A Level Biology: Membranes 3 – Passive Movement
Hi! Welcome to my third video on this series about Membranes. Today, we are going to be
looking at the passive movement of molecules.
The first thing to know is that some lipid soluble molecules and some very, very small
molecules, things like oxygen and CO2 can directly diffuse through the lipid bilayer.
They don’t require a protein channel at all and they can move freely across the membrane.
Slightly polar molecules are the ones with specific charges such as water. Some of the
hydrogens and parts of the oxygen actually have a specific charge on the water and slightly
larger molecules would need, basically, protein channels in order to move through and hence
that.
Now, molecules that passively diffuse from an area of high concentration to an area of
low concentration – that is referred to as diffusion. That’s essentially the definition
and what happens is as it moves down the concentration gradient, you end up starting with an area
of high concentration until they are equally spread out. This diagram doesn’t take into
account the fact that these molecules would still be moving. But they will be evenly distributed
around.
One more time, you start from an area of very high concentration and they spread out until
they reach an equilibrium and that’s effectively what’s diffusion is. It is passive because
it doesn’t require any energy.
It is the amount of kinetic energy of the molecules and usually, the temperature of
the molecules determines how quickly this happens. So the warmer the molecules are,
the more heat they’re exposed to, you have seen this instance with the cold air that
are moving around but they are not moving too quickly. Whereas, as you apply heat, this
chart here, it should show that it is moving around a lot quicker speed and hence, they
have great kinetic energy and this will speed up diffusion. So if you think about it, if
you were for instance, cooking a meal downstairs and you are cooking the meal in summer and
in winter, you would expect that the particles that you can spell would move quicker during
the summer because the particles have more energy and hence, they move around quicker.
Larger molecules and polar molecules can enter by facilitated diffusion. The difference between
that and normal diffusion is the fact that it requires a carrier or a channel protein
in order to move down a concentration gradient. It is exactly the same as diffusion, where
it moves down a concentration gradient from an area of high to low, but with facilitated
diffusion, it requires a channel or carrier proteins.
In some cases, some carrier proteins can actually physically change shape and they enable the
molecule to pass through them. In this example, they’re using amino acids. The amino acid
will bind into the carrier protein, will change the shape of the carrier protein, enabling
the amino acids to pass through and then, it will revert back to its original shape.
So another form of facilitated diffusion is where the protein channel physically rotates
and moves through the phospholipid bilayer and hence, the molecule binds to the protein,
it rotates and then enables and carries the molecule through with it to the other side.
I have summarized some of the different forms of diffusion. So we have the ones carrying
things like amino acids. With this one, the green one here physically changes shape as
the amino acid moves through, whereas this one will rotate and actually move through
the plasma membrane. Channel proteins like this one would enable polarized molecules
at water that can move through and we can also get the movement of things like oxygen
directly through the phospholipid bilayer.
The thing that all of these have in common is the fact that they’re passive so they
require no energy and they move from an area of high concentration to an area of low concentration.
[end of audio – 04:12] A Level Biology: Membranes 3 – Passive Movement
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