All cells are enclosed by a cell membrane. This structure has two layers, and is represented in the diagram below.
The extracellular environment is the area outside of the cell. Each layer has two main components, phospholipids and proteins.
The phosphlipids in the two layers are arranged like this:
The phospholipid molecules are able to move around within the layers and give the cell membrane flexibility.
Protein molecules are found embedded in the two layers of phospholipids. Membrane proteins have a wide variety of functions.
Some allow a cell to respond to specific chemical signals from other cells, others are enzymes and some proteins are involved in the transport of substances across the cell membrane.
The cell membrane is selectively permeable. It lets some substances pass through rapidly and some substances pass through more slowly, but prevents other substances passing through it at all.
Some small molecules such as water, oxygen and carbon dioxide can pass directly through the phospholipids in the cell membrane. Larger molecules such as glucose require a specific transport protein to facilitate their movement across the cell membrane.
Very large molecules such as proteins are too big to move through the cell membrane which is said to be impermeable to them.
The type of transport proteins present in a cell membrane determines which substances the membrane is permeable to.