What does metabolism mean?

Metabolism is defined as the group of life-supporting, enzyme-mediated chemical reactions in organisms. A metabolic pathway is a set of these chemical reactions that involve anabolism (synthesis of complex biomolecules) and catabolism (breakdown of biomolecules into small molecules) that occur inside a cell. Metabolites are the reacting substances, products, and intermediate substances of an enzymatic process that are altered by a series of chemical events catalyzed by enzymes. 

What is the importance of metabolism in organism?

For the formation of biological molecules and to carry out biological functions, all living cells require a steady supply of energy. The energy is obtained by the breakdown of chemical bonds present in organic molecules available from food. The chemical events of metabolism are regulated by certain proteins present in the body. The conversion of chemical energy stored in food into adenosine triphosphate (ATP) to power cellular processes; the conversion of food into lipid molecules, nucleic acids, amino acids, and certain carbohydrates; and the metabolic waste removal by enzyme-mediated metabolic pathways are the three main functions of metabolism. Organisms undergo growth, development, and reproduction, respond to their surroundings and maintain their cell structure due to the energy produced from the metabolic reaction pathway.

In metabolism, the rate of breakdown of metabolites is termed a steady state. The metabolic flux, or flow rate, varies based on the metabolic demands. In a metabolic pathway, however, a steady state is maintained by balancing the rate of the substrate provided by a previous step with the rate at which the substrate is converted into a product. As a result, the concentration of substrate remains rather constant.  

Types of metabolic pathways

There are two types of metabolic pathways: anabolic and catabolic. Anabolic pathways are involved in the synthesis of macromolecules using energy, while catabolic pathways are used to break down the large molecules into small molecules by releasing energy. The energy released by one is used up by the other; the two paths are mutually beneficial. In addition to the two basic pathways, there is an amphibolic pathway. It may be either anabolic or catabolic based on the requirement for energy.

The image is about the anabolic and catabolic pathway
CC BY | Image credits: https://sites.google.com

Anabolic pathway

The anabolic pathway helps to produce macromolecules like proteins, polysaccharides, and nucleic acids. The condensation reactions are utilized to link monomers. Enzymes and their cofactors are involved in the production of macromolecules from smaller biomolecules. Metal ions and the reducing agents NADH (nicotinamide adenine dinucleotide hydrogen), NADP (nicotinamide adenine dinucleotide phosphate), and FADH2 (flavin adenine dinucleotide) have a role as cofactors in anabolic processes at different stages. The formation of glycogen from glucose is an anabolic process. For glycogen synthesis, the overall equation is:

Glycogen (n glucose residues) + glucose + Two ATP → glycogen (n+1 glucose residues) + Two ADP (adenosine diphosphate) + Two Phosphates.

Catabolic pathway

Catabolism is a series of metabolic pathways that are involved in the breakdown of macromolecules such as polysaccharides, lipids, proteins, and nucleic acids into smaller molecules (monomers), such as monosaccharides, fatty acids, amino acids, and nucleotides. These monomers are either used as a substrate during anabolic reactions or are oxidized to generate energy for cellular processes.

Cellular respiration is the oxidation of biomolecules in cells to release energy. It is a catabolic event in which the respiratory substrates (starch, sugar or other sugars, lipids, proteins, organic acids, etc.) are broken down by a sequence of enzyme processes to release energy in the form of ATP (adenosine triphosphate). Glycolysis, the tricarboxylic acid cycle (TCA cycle), and the disintegration of muscle protein to use amino acids as substrates for gluconeogenesis are examples of catabolic processes.

The key metabolic pathways

The image shows the metabolic pathway in cell
CC BY | Image credits: http://epgp.inflibnet.ac.in

The key metabolic pathways and their primary control sites are as follows:


Glycolysis is a catabolic process where glucose is broken down into two pyruvate molecules and produces NADH, ATP, and water. This reaction occurs in the cell cytoplasm and can occur in the presence or absence of oxygen.

Krebs cycle

The Krebs cycle, also termed the tricarboxylic acid cycle (TCA cycle) or citric acid cycle, is a set of enzyme-catalyzed reactions that takes place in the mitochondrial matrix. In the Krebs cycle, the two pyruvate molecules generated in glycolysis are used to form acetyl CoA in the mitochondria. The acetyl-CoA is then converted into CO2 and reduced coenzymes to form adenosine triphosphate (ATP) in the electron transport chain. This is an eight-step procedure during which the acetyl group of acetyl-CoA is oxidized to produce two molecules of CO2 and one ATP during the process. FADH2, NADH, and the reduced high-energy molecules are also created.

