Metabolism, ATP and the Electron Carriers

Metabolism is the sum of all chemical reactions in an organism. It happens in every living cell: plant, animal, fungal and bacterial.

• Catabolism: large molecules are broken into smaller ones, mostly by oxidation and hydrolysis. Overall it releases energy. Examples: glycolysis, the Krebs cycle, digestion, the oxidation of NADH.
• Anabolism: small molecules are built into larger ones, mostly by reduction and condensation. Overall it needs energy. Examples: protein synthesis, DNA replication, photosynthesis, glycogen synthesis.
• An exergonic reaction releases free energy ($\Delta G < 0$) and is spontaneous: it can go on without an energy input.
• An endergonic reaction needs an input of free energy ($\Delta G > 0$). Cells drive it by coupling it to an exergonic reaction, usually the hydrolysis of ATP.
• Carbohydrates are the main fuel of catabolism: they are broken down to release energy for cell work, and they are also stored (glycogen, starch) and used as building material.

ATP (adenosine triphosphate) is a nucleotide: the base adenine, the five-carbon sugar ribose, and three phosphate groups. Adenine plus ribose is called adenosine.

• ATP hydrolysis: $\mathrm{ATP + H_2O \rightarrow ADP + P_i}$. It is exergonic: about $-30.5\ \text{kJ/mol}$ under standard conditions, and more in a real cell. It is a hydrolysis, not a condensation and not a redox reaction.
• The energy is released because the products are more stable than ATP. Breaking any bond on its own absorbs energy, so a "high-energy bond" is only shorthand.
• ATP is made from ADP and phosphate in three ways: substrate-level phosphorylation (a phosphate passed directly from a substrate, in glycolysis and the Krebs cycle), oxidative phosphorylation (mitochondria) and photophosphorylation (chloroplasts).
• ATP is not stored in large amounts; each molecule is recycled many times a minute. Muscles keep creatine phosphate as a quick reserve: it passes its phosphate to ADP to remake ATP during the first seconds of intense effort.

• Oxidation is the loss of electrons (often a loss of hydrogen or a gain of oxygen). Reduction is the gain of electrons (often a gain of hydrogen). Memory aid: OIL RIG.
• The two always happen together: the molecule that loses electrons is oxidised, the one that gains them is reduced. Biological oxidations that remove hydrogen are catalysed by dehydrogenases.
• NAD⁺ accepts two electrons and one proton to become NADH (written NADH + H⁺, since a second proton goes into solution). It is used in glycolysis, the link reaction and the Krebs cycle.
• FAD accepts two electrons and two protons to become FADH₂, in the Krebs cycle and fat breakdown.
• NADP⁺ is reduced to NADPH in the light-dependent reactions of photosynthesis. NADPH is the reducing carrier of anabolism (Calvin cycle, fatty acid synthesis).
• These carriers are coenzymes, made from B vitamins. They are not enzymes: they work with enzymes.