Fundamentals of Molecular Biology. Part 1: Molecular Cell Biology - A. N. Ogurtsov 2011

Oxidation of Glucose and Fatty Acids

The most common way to store energy and deliver it to where it is needed is through the synthesis of ATP molecules.

In Cells, energetically unfavorable (endergonic) processes are coupled with the exergonic Hydrolysis of the phosphoanhydride bond between phosphate groups in ATP. This occurs, for instance, during the synthesis of Proteins from Amino Acids and Nucleic Acids from NUCLEOTIDES, Active Transport across membranes, Muscle sarcomere contraction, and so forth.

The ATP molecule serves as the universal cellular "fuel"; it is utilized across all types of organisms. The processes of ATP synthesis and consumption rank among the fundamental biological mechanisms, alongside the storage and utilization of Genetic information and Cell Division.

The ATP synthesis reaction is endergonic and can be expressed in its general form as

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where P_i^2- represents inorganic phosphate (HPO^2-₄), commonly designated as P_i. The energy required to drive this reaction (7.3 kcal/mol) is supplied to The Cell primarily through two processes:

✵ aerobic oxidation, which takes place in virtually all types of cells,

✵ Photosynthesis, which occurs exclusively in plant leaf cells and certain unicellular organisms.

During aerobic oxidation, Fatty acids and sugars—principally glucose—are oxidized to CO₂ and H₂O, and the released energy is converted into the energy of the phosphoanhydride bonds in ATP. It is precisely through aerobic oxidation that ATP is synthesized in animal cells and the majority of other non-photosynthetic cells.

The initial stage of glucose oxidation, known as Glycolysis, takes place in the Cytosol of both eukaryotes and prokaryotes and does not require molecular oxygen O₂. ATP is generated during The final stage, which proceeds with the involvement of molecular oxygen.

In eukaryotes, this concluding stage of aerobic oxidation takes place within the Mitochondria.

In prokaryotes, which possess only a single Plasma Membrane, the final stages of aerobic oxidation occur directly on this membrane.

Although the final stages of Fatty acid oxidation and ATP synthesis sometimes take place in mitochondria, in most Eukaryotic cells Fatty acids are oxidized to CO₂ and H₂O within Peroxisomes without the synthesis of ATP.

During photosynthesis, light energy is converted into the chemical energy of ATP phosphoanhydride bonds and stored within the chemical bonds of CARBOHYDRATES (primarily sucrose and starch). Photosynthesis also generates oxygen as a byproduct.

In plants and unicellular Algae (eukaryotes), photosynthesis takes place in METABOLISM/14.html">Chloroplasts. Certain prokaryotes also carry out photosynthesis on their Plasma Membranes or specialized plasma membrane invaginations, via a mechanism similar to that operating in chloroplasts.

The evolution of oxygen during photosynthesis serves as virtually the sole source of oxygen in Earth's atmosphere, while the carbohydrates produced in this process provide practically the primary energy source for all non-photosynthetic organisms.

However, it should be kept in mind that some primitive organisms utilize Energy Sources other than sunlight; for instance, deep-sea Bacteria obtain the energy required to convert CO₂ into carbohydrates and other cellular metabolites by oxidizing Inorganic Compounds dissolved in ocean Water.