- [Instructor] So potential
energy is stored energy. We find this in our gasoline, we find this in the biological molecules that we eat on a day-to-day basis, pretty much anytime you have atoms sharing electrons with one another, you have potential energy. All right, so what's kinetic energy? Well, kinetic energy is
when it's being used. So the use of potential energy, or the manifestation of energy, or the movement, as we've talked about, the actual work is being done. So when we see any type
of movement whatsoever, that's kinetic energy, whether it's movements of tiny atoms, or whether it's movement of a
large conglomeration of atoms, like you and I, so any type
of movement in our universe is kinetic energy, the ability
to create that movement, but not being used,
that's potential energy. Now, there's other types of
particles that exist in nature, heat, light, electricity, this deals with different manifestations of subatomic particles within atoms, or the actual electrons themselves. So light is photons of energy, they're pure energy that
get emitted by the sun and other types of reactions that can release that type of energy. Heat is also radiant energy where molecules can dissipate
those types of particles and they can be absorbed,
and sometimes be reused. But that's really what kinetic energy is, it's just movement of any
and all types of particles, and wave forms, and whatnot. So that's it, it's either
potential or kinetic, if it's stored, it's potential, if it's being used, it's kinetic, and you can convert one
back into the other. So here we have a bicyclist who probably ate a good
breakfast in the morning, so he ate a lot of molecules,
had a lot of potential energy. Well, now he burns that
by undergoing metabolism, he breaks covalent bonds of
that oatmeal or whatever he ate, and that provides the kinetic energy for him to go up the hill. Well, now that he's at
the top of the hill, he has potential energy
primarily due to gravity, and then that potential
energy can be converted back into kinetic energy
as he goes down the hill. So that's really all it is, whether it's bicycling or
whether it's biological, or whatever the case may
be, it's one or the other. So it's always being converted into kinetic, back to potential, back to kinetic, and so on and so forth. Well, we know many
different sources of energy, we've got batteries that
have potential energy, we've got gasoline which has a wide variety
of potential energy, and we have basic biological molecules, and we use them in a variety
of ways to fuel what we need, and you turn the light on, it starts using that potential energy and turns it into photons
of light and such, the car can use it for pretty much everything
we've talked about, movement, heat, light, electricity, are all generated by the
combustion of this fuel. And then, of course, us, as we eat glucose, and
lipids, and proteins, our mitochondria break
these down into ATP, which is our source of potential energy that fuels our muscles, and
our neurons, and the like. Now, let's talk about thermodynamics because these are the laws that govern energy in our universe. Now, remember we talked about laws. These are progression
of the scientific method that we know so much about, we can make predictions
on what's gonna happen. So these are very well-understood concepts that we know quite a bit about and can make these predictions
based upon these laws. The first law is pretty easy. Our universe is
essentially a closed system where we have no input nor exit of energy. So the first law is where we call it the law of energy conservation, energy cannot be created nor destroyed, it doesn't come into existence, nor does it go out of existence,
as far as we can tell, there are still unknowns about dark matter and dark energy, and whatnot,
and once we figure those out, we might have to adjust
and modify a few things. But in all the observable universe, as far as how things work, energy doesn't pop into existence, nor does it pop out of
existence, it's always there. So only you can change it
from one form to another. I mean, if you take a block
of wood, and you burn it, you may think you've destroyed
the energy, you haven't, you've merely converted it
into another form of energy, namely, heat, light, and whatnot. So you've broken the covalent bonds, but you haven't destroyed the energy, you've merely transformed it. So that's the first law of thermodynamics, energy is conserved, always, it always remains constant. Now, there's a second law
that's a little more difficult, this is the one that we're
gonna focus on for biology, because it plays a key
role in how the cells and other systems maintain homeostasis. Even though all energy is
conserved in our universe, when you transfer energy from
one molecule to the next, from one system to the next,
it is always inefficient. Now, some systems, some energy transfers are more efficient than others, but there's always going to
be that inefficiency going on. So what do we call that? There's actually one
word that could be used to describe the second
law of thermodynamics, I mentioned it before, do
you remember what it is? It's up there too, entropy. So entropy is the second
law of thermodynamics, it basically states that when you transfer energy
from potential to kinetic, or kinetic back to potential,
or potential back to kinetic, and so on, you always lose
some to the environment. Now, some things are more
efficient and therefore lose less, but that's always in play. Entropy causes things
to become more random, is another way of putting it. Our daughter's room never stays like this, it's much easier to have
a high degree of disorder than it is to have a high degree of order. In order to get this, you've
got to put energy into it. This is what occurs naturally, but on a scale of what is
going on in our universe, there's another way of describing this. When we say things have
a high degree of order, one way of describing that is saying that they're very highly concentrated. Let's look at an easy example, let's say you have a perfume bottle, and as long as it stays closed, the perfume will remain in
there for a long period of time. But as soon as I open up
the top, what happens? You get evaporation into
this outside environment. And so what happens is, when you allow for the
molecules to escape, then this is the natural order of things, things tend to become less
