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Dynamic State of Body Constituents - Concept of Metabolism, its Pathways, Metabolic Basis for Living and the Living State

Do you know how a living system works? How do we get energy from the food we eat? There are millions of chemical reactions taking place in our cell. These are called metabolic reactions. Every metabolic reaction requires a catalyst. Even the physical process of CO2 dissolving in water is a catalysed reaction in living systems. But do you know what these catalysts do?

Biological catalysts or enzymes increase the amount of product created per unit time by speeding up the rate of metabolic conversation.

Thousands of organic chemicals can be found in a single bacterial cell, protozoans, plant, or animal. These substances or molecules exist at specific concentrations (measured in moles per cell or moles per litre). All these substances have a turn over, which means that they are continually changing into and being created from other biomolecules. This means that there are breakdown and synthesis reactions occurring simultaneously within cells. Do you know what is the sum effect of all these reactions on the cell? How it affects the existence of the cell?? Come, let’s learn about it.

List of Contents


Metabolism is the collective term for all of the chemical reactions that occur in living organisms and are responsible for the breaking and making of biomolecules. Biomolecules are transformed as a result of each metabolic reaction. Examples of metabolic reactions are:

  • Removal of carbon dioxide gas when an amino acid is converted into an amine
  • Breakdown of a nucleotide base to remove amino acids
  • Breakdown of a disaccharide into monosaccharides by the hydrolysis of glycosidic bonds.

Every metabolic reaction is a catalysed reactions in which the catalysts speed up metabolic conversion.

Metabolism is of two types: Anabolism and Catabolism


The metabolic reactions in which complex molecules are formed from simpler molecules are called anabolic reactions.

anabolic reaction

The metabolic pathways involving anabolic reactions are called Anabolic pathways. These pathways are endothermic, that is, they consume energy by the hydrolysis of ATP.

For example:

  • Assembly of amino acids to form a protein.
  • Synthesis of cholesterol from acetic acid.

example of catabloic reactions


The metabolic reactions in which complex molecules break down into simple molecules are called catabolic reactions.


The metabolic pathways involving catabolic reactions are called Catabolic pathways. These pathways are exothermic, that is, they evolve energy in the form of ATP.

For example:

  • Formation of lactic acid from glucose in our skeletal muscles.
  • Breakdown of glucose to pyruvic acid (glycolysis)
  • Breakdown of polypeptide into amino acids

example of catabloic reactions

Storage of Energy by Living Organisms

Living organisms capture the energy released during breakdown of complex molecules in catabolic reactions and store it in chemical bonds of high energy molecules such as adenosine triphosphate (ATP). These high energy bonds are hydrolysed to release energy which is used as needed for osmotic, mechanical and biosynthetic tasks that we do. 

role of atp in anabolic and cataboloic reactions

The most essential kind of energy currency in living systems is the bond energy in a molecule called adenosine triphosphate (ATP).

structure of adenosine triphosphate

ATP provides energy by the hydrolysis of phosphoanhydride bonds between the phosphate groups.

hydrolysis of atp

Hydrolysis of one mole of ATP to ADP provides 7.3 kcal/mol energy under standard conditions.

Metabolic Pathways

The majority of the metabolic events are always related to other reactions and do not occur in isolation. Metabolites are transformed into one another through a series of interconnected processes known as Metabolic Pathways. These metabolic pathways are comparable to city traffic in terms of complexity. These metabolic pathways can be linear (glycolysis) or cyclic (Kreb’s cycle). These pathways have traffic junctions, that is, they criss-cross each other.

criss-crossing of metabolic pathways

Like automobile traffic, the flow of metabolites along the metabolic pathway has a specific rate and direction. This flow of metabolites constitutes the dynamic state of the body.

