What is Metabolism in Biology?

In Biology, Metabolism is the reaction that converts the food we eat into energy by the body and the cells. We need this energy for every exercise our bodies do, from moving to thinking to developing. Specific proteins in the body control the chemical reactions occurring during metabolism. 

In this detailed guide, we will learn about the functions of metabolism, Enzymes, Metabolic pathways, metabolism in the purview of the laws of thermodynamics, etc. We will also discuss the chemical reactions for energy transfer and 10 interesting facts about metabolism in biology.

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The 3 vital functions of metabolism

  1. The conversion of food energy into energy available to power cellular operations
  2. The conversion of food into building blocks for lipids, proteins, carbohydrates, nucleic acids and
  3. The excretion of the metabolic wastes.

Enzymes and Metabolism

Metabolism in Biology can not be completely understood without a detailed discussion of enzymes.

The enzyme is a protein that functions as a catalyst in living organisms, controlling the pace at which chemical reactions occur while remaining unaffected.

All biological activities in living creatures involve chemical reactions, and enzymes regulate the majority of them. Many of these processes would not occur at all if enzymes weren’t present.

Role of enzymes

All components of cell metabolism are catalyzed by enzymes. This comprises

  • food digestion, which breaks down major nutritional molecules (such as proteins, carbs, and lipids) into smaller ones
  • chemical energy conservation and transformation and
  • the creation of cellular macromolecules from smaller precursors.

We already mentioned that enzymes are proteins that aid in the speeding up of metabolism or the chemical reactions that occur in our bodies. Some substances are created while others are destroyed.

Some enzymes break down large nutrition molecules like proteins, lipids, and carbohydrates into smaller molecules. This occurs in animals’ stomachs and intestines during food digestion.

Other enzymes move the smaller, broken-down molecules into the bloodstream via the gut wall.

Also, some other enzymes aid in the production of cellular constituents by promoting the creation of large, complex molecules from small, simple ones.

Enzymes are also responsible for a variety of other tasks, including energy storage and release, the course of reproduction, respiration processes, and eyesight. They are necessary for survival.

Each enzyme can only encourage one sort of chemical reaction at a time. Substrates are the chemicals that the enzyme reacts with.

Enzymes work in metabolic pathways, which are highly organized metabolic systems. A seemingly simple biological phenomenon, such as muscle contraction or nerve impulse transmission, consists of various chemical steps, which include the reaction of one or more chemical substrates, which are then converted to chemical substances called products. The product of one step in a metabolic pathway serves as the substrate for the next step in the pathway.

Importance of Enzymes

Enzymes are essential in metabolism because they allow organisms to drive desired reactions that require energy but will not occur independently by linking them to spontaneous energy-releasing reactions. Enzymes function as catalysts, allowing a reaction to progress more quickly. They also allow the rate of a metabolic reaction to be regulated, for example, in response to changes in the cell’s environment or signals from other cells.

Living entities require enzyme-mediated reactions to reproduce and grow, structural maintenance, and respond to their surrounding. We call all the chemical reactions in living organisms intermediary (or intermediate) metabolism. Some examples are digestion and the movement of chemicals into and between cells.

Enzyme deficiencies and their effect

Many hereditary human disorders, such as albinism and phenylketonuria, are caused by enzyme deficiencies.

Most chemical reactions have an energy barrier that must be overcome for the reaction to take place. This barrier keeps complex molecules like proteins and nucleic acids from decomposing on their own, which is crucial for life to exist.

However, when a cell’s metabolism requires modifications, some of these complex molecules must be broken down, and this energy barrier must be overcome.

Heat may provide the additional energy required (known as activation energy), but the cell would perish as a result of the increased temperature. The employment of a catalyst to lower the activation energy level is an alternative. This is the function of enzymes. They react with the substrate to generate an intermediate complex—a “transition state”—that allows the reaction to proceed with less energy. The unstable intermediate chemical degrades swiftly into reaction products, leaving the enzyme free to react with other substrate molecules.

The active site of the enzyme is the only part of the enzyme that binds to the substrate. The active site is a groove or pocket created by the folding structure of the protein. The electrical and chemical properties of the amino acids and co-factors-factors in the active site, as well as the three-dimensional structure, allow only a specific substrate to bind to the site, determining the enzyme’s specificity.

So we see that a deficiency of enzymes may create problems with metabolism and hence disorder/diseases.

Metabolism in Humans

Foods are primarily composed of carbohydrates, lipids, and proteins, which act as fuel molecules for the human body. The digestive process (breaking down these nutrients into smaller pieces) and subsequent absorption (entrance into the bloodstream) of the digestive end products allow tissues and cells to convert the potential chemical energy of food into useful work.

