- [Narrator] The plasma membrane is made up
of several organic molecules.
I will concentrate on two of them in this presentation.
The first molecule is a lipid.
It is called a phospholipid, because instead of the usual,
three fatty acid chains, there are only two.
The third bonding position is with a phosphate group.
The phospholipid molecule is divided
into a head region and a tail region.
Th head region is the glycerol and phosphate group.
The tail region is the fatty acid chains.
The head is hydrophilic, attracted to water.
The tail is hydrophobic, repelled by water.
It is this property which allows for the unique
molecular alignment that makes the plasma membrane.
If a droplet of phospholipid was put into water,
a micelle would form.
A micelle is a sphere with the hydrophilic heads
on the surface, the tails inside, away from the water.
Our bodies are made up of approximately 60% water
and trillions of cells.
The water can be found inside the cells,
outside the cells, and in the blood stream.
The location of water and the phospholipid,
hydrophilic and hydrophobic ends
enable the membrane to form a bi-layer of lipids.
The currently accepted model of the cell membrane
was developed by Singer and Nicolson in 1972
at the University of California San Diego.
Pictured here, the plasma membrane looks like
a sea of lipid with icebergs floating in it.
The membrane is very flexible and has a fluid motion.
This design of the plasma membrane
is called the fluid mosaic model.
Quite often these proteins have sugar molecules
attached to them, forming glycoproteins.
These molecules extend from the membrane's surface
and act as an identification marker.
The proteins are used by the immune system to identify
a cell as being self, or foreign, called non-self.
Each individual has their own biochemical makeup.
In other words, each human being
has their own set of cellular proteins.
But if a cell comes into the body
that has a different set of proteins,
whether it is a bacteria or an organ transplant,
the immune system may identify it as self,
or as non-self and attack.
In order for the cell to survive, many different materials
must be able to cross the plasma membrane.
When water passes through a cell's membrane,
the type of diffusion is called osmosis.
When solid particles pass through a cell's membrane,
the type of diffusion is called dialysis.
Before looking at this movement through the membrane
it is important to understand
the movement of molecules in general.
Atoms and molecules are in constant motion.
This random motion will disrupt the cluster
rather than maintain it.
This motion is referred to as Brownian movement.
This motion is always present in some degree
in gasses, in liquids, and even in solids.
For instance, if a bottle of perfume is left open in a room,
eventually, no matter where you are in that room,
you are able to smell the perfume.
The molecules of air hit the liquid,
causing the liquid molecule to move into the air.
At the time the bottle is first opened,
there is a high concentration of perfume molecules
in the bottle, but very low concentration
of perfume in the air.
As the perfume molecules are bombarded by the air,
the concentration of perfume is dispersed into the air.
In other words, the perfume is moving
down a concentration gradient, high to low.
This movement is called diffusion.
It uses Brownian movement.
Now let's take a look at another example of diffusion.
In this aquarium of water we dropped some blue dye.
As the water molecules move,
they bump into the dye molecules.
The dye molecules at first, in a high concentration,
start moving and bumping into each other
and the water molecules,
moving into the area of lower concentration.
As this movement continues, the dye spreads out
and equilibrium is attained.
Equilibrium is equal concentrations of the dye
and water in every area of the aquarium.
Remember, Brownian movement continues
even after equilibrium is reached.
although extremely thin, the plasma membrane
separates the contents of the cell from the environment.
The membrane has several functions.
This video concentrated on the movement
of materials through the plasma membrane.
Remember that this membrane is selectively permeable
and can prevent materials from passing through
or keeping materials is, and it will permit,
even facilitate, the entrance and exit of materials.
Materials may move through membranes passively or actively.
In this video segment, passive transport,
which uses Brownian movement, was introduced.
Passive transport includes diffusion,
defined as the movement of materials from a region
of high concentration to low concentration.
Two specific forms of diffusion are osmosis and dialysis.
These processes are essential to the survival of the cell.
Nutrients must enter, wastes must exit.
The structure of the membrane allows for the processes
to take place, which enables the cell to live.