Metabolism Living State Basis


Living organisms undergo a series of chemical reactions called metabolism. Metabolic reactions help the growth and reproduction of organisms. Every chemical reaction from assimilation to transportation comes under metabolism. Metabolism includes the breaking of compounds that releases energy and the making of compounds that consumes energy. Several metabolic reactions together help the organism maintain its living state.

Metabolic Basis for Living

There are two types of chemical reactions involved in metabolism: catabolism and anabolism.

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The process of converting complex compounds into simpler compounds is called catabolism. Catabolic reactions generate power and elements required by anabolic reactions. Compounds like proteins, lipids, and carbohydrates are digested to form simpler molecules like amino acids, fatty acids, and monosaccharides. Simpler molecules are absorbed by cells and converted into even simpler molecules, such as acetyl Coenzyme A, where energy is released.

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There are three stages of catabolism. They are as follows −

  • Digestion of Large Molecules &minus Cells cannot take up larger molecules directly. Therefore, enzymes are needed to break down larger molecules into smaller ones. Natural enzymes are secreted by microorganisms, whereas animal enzymes are secreted by the pancreas, stomach, and salivary glands.

    When we consume food, the digestion process begins. As food particles pass through the stomach, they reach the large intestine through the small intestine. During this process, food particles undergo many chemical reactions carried out by enzymes. For example, starch is broken down into simple sugars by amylase, lipids are digested by lipase into fatty acids and glycerol.

  • Releasing of Energy − As food particles break down into smaller units, they are taken up by cells and converted into smaller molecules such as acetyl Coenzyme A.

  • Storage of Energy − Acetyl Coenzyme A produces water and carbon oxide by undergoing the process of oxidation. During the reduction of NAD+ into NADH, the energy released is stored.

Let us go through some examples.


The set of reactions through which glucose is converted into pyruvate is called glycolysis. It is a ten-step reaction that involves many enzymes. The initial five steps are called the preparatory phase which involves the conversion of the six-carbon compound glucose into two glyceraldehyde-3- phosphate units (three-carbon compounds). Energy is consumed during these reactions.

The last five steps together are called the pay-off phase, which includes the gain of NADH and ATP which are energy-carrying molecules. In the end, the enzyme pyruvate kinase converts phosphoenolpyruvate to pyruvate. Glycolysis involves several reactions like phosphorylation, isomerization, oxidation, and alcohol dehydration.

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The Citric Acid Cycle

Enzyme-mediated oxidation of acetyl coenzyme A found in fats, proteins, and carbohydrates to release stored energy takes place in this cycle. The citric acid cycle involves several chemical reactions like alcohol condensation, dehydration, hydration, oxidation, decarboxylation, and phosphorylation. During the series of reactions, enzymes like citrate synthase and fumarase are involved. In the end, it releases oxaloacetate which is a fourcarbon compound.

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Anabolism is the energy-consuming process that involves making up larger molecules from simpler units. Anabolism gets energy from catabolic pathways. In many cases of anabolic pathways, ATP acts as a source of energy. Species are classified according to their energy requirements as follows,

  • Autotrophs − Autotrophic organisms use light energy. Examples include plants and algae.

  • Heterotrophs − Heterotrophs use organic compounds as their energy source. Examples include animals and birds.

  • Chemoautotrophs − Chemoautotrophs obtain energy from chemical reactions. For example, some bacteria and archaea.

  • Chemoheterotrohs − Chemoheterotrophs obtain energy from chemical reactions. For example, animals and fungi.

  • Photoautotrophs − Their energy source is light. For example, mango tree and euglena.

  • Photoheterotrophs − These organisms use solar light for their energy. For example, some bacteria like Rhodospirillum and Heliobacteria.

There are three stages in anabolism.

  • Precursors like amino acids, monosaccharides, isoprenoids, and nucleotides are produced in the first stage.

  • Energy is used in stage two to activate these precursors into reactive forms.

  • Polysaccharides, proteins, nucleic acids, and lipids are formed from reactive precursors.

Functions of Anabolism

Anabolism includes joining smaller units to form a macromolecule. Anabolism helps to grow muscles and bone density. It also helps the growth and development of living organisms. Anabolic pathways cause cells to differentiate and increase body size. The anabolic steroid hormone testosterone is required to maintain male features in men. Another anabolic hormone insulin helps in protein synthesis and glucose uptake.

Examples of Anabolism

Some examples of anabolism are -

  • Lipid formation − Here, lipids are synthesized from glycerol and fatty acids.

  • Protein synthesis − In this process, proteins are formed from smaller molecules such as amino acids.

  • Carbohydrate formation − This involves the formation of carbohydrates from simple sugars.

Metabolism and the Living State

As per the law of physics, if a system is in an equilibrium state it cannot work. This rationale can be applied to biology as well. Living organisms cannot attain equilibrium since they constantly undergo metabolic reactions. Hence living organisms are in a nonequilibrium steady state. This indicates that biological processes are continuously ongoing.


This tutorial gives a brief idea about the metabolic reactions in the living world. This tutorial explains the detailed description of catabolic and anabolic reactions. The relationship between the living state and the metabolism has been described in this tutorial. In conclusion, this tutorial may be useful for understanding the basic concepts of metabolism.


Q1. State two reactions of glycolysis with enzymes.

Ans. Reactions of glycolysis include, isomerization with the enzyme phosphoglucose isomerase and dehydrogenation reaction with glyceraldehyde-3-phosphate dehydrogenase.

Q2. Write down the different stages of anabolism.

Ans. Production of precursors, conversion of precursors into reactive form, and formation of larger molecules are the three stages of anabolism.

Q3. What are the different stages of catabolism?

Ans. Different stages of catabolism are the digestion of larger molecules, the release of energy, and the storage of energy.

Q4. State two examples for chemoautotrophs and photoautotrophs.

Ans. Examples of chemoautotrophs are Nitrosomonas and Nitrobacter and examples of photoautotrophs are nitrogen bacteria and cyanobacteria.

Q5. What is the function of amylase and what are the types of amylases?

Ans. Amylase digests complex polysaccharide starch into simple sugars. There are three types of amylases. They are as follows, alpha amylase, beta amylase, and gamma amylase.

Updated on: 09-Jan-2023


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