B2.1.4

• Membrane proteins can be divided into two groups because of their structure and position.
• Integral proteins are hydrophobic and are embedded in the hydrocarbon chains in the centre. They can be transmembrane proteins - extended across the membrane, but they can also pit into one of the bilayers.
• Peripheral proteins are hydrophilic on their surface. They are often attached to the surface of integral proteins and this attachment is often reversible. Some have a single hydrocarbon chain attached to them which is inserted into the membrane.
• Integral proteins can be carrier molecules or channels. They help transport substances that can’t diffuse across the membrane. They are often receptors for hormones and neurotransmitters or enzymes for catalysing reactions.
• Peripheral proteins act as receptors for hormones or neurotransmitters or they are involved in cell recognition. On the inside of the cell they are responsible for cell signalling or chemical reactions. They can dissociate from the membrane and move into the cytoplasm.