Redox Reactions & Glycolysis

• Redox reactions involve the transfer of electrons (gain or loss) between species.
• OIL RIG (Oxidation Is Loss, Reduction Is Gain), ELMO (Electron Loss Means Oxidation), and LEO goes GER (Loss of Electrons is Oxidation, Gain of Electrons is Reduction) are mnemonics for redox reactions.
• Reduced species have the ability to reduce other species, this is the main role of hydrogen carriers.

Oxidation vs. Reduction

• Oxidation involves the loss of electrons, frequently gaining oxygen or losing hydrogen.
• Reduction involves the gain of electrons, frequently losing oxygen or gaining hydrogen.

Glycolysis

• Glycolysis is the first stage of cell respiration where glucose is broken down into two molecules of pyruvate.
• It is an anaerobic reaction that happens in the cytoplasm.

Four Main Parts of Glycolysis

• Phosphorylation: A hexose sugar becomes hexose biphosphate using two ATP.
• Lysis: Hexose biphosphate splits into two triose phosphates.
• Oxidation: Hydrogen is removed from the triose phosphates (NAD reduced to NADH + H+).
• ATP Formation: Four ATP molecules released as triose phosphates convert to pyruvate.
• One glucose molecule results in 2 pyruvate, 2 (NADH + H+), and a net gain of 2 ATP.

Aerobic Respiration

• Aerobic respiration happens in the mitochondria, using pyruvate from glycolysis.
• It produces large amounts of ATP in the presence of oxygen using three main processes:

The Link Reaction

• Pyruvate is transported to the mitochondrial matrix, producing one NADH + H+ via oxidation.
• The pyruvate loses a carbon (as CO2) and the remaining two carbons complex with coenzyme A (CoA) to form acetyl CoA.

The Krebs Cycle

• In the matrix, acetyl CoA combines with a 4C compound to form a 6C compound.
• The 6C compound is broken back down into the original 4C compound over a series of reactions.
• Results in the formation of 2 CO2 molecules, 1 ATP molecule, 3 (NADH + H+), and 1 FADH2.

The Electron Transport Chain

• Hydrogen carriers (NADH + H+ and FADH2) provide electrons to the electron transport chain on the inner mitochondrial membrane.
• Electrons lose energy as they cycle down the chain, which is used to translocate H+ ions to the intermembrane space (creating a gradient).
• Hydrogen ions return to the matrix through ATP synthase, producing multiple ATP molecules via chemiosmosis.
• Oxygen acts as the final electron acceptor, combining electrons with H+ ions to form water molecules.
• The electron transport chain produces the majority of ATP (~32 out of 36 ATP molecules).

Oxidative Phosphorylation and Chemiosmosis

• Oxidative phosphorylation describes ATP production from oxidised hydrogen carriers.
• Electrons lose energy as they are passed between carrier molecules in the electron transport chain.
• This energy is used to translocate H+ ions from the matrix to the intermembrane space against the concentration gradient.
• The buildup of H+ ions creates an electrochemical gradient, known as proton motive force (PMF).
• Protons return to the matrix via ATP synthase, releasing energy, which is used to produce ATP.
• This process is called chemiosmosis and occurs in the cristae.
• The H+ ions and electrons are combined with oxygen to form water, allowing the process to be repeated.

Mitochondria Structure and Function

• Inner membrane: Folded into cristae to increase the surface area for the electron transport chain.
• Intermembrane space: allows accumulation of protons (increasing PMF).
• Matrix: Contains enzymes and suitable pH for the Krebs cycle.
• Outer membrane: Contains transport proteins for moving pyruvate into the mitochondria.