The cell membrane

All cells are enclosed by a cell membrane. This structure has two layers, and is represented in the diagram below.

The composition of a cell membrane with the cytoplasm within the membrane magnified.

The extracellular environment is the area outside of the cell. Each layer has two main components, phospholipids and proteins.

Phospholipids

The phosphlipids in the two layers are arranged like this:

Lipids and their structure within a cell membrane showing how the molecules have two layers.

The phospholipid molecules are able to move around within the layers and give the cell membrane flexibility.

Membrane proteins

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.

Protein molecules embedded within the two layers of lipids.

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.

Small carbon dioxide molecules pass directly through the lipids. Larger glucose molecules travel through the proteins within the lipids. CO2 molecules pass directly through phospholipids. Glucose molecules travel through the proteins.