Cells 3.9 bya?

Cell Membrane

Soon after life began there evolved a thin oily skin around a group of living molecules. This skin is called the cell membrane. It is not known when the membrane first appeared on Earth. In this story we guess it was around 3.9 billion years ago, although no fossils have yet been found from this date. Indeed, it precedes the oldest known rocks.

It is not clear how or why the membrane was first formed, but it was a vital step forward for life. The membrane kept life's molecules together and also protected them from harmful chemicals which could not pass through the membrane.

The ability of a membrane to protect life's molecules depends on its structure, and its structure depends on its constituent parts. This is an example of one of the themes of this story, the self-assembly of matter. In this case, the constituent parts were molecules called phospholipids.

Phospholipid

Main parts of phospholipid molecule

At one end of a phospholipid is a structure able to make hydrogen bonds and therefore stick to water. Joined to this are two long molecules which do not stick to water and are therefore slippery.

Self assembly of phospholipids when placed in water

When placed in water, the phospholipids form themselves into two layers with their heads facing outwards (sticking to the surrounding water) and their slippery tails pointing inwards (to escape from the water). The membrane forms a flat flexible structure.

Size of cell membrane

If a soccer ball were inflated to the size of the Earth (what I call Soccearth) then the cell membrane would be a soft flexible wall about 15 cm thick. That is about the same thickness as the wall of a "bouncy castle".

Liposome

A membrane was capable of forming itself into a round ball containing a drop of water and the molecules of life. Such an object is called a liposome.

Structure of a liposome in water

Perhaps this was the way which the first cells formed.

Membrane Proteins

As life developed, new proteins evolved which could use the special properties of the membrane. If a protein had sticky amino acids at one end and slippery ones at the other, it could float in the membrane with the sticky half in the watery cell and the slippery half in the membrane.

Protein spanning a membrane

This protein could then do special jobs, such as detecting food molecules in the outside world.