A Level Biology: Membranes 1 – Plasma Membranes
Hi!
Welcome to my first video on the series about Membranes.
Today, we are going to be looking at the structure of the Plasma Membrane.
The first thing is that we really realize about cells is there are a number of different
membranes found within the cell and so, some of them are just listed down here.
So you have the plasma membrane which is labeled here on the outside.
The tonoplast.
We also have one on the mitochondria.
We have two on the mitochondria, for instance.
There is one on the inside structure called the crustae and we have one on the chloroplast
and also there is some form of a membrane on the nuclei.
So membranes are found throughout.
Today, we are mainly going to be looking at Plasma Membranes.
The major function of a membrane allows a cell to be divided into different parts and
so you can have varied conditions in those different parts.
A good example of that is a lysosome.
A lysosome is an organelle that contains digestive enzymes.
Hence, you wouldn’t want those digestive enzymes freely swimming around the rest of
the cell since the rest of the cell would get digested.
So membranes act as a barrier and therefore stop digestive enzymes getting out and therefore,
digesting the cell.
The major constituents of membranes are these things called phospholipids.
From the name, “phospho”, you probably guessed that it’s a phosphate and “lipids”
and hence, a fat.
Sometimes, you see the diagram drawn out this and this is referred to as a phospholipid
bilayer as there are two sections of it, hence bilayer.
If we magnify one of these sections here, you can these different parts.
The first part of the top here is the phosphate group.
As you would expect with phospholipids, you have a phosphate which is made of phosphorous
and oxygen.
This second blue section here is called glycerol; it’s another chemical and the phosphate
and the glycerol held together by this phosphoester bond and then at the bottom, these two green
sections in here, you have two fatty acids.
The fatty acids are held to the glycerol via an ester bond and this is basically what a
phospholipid actually looks like.
There are two distinct regions to a phospholipid.
First, the top part where the phosphate group is hydrophilic head.
Now, hydrophilic – “philic” means to like and hence, the phosphate group at the
top there likes water, hence “hydro”.
The bottom end, where the fat is hydrophobic, so “phobic” means does not like, so it
does not like water.
You already probably know this that when you mix things like fats, oils with water, the
two don’t mix or emulsify and so, the top part likes water, the bottom section does
not.
So when phospholipids are placed into an aqueous solution like they are here, what you got
is a polar hydrophilic head.
Now, polar just means that there is a charge involved on that section.
Hydrophilic means to like water and then you got the tails which are hydrophobic and what
ends up happening is the hydrophilic section, the bit that likes the water gather around
the outside and then the hydrophobic tails that do not like the water, gather in the
inside and they form the structure called a micelle.
So the plasma membrane ends up looking like this structure or a phospholipid bilayer because
what you end up having, again, is the phosphate group which likes the water on the outside
and then, the tails which don’t like the water on the inside, so they are not exposed
to them.
So the conditions of the cell: in the outside of the cell, it will be aqueous and the inside
of the cell will be aqueous as well.
This set-up is preferential and that’s what’s referred to as phospholipid bilayer.
In summary, a unit membrane consist of a phospholipid bilayer and phospholipids are made up of polar
hydrophilic phosphate head, that means they like the water and then the non-polar hydrophobic
tail consisting of fatty acid chains.
So hydrophobic means they don’t like the water and hence, they find a way from any
aqueous solutions.
[end of audio – 04:09] A Level Biology: Membranes 1 – Plasma Membranes
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