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- [Voiceover] You already discussed

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about extracellular
receptors a little bit,

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how to use a protein
in the plasma membrane.

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Well, this is showing how
that purple oval here,

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which is the protein, is
embedded in the plasma membrane,

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but all it has is a place
where the receptor can bind.

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That receptor, that little site there,

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just is waiting for a signal.

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The signal is gonna tell
it to perform an action.

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In this case, it's time to form the action

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to change the substrate into the product.

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Well the term substrate is
referring to any particle

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or any substance that is
gonna be used in a reaction.

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The product is the end
result of a reaction.

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So without anything even
leaving or entering the cell,

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this receptor that receives a signal

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on the outside of the cell

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can trigger a reaction on
the inside of the cell.

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So let's get into a
little more detail here.

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The sodium-potassium pump.

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This pump is probably one
of the most important pumps

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in your body.

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It's going to help expel unwanted ions

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but keep the ones you need.

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It helps keep the cell
volume where it should be

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to make sure the cell is the
proper size, proper shape.

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It helps with overall functionality.

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This sodium-potassium
pump is gonna utilize ATP,

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the energy source,

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and it will expel three sodium ions

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for every two potassium
ions it brings in the cell.

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So each cycle will take three sodium ions

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from inside the cell and push them out.

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And then it'll take two potassium ions

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from the outside and bring them in.

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So let's go through a step by step

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to kinda show what that was again.

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First off, sodium ions
are going into the pump.

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If you notice, they're coming
from the inside of the cell.

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Then you apply some ATP,
give it some energy.

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When you break that bond,
you release the energy.

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That purple spheric structure
there, that protein,

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will then change shape
and allow the sodium ions

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to be moved to the outside of the cell.

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Once it's open to the
external environment,

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or the outside environment,

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you'll then pick up two potassium ions.

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K+ is potassium.

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Those then will go
through, again, a change,

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leading to them being
released into the cell.

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This is necessary because typically

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potassium has a high concentration inside.

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And that high concentration
slowly leaks out.

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But sodium has a high
concentration on the outside

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and that slowly leaks in.

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So because these are slowly going down

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the concentration gradients,

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high concentration sodium outside,

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gotta keep pushing back out there

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and keep that concentration.

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High potassium concentration inside.

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It'll keep pulling the potassium in

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to keep it as high concentration.

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So this is very important
to keep the proper volume

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of your cell.

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These sodium-potassium pump.

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So a study states in normal,
every day equilibrium,

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no problem.

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The amount of potassium going out

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and the amount of potassium
coming in are the same amount.

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Amount of sodium going out,

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the amount of sodium
going in, same amount.

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But now when the rate of
outward sodium transport

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is faster than the inward,

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all of a sudden, the cell is shrinking.

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But if the sodium transport going out

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is less than the sodium coming in,

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now the cell is ballooning up, expanding.

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The reason being water follows sodium.

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Well, where do we find
sodium in our bodies?

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And the environment around us?

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Sodium is part of table salt.

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Table salt is NaCL,

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so sodium is part of table salt.

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This more salt you take in,

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a lot of times, the thirstier it gets.

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Think about if you ever
went to a ball game

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and ordered those big, soft pretzels,

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the ones loaded with salt.

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After you're done eating
that, what do you feel?

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More often than not, you feel thirsty.

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Your body's trying to take in more water

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to balance out or reach
equilibrium with the sodium.

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This movement in and out
of fluid from the cell,

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controlling the fluid volume,
is referred to as tonicity.

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Now tonicity itself is the
concentration of solutes

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in two different fluids.

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As far as the cell goes,

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we can look at the fluid outside the cell

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and the fluid inside the cell.

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So your tonicity is comparing
these two different fluids.

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If you have an isotonic solution,

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the fluid inside, intracellular,

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and the fluid outside, extracellular,

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both have equal ionic concentrations.

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You may have heard of
the term isotope before,

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if you ever had a stay in the hospital.

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You're in the hospital with
an IV fluid bag, a saline bag.

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That is an isotonic solution.

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It's meant to help
replace fluid in your body

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with the same exact ionic concentrations.

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So isotonic.

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Nothing's really changing much.

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They're both equal amounts.

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Now in hypertonic,

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the extracellular, the fluid outside,

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has a high ionic concentration
than what's inside.

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So hyper means high.

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So the extracellular,

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the fluid outside has a
higher ionic concentration.

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Because the outside fluid,
or the extracellular fluid,

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has a higher concentration,
water diffuses out.

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The water will leave the cell

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and go out of the cell to try and dilute

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that extracellular higher
ionic concentration.

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So the water flows out.

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As it leaves the cell,

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the cell will start to shrink and shrivel.

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If it shrinks and shrivels
from enough water loss,

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it could potentially die.

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But the flip side is hypotonic.

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Hypo means low or below.

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So now the extracellular
ionic concentration

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is lower than the intracellular.

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So the fluid outside
the cell has less ions

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than the fluid inside the cell.

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Because the internal concentration
of ionic ions is higher,

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water goes into the cell to try

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and diffuse the higher concentration.

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The whole idea with both of these

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is water is going towards
the higher concentration.

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Try and dilute it.

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So in this hypotonic solution,

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as the water goes into the cell,

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it'll cause the cell to
swell up, get larger.

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If enough fluid forces
its way into the cell,

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it could actually cause the
cell to rupture or break open.

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Obviously, breaking open
the cell will kill it.

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So hypertonic, hypotonic.

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Both deal with tonicity.

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Hypertonic, the outside fluid
has a higher concentration.

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Hypotonic, the inside fluid
has a higher concentration.

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Let's look at this diagram.

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This is symbolic of red
blood cells in a fluid.

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The one on the far left,

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where you see it has isotonic at the top,

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the red blood cell is put in there.

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Inside and outside the red blood
cell are the same solution.

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Nothing happens.

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But in the case of a hypertonic solution,

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well hypertonic, the outside environment

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has a higher solute that
causes the cell to shrivel up.

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And the last one, the
far right, hypotonic.

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Well hypotonic means
the inside of the cell

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has a higher concentration

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because the outside around the cell

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has a lower, hypo, lower concentration.

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So what happens here?

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Water flows into the cell,

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causing the cell to swell up, get larger.