Cellular Metabolism: Catabolism, Anabolism, and ATP Production

Metabolism

Metabolism is the set of chemical reactions that occur inside the cell. The different sequences of reactions or routes are called metabolic pathways, and the molecules involved are metabolites.

There are two phases:

1. Catabolism: Transformation of complex organic molecules into simpler ones (degradation). This process releases energy stored in ATP's phosphate bonds.
2. Anabolism: Synthesis of complex molecules from simple ones (construction), requiring energy provided by ATP.

Structure of Metabolic Pathways

• Linear: Intermediates are only used in a particular pathway.
• Branched: Metabolic routes intersect or cross at two or more points.
• Cyclical: After a series of transformations, the initial molecule is regenerated.

Types of Metabolism Based on Carbon Source

1. If CO₂ is used as the carbon source, it's called autotrophic metabolism.
   • If light is used as the energy source, it's photosynthesis.
   • If chemical reactions are used as the energy source, it's chemosynthesis.
2. If organic matter (methane, glucose, fats, etc.) is used, it's called heterotrophic metabolism.

ATP

ATP (adenosine triphosphate) is a nucleotide of great importance in metabolism, as it stores or releases energy in its phosphoester bonds. These bonds are said to be energy-rich. Enzymes called ATPases catalyze ATP hydrolysis. ATP synthesis is carried out in two ways:

1. Substrate-level phosphorylation: ATP synthesis occurs using the energy released from a molecule.
2. Oxidative phosphorylation: In oxidation-reduction reactions, detached electrons are captured by a carrier molecule. This step leads to the release of energy, which is used to phosphorylate ADP and form ATP.

Electron Transport

Many metabolic reactions are redox reactions. The released electrons are captured by a molecule, which is then reoxidized and returns to its original state by giving electrons to other compounds.

Catabolism

Catabolism is the degradative phase of metabolism aimed at obtaining energy. Organic molecules are transformed into simpler ones, and the final products are expelled by the cell. The energy released in these reactions is stored in ATP to be subsequently used for organic synthesis reactions and other cellular functions. Catabolism is similar in autotrophic and heterotrophic organisms.

Types

Depending on the nature of the substance that is reduced:

1. Fermentation: The molecule that is reduced is organic.
2. Respiration: The molecule that is reduced is inorganic.

Carbohydrate Catabolism

Glycogen and starch reserves are hydrolyzed to glucose, which enters the degradative pathway. Two phases are distinguished:

1. Glycolysis: Glucose is split into two molecules of pyruvic acid, producing 2 ATP in the cytoplasm. It occurs anaerobically and consists of a sequence of 10 enzymatic reactions summarized in two stages:
   1. Stage 1: Glucose is phosphorylated and fragmented, producing two molecules of glyceraldehyde 3-phosphate. This process consumes 2 ATP.
   2. Stage 2: Two molecules of glyceraldehyde 3-phosphate are oxidized and converted into pyruvate, producing 4 ATP. Therefore, the energy efficiency of glycolysis is 2 ATP molecules per glucose molecule.
2. Fermentation: Fermentation is an anaerobic process that obtains partial oxidation of organic molecules, yielding a low amount of ATP. There are strict and facultative anaerobic organisms.
   • Lactic acid fermentation: Bacteria (lactobacilli) convert pyruvate to lactate. NADH produced in glycolysis is regenerated.
   • Alcoholic fermentation: Saccharomyces convert glucose to ethanol and CO₂, with the production of 2 ATP. NADH produced in glycolysis is regenerated in the reduction of ethanol. For the cell, the major product is ATP; ethanol and CO₂ are waste products.
3. Respiration: Pyruvic acid produced in glycolysis enters the mitochondria and undergoes oxidative decarboxylation catalyzed by the pyruvate dehydrogenase enzyme system into acetyl-CoA, which enters the Krebs cycle.