organized and more random. Another way of describing that is things go from a high concentration to a low concentration, the perfume is at a very low concentration out here in the atmosphere,
and so therefore, its natural tendency is to evaporate and go into this very
low concentration, okay? Think about a canister of carbon dioxide, maybe like we have in a
variety of different places, you don't naturally get carbon dioxide coalescing from the atmosphere and compressing into this file
that does not what happens. But let's say you have
a high pressure system of carbon dioxide here and
you open it up, what happens? It'll escape to where you have lower amounts of carbon dioxide, okay? So the same thing is true in biology, when we have molecules like ions that are unevenly distributed on one side of the membrane or another, then they tend to flow
from a high concentration to a low concentration until
they're equally distributed. Once they're equally distributed,
then the net flow is zero. And that's really what
entropy is all about as it applies to biology, is that if you have different
concentrations of things, then they will try to equalize. That's really what it comes down to, what entropy comes down to. Okay, so in chemistry, we learned that if you
have certain reactants, but no products, then the natural tendency is that two things will
react with one another to start generating some product. Well, as the product increases,
and as these decrease, then this essentially, equalizes. So it's the same process in
metabolism in our bodies, if we have substrates, which we call them, or things to interact and
react with one another, and we don't have any of the products, this will naturally occur
to create these products. But once we have the
same amount of products as the reactants, then the equation stops. Now, most of the time,
we get rid of the product so that this keeps flowing this way, we go from a high to a low concentration. So let's show how this works. And this is how kinetic and potential energy work
in a biological system. Some reactions absorb energy,
some reactions release energy. So let's look at the absorption. When a reaction absorbs energy, it is said to go from
kinetic to potential, and this is exactly
what photosynthesis is, light, which is kinetic
energy, in chloroplasts, is converted into covalent bond energy by covalently bonding
carbons, and oxygens, and hydrogens together to form glucose. So this is taking the kinetic energy, which is just constantly flowing
to our planet from the sun, and turn it into potential energy. That's really what plants do. This, we have a special name for, we call it an endergonic reaction. So an endergonic reaction is a reaction that absorbs energy and
is turned into potential, so it's kinetic to potential. This is how we build our structures, your body has a substantial
amount of potential energy that is built up over your lifetime, okay, you have a lot of energy
in all of your molecules. Exergonic reactions are
reactions that release energy, for example, in your mitochondria, when your cells absorb the sugars, and the fats, the proteins, the mitochondria will
break builds covalent bonds and release the energy that
was stored in the glucose, in the lipids, in the proteins. So this is the equivalent of
going from potential to kinetic an exergonic reaction
is breaking the bonds, releasing energy by converting
it into kinetic energy. So you can see how these two reactions essentially worked hand in hand. Some reaction releases
energy, an exergonic, another reaction absorbs
that energy, endergonic. When we put those two together, lemme jump ahead real quick, we actually call it a coupled reaction. So what is a coupled reaction? A coupled reaction is nothing
more than one reaction releasing energy, exergonic, and another reaction absorbing
that energy, endergonic. That's it. So here's an example. ATP, that battery that I told you about, and I'll show you how ATP
works in just a second, ATP provides the energy by when its covalent bond is broken, it gives the energy necessary for actin and myosin,
which is in your muscle, to interact with one another, that's what causes muscle tension. So when ATP uses its energy, then the muscles absorb that energy. The muscle contraction is
the endergonic reaction. When you break ATP, that's
the exergonic reaction. And so something as simple
as flexing your muscle is a coupled reaction, something gives up energy, the
other absorbs it, all right? I might add and point out
how entropy plays here. Heat is mostly the byproduct of inefficiency of energy transfer. Remember, I told you that entropy, when you transfer energy
from one system to the next, there's always gonna
be some loss of energy. So the muscle doesn't fully claim all of the energy from the ATP, some of it dissipates to
the environment as heat, which is why when you
exercise, your body heats up because every time you
break covalent bonds, you generate heat as a
byproduct due to entropy. All right, so let's look at why ATP, why does the cell use ATP instead of just directly using glucose, or lipids, or proteins, or whatnot? Well, remember what we talked
about with covalent bonds, the reason why atoms form covalent bonds is because it provides stability, valence electron shell theory, when the atoms have a full
valance electron shell, then the atoms are stable. So in order to break a covalent bond, you have to overcome the resistance that these atoms have
to becoming unstable. They don't like that, they like to hold on to those electrons, they like to stay close together and share the electrons to
form those covalent bonds. That's why it's a good
source of potential energy. Well, glucose, fats, proteins,
they're all the same, they're very stable molecules when it comes to the covalent bonds. So your mitochondria are the organelle that are specifically designed
to take any organic molecule, and turn it into this. So why does this become
the universal battery that the cells need to be
able to transfer energy? Well, it comes down to
this portion right here, the Triphosphate portion of the ATP, remember, it stands for
Adenosine Triphosphate. So why is that so important,
why is this so special? Because energy can be found anywhere. You got energy here, there,
there, there, everywhere. Why this? Well, look at these phosphates. What do you notice about
them, what do they have? What do they have in the top part of this? What does this little negative sign mean? - Charge.
