so let's switch gears and talk about the
fluidity of the membrane
so there are several structures as we
said
embedded throughout the membrane and a
lot of these structures
need to be mobile they need to be able
to move around the cell
and so the dynamic flow of the membrane
allows for this to happen
as well the membrane proteins and the
lipids within it usually stay in their
half of the bilayer
so if you think about the inner half of
the bilayer versus the outer half of the
bilayer
the membrane proteins are mostly moving
lateral
although some flipping can occur
as well embedded within our membranes we
have cholesterol cholesterol is actually
there to
stabilize the membrane and make it so
that it's not
too fluid so while we do want our
membranes to be dynamic
we also need it to have some type of
structural integrity
and the cholesterol acts as kind of like
the support beams
in our membranes so that there is
something there
that makes it have a little bit of
rigidity
now the purpose of the plasma membrane
is of course to separate our external
and internal environment
but because of the structure of the
plasma membrane
where you have the phosphate heads and
the fatty acid tails
it is selectively permeable why
it's selectively permeable because
everything cannot pass through
the fatty acid tails they almost act as
like a gatekeeper
so it is permeable to small
nonpolar uncharged molecules because
those things
can pass through the gate they can pass
through fatty acid tails
because they do not have a charge and
they are not polarized in any manner
but if you are trying to get something
across the membrane whether it's getting
it in
or getting it out you it usually
requires some type of trans
membrane protein that can act as a
channel
or a transporter in order to allow these
substances to move across this gate
or fatty acid tails that block
everything
and as well sometimes things are so big
that the only way they can actually get
in is through
vesicular transport where it's brought
in through a vesicle
or it is released from the cell through
a vesicle and we'll talk about that a
little later
so when it comes to movement across the
plasma membrane
it usually involves a concentration
gradient
so concentration gradient is the
difference in concentration
of a chemical between one side of the
plasma membrane
and the other side of the plasma
membrane so think about it from the
standpoint of
one side might be really crowded while
the other side
is less crowded and things are going to
try to move away from being really
crowded
to less crowded the same way you would
do let's say
in any situation where you're in a
really crowded room and there's an
opportunity to move away from the crowd
as well as concentration gradients we
also have electrical gradients
this is especially important in cells in
the nervous system
and in the muscular system which we will
talk about later
and if you put those two together you
can also have a gradient known as an
electrochemical gradient
which becomes important when we talk
about cellular respiration
so in order to move substances across
the cell there's usually two ways that
this is done
it can be a passive process where things
just move
down their concentration gradient
examples of passive processes include
simple diffusion where nonpolar
uncharged substances can move passively
through the plasmid brain down their
concentration gradient
without any help from anything else
however if you're trying to move things
like
water or ions you might need a little
bit of help
and this we use facilitated diffusion
facilitated diffusion involves the use
of channels
and transport proteins in order to move
things down
the concentration gradient in both of
these processes
it's passive because there's no energy
required
the same way if you put a ball at the
top of the hill and let it go without
actually pushing it
it will roll down the hill on its own
these processes are the same way
a third passive process is osmosis
osmosis is specific for the movement of
liquid
across the plasma membrane and in this
it's also a passive process and involves
movement of liquid from a higher liquid
concentration
to a lower liquid concentration
active processes involve energy input in
order to move things from one side of
the membrane to the other
there are multiple types of active
processes including primary and
secondary transport
as well as vesicular transport in both
of these
we are usually moving something against
their concentration gradient
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