The Electron Transport Chain (College Board AP® Biology): Study Guide

Transferring energy from electrons

• Energy is transferred from electrons as they pass down the electron transport chain (ETC); this involves a series of coupled reactions that establish an electrochemical gradient across a membrane

• The energy transferred from electrons drives chemiosmosis, which enables the production of ATP

• The ETC reactions occur in:

  • mitochondria during cellular respiration

  • chloroplasts during photosynthesis

  • prokaryotic plasma membranes

Cellular respiration

• The transfer of electrons results in production of ATP in cellular respiration as follows:

  1. electrons from NADH and FADH₂ are passed to a series of electron acceptor proteins on the inner mitochondrial membrane; these proteins make up the ETC

     • NADH and FADH₂ come from the Krebs cycle and glycolysis

  2. the transfer of electrons along the ETC drives the formation of a proton (H⁺) gradient across the inner mitochondrial membrane

     • Protons are formed from hydrogen atoms which are donated by NADH and reduced FADH₂ 

     • The energy from the ETC is used to transport protons across the inner mitochondrial membrane from the matrix into the intermembrane space

     • This generates a region of high proton concentration on one side of the membrane and a region of low proton concentration on the other

  3. protons diffuse back across the membrane through a membrane-bound protein channel associated with the enzyme ATP synthase

     • This is chemiosmosis

  4. chemiosmosis drives the formation of ATP from ADP and inorganic phosphate

     • The addition of phosphate to ADP via this mechanism is known as oxidative phosphorylation

• At the end of the ETC the electrons pass to oxygen; oxygen acts as the final electron acceptor

  • The electrons combines with protons and oxygen to form water

Decoupling electron transport and ATP production

• Some organisms can decouple the electron transport from oxidative phosphorylation

  • In this case the protons diffuse back across the membrane via a channel protein that is not associated with ATP synthase, meaning that ATP is not produced

• The energy that would otherwise have gone towards ATP production is instead released as heat

• Endothermic organisms can use this heat to regulate their body temperature and maintain metabolic reactions

Photosynthesis

• The electron transport chain is also an important feature of the light-dependent reactions of photosynthesis

  • The ETC in photosynthesis also drives the production of ATP from ADP and P_i

• There are several differences between the ETC in photosynthesis and cellular respiration; in photosynthesis:

  • electrons are donated by chlorophyll rather than by coenzymes

  • the electron acceptors are within the thylakoid membrane rather than the inner mitochondrial membrane

  • the proton gradient forms across the thylakoid membrane rather than the inner mitochondrial membrane

  • the terminal electron acceptor is NADP⁺ rather than oxygen

  • the process is known as photophosphorylation rather than oxidative phosphorylation

Prokaryotic plasma membranes

• The reactions of the ETC occur across folded regions of the plasma membrane in prokaryotic cells

  • Prokaryotes do not have internal membrane-bound organelles

• The passage of electrons through the ETC in prokaryotes is accompanied by the movement of protons across the plasma membrane

• Aerobic prokaryotes use oxygen as a terminal electron acceptor, while anaerobic prokaryotes use other molecules, such as nitrate and sulfate