- They're charged, okay. What happens when you put molecules of the same charge near one another, do they attract or repel? (students mumbling)
They repel one another. It's not like an ionic bond where you have two opposite-charge ions, you've got three ionic charges here that are very strong
repelling one another. Well, due to that fact, the covalent bonds that hold these phosphates
together become weakened, and as such become the easiest
covalent bonds to break in any biological molecule. And that's why your cell converts all organic molecules
into ATP because the cell, the machinery of the cell, can tap into this energy anywhere, okay, doesn't need any special type of thing, this is easy energy, the easiest. This also makes the ATP
molecule very unstable. So your cell usually doesn't make ATP except for in the moment when it needs it. So your mitochondria is
constantly pumping out ATP and your cell is constantly using that, especially your muscles
and your neurons, okay? So that's why, and that's one
of the questions I'll ask you, why does the cell use ATP
instead of directly using glucose and other sugars for their
covalent bond energy? Because of the nature of
these three phosphates, they create this repulsion
field that makes it so that that covalent bond
is very easy to break. And the trick is has just as much energy as any other covalent bond,
it's just easier to break, and therefore easier to
extract the energy from it. So guess what, ATP acts as an intermediary between most reactions, for example, you eat sugars,
and fats, and proteins, first, your mitochondria
is gonna break them down, that's an exergonic reaction, and assemble used-up fuel molecules, which we call ADP, Adenosine Diphosphate, which has two phosphates instead of three, and it just reattaches that phosphate. So this is an endergonic reaction. Remember what happens when
we put the two together, we call it a what? A coupled reaction, so you release energy
and it gets absorbed. Now, this becomes the exergonic reaction, now the ATP releases the
energy somewhere in the cell, and something else absorbs it, and that becomes the coupled reaction. So you can see that all it
is is just energy transfer, so it's going from one molecule
to the next, to the next, okay, as I just explained it,
your cells absorb glucose, your mitochondria will
break the covalent bonds of that glucose molecule into
its fundamental molecules, carbon dioxide and water. We'll learn that this is exactly what plants need to make glucose. So when the mitochondria
break glucose back down, they turn it back into
the fundamental molecules that the plant needs to, again,
repackage it into glucose. So the cycle just keeps going
round and round, and round, we consume glucose, plants make it, we consume it, they make it. Notice though, however,
that as we transfer energy from glucose to ATP, and from
ATP to say, making a protein, which requires energy to
assemble the amino acids together to form a polypeptide chain,
notice energy is lost. What's that called again? What law? - Entropy.
- Entropy. That's entropy right there. You can never fully
transfer all of the energy from one molecule to the next, there's always going to
be some loss of energy. Which brings us to the fact that if we don't have
photosynthesizers on this planet, you and I wouldn't be here
because we'd run out of energy, without something to absorb the sunlight and turn it into food for
you and I, we're not here, we don't exist, we cannot survive, because we don't have the capacity to generate sugars, and fats,
and proteins, from light. Plants and other
photosynthetic organisms do, hence our dependence
upon them for our food, because you can't recycle
all the energy in your body, if you did, you wouldn't
have to eat hardly at all if you could recycle all
the energy without any loss. All right, ATP is very,
very short-lived, okay, but the cell can pump
out mass amounts of it for your glucose, I mean, it can take a single glucose molecule and almost pump out 40 of
these, okay, so you get a lot, you get a lot of ATP for
your organic molecules. Okay. Now, last concept today, enzymes, and we probably won't get
all the way through this, but I've got some videos to show you, as well as some things we did here at UBU, that really help out with this thing. Now, when we talked about dehydration
synthesis and hydrolysis, really we were talking about
these two processes right here. For example, when you break covalent bonds and split polymers down into
monomers, that's called what? Is it dehydration
synthesis, or hydrolysis? - Hydrolysis.
