Identify the incorrect statement regarding aquaporins:
1. Water channels are transmembrane proteins made up of eight different types of aquaporins.
2. Water moves most rapidly in tissues that have aquaporin water channels.
3. The presence of water channels increases membrane permeability to water.
4. Aquaporins selectively conduct water molecules in and out of the cell but allow some passage of protons.

What do A, B, C and D represent in the following figure?