Monosaccharides, primarily glucose (from carbs)

Monoacylglycerol and long-chain fatty acids (from lipids), and

Short peptides and amino acids (from proteins) are the most often absorbed end products of food digestion (from protein). Once in the bloodstream, different cells can utilize these substances. These three types of molecules have long been recognized as sources of energy for human metabolism.

The metabolic system of an organism determines which substances are beneficial and which are detrimental. Hydrogen sulfide is used as a source of sustenance by some prokaryotes, although it is hazardous to vertebrates.

The basal metabolic rate of an organism measures the amount of energy required by all of these chemical reactions.

Catabolic reactions are those in which chemicals are broken down (for example, cellular respiration converts glucose to pyruvate), while anabolic processes are those in which compounds are built up (synthesis) (such as proteins, carbohydrates, lipids, and nucleic acids).

Catabolism usually releases energy, whereas anabolism absorbs it. Metabolism’s chemical reactions are arranged into metabolic pathways, in which one molecule is changed into another by a sequence of stages, each aided by a unique enzyme.

Metabolic pathways

To understand Metabolism in Biology, we must understand Metabolic pathways.

Each metabolic pathway is made up of a sequence of biological reactions linked by intermediates: the products of one reaction serve as substrates for succeeding reactions, and so on. Metabolic pathways are frequently thought to move in a single direction. Even though all chemical processes are technically reversible, the conditions in the cell frequently make it more thermodynamically advantageous for flux to go in one direction.

For example, one pathway may be responsible for producing specific amino acids, yet another unique pathway may be responsible for its breakdown. Feedback inhibition is frequently used to regulate metabolic processes. Some metabolic processes, such as the Krebs Cycle, flow in a ‘cycle,’ with each component of the cycle serving as a substrate for the succeeding reaction in the cycle. In eukaryotes, anabolic and catabolic pathways often occur independently of one another. Either they are divided physically by organelle compartmentalization or biochemically by the various enzymes and co-factors.

Types of metabolic pathways

There are mainly three types of metabolic pathways

  • the catabolic pathway (catabolism)
  • the anabolic pathway (anabolism), and 
  • the amphibolic pathway

Catabolic pathway

A catabolic pathway is a set of processes that result in a net release of energy in the form of a high-energy phosphate bond created between the energy carriers adenosine diphosphate (ADP) and guanosine diphosphate (GDP) to produce adenosine triphosphate (ATP) and guanosine triphosphate (GTP). As a result, the net reaction is thermodynamically advantageous, as the final products have lower free energy. A catabolic route is an exergonic mechanism that uses energy from carbs, lipids, and proteins to yield chemical energy in ATP, GTP, NADH, NADPH, FADH2, and other compounds. Carbon dioxide, water, and ammonia are the typical end products.

When combined with an endergonic anabolic process, the cell can manufacture new macromolecules from the original substrates. The phosphorylation of fructose-6-phosphate by the enzyme phosphofructokinase generates the intermediate fructose-1,6-bisphosphate, which is followed by the hydrolysis of ATP in the glycolysis pathway, is an example of a linked reaction. The chemical reaction that results within the metabolic pathway is very thermodynamically favorable and irreversible in the cell.

Anabolic Pathway

In contrast to catabolic mechanisms, anabolic pathways require energy to build macromolecules such as polypeptides, nucleic acids, proteins, polysaccharides, and lipids. Due to a positive Gibbs Free Energy (+G), the isolated anabolism reaction is unfavorable in a cell. As a result, a chemical energy input via coupling with an exergonic reaction is required. The thermodynamics of the process is affected by the coupled reaction of the catabolic route, which lowers the overall activation energy of an anabolic pathway and allows the reaction to occur. A non-spontaneous endergonic response, on the other hand, is an endergonic reaction.

An anabolic pathway is a biochemical pathway in which smaller molecules get combined to generate larger, more complex molecules. The reversed process of glycolysis, also known as gluconeogenesis, occurs in the liver and occasionally in the kidney to keep blood glucose levels stable and provide enough glucose to the brain and muscle cells. Although gluconeogenesis is similar to glycolysis’s reverse process, it has three separate enzymes that allow the pathway to happen on its own. 

Amphibolic pathway

The availability of or necessity for energy determines whether an amphibolic pathway is catabolic or anabolic. Adenosine triphosphate (ATP), which stores its energy in phosphoanhydride bonds, is the currency of energy in living cells. The energy is used to carry out biosynthesis, move around the cell, and control active transport. The glyoxylate cycle and citric acid cycle are two examples of amphibolic processes.