- Hydrolysis. Guess what, hydrolysis
and an exergonic reaction are the same, they're exactly the same. That's all hydrolysis is, hydrolysis is the
breaking of covalent bonds and the releasing of energy. So guess what dehydration synthesis is? It's an endergonic reaction. When you covalently bond monomers
together to form polymers, you are building complex molecules and storing energy as potential energy. So now you know that dehydration synthesis and hydrolysis are equivalent to an endergonic and
an exergonic reaction, that's really all it is. But these two processes
in biological systems don't occur naturally. The reason for that is because
remember covalent bonds are stable bonds, in order to break one, you have to do one of two things, you either have to provide enough energy, usually by heating something up, to cause them to break apart,
they absorb that energy, and eventually they'll break apart, or you just lower the requirement for the atoms to break that covalent bond. Well, guess what, we have
biological structures that do the latter,
they're called enzymes. So what do enzymes do? Enzymes are proteins,
which are essentially, as we learned in the
Organic Molecule lecture, polypeptide chains of amino
acids that fold into a tertiary, or sometimes quaternary, depending upon how many
there are, structure, and based upon that structure,
they each have a certain job. Some enzymes undergo dehydration synthesis where they assemble molecules together. We've gone over a major
enzyme in the last lecture, and we think about it. It's an enzyme that actually
makes enzymes, it's ribosomes. Oh, I thought you said lysosomes. Ribosomes, sorry, again, ribosomes are enzymes,
'cause what do they do? They assemble amino acids together to form polypeptide chains. So they're literally enzymes
that make other proteins. Which brings us to a good question, which came first, the
enzyme or the protein? And that's a good question,
we've asked that one, and there's an answer,
but not for this class. Well, later along we'll get into that. So some enzymes have
a particular structure that undergoes hydrolysis. For example, in your mouth, you have an enzyme called amylase, and when you start eating foods that are especially high
in starch or carbohydrates, that amylase starts breaking down the starches into disaccharides, and then once they enter in your stomach, there are other enzymes that break the disaccharides
down into monosaccharides. And then when it gets
into the mitochondria, there are enzymes there that
break down the monosaccharides and turn them into ATP. So for every stage of
breaking covalent bonds, there's an enzyme, okay? Now, what do enzymes actually do? Well think about it, imagine
that you are trying to assemble your kid's bike for Christmas. I didn't buy a bike for
my kids this Christmas, so I didn't have to
assemble anything together, but imagine back in the day,
when you had to assemble them, now they come prefabricated, let's say that you had
to tighten the nuts, and the bolts, and whatnot, and you didn't have any tools whatsoever, could you do it safely, or your kid wouldn't
fall apart on his bike? Probably not, why, 'cause
without the tools like a wrench, or screwdriver, or whatever, you wouldn't be able to
facilitate the necessary torque to tighten the nuts, to
tighten the bolts, and whatnot. Well, the same thing is true for enzymes, if you don't have an enzyme, then the job that that
enzyme was supposed to do really can't be done in your body. Because in order to get it
done, without an enzyme, you have to heat your
body up so much that, what starts happening when
you heat proteins up too much? They denature. So you can accomplish the same thing by generating lots of energy, but it would normally kill you to do so. So what happens is, when you
either break a molecule apart or put molecules together, there's an inherent
amount of energy required to accomplish that. And this is a key term
you're gonna have to know, it's called energy of activation. It's not a new type of energy, it's just the amount of energy required to break or make a covalent bond, that's what activation energy
is, or energy of activation, the amount of energy required to break or make a covalent bond. I have a very simple example
that illustrates this concept. Let's say that you want to study for one of my quizzes with
someone here in the class and they've been very reliable in the past and you really need their help. Well, one day you say, "Hey, let's meet at the
top of the hill and study and do what we need to do." Well, you get to the top of the hill, and you notice that they found some new person to hang out with, right? So they decided to go
off and leave you alone and to fail in my quiz. Well, there just happens
to be a boulder up here, conveniently, in this position,
and you decide to push that and ultimately end your
misery by ending theirs. Now, that's a hard thing to
do, to push that large boulder, I'd rather get my revenge an easier way, how can I do this easier? What should I do? What is that? Leverage, fulcrum, a lever,
that makes it really easy to get that ball rolling and kill them. So, in the end, what is this? That's the enzyme. The enzyme allows you to do the same job with less energy input. It's like trying to screw
a nut with your fingers versus a wrench, one substantially
easier than the other, requires a substantially less energy to get the same job done. That's what enzymes are. You can think of it as
the energy required, say, to push a boulder down a hill
or to get the ball rolling. Once you get the ball rolling, then the energy releases itself and you get more out of it
than what you put into it. But you still have to overcome that initial resistance of these molecules to break their covalent
bonds, and whatnot. So that's where enzymes come in. Because at normal biological temperatures, the amount of energy required to spontaneously break covalent
bonds is not sufficient. As such, enzymes are proteins that have a particular tertiary structure that lower the amount of energy required to break the covalent bond, but they are specific to
what we call the substrate, which is what they metabolize, what they break down,
what they put together, this is not just breaking things down, it's also putting things together, the same principle applies. So that's what a catalyst does, it essentially lowers the
amount of energy required to cause some reaction to occur. So it's like I mentioned
before with the wrench and the bolt, and the
screwdriver, and the screw, the wrench can't do the
screwdriver's job, and vice versa. And if you try to do
it without either tool, the amount of energy required
is substantially more than if you have the proper tool to give the necessary torque,
and other things of that sort. This was what I explained is how you know that if
you have too much heat, too much change in the pH, and too much salt, or a
different salt concentration, that ultimately denatures the enzyme, the enzyme cannot function, as you saw many examples
of the liver in that. Now, we're gonna describe and
explain why cyanide kills you, as well as many other
various toxins or whatnot, because the things that
are toxic to you and I aren't necessarily toxic
