It is a multistage process. There are three main stages of catabolism: STAGE I: Conversion of complex molecules into their building blocks… Complex organic molecules like proteins, lipids, and polysaccharides are catabolized to their smaller components or monomers, outside cells… polysaccharides are broken down to monosaccharides, lipids to free fatty acids and glycerol, proteins to amino acids. STAGE II: Formation of simple intermediate. The building blocks produced in stage I are degraded to simple intermediates such as pyruvate and acetyl CoA… A small quantity of energy (as ATP) is captured in stage II. STAGE III: Final oxidation of acetyl CoA. Acetyl CoA is completely oxidized to CO2, liberating NADH and FADH2 that finally get oxidized to release large quantity of energy (as ATP). Krebs cycle (or citric acid cycle) is the common metabolic pathway involved in the final oxidation of all energy-rich molecules.

Catabolism includes various processes such as glycolysis, the Krebs cycle, breakdown of muscle protein to release amino acids needed as substrates for gluconeogenesis, as well as breakdown of fats to fatty acids in adipose tissue.[1] In catabolism, large macromolecules such as polysaccharides, lipids, nucleic acids and proteins are degraded to their constituent monomers – monosaccharides, fatty acids, nucleotides and amino acids, respectively.[1] This degradation releases energy, part of which is lost as heat and the rest is used for synthesis of ATP, the energy currency of the cell.[1]

Catabolism can be broken down into 3 main stages. Stage 1 – Stage of Digestion. The large organic molecules like proteins, lipids and polysaccharides are digested into their smaller components outside cells… Digestive enzymes include glycoside hydrolases that digest polysaccharides into monosaccharides or simple sugars. Stage 2 – Release of energy. Once broken down these molecules are taken up by cells and converted to yet smaller molecules, usually acetyl coenzyme A (acetyl-CoA), which releases some energy. Stage 3 - The acetyl group on the CoA is oxidised to water and carbon dioxide in the citric acid cycle and electron transport chain, releasing the energy that is stored by reducing… NAD+ into NADH. Once inside these sugars undergo glycolysis, where sugars such as glucose and fructose are converted into pyruvate and some ATP is generated. Pyruvate… is converted to acetyl-CoA and fed into the citric acid cycle or the Kreb’s cycle.

Glycolysis represents the first stage of glucose catabolism in organisms that perform cellular respiration. The glycolytic pathway involves the breakdown of glucose to two pyruvate molecules in ten sequential enzymatic reactions within the cytosol. The enzymatic process of glycolysis occurs within the cytosol and can be divided into two definitive stages of energy investment and energy recovery.

We can think of catabolism as occurring in three stages. In stage I, carbohydrates, fats, and proteins are broken down into their individual monomer units: carbohydrates into simple sugars, fats into fatty acids and glycerol, and proteins into amino acids. One part of stage I of catabolism is the breakdown of food molecules by hydrolysis reactions into the individual monomer units—which occurs in the mouth, stomach, and small intestine—and is referred to as digestion. In stage II, these monomer units (or building blocks) are further broken down through different reaction pathways, one of which produces ATP, to form a common end product that can then be used in stage III to produce even more ATP.

We can think of catabolism as occurring in **three stages** (Figure 20.4 "Energy Conversions"). In **stage I**, carbohydrates, fats, and proteins are broken down into their individual monomer units: carbohydrates into simple sugars, fats into fatty acids and glycerol, and proteins into amino acids. One part of stage I of catabolism … is referred to as **digestion**. In **stage II**, these monomer units (or building blocks) are further broken down through different reaction pathways… to form a common end product that can then be used in **stage III** to produce even more ATP. The further metabolism of monosaccharides, fatty acids, and amino acids released in stage I of catabolism occurs in **stages II and III of catabolism**.

Catabolic processes are degradation processes in which complex organic molecules are broken down to simple and few end products.[2] Aerobic metabolism consists of three phases: (1) degradation of biopolymers in the digestive tract to monomers – in this phase there are no oxidation reactions and no macroergic compounds are synthesized; (2) oxidation of these monomers to acetyl-CoA, predominantly via glycolysis and related pathways; (3) degradation of acetyl-CoA in the citric acid (Krebs) cycle to CO₂ and H₂O, with the hydrogen transferred to the respiratory chain where oxidative phosphorylation and ATP synthesis occur.[2] The processes that supply energy are oxidative, catabolic and exergonic, such as glycolysis, β-oxidation of fatty acids and oxidation in the respiratory chain.[2]

Catabolic pathways involve degrading (or breaking down) complex molecules into simpler ones. An example of a catabolic pathway is the breakdown of glucose during cellular respiration. Molecular energy stored in the chemical bonds of glucose are harvested in such a way that it can produce ATP. Other energy-storing molecules, such as fats, also break down through similar catabolic reactions to release energy and make ATP.

**Catabolism** is the metabolic pathway that breaks down complex molecules into simpler ones, releasing energy in the process. This energy is often stored in the form of adenosine triphosphate (ATP). Catabolic processes include **glycolysis in which glucose is broken down to pyruvate**, producing ATP and NADH. The **Citric Acid Cycle (Krebs Cycle)** is also a catabolic process in which the **oxidation of acetyl-CoA to carbon dioxide and water** generates ATP, NADH, and FADH2.

You will find explanations, schemes and exam questions that will help you understand the main stages of catabolism – from glycolysis and respiration in mitochondria to fermentations.[4] The material presents catabolic processes as a sequence starting with glycolysis, followed by mitochondrial respiration (citric acid cycle and electron transport chain with ATP synthesis), and alternative pathways like fermentation.[4]

The process of catabolism is divided into three distinct phases or steps. The three steps are: Digestion; Releasing Energy; Storing Energy. In the digestion stage of catabolism, large, polymeric organic molecules are broken down into smaller molecules called monomers. In the second step, examples include glycolysis, in which glucose is broken down into pyruvate, and beta-oxidation. In the third step, the products from stage two start by being further oxidized in the citric acid cycle or Krebs cycle. Large amounts of energy are generated and stored in the form of ATP. This phase of step three is known as oxidative phosphorylation or terminal respiration.

