Energy Generation In Mitochondria and Chloroplasts

The energy released by the hydrolysis of ATP to ADP can be harnessed to drive many otherwise energetically unfavorable chemical reactions in cells. In other words, ATP is the activated carrier which contains a high-energy bond and whose hydrolysis releases a large amount of free energy.

MITOCHONDRIA AND OXIDATIVE PHOSPHORYLATION

As protons pass through the transmembrane carrier, the stalk spins rapidly and rubs against proteins in the enzyme’s lollipop-shaped head, altering their conformation and causing them to produce ATP.

The transfer of electrons to O2 at the end of the electron-transport chain involves nearly all the oxygen we breathe, in other words, in mitochondria, the final electron acceptor in the electron-transport chain is oxygen (O2)

MOLECULAR MECHANISMS OF ELECTRON TRANSPORT AND PROTON PUMPING

NADH is an excellent molecule to donate electrons to the electron-transport chain because…

  • it has a weak affinity for electrons

  • It has a negative redox potential

Most of the energy for the synthesis of ATP comes from NADH produced by the citric acid cycle

  • One turn of the citric acid cycle produces three molecules of NADH

  • Because glycolysis produces two molecules of pyruvate, six molecules of NADH will be produced per molecule of glucose oxidized

CHLOROPLASTS AND PHOTOSYNTHESIS

As the human population grows, it becomes increasingly important to maximize crop yield. Therefore, scientists are searching for more efficient ways for plants to convert CO2 into biomass. One approach is to genetically modify plant enzymes involved in photosynthesis to increase their efficiency. The enzyme Rubisco (ribulose 1,5-biphosphate carboxylase/oxygenase) catalyzes the carbon fixation step of the Calvin cycle, covalently attaching carbon dioxide to ribulose 1,5-biphosphate.

  • Photosynthesis in the chloroplast splits water, releasing O2 as a byproduct

  • Some of that O2 is used by mitochondria during cellular respiration, specifically in oxidative phosphorylation as the final electron acceptor in mitochondrial electron transport chain, forming H2O

  • O2 cannot be converted to CO2 used in carbon fixation in chloroplast stroma because CO2 is fixed

  • The final electron acceptor in the photosynthetic electron transport chain is NADP+ which then becomes NADPH

  • O2 accepts electrons, it doesn’t donate them

THE EVOLUTION OF ENERGY-GENERATING SYSTEMS

  • Membrane-based mechanisms for the generation of energy appeared very early in the history of life, more than 3 billion years ago

  • Electron-transport systems likely provided energy for the earliest cells on Earth

  • The first living cells on Earth are suspected to have generated ATP through fermentation

  • In fermentation, organic molecules are broken down to generate energy without the involvement of oxygen