What type of molecule forms a lipid bilayer within a cell membrane?

Phospholipid molecules.

Phospholipids are a type of ; that much is obvious from the name.

A phospholipid molecule consists of one head that loves water( hydrophilic ), and two tails that hate water( hydrophobic ).

Aother thing is obvious from the name. To see what it is, let's break it down into two parts.

"Phospho" - "Lipid". Notice that Phospho comes from the word Phospho(rus) , the element with the atomic number 15.

Phosphorous is present in the head . That's what we'll get to know first.

Phosphorous isn't present in the head on it's own. It's present as a part of a phosphate group .

The phosphate group is just the name given to an arrangement where four oxygen atoms surround the phosphorous.

When phosphorous is in the head of a phospholipid, he becomes really proud to be at the top of the molecule. So he thinks he is too cool to make bonds with anyone else directly. But as oxygen is an old friend, he asks him to make the bonds with others for him.

This is the phosphate group:

To complete the phospholipid molecule, the phosphorous containing head needs to attach to the tails. Let's get to know the tails a little more.

They are just a long chain of hydrocarbons:

These chains need to be attached to the bottom of the head. But because of phosphorous's pride, this can't happen. Both the chains cannot be attached to one oxygen directly.

Therefore, an intermediate molecule is required that will attach with both the tails at one end, and attach with one of phosphorus's oxygen at the other end. This is where glycerol comes in.

Notice the attachment below. Glycerol(in pink, plus two carbons outside pink that have double bonded oxygen) takes one tail on each carbon at one end, and one oxygen on a carbon at the other end.

Some random group then gets attached to the oxygen at the other end because the oxygen feels lonely.

BEHOLD! THE PHOSPHOLIPID MOLECULE!

All the colored part is the head .

There are a lot of phospholipid molecules in a . Their arrangement makes up the Lipid Bilayer .

The heads prefer to be in the direction of the water. And the tails like that because then, they can stay away from the water.

• There is water outside and inside the cell.
• If there was only one layer of phospholipids, the tails would have to face the water on the opposite side of the head.
• This problem is resolved when there are two layers of phospholipids.
• One with heads pointing towards the outside of the cell and one with head pointing towards the inside of the cell.

Hence, the name is given as Bi Layer. Bi means two.

And so, this is how the structure of the cell membrane comes to be:

Extracellular=Outside cell Intracellular=Inside cell

This is just one part of the structure of a cell membrane. It contains so much more and actually looks something like this:

Learning about the phospholipids is just like laying the foundations of a building. Obviously, you've got to build the structure on top of the foundations. In the same way, if you choose to accept it, you can do that by exploring more about the cell membrane here(after all, you yourself are just a collection of trillions upon trillions of cells):

http://www.biologycorner.com/APbiology/cellular/notes_cell_membrane.html

http://www.mhhe.com/biosci/genbio/maderinquiry9/etext/chapt04.pdf