Catabolic pathways involve the breakdown of nutrient molecules (Food: A, B, C) into usable forms (building blocks). In this process, energy is either stored in energy molecules for later use, or released as heat. Examples of catabolic processes include glycolysis and cellular respiration, in which nutrients are broken down to produce usable cell energy. Cellular respiration itself involves a series of pathways, including glycolysis, the citric acid cycle, and oxidative phosphorylation, that together oxidize nutrient molecules and generate ATP.

The Krebs cycle, also known as the citric acid cycle, is a key stage in metabolism that converts acetyl-CoA into CO₂ and electron carriers (reduced coenzymes), which then feed into the electron transport chain.[6] In this cycle, the acetyl group of acetyl-CoA is completely oxidized, and the released energy is conserved in the form of NADH and FADH₂ that drive oxidative phosphorylation and ATP synthesis in mitochondria.[6]

Catabolic pathways are those that generate energy by breaking down larger molecules… An example of a catabolic pathway is cellular respiration, which breaks down glucose and produces ATP. Cellular respiration includes glycolysis, the citric acid cycle, and oxidative phosphorylation. In glycolysis, glucose is broken down in the cytosol to pyruvate. The citric acid cycle and oxidative phosphorylation occur in the mitochondria and oxidize acetyl-CoA and reduced electron carriers to produce ATP.

Catabolism is one of the two main types of metabolic processes in living organisms, along with anabolism. It includes chemical reactions that break down large, complex molecules into smaller units, releasing energy that can be captured in ATP.[9] The body’s need for energy is satisfied through digestion, during which molecules from unprocessed energy sources (food) are broken down. This breakdown provides the organism with nutrient building blocks necessary for constructing desired molecules, which are then used as energy and structural elements.[9]

Catabolism is the set of metabolic pathways that breaks down molecules into smaller units that are either oxidized to release energy or used in other anabolic reactions. Examples of catabolic processes include glycolysis, the citric acid cycle… and oxidative deamination of neurotransmitters by monoamine oxidase. In aerobic respiration, glucose catabolism proceeds through glycolysis (in the cytosol), the citric acid cycle (in the mitochondrial matrix), and the electron transport chain/oxidative phosphorylation (in the inner mitochondrial membrane), generating ATP.

Figure 14.5 **The three stages of catabolism**. Stage 1: Proteins, polysaccharides and lipids are broken down into their constituent amino acids, monosaccharides, and fatty acids. Stage 2: These diverse building blocks are degraded to a few simple intermediates, such as **acetyl-CoA, pyruvate, and some other simple molecules**. Stage 3: **Catabolism converges to three principal end products: water, carbon dioxide, and ammonia.**

Catabolic processes are biochemical reactions of oxidative degradation of organic compounds.[3] Glycolysis is the glycolytic chain (anaerobic degradation of glucose) in which glucose is converted through a preparatory phase and an energy-yielding phase to pyruvic acid (pyruvate), with simultaneous biological oxidation and phosphorylation of ADP to ATP.[3] During the energy-yielding phase, two molecules of glyceraldehyde phosphate are degraded in successive reactions to two molecules of pyruvic acid, and the released energy is used to form ATP.[3]

The basic stages of catabolism are often described in three steps: (1) breakdown of dietary macromolecules to monomer units (such as glucose, fatty acids, amino acids) during digestion; (2) conversion of these monomers to a limited set of simple intermediates, most notably acetyl-CoA, in processes such as glycolysis; (3) complete oxidation of acetyl-CoA to CO₂ and H₂O in the citric acid cycle with transfer of electrons to the electron transport chain where oxidative phosphorylation produces ATP.[7] The first two stages are common to almost all organisms, whereas the third stage (mitochondrial respiration with oxidative phosphorylation) is characteristic of aerobic metabolism.[7]

Metabolism refers to all chemical reactions involved in maintaining the living state of the cells and the organism. Catabolic pathways break down complex molecules into simpler ones and release energy; anabolic pathways build complex molecules from simpler ones and require energy.[10] The breakdown of glucose through glycolysis, followed by the citric acid cycle and oxidative phosphorylation in the mitochondria, is a central catabolic pathway that cells use to harvest energy in the form of ATP.[10]

Glycolysis, the citric acid cycle, and the electron transport chain are the three main stages of glucose catabolism in cellular respiration. Digestion of food polymers into monomers such as sugars, amino acids, and fatty acids occurs in the gastrointestinal tract and is often described as a preliminary stage of catabolism that supplies substrates for these cellular pathways. Oxidative phosphorylation refers to the coupling of the electron transport chain in the inner mitochondrial membrane to ATP synthesis via ATP synthase, using the proton gradient generated by electron transfer from NADH and FADH2 to oxygen.

Catabolism participates in metabolism through a series of chemical reactions that increase the availability of natural molecules in the body. Catabolism is observed in stages of undernutrition – tissue structures break down to provide, for a specific time, the necessary physiological reaction and process in the body.[5] The need of the body for energy is met through digestion, during which the molecules from the unprocessed energy (food) are broken down. The breakdown of food molecules is a complex process that provides the body with nutritional building blocks needed for the construction of more desired molecules, which are then used for energy and building elements for anabolism.[5]

What is the purpose of metabolism? Learn about the two major divisions in metabolism: anabolism (building up) and catabolism (breaking down).