to every living organism and vice versa, antibiotics, why do antibiotics hurt bacteria, not us, barring a allergic reaction, which I'll explain the
difference here in a second. So, enzymes, in order
for them to do their job, that tertiary structure must
be unbound to something else, it has to be free to be able
to break something apart or put something back together. So there are some poisons for any organism that essentially mimic the substrate that the enzyme is supposed to metabolize, but because they're not
quite the same structure, then they end up sitting where the enzyme is
supposed to do its job, and they are not metabolized,
so they just sit there and they prevent the metabolism
of that particular product. And the enzyme needs to
literally be destroyed and remade before it can function again, which takes a little while. So why does cyanide kill you? Well, there's an enzyme
in your mitochondria called ATP synthase. Guess what it makes, ATP. Cyanide blocks that enzyme
from being able to make ATP. And so when you get sufficient amounts into your bloodstream and into your cells, your cells stop producing
ATP, and you die. So that's why cyanide kills you, as well as many other substances can actually do the same
thing to another enzyme that's critical for your cell survival and energy production,
and things of that sort. Let's look at penicillin. This was actually secretion from a fungus that fungi used, to kill bacteria and cause them to die
and then absorb them. So this is how we first
discovered antibiotics, is actually fungal secretions that they use for decomposition. Penicillin was one of these. So why does penicillin
kill bacteria but not us? Well, penicillin targets enzymes that synthesize the bacterial cell wall. Now, do animals have a cell wall? Do we produce a cell wall? No, so because we don't have this enzyme that's key for the function
or survival of our cells, penicillin doesn't kill us. Now, there are people who can
have an allergic reaction, but it's no different than
like a peanut allergy, where the body thinks that
it's some toxic substance and accidentally creates
an immune response. So for people who are
allergic to penicillin, it's not that penicillin
is actually dangerous, no, no more dangerous than a peanut is, but you have these
different types of allergens that the body can respond to, and they can be life-threatening
for some people, but not be because we have an enzyme that this product can bind to. So this is why cyanide kills
us, but doesn't kill bacteria, because they don't have mitochondria, they don't make ATP this way,
so they're like, "Whatever." But penicillin kills them, that's toxic to them, and not toxic to us. So this is where we get into our medicines and our various chemicals
that when we treat, that's why we can have such
high doses of antibiotics coursing through our
bloodstream, and not even hurt us because we don't even have
that enzyme that it affects. All right, so I mentioned too, that some enzymes are
more key than others, but every metabolic reaction in your body pretty much requires an enzyme, whether it's putting proteins together, whether it's breaking glucose down, every step has different enzymes. And so these are key proteins that are necessary for metabolism and if you don't have one, it sometimes can just
cause mild irritation, like lactose intolerance,
or it can cause death if you don't have particular enzymes like with the Tay Sachs disease, that enzyme doesn't metabolize lipids, it builds up in the brain and the child dies by about age three. Now, this second part is more fun because we look at its application. This second part actually addresses how cells maintain homeostasis by either working with
or against entropy, okay? So remember, when I say with entropy, or we say high to low
concentration, that's with entropy. Another thing you're gonna see is, in some of the vernacular they use, is what we call concentration gradient. What does that mean? It means that when you have a membrane, and you have a high
concentration on one side and a low concentration on the other side, that's its concentration
gradient, kind of like a hill, it goes from high to low, one side is high, the other side is low. If we talk about going low to high, that's what we call against
its concentration gradient, and that requires energy. So that's a lot of the
terminology I'm gonna use. If I say with the concentration gradient, I mean, with entropy,
with the natural process as it goes from high to low. It's why I use the example of the hill. If I say against the
concentration gradient, this is where energy is required, because it's like trying to
push a boulder up on the hill, it's trying to go against
what naturally occurs. This is where you take molecules
and you pressurize them, you put them into a high concentration, it requires energy to do that. So the natural tendency of
all molecules in our universe is to undergo a process
which we call diffusion. Diffusion is just the inherent movement, which is kinetic energy, within an area to where the molecules try to
distribute themselves evenly. So a prime example of this
is just making the tea, you put the teabag in the water, and over time the
molecules start diffusing until you have a nice homogenous solution of the tea chemicals, and
that's diffusion, okay, that's the natural process. It's evaporation of a
perfume into a larger area, it's just how everything works, everything goes from a high concentration to a low concentration, that's diffusion. But when dealing with cells, because they have a membrane, that membrane selectively allows what comes in and out of the cell. So there's about six modes of transport that you're gonna learn
today of how a cell is able to regulate access to a cytoplasm. Now, some things occur naturally, which is, as we talked
about, with entropy, and these are actually key
and important processes, you really don't give a second thought to on a day-to-day basis, but they're just constantly happening. So let's talk about the first three. The first three belong
to a group of transport, which we call passive transport. The reason why we call
it passive transport is because all three of these
require no energy to occur. Why, because it's the natural
order of the universe. All of these occur when you have different
gradients of molecules and they go from a high to a low, okay, so that's why we call it passive transport is because it occurs passively, or without any type of effort, and it's just one of the
laws of our universe. All right, so let's talk
about the first one. We don't just call it diffusion, because diffusion is just the
natural process of molecules to go from a high to a low, when it is across the cell membrane, we call it simple diffusion. So in simple diffusion, the membrane, lemme pull
up some of my animations that I've created here, the membrane is very porous in the sense that there is
space that allows some molecules to go through the phospholipid bilayer. Okay, there we go. So remember, phospholipids
are these molecules that have the hydrophilic calves
and the hydrophobic tails. And they form this bilayer that encases all of the
