Which type of membrane protein extends completely across the phospholipid bilayer?

The correct answer is Integral protein.

- Membrane proteins are broadly classified based on their association with the phospholipid bilayer.
- Integral proteins are embedded within the membrane and often span the entire phospholipid bilayer, extending from one side to the other.
- This characteristic allows them to function as channels, transporters, or receptors, facilitating communication and the movement of molecules across the cell membrane.

- In contrast, peripheral proteins are located only on the surface of the membrane and do not penetrate the hydrophobic core.
- They are loosely attached to the membrane or to integral proteins.
- Lipid-anchored proteins are covalently attached to lipid molecules within the membrane but also do not span the bilayer.
- Glycoproteins have carbohydrate chains attached, which can be part of either integral or peripheral proteins, but the defining feature is not their membrane-spanning nature.

Reference: Alberts et al., Molecular Biology of the Cell, 6th Edition, Chapter 10: Membrane Structure