The Living State

In a living organism metabolites or biomolecules are present at amounts unique to each of them. For example, blood glucose concentration for a healthy adult human being ranges from 4.2 to 6.1 mmol/L, while hormone concentrations are measured in nanograms/mL. The most significant truth about biological systems is that they all exist in a steady-state characterised by concentrations of these biomolecules. These biomolecules are undergoing metabolic changes continuously and are in a state of metabolic flux. Any chemical or physical process would reach equilibrium on its own but systems in equilibrium can't do work. Living organisms cannot afford to be in a state of equilibrium since they are constantly working. The steady state of the living body is a non-equilibrium state. To be able to conduct work, the living state is a non-equilibrium steady state and the life processes and metabolic reactions constantly attempt to avoid reaching the state of equilibrium. This is accomplished by the continuous input of energy into the living system which is obtained fom metabolic reactions. As a result, the terms "living state" and "metabolism" are interchangeable. Thus, there can be no living condition without metabolism.

Practice Problems of Metabolism

1. Select the statement which correctly defines the living state.

a. It is an equilibrium non-steady state which can perform work
b. It is a non-equilibrium non-steady state which can perform work
c. It is an equilibrium steady state which can perform work
d. It is a non-equilibrium steady state which can perform work

Solution: Living state is a non-equilibrium state which can perform work. This is because if a living system is in equilibrium state, it will not be able to perform work. Additionally, living states cannot be in a state of equilibrium because biomolecules are continuously undergoing metabolic changes. Thus, the living state is a non-equilibrium steady state that is capable of working.
Hence, d is the correct option.

2. Read the given assertion and reason and select the correct option.

Assertion: Breakdown of glucose to pyruvate is a catabolic reaction.

Reason: Catabolic reactions liberate energy in the form of ATP by breaking down substances.

a. Both assertion and reason are correct, reason is the correct explanation of assertion.
b. Both assertion and reason are correct but reason is not the correct explanation of assertion.
c. Assertion is correct, Reason is incorrect.
d. Both assertion and reason are incorrect.

Solution: Glucose is a 6 carbon compound while pyruvate is a 3 carbon compound. Breakdown of glucose (complex biomolecule) to pyruvate (simpler molecule) results in the release of energy which is stored in the form of ATP. Hence, it is termed as a catabolic reaction. Thus, both assertion and reason are correct and the reason is correctly explaining the assertion. Hence, option a is correct.

FAQs of Metabolism

Question 1. Define Metabolism.

Answer: Metabolism is defined as a collection of all of the chemical reactions that take place in living organisms. It helps in the breaking and making of biomolecules.

Question 2. What are Metabolic pathways?

Answer: The transformation of metabolites from one molecule to another through multiple series of interconnected steps are called metabolic pathways.

Question 3. Mention the difference between Anabolism and Catabolism.




Anabolism involves the chemical reactions which require energy to form a complex molecule from simple molecules.

Catabolism involves the chemical reactions in which a complex molecule breaks down into one or more simpler molecules with the release of energy.

Question 4. What is Metabolic flux?

Answer: Metabolic flux means the continuous changing of one molecule into another leading to metabolic changes in the living organism.

Question 5. What do you understand by the statement that the living state of a body is a non-equilibrium state?

Answer: Non-equilibrium state is a state in which work can be done. Living organisms are usually constantly working because the biomolecules are undergoing metabolic changes. Thus, the living system cannot be in equilibrium state. Hence, the living state is a non-equilibrium steady state and the life processes and metabolic reactions constantly attempt to avoid reaching the state of equilibrium. 

Question 6. How do living organisms store energy?

Answer: Living systems store energy in the form of a chemical compound called Adenosine Triphosphate (ATP). Breakdown of a complex biomolecule results in the liberation of energy which is stored as ATP. ATP has high energy phosphoanhydride bonds. The breakdown of these bonds leads to the liberation of a large amount of energy which can be used in anabolic reactions.

Other Related Topics

The Living World Biological Classification Plant Kingdom
Animal Kingdom Morphology of Flowering Plants Anatomy of Flowering Plants
Structural Organization in Animals Cells: The Unit of Life Biomolecules
Cell Cycle and Division Transport in Plants Mineral Nutrition
Photosynthesis in Higher Plants Respiration in Plants Plant Growth and Development
Digestion and Absorption Breathing and Exchange of Gases Body Fluids and Circulation
Excretory Products and their Elimination Locomotion and Movement Neural Control and Coordination
Chemical Coordination and Integration

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