cell but they are low concentration
outside of the cell, but the cell wants to get rid of them.So in this case, if you want
to put a molecule against a concentration gradient, that requires energy. And the energy
is usually provided by ATP so ATP will hydrolyze to ADP and phosphate and this energy leads
to a confirmational change that will be able to pump the molecules out.And so one example
is the sodium potassium pump. So here, I show this in a movie again, you have essentially
the situation where the sodium concentration in the fluid of your body is very high, the
sodium concentration in the cell is low, but the cell really would like to expel even more
sodium into the outside. So it's against the gradient. On the other hand you have potassium
this, is very low outside, but it's very high inside and potassium has many physiological
roles so you want the potassium concentration even getting higher inside of the cell. So
also potassium needs to go against the gradient. Because the cell tries to suck it up and put
it inside of the cell.And so for that purpose we have the sodium potassium pump, and this
works like this... So 3 sodiums bind to the pump, ATP phosphorylates the protein it changes
the confirmation, the sodium is released and then potassium binds the phosphate is cleaved
off and the potassium can fuse inside of the cell.So I'm going to show this again. So here's
a confirmation where sodium binds, ATP is hydrolyzed, the protein is phosphorylated,
changes confirmation, open up to the out sigh, the potassium is released, now it has an affinity
forÊ sodium is released now it has an affinity for potassium and for the confirmation to
go back the phosphate has to be cleaved off and then it changes to the other way and potassium
can flow inside.So as you can see here's the way how energy or ATP molecules drive confirmational
changes and therefore molecules can move across the membrane against a concentration gradient
and that requires as I said energy, so for this you actually do needÊ oops.You do need
energy.Any questions to that?>>STUDENT: [Indiscernible]>>INSTRUCTOR: Yes?>>STUDENT: [Indiscernible]>>INSTRUCTOR:
Protein.>>STUDENT: [Indiscernible]>>INSTRUCTOR: So, the question was, if theÊ that the potassium
only goes inside when the sodium concentration gets too high outside. No. So it's independent
of what the concentration outside is.The key is that the protein undergoes a confirmational
change and this confirmational change opens up to the outside and the sodium will diffuse
anyway out, and that then opens up the space for the potassium to diffuse and that diffuse
in. And so it is not the concentration outside that dictates if potassium goes in or out
it's just the confirmation of the protein.Yes?>>STUDENT: [Indiscernible]>>INSTRUCTOR: So, what we have
then here is a diffusion, if you want, of ions. And so, you haveÊ when you think about
ions, in particular, not sugars but charge molecules, ions you have two forces that dictate
whether a molecule goes in or out. So the one that we talked already is the concentration
gradient.So this is considered a chemical force.So that's purely concentration and it
doesn't matter what type of molecule it is. But if you have ions you have an additional
force and that is the force of the electricity, so you have electrical force.Because ions
are charged, if you translocate them across the membrane, you're not only change the concentration
of that ion you also change the overall charge in comparedÊ in each space.So for example
here, in the sodium potassium pump you put 3 sodiums out, but you put only 2 potassium
in.Yeah. So because therefore you basically increase the charge of the outside space,
so on the outside, you a positive potential on the inside you have a negative potential,
because you constantly shovel a net E flux of positive charges to the outside and therefore
this here, the positive/negative along a membrane is called the membrane potential.So only in
the case of ions you also have an electrical force.Besides the chemical force, the concentration
force.And so, when you think about it, the pump here, not only has to move the molecules
against the concentration gradient, it also has to move the molecules against an electric
gradient because every time it puts sodium out it needs to even overcome the positive
effect on the outside of the membrane.So it needs even more power to do this.And this
membrane potential is very important for example, the [Indiscernible] in neurons, in your nerve
cells because this pump can generate such potential and that can move along the neurons.>>STUDENT:
[Indiscernible]>>INSTRUCTOR: Say it again, please.>>STUDENT: [Indiscernible]>>INSTRUCTOR:
How is the plus you have an overall increase in positive charge.>>STUDENT: [Indiscernible]>>INSTRUCTOR:
So, because it takes energy to take ions and move them against a concentration, against
a gradient, against a concentration against the chemical force, and ions depending ton
situation you also need energy to move them against an electrical force. This energy can
actually be utilized in the reverse process so we have here a proton pump, so a proton
pump takes protons and pumps it outside. Usually the proton concentration outside is higher
than the inside. So for protons to go outside they need to overcome the concentration gradient
and they need to overcome the charge, the membrane potential, there is more charge here,
that's positively charge and that is relatively speaking negatively charged.So this would
be again activity transport, energy is required to pump the protons out.Yeah, so you can do
this. But, because then there is a natural tendency for these protons to go backwards
to lower concentrations and to the more negative charge you can take advantage of this, and
couple it with something that you would like to do, and this is what is called this cotransport.
