Bacteria and protists use two processes to extract energy from carbon chains into a usable form of ATP. The first process is glycosis. In glycosis, the organisms break down lactose and other complex sugars into smaller carbon chains. These smaller carbon chains pass through the cellular membranes of bacteria and the mitochondrial membranes of protists. Within the mitochindria or the cellular membrane, the smaller carbon chains are broken down into glucose, the simplest form of sugar. Bacteria and some species of protists can use free-floating glucose as a primary energy source. The second process is ATP synthesis. ATP synthesis occurs when the mitochondria or the cell as a whole absorbs light to generate a charged proton gradient on one side of the membrane, and then pushes that gradient through to the other side of the membrane. When the protons are expelled to the opposite side of the membrane, they can re-enter the cell only through specific locations on the membrane called channels. These channels are where ADP is stored. As ADP transfers the protons back to their original side of the membrane, its chemical properties change and it is transformed into ATP. All cellular organisms can use ATP as a primary source of energy.