LESSON 1: THE PATHWAYS OF RESPIRATION

Explanation: Electron transport chains are associated with membranes, such as the mitochondrial membrane in eukaryotic organisms, the thylakoid membrane of photosynthetic organisms, and the plasma membrane of prokaryotic organisms. While some of the details differ, electron transport chains in all organisms operate on the same basic principles.

Excited electrons are brought to the electron transport chain by electron carriers such as NADH and FADH2. In photosynthetic organisms the electrons are donated by water and excited by light energy. The electron donor is oxidized as the electrons are passed to the first protein complex in the chain. As it accepts the electrons, the protein complex is reduced. Next the electrons are passed from the first protein complex, which becomes oxidized, to the second protein complex, which becomes reduced. Each of these redox reactions is slightly exergonic, or energy releasing. Some of this energy is used to move protons across the membrane against their concentration gradient. With each successive transfer of electrons, the original excited electrons lose some of their energy. After several successive redox reactions, the electrons are donated to a terminal electron acceptor. In aerobic respiration the terminal electron acceptor is oxygen. In photosynthesis the terminal electron acceptor is a compound called NADP.

The energy in the excited electrons has been transformed into an electrochemical gradient represented by the high concentration of protons on one side of the membrane. These protons can diffuse back across the membrane by passing through a protein complex called ATP synthase. As these protons pass through ATP synthase, down their concentration gradient, they provide the energy needed to make ATP from ADP and inorganic phosphate within the cell.