today we're going to be talking about diffusion
osmosis and water balance
diffusion is the movement of a solute in solution
from an area of high solute concentration
to an area of low solute concentration
so for example, if we have a solution
and we dump in some salt
we're gonna have a high solute concentration
in the corner of the beaker
as time goes by.... put time across the bottom
because as time goes by
we are still going to have a relatively high concentration of solute molecules in the corner
but we're also gonna have
some solute molecules diffusing moving from an area of
high solute concentration to an area
of low solute concentration until eventually
they reach an equilibrium where they are randomly
and evenly spaced throughout the solution
in this example we have
solute molecules in the extracellular space
moving cross a cell membrane or lipid bilayer
to an area of low solute concentration
so over time again
we have molecules
of solute moving from
an area of high concentration to an area of low concentration
until we have equivalent
solute concentration
on both
sides of the lipid bilayer
so there are 14 solute molecules
on this side of the bilayer and 14 on this side
and in the same volume giving them equal concentrations
so thats diffusion whether diffusion happens
as it did in these beakers without a membrane
or in your cells
where diffusion may happen across the lipid bilayer
diffusion is movement of a solute from an
area of high solute concentration to an area of
low solute concentration
in contrast, osmosis is the movement
of solvent, usually water
across a membrane from an area
of high solvent concentration to an area
of low solvent concentration
let's take a look at an example of osmosis
on the right side of the membrane we have a high concentration of solute molecules
and a low concentration of solvent
so since osmosis deals with the movement of solvent
we're going to write down that we have low solvent concentration
on that side of the beaker
and on the other side of the membrane represented in red
we have a high solvent concentration
and a relatively low solute concentration
so again we are going to look over time
at what happens
in osmosis
so we're still going to have
the same
same number of solute molecules
on each side of the membrane because
the solute molecules are not moving
but the solvent is going to move
and so over time
we're going to have
a movement
from an area
of high solvent concentration
to an area of low solvent concentration
until they reach an equilibrium
Talking about osmosis
especially as it relates to ourselves we need to know
three more definitions
hypertonic, hypotonic, and isotonic
it's important to remember
when we're looking at hypertonic solutions hypotonic solutions
or isotonic solutions they're always hypertonic
to another environment
hypotonic to another environment or isotonic to another environment
you always are comparing the solute
and solvent concentration from one
solution to the next
so if a solution is hypertonic to a second solution
it has a high solute concentration
relative to the other solution
hypotonic solutions have low solute concentrations
relative to another solution
isotonic solutions have equal
solute concentrations
relative to each other
tonicity are osmosis are very important when it comes to cells
in a hypertonic solution the solute concentration
outside the cell is greater than the solute
concentration inside the cell so that means that
solvent or water concentration is low
outside of the cell and high
inside at the cell so in this case
water is going to flow from high
water concentration to low water concentration
and hypertonic solution
water is going to flow out up the cell
in a hypotonic solution
the water concentration outside of the cell
is high and the water concentration
inside of the cell is low so we're going to have water flowing
from it high concentration to its low concentration
water's gonna flow in to the cell
In an isotonic solution solute concentration
and solvent concentration equal
so it's tempting to say that no water would move
across the cell membrane however water is
always flowing across the cell membrane it's just going to flow
so that the solvent concentration remains equal
it's going to flow into and out of the cell
at the same time so let's take a look at how that might affect plant cells
so we said in a hypertonic solution, we're going to have
water going from an area of high water concentration
to an area of low water concentration
leaving the cell
in plant cells
we can see that they have a cell wall
and
cell membrane which
surrounds all of their cytosol and cytoplasm
and so if water leaves a plant cell
that cell membrane
pulls away from a cell wall
Let's look at an isotonic solution and see how that effects plants cells
so as we said water would be
entering and exiting the cell and again we see that the
cell membrane of this plant cell
has pulled away a little bit from
the cell wall... That leads to a wilted
or flaccid looking plant cell
However if we are in a hypotonic solution and water is entering the cell
water fills up this vacuole
inside the plant cell
and the water pressure pushes the cell membrane
against the cell wall creating a more
rigid or healthy appearance for the plant
so for plants cells the ideal solution
is hypotonic
and let's take a quick look
at some plants that are either wilted
or healthy
so we just said plant cells tend to be wilted
in an isotonic solution because there is
less water inside of the cell inside of those vacuoles
pushing against the cell wall so those cells
tend to be wilted
healthy plants cells exist in a hypotonic solution
hypotonic again
we said has a lower solute concentration
outside of the cell than inside of the cell
So the way to get from an isotonic to a hypotonic solution
is to add water
that's why if your plants are looking wilted
and you water them you can get a healthier
looking plant cell
So plant cells
ideally are in hypotonic solution but what about
animal cells let's take a look at what happens to some
animal cells these are red blood cell in different solutions
and determine maybe what is the best for animal cells
and whether that is different from plant cells
in a hypertonic solution
we see water leaving the red blood cell
this gives these red blood cells a sort of shriveled appearance
in an isotonic solution
water is entering and leaving the red blood cell
at an equivalent rate and they have their typical
doughnut shape and they look pretty healthy
but let's take a look at hypotonic and determine if that might be
the best because that is the best for
plants cells remember
In a hypotonic solution water is going to
enter the red blood cells
so when water enters the red blood cells they swell up
kind of looking more like balloons and what happens with a balloon
when you put too much in it
it bursts
so this is called lyses
when cells are placed in hypotonic solution they burst
so obviously that is not ideal
for our red blood cells and animal cells
an ideal solution is gonna be isotonic
so animal cells are a little bit
different than plant cells when we're thinking about water balance
and tonicity