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- [Voiceover] When molecules
go across the plasma membrane,

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there's more than one way for it to occur.

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They can go through what's
called passive transport.

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In the case of passive
transport, the particles moving

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and the membrane itself
does not need any energy.

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The energy has been stored or has

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already been exerted before.

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So, types of transport
are diffusion and osmosis.

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Diffusion you might have heard of already.

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Think of diffusion, you might start

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thinking of someone baking cookies.

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Do you have to have your head right

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in the stove to smell the cookies?

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No, the smell has diffused from the stove

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and the oven over to where you are.

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Osmosis, we'll touch on later.

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Active transport, cells
have to expend energy.

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So, you have to put energy into the system

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to make the system work.

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Then there's this bulk transport.

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It involves the vesicles.

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What actually happens is, think about

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endo and exocytosis, is the way

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of moving large particles or
large things back and forth.

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What if they can't fit through
those protein channels?

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In endocytosis, E-N-D-O,
you're bringing substances in.

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Endo, within.

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While exocytosis, E-X-O,
exo, think of exit.

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The substances are leaving.

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Endo and exocytosis.

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Does passive transport,
okay, remember, no energy

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at the time is moving, is all powered

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by a concentration gradient.

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The simple basic idea of diffusion,

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a particle moving though the lipid layer.

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Well, you also have simple basic diffusion

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through a protein channel,
but then sometimes

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you have this facilitated transport.

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That means you have to have transport

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or carrier protein in the membrane.

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Something's helping or opening a port.

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Think of that smell
back in the oven again.

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Baking those nice chocolate chip cookies.

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You can smell it in the kitchen,

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but if all the windows are shut

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can you smell it outside
the kitchen window?

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No, you shouldn't be able to.

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Unless your windows have a
whole lot of leaks in them.

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But now, if you open that window,

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and you're standing outside, would

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you be able to smell
the chocolate cookies?

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Most likely.

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That window, by opening
it, you're facilitating,

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or helping, the transport.

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This is meant to show how particles

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can move through various
types of passive transport.

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You can see diffusion,
diffusion through channels,

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and facilitated transport.

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So, this idea of diffusion
right though the lipid bilayer.

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Particles small enough, going from

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a high concentration
to a low concentration.

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The reason here why it goes
through, from high to low,

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is every system is trying to reach

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what's called equilibrium.

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Equilibrium is when both environments

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are equal or the same.

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In this particular
figure, the top portion,

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the blue, has a much higher concentration

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of all three spheres.

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The bottom part, the yellow, has

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a much lower concentration.

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That means this particular membrane

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will have transport going across it

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until you reach equilibrium
of all particles.

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Or, equilibrium cannot be
reached, one of the two.

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So, we go from high to low,
right through the membrane.

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We can go from high to low right

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through a channel, the middle one.

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The channel helps just get a little bit

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easier passageway, or
allows it to float through.

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Well, the far right,
facilitated transport.

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The black sphere, this is too large to

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go through the protein channel,

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and definitely too large to go
through the plasma membrane,

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so it goes into this protein channel

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which then will change shape and

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release it to the inside.

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So we have three different ways here.

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What's not shown is the idea of osmosis

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I said we'd get back to.

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Osmosis works a very similar principle,

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only difference is the membranes
are not semi permeable.

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Everything's kept inside for osmosis.

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That means the only thing
left to move is water.

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Osmosis is the movement of water

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from high to low concentration gradient.

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But it's all based on
the initial concentration

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of the liquid, both sides.

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Switching gears to active transport.

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Active transport does
exact opposite of passive.

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You have to have ATP, which is energy.

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You need to have a protein, and it's

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moving in the reverse action
of your passive transport.

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Passive transport was
high concentration to low,

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active is going from a low concentration

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up to a high concentration.

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It's helping to reset the gradients.

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So, here we have active transport.

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Left hand side, letter A, you can see that

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the ATP or the energy has been converted

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to ADP and inorganic phosphate.

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This is necessary because
you have to have energy

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to push against the gradient.

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Notice that black sphere
is going up into the blue?

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There's already more of
those spheres up there.

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Right hand side, carrier proteins.

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The carrier proteins you see enter in,

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they then are all set and ready to go,

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tucked in, and released on
the inside, carrier proteins.

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Endo and exocytosis.

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Does anyone remember what the
term "endo" was referring to?

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Endo, or within.

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Exo, or, what's also for exo?

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Exo is exit, or out.

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This is how you move
large amount of particles

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or very large particles
all at the same time.

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Endocytosis brings things in.

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Exocytosis takes them out.

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There's kind of this round robin,

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something comes in, something goes out,

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something goes in, something goes out.

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Here are two more pictures depicting

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endo and exocytosis.

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The top left, which is A,
is showing you endocytosis.

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You can see how many particles are

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being brought in at one time.

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It's like a bulk delivery.

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While the next one down, B, is exocytosis.

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The particles are exiting.

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So, the vesicles open up wide, releasing

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all the particles out and allowing

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itself to get reabsorbed, taken back in.