cytoplasmic components of the cell, its organelles, its fluid, and the like. Well, molecules, if they're small enough, and they're not polar, can
easily just squeeze in between and move through the membrane. The membrane doesn't do anything, in fact, it can't do
anything against this. So oxygen is key for our cells to be able to undergo certain metabolism, or what we call aerobic respiration. Every time you expand your lungs and take in a large concentration of air, what you're doing is you're increasing the oxygen concentration around the cells in your lungs. Well, when the blood reaches the lungs, the oxygen content is very low, and so by having a high concentration of oxygen in your lungs, and a low concentration
of oxygen in your blood, the oxygen naturally diffuses
through the membrane, because it's trying to
equalize, it's trying to diffuse so that both sides of the membrane have equal amounts of oxygen. But we keep breathing
out and re-establishing that high concentration
of oxygen in our lungs, so every time we breathe in
and out, we keep making it so that more oxygen is on
the outside of our blood, and so it diffuses into
our blood naturally. So this is what it's illustrating. When you have high concentration
of oxygen on one side, it just leaks right through the membrane to try to equalize and become equal, but because we keep breathing out the air and breathing in new oxygen,
it'll never equalize. So that's why breathing
works the way it does, why oxygen is able to
diffuse into our blood through what we call simple diffusion. Well, the same process occurs, but in the opposite
direction, for carbon dioxide, when we undergo metabolism,
the glucose, and the sugars, and the fats, and the
proteins, that get broken down, get turned into carbon dioxide. Well, our cells pick up that, our blood picks up the carbon
dioxide and take your lungs. Well, as long as you're in an environment where the carbon dioxide you're
breathing in is much lower, then carbon dioxide leaves your blood. And this is the reason why you don't enclose
yourself in a sealed room, because not only does the oxygen run out, but the CO2 starts building up as well. If the CO2 builds up enough,
then what ends up happening is you don't get simple diffusion of the carbon dioxide out of your blood, it stays in your blood and it starts causing massive problems. So you need to be in an environment
where the carbon dioxide is substantially lower
than what's in your blood so that it can naturally diffuse out. There's no other mechanism to do this, carbon dioxide just will
go from high to low. So as the blood reaches your lungs, it essentially diffuses
in the other direction. So it all just depends upon
the concentration gradient, if oxygen is higher on
the outside of the cell than on the inside, it
diffuses into the cell, if carbon dioxide is higher
on the inside of the cell than on the outside, it
diffuses out of the cell. Now, here's another thing
you have to be aware of, every molecule has its own
concentration gradient. What I mean by that is, let's say you have your
phospholipid bilayer here, you have high levels of oxygen, and high levels of carbon dioxide. So carbon dioxide is at high levels inside and oxygen is at high levels outside, they don't look at each other and say, "Oh, well, because we both are high, then we'll just stay where we're at." Oxygen merely looks at other oxygen in terms of its diffusion, it
only deals with other oxygen. And same thing with carbon
dioxide, it only moves in accordance to its own
concentration gradient. So that's why you can have
these different gradients and simultaneously, oxygen will move in as carbon dioxide is moving out. So you don't have to breathe twice for every type of exchange, you breathe in, oxygen
diffuses into your blood, carbon dioxide diffuses out in your blood, you get rid of that air,
you breathe in a new batch, same thing occurs. All right, so that's what
we call simple diffusion, does not require any energy
on the part of the cell, the membrane is there and the molecule simply work with entropy
to go across the membrane. Now, I mentioned that these molecules must be small and not charged. The reason because this
region right in here, where the phospholipids are
at, is extremely hydrophobic, which means if you have
any charged particles, ions, polar molecules
like glucose, and whatnot, then they're actually impermeable to going through this membrane. Let me show you another video. If there is these charged particles, they essentially bounce
right off the membrane 'cause they can't squeeze in between, because of how charged they are, they get trapped on one
side of the membrane. So this brings us to our
second mode of transport, we call it facilitated diffusion. This requires help, these molecules can't naturally go through this phospholipid bilayer, so they need some type of channel by which they can go through
naturally from a high to a low. These proteins are usually
what we call ion channels. We also have channels for other molecules like glucose channels, and whatnot. So there's a lot of proteins that each are specific
for a particular molecule. You have hydrogen ion
channels, sodium ion channels, potassium ion channels, glucose channels, so each protein is specifically designed to allow a certain molecule
through the membrane. But the key here too, is it's like a revolving
door in the hotel, the door doesn't move
itself, it merely moves when the molecules are
putting pressure against it. And so it naturally occurs due to the diffusion of these molecules. So the only difference
between facilitated diffusion and simple diffusion is facilitated, the molecules need some help to go through because they're charged, or they're large, or they're polar, and
they can't go through that membrane barrier, because
it's so hydrophobic, okay? So you might have ions
that are much smaller than molecules like
oxygen and carbon dioxide, but they'll bounce right off the membrane, they can't go through unless
they have some type of pore, or channel, in the membrane
that allow them to go through. That's what this video is illustrating that they will essentially
just travel through the channel from a high to a low concentration, they're at a high
concentration on one side, you give them a way out, and
they will take that way out because that's the natural
order of process called entropy. All right, now, the last mode of transport is actually the same as simple diffusion, but it's called osmosis. So why do we distinguish between the two? Simple diffusion is the diffusion of any molecule across the
cell membrane, except water. Now, why do we have to make
the distinction between water? Well, it's because simple
diffusion is occurring in the watery environment, and osmosis is the simple
diffusion of water, okay? But there's some strange things that kind of go on with
osmosis when you look at it, so that's why we kind of separate it in terms of talking about it instead of just simple diffusion. So the same principle
applies here with osmosis, if you have high concentrations of water on the outside of a cell and low concentrations of water
on the inside of the cell, Which way will the water flow, it will flow out or in? It's low here and higher outside the cell. - Flow in.
