Redox Reactions

• During a redox reaction, energy is transferred from one molecule to another through the transfer of electrons. When a molecule loses electrons, it is losing energy and said to be oxidized. When it gains electrons, it is gaining energy and said to be reduced. (You can remember this by thinking of the charge on the molecule; electrons are negatively charged, so when you gain electrons, charge is reduced.) Cellular respiration involves the complete oxidation of glucose, converting it into carbon dioxide, while other molecules pick up the energy from the glucose by taking its electrons.
  
  

Glycolysis

• In glycolysis, glucose (a six-carbon sugar) is oxidized and broken into two molecules of pyruvate (a three-carbon molecule). This occurs through a series of 10 different chemical reactions, each of which is catalyzed by a different enzyme. When glucose is oxidized, electrons and protons are transferred to NAD molecules, converting them to NADH. NADH is an electron carrier, carrying the energy from the sugar in the form of the electrons that are taken from glucose. Glycolysis results in the production of two ATP molecules and two NADH molecules from each glucose molecule.
  
  

The Citric Acid Cycle

• After glycolysis, the pyruvate is converted into acetyl coenzyme A. Acetyl coenzyme A is brought inside the mitochondrion to begin a series of chemical reactions called the citric acid cycle. During the citric acid cycle, each pyruvate molecule is fully oxidized and broken into three carbon dioxide molecules. The citric acid cycle produces two more ATP for each glucose molecule but stores most of the energy that is released in the form of electron carriers NADH and FADH2.
  
  

Oxidative phosphorylation

• Oxidative phosphorylation includes the flow of electrons down the electron transport chain and the use of chemiosmosis (the flow of protons across a membrane) to generate ATP. The electron carrier molecules NADH and FADH2 drop off their electrons at the electron transport chain, a series of increasingly electronegative molecules embedded in the folded membranes inside the mitochondria. The electronegativity of a molecule is a measure of how strongly it attracts electrons and how close it holds them. Every time the electron is transferred, a step down the chain to a more electronegative molecule, energy is released, analogous to the potential energy released when a ball rolls downhill. The last step for the electron is oxygen, which is very electronegative. Once the electron is bound to oxygen, it has released all of its energy and can't go any further, which is why cellular respiration uses up oxygen.
  
  The energy released during each transfer down the chain is used to actively pump protons from the mitochondrial matrix into the intermembrane space, resulting in a concentration gradient. The protons flow back out due to diffusion, but as they flow, they pass through a molecular wheel called ATP synthase. The flow of protons causes the wheel to turn, which provides the energy for the synthesis of ATP molecules. Oxidative phosphorylation produces about 34 ATP molecules for each glucose molecule.