Because one molecule of glucose in the reaction produces two acetyl-CoA molecules, the Krebs cycle must be repeated twice, yielding four CO2, six NADH, two FADH2, and two ATPs.

Oxidative phosphorylation

It occurs in the mitochondria. The mitochondrial inner membrane houses the electron transport chain system. It is connected to the electron transport chain process. Through a set of redox processes, electrons are transported from one member of the transport chain to another to generate energy (ATP) in the presence of oxygen. Oxidative phosphorylation is the last step in biological oxidation.

Pentose phosphate pathway

The pentose phosphate pathway also termed as phosphogluconate pathway results in the formation of NADPH, five-carbon sugars, and ribose 5-phosphate. This process runs parallel to glycolysis. The pathway takes place in the cytoplasm of liver cells, adrenal cortex, and glands of lactating mammary. In plants, the majority of processes occur in plastids.

Fatty acid synthetic pathway

It is the route for producing fatty acids using acetyl-CoA & NADPH. The enzymes used for this pathway are termed fatty acid synthases. FAS (fatty acid synthase complex) is an enzyme complex that is accountable for the synthesis of fatty acids in the cytosol. 


Glycogenesis forms the reaction pathway of synthesizing glycogen and adding glucose monomers to glycogen chains for storage. When dietary carbohydrates are available, glycogenolysis restores glycogen reserves in the skeletal muscle cells and liver. It is triggered by both increased glucose availability and insulin.

Fatty acids β-oxidation

Fatty acids are broken down through a separate process called beta-oxidation, which occurs in the mitochondria. Fatty acids, which are activated before entering the mitochondria. The activation process occurs in the cytoplasm and involves the fatty acid conversion into acyl-CoAs. For further processes to take place, acyl-CoA must be entered into the mitochondria. The mitochondrial inner membrane is impervious to acyl-CoAs. The acyl-CoA acts with a "special" amino acid, carnitine, allowing it to enter inside. Carnitine transfers the acyl group to another CoA molecule inside the mitochondrion.

Metabolic pathways regulation

The metabolic pathway reactions are interconnected and complex. To maintain homeostasis within cells, metabolic reaction events must be precisely regulated in response to changing environmental conditions. Metabolic pathway regulation entails increasing or lowering the response of a specific enzyme in a reaction pathway to signals.

  • Intrinsic regulation: It occurs when reactions self-regulate in accordance with variations in substrate or product levels.
  • Extrinsic regulation: It occurs when the metabolism of a cell changes in response to signals from other cells. Soluble messengers, like growth factors and hormones, carry the signals along the routes and are sensed by certain cell surface receptors. Second messenger systems then transmit these signals inside the cell.

Context and Applications

This topic is significant in the exams for both school level, graduate, and postgraduate levels, especially for bachelors of science in zoology and botany, and masters of science in zoology and botany.

Practice Problems

Question 1: The rate of metabolite breakdown is known as ______.

  1. metabolism
  2. steady state
  3. metabolic state
  4. homeostasis

Answer: The correct answer is 2.

Explanation: The rate of metabolic flux, or flow, varies according to metabolic demands. In a metabolic pathway, however, a steady-state is maintained by balancing the rate of the substrate provided by a previous step with the rate at which the substrate is transformed into a product, as a result, the substrate concentration remains rather constant.

Question 2: Under extreme starvation conditions, which of the following kinds of metabolites is utilized to generate glucose?

  1. Starch
  2. Fats
  3. Amino acids
  4. Glycogen

Answer: The correct answer is 4.

Explanation: The liver releases the significant amount of glucose formed by the breakdown of glucose 6-phosphate formed from glycogen into the bloodstream.

Question 3: What is glycolysis?

  1. Breakdown of glucose
  2. Synthesis of glucose
  3. Synthesis of glycogen
  4. Breakdown of glycogen

Answer: The correct answer is 1.

Explanation: Glycolysis is the reaction of breaking down glucose to form energy.

Question 4: The citric acid cycle occurs in ____.

  1. ribosomes
  2. cytosol
  3. nucleus
  4. mitochondrial matrix

Answer: The correct answer is a 4.

Question 5: The acetyl-CoA is obtained from

  1. Pyruvate
  2. Glucose
  3. Fructose
  4. Lactate

Answer: The correct answer is 1.

Explanation: The pyruvate from glycolysis produces acetyl-CoA by oxidative decarboxylation.

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