So in this case we have a high concentration of protons now with a lot of positive charge
so there's a tendency if you is have a carrier protons to get back in so you take a sucrose
molecule and move it against a concentration gradient. Here sucrose is high and here low,
so it requires energy to get the sucrose inside of the cell and that energy is obtained not
by hydrolyzing ATP but by taking advantage of the proton gradient and the protons and
the sucrose are together inside of the cell. So this is called a cotransporter so this
is called the sucrose, proton cotransporter.So here then, a brief summary, again of the secretion
pathway in cells. And when we think of membranes they have a specific excitedness, I guess
that's what it's called in English, so again we have the ER it pinches off vesicles it
moves to the golgi, go from the cis to the transÊgolgi and they're formed and go to
the plasma membrane and in the plasma membrane they will be suited. But if you have transmembrane
proteins they will stick the entire time in these membranes.And for the function of these
transmembrane proteins in the cell it's very important that these proteins are there in
the right direction so you do not want to have a sodium potassium pump being inserted
in the membrane in the opposite direction because that would wreak havoc in your cells.
So to make sure that the proteins are there in the light side, the sidedness of the lipid
bilayer is maintained here. So you can see there is in the ER you have an inside lipid
layer and the outside lipid layer and the golgi that remains the inside and outside
and it fuses to the plasma membrane the inside become it is outside.So if you modify your
protein in the golgi or even in the ER, for example here with [Indiscernible], you can
also be sure that they're facing the outside.Yeah? And so, also, we have learned earlier the
flipflopping is extremely rare so the flipping of these proteins hardly ever happens so this
is how the cell makes sure that there is a certain sidedness so it can be sure that the
membrane proteins have the right orientation to be functional.Another point is also because
you have now two different layers the composition of the two layers the lipid composition can
also vary. So it is not only the protein direction it's also the lipid composition that can vary.And
so very briefly to going back to this function of the membranes, we had already this case,
where you take food particles and they're engulfed in a membrane, in a food organelle
and then the lysosome comes along and spits out the digestive enzymes how was this called?
Somebody.Phagocytosis. Yes.And so phagocytosis is a form of what is called endocytosis. It
means that the membrane is [Indiscernible] inward and is pinched off from the plasma
membrane and one is phagocytosis if you have food particles if you just take soluble proteins
or ions or something, then there this is called pinocytosis. Pino is Greek for linking, this is intake of licked
everyone though this is what they care about molecules. This is third one, this is receptor
mediated endocytosis. So you have [Indiscernible] in the membrane, so this is called a ligand.
And only when the ligand binds to the receptor then endocytosis occurs you have these coated
proteins that make sure that you pinch in the plasma membrane, they take the vesicle
inside of the cells and then they can release the receptor and the molecule and they can
do with it whatever they like to do depending on what molecule it is.So some examples of
this is for example, the cholesterol in humans.So the cholesterol when you eat it with your
food, it's transported through your system as LDL, low density lipoproteins so there
are little units that float around in your blood and so when they bind to a receptor,
the LDL they're taken up by the cell, they move into a vesicle and there the LDLs, the
low density lipoproteins are degraded and the cell can take it up.So there are some
diseases whether you have very few of those receptors an as you can image, if you have
few of these cholesterol body receptors, cholesterol will not be taken up by the cells and that
causes arthrosclerosis in your blood, so this is an example of the receptor mediated endocytosis
and we continue on Friday. Thank you.