- It'll flow in, okay, just like all these other molecules, it'll go from a high to a low. Well, the confusing part
is, we look at the cells, and we look at the
solution that they're in, and we don't actually look at the water, we look at how much
salt you turn in there. For example, let's say you had
a 1% salt solution out here. Well, what percentage
of the water then is it? If you're looking for 100%,
what would the water be? - 99.
- 99%. Okay, so that's the
percentage of water, 99%. Let's say inside of the cell, it's at a much higher salt
concentration like 5% salt. How much water do you have? 95% water. So 99, 95, which way does it go? It goes in, again, this has less water. So it's the concentration gradient, if the outside solution is 99% water and the inside solution
is 95% water, it goes in. What if I reverse this,
what if this were 99% and this were 95% because the
outside was extremely salty, and the inside was not so salty, now which way did the water go? It leaves the cell. Okay, so that brings us to a key aspect of what cells do to try
to maintain homeostasis, especially where we're
just completely surrounded with these different
concentrations of water. We call it tonicity, which is essentially the
relative concentration of water out and inside of the cells. Now, if the solutions on
the outside of the cell is isotonic, that means that
the concentration of water is equal on both sides of the membrane. When that happens, the net
flow of water is zero, okay? So iso means the same. So in an isotonic solution, which is what our body tries
to maintain in our blood, and in other areas, the net flow of water is absolutely zero. Because what happens is,
if you have these two, which we call hypertonic, and hypotonic, then problems start
happening with the cell. What am I talking about? Well, here's an example in your blood, your blood, if you look
at it from the side, looks kind of like a doughnut without the hole in the
middle, just looks like this. You can kind of see that groove in there. It's not an actual doughnut shape, it doesn't have a hole
in the middle of it, but it is more round on the outsides and less round on the inside,
that's just the shape of it. Okay, well, when your blood is isotonic with your blood plasma, which is the fluid that it bathed in in your
cardiovascular system, then the cell functions normally. But let's say, for example,
let's say you lose some blood, and you go into the hospital, and they put an IV into your blood because they need to
restore some of the fluids, the IV must be isotonic with your blood, it cannot be pure water,
because if it were pure water, this is what would happen,
your red blood cells would literally blow up
like a balloon, and burst. So you could kill someone by pumping pure water
into their bloodstream. Water kills (chuckling)
in many different ways. But this is one of those scenarios is that an IV actually
has salts and sugars in it so that it's not 100% water
because the inside of your cells aren't 100% water, they're salts, there's other things in there. And so we've measured it and we know exactly what percentage, it's usually about 2.7% or so, of solutes. So that is 97.3% water, or whatnot, and that's isotonic with your blood. So if a nurse were to come in and be like, "Oh, your IV is out,
or we're out of stock. Well, you still need fluid, so I'm just gonna fill
your IV up with tap water," you better get out of there 'cause she's gonna kill you, okay? So, on the flip side,
if you put a solution that has a high concentration of solutes, like salts, and sugars, and
whatnot, and it's too high, meaning it has more salt and
less water, then this happens, water leaves your cells,
the cells shrivel up, and they die and they don't
function properly as well. So that's why if you have pure
water put around your cells, or salty water put around your cells, you got a big problem on your hands. Now, plants tend to like being in what we call a hypotonic surrounding. Why, because they don't
burst like our cells do. Why, they have a cell wall, and that cell wall allows the cell to actually engorge itself with water, which is actually critical for its ability to undergo
photosynthesis, without dying, the cells don't burst open, so it loves that hypotonic surrounding. If you wanna see this at home,
and most people know this that if you wanna keep
lettuce from wilting, then you put it in water,
you put it in your fridge, you put it in water, why, because when you put it
in water, the cells swell up and it allows them to remain very turgid. However, if you don't give
your plant a lot of water or if you give it water with too high of a solute concentration, the water actually leaves the cells and you have a very sad-looking plant. So one of the confusing things
that people have with this is when they see the word
hyper, hyper usually means what, more or less? - More.
