All cells in nature are surrounded by Biological Membranes, which all have the same basic structure. Some organelles found in Eukaryotic Cells also have membranes.
Membranes separate their contents from the environment. Cell membranes separate the cell contents from its environment, and organelle membranes separate the organelle contents from their environment. Membranes regulate the movement of materials through them. For example, cell membranes might not allow starch molecules to leave the cell.
Cell membranes are also involved in cell communication and recognition, and in holding some components of metabolic reactions in place.
The Phospholipid Bilayer
All Biological Membranes are made of the same basic structure. This is composed of molecules called Phospholipids, which form a Phospholipid Bilayer.
Phospholipids are fats. They are composed of two Fatty Acid 'tails' and a Phosphate 'head'. The Phosphate 'heads' are Hydrophilic whereas the Fatty Acid 'tails' are Hydrophobic, meaning that Phospholipids are Amphipathic.
When placed in water, the 'heads' orientate themselves towards water molecules and the 'tails' away, meaning that phospholipids will form a layer above water if left. If Phospholipid Molecules are completely surrounded by water, they may form a Bilayer.
A Phospholipid Bilayer consists of two layers of Phospholipids where the 'tails' point inwards and the 'heads' point outwards, towards water. One layer is like a mirror image of the other.
The Phospholipid Molecules are not bonded together, however, their Amphipathic Nature gives the Bilayer a degree of stability, since the Hydrophilic 'head' cannot easily through the Hydrophobic region created by the 'tails'. The molecules can however move freely as a fluid in the plane of the Bilayer.
Small, non-polar molecules can pass through the Phospholipid Bilayer since they can 'squeeze' between the Phospholipid Molecules and are not repelled by the hydrophobic region. Water molecules can also move through the Bilayer, despite being polar.