Integral membrane proteins are embedded within one or both layers of the phospholipid bilayer, whereas peripheral membrane proteins are attached to the inner or outer surface of the membrane. This distinction is required at both SL and HL in IB Biology subtopic B2.1.
The Reasoning
A membrane has a hydrophobic interior formed by the fatty acid tails of phospholipids and hydrophilic surfaces formed by their phosphate heads. The location and amino acid composition of a membrane protein must therefore suit its chemical environment.
Integral proteins contain hydrophobic regions that interact with the bilayer’s hydrophobic core. Some extend only into one lipid layer, while transmembrane proteins cross the entire bilayer and have hydrophilic regions exposed to aqueous solutions on both sides.
Peripheral proteins do not enter the bilayer’s hydrophobic core. They are attached to a membrane surface, often through interactions with phospholipid heads or integral proteins, and can usually be removed without disrupting the bilayer.
| Feature | Integral membrane proteins | Peripheral membrane proteins |
|---|---|---|
| Position | Embedded in one or both lipid layers | Attached to the inner or outer membrane surface |
| Interaction with bilayer | Hydrophobic regions interact with fatty acid tails | Mainly interacts with hydrophilic heads or other proteins |
| Typical roles | Channels, pumps, receptors and cell adhesion | Enzymes, signalling components and cytoskeletal attachment |
| Removal | Usually requires disruption of the bilayer | Usually removed more easily without disrupting the bilayer |
A common misconception is that every integral protein crosses the whole membrane. This is incorrect: all transmembrane proteins are integral, but some integral proteins are embedded in only one layer.
Exam Technique
For a distinguish question, compare both protein types directly: state that integral proteins are embedded in the bilayer, while peripheral proteins are surface-attached. In a membrane diagram, place hydrophobic parts of integral proteins among the fatty acid tails; do not draw peripheral proteins inside the hydrophobic core.