- More. And they think, oh, it
has more water in it. No, this is referring to
the solute concentration. So a hypertonic solution is one that has a high
solute concentration, and as such, less water
on the outside of it. A hypotonic solution is one that has a very low
concentration of solute and therefore has a higher
concentration of water. This is the one that just people tend to struggle with a lot, okay? The cell actually does a number of things that work against entropy to
try to maintain homeostasis. And so here, these last set of processes actually work against entropy, the first of which is
called active transport. So whereas passive transport
just happens naturally, active transport is like
pushing a boulder up a hill. This requires energy because you're working against
what would naturally occur. What am I talking about? Well, the main thing that
cells do in certain situations to work properly, is they'll
actually take molecules from a low concentration and push them against their
concentration gradient. So why would the cell ever wanna do that? Well, let me give you two examples. The first example is
how your neurons work. Your neurons actually have, if
you were to put an electrode on the cell membrane of your neurons, it would measure negative 70 millivolts. There is actually a voltage
of membrane potential to these neurons, and the reason for that is because it maintains
different gradients of sodium and potassium on
each side of the membrane. Well, remember that each
one only cares about itself, and so this enzyme here will push proteins against the concentration
gradient here for sodium, and it pushes potassium against its concentration
gradient the other way. So this one actually has a dual role, it actually shoves sodium out
and then shoves potassium in, both of which are against
their concentration gradient. So it requires energy to do this process, but by pushing sodium and potassium in, it creates like a battery,
an electrical potential that allows your neurons to do their job. This is why your neurons in
your brain use so much ATP is in order for them to fire and function the way that they do, it has to constantly
maintain this gradient, and it reestablishes this polarity every like five milliseconds,
okay, so that's pretty fast. Now, your muscles are another example of how active transport is used in the functioning of your cells. When calcium is present
in your muscle fibers, your muscles contract, but you need to let your
muscles relax as well. So the only way in which you can do that is to remove the calcium. So your muscles actually take the calcium that's surrounding your muscle fibers and push them into these
membrane compartments where they're not touching
the muscle proteins. That's active transport. So they push it from a low to a high, they concentrate your calcium
into these compartments and then when the muscle
gets the signal to contract, then you get facilitated diffusion, it naturally goes from a high to a low. So in one situation, when
your muscles contract, you get facilitated diffusion, when your muscles relax,
you've got active transport. This one requires energy,
like ATP, for that to occur, this one, no ATP is required because it naturally
occurs from high to low. So that's how your muscles work, every time your muscle contracts, that's facilitated diffusion of calcium, every time your muscles relax,
that's actually using energy. Now, the last two are
actually pretty simple, they're what we call bulk transport. We don't always just pump ions, and water, and molecules, across the membrane, sometimes you just need
to take out the garbage, you just need to have these
large mass of molecules that are taken out all at once. So what the cell will do
is surround these molecules in a membrane vesicle, and then it fuses these membranes together and releases content outside of the cell. Again, this requires energy for one main reason, when you fuse these membranes, because the hydrophobic
region doesn't like the water, there are proteins in here
that kind of come together and then they cause the membranes to fuse, that takes a substantial
amount of energy to do that, it doesn't occur naturally, the membranes don't just form together like bubbles necessarily do, okay? So there are proteins here
that facilitate this process but that's where the
energy requirement is here, this vesicle must fuse with the membrane, that requires a substantial
amount of energy to do so. So exo stands for exiting,
or leaving the cell. So what's an example of this? Well, when your neurons
communicate with one another, if you've ever heard of what we call neurotransmitter release,
that's exocytosis. So when a neuron synapses with another neuron, at the end of this neuron,
they have these little vesicles that have what we call
neurotransmitters in them. Well, when the signal tells
this neuron to release that, then these fuse with the membrane, release the neurotransmitter
into this area, it binds to this, and then
the signal continues on. You've got things like dopamine,
and serotonin, and whatnot, these are neurotransmitters,
that's exocytosis, that's how your neurons
communicate with one another. This is also used in like lactation where the breast cells will
produce the milk proteins that then get released and
go into the lacteal ducts. You actually saw this
in your previous lecture on the organelles. Remember the endomembrane system? Right here, this is a prime example, where the proteins get made, they get packaged up in the smooth ER, modified in the Golgi apparatus, and then they fuse with the membrane and they're released into the
milk ducts, that's exocytosis. So that's what we call bulk transport. Now, what about bringing things in? Well, that's what's called endocytosis. So endocytosis is also bulk transport. Prime example of this is one
cell eating another, okay? White blood cells eat bacteria,
they even eat cancer cells. Notice that they literally
have to form their membrane around a large molecule, or a
smaller series of molecules, but the fact of the matter is, it requires a substantial amount of energy to rearrange the cytoskeleton
to be able to do this, and to form this membrane vesicle. So endocytosis also requires energy for this process to occur. And this is an electron
scanning micrograph showing you a lot of the distortion. This is a white blood
cell eating a cancer cell, to engulf it, and then it will
digest it with its lysosomes and then use that material
and break it apart. So exocytosis is bulk
transport out of the cell, endocytosis is bulk
transport into the cell.