The energy-producing and energy-using pathways are included in both these chemical processes. The glyoxylate shunt pathway is a substitute for the tricarboxylic acid (TCA) cycle because it redirects the TCA pathway to prevent the complete oxidation of carbon molecules and maintain high-energy carbon future energy sources. This mechanism is found solely in plants and microorganisms, and it transpires even when glucose molecules are not present.

Metabolism in terms of thermodynamics

The rules of thermodynamics, which describe the flow of heat and work, must be followed by living creatures.

Metabolism and the first law of thermodynamics

Energy cannot be created or destroyed, according to the First Law of Thermodynamics, often known as the law of conservation of energy.

The energy in a closed system may shift from one form to another, but it remains constant. The first law of thermodynamics applied to metabolism states that heat transferred out of the body (Q) and work done by the body (W) removes internal energy, while food intake replaces it. (We may consider food intake as work done on the body.)

Metabolism and the second law of thermodynamics

The second law of thermodynamics asserts that the amount of entropy (disorder) in an isolated system cannot decrease. Despite the incredible complexity of living organisms, life is conceivable because all organisms are open systems that interchange matter and energy with their surroundings.

Living systems are not in equilibrium; instead, they are dissipative systems that maintain their high complexity by increasing the entropy of their surroundings. It is accomplished by connecting the spontaneous processes of catabolism with the non-spontaneous processes of anabolism in a cell’s metabolism.

Metabolism, in thermodynamic terms, preserves order by causing disorder.

Adenosine triphosphate (ATP) and Metabolism

Adenosine triphosphate (ATP) is an organic hydrotrope that supplies energy for various tasks in living cells, including muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis.

ATP is the “molecular unit of currency” for intracellular energy transfer, and it may be found in all forms of life.

When consumed, it converts to either adenosine diphosphate (ADP) or adenosine monophosphate (AMP).

Other ATP recycling systems allow the human body to recycle its own body weight in ATP every day.

It is also a coenzyme and a precursor of DNA and RNA. Adenine’s 9-nitrogen atom is linked to the 1′ carbon atom of a sugar (ribose), which is then linked to a triphosphate group at the sugar’s 5′ carbon atom. The adenine and sugar groups remain unchanged throughout the metabolic processes, whereas the triphosphate is converted to di- and monophosphate, yielding ADP and AMP, respectively.

 The three phosphoryl groups are denoted by the letters alpha (α), beta (β), and gamma (γ) (for the terminal phosphate). ATP is stable in aqueous solutions between pH 6.8 and 7.4 in the absence of catalysts. At higher pHs, it quickly hydrolyzes to ADP and phosphate. Living cells maintain a ten-order-of-magnitude difference in the ATP-to-ADP ratio, with ATP concentrations five times higher than ADP concentrations.

The majority of practical ATP mimics cannot be hydrolyzed like ATP; instead, they trap the enzyme in a configuration comparable to that of an ATP-bound state. Adenosine 5′-(-thiotriphosphate) is a common ATP counterpart in which a sulfur atom replaces one of the gamma-phosphate oxygens; this anion is hydrolyzed at a considerably slower rate than ATP and functions as an ATP inhibitor. In the crystallographic study, the bound vanadate ion is employed to model hydrolysis transition states.

Chemical reactions for energy transfer

When a chemical reaction occurs, energy often moves to or from the environment. Fireworks, for example, emit energy in the form of light, heat, and sound when they ignite. Breaking a bond requires energy. When bonds are established, energy is released.

An energy level diagram can depict these energy shifts.

Exothermic Reactions

The energy produced by creating new bonds is more than the energy required to break existing bonds in an exothermic reaction.

Energy is given off in an exothermic reaction, usually in the form of heat. A spike in temperature is generally the first sign of this. Exothermic reactions are a regular occurrence. For example, when water is added to anhydrous copper sulfate, it can be used in neutralization reactions such as neutralizing the acid with a metal or an alkali.

Endothermic Reactions

The energy required to break existing bonds is more than the energy released when new bonds are formed in an endothermic reaction.

In an endothermic reaction, energy is taken from the environment, usually as heat. A drop in temperature is a common indicator of this. Endothermic reactions are a rare occurrence. Some examples include photosynthesis, which absorbs light energy, and thermal decomposition, which requires a lot of heat to disintegrate calcium carbonate and dissolve some salts in water.

Reversible Reactions

When a reaction is exothermic in one direction, it becomes endothermic in the opposite. In each situation, the same quantity of energy is transferred. It is necessary to heat the forward reaction, and it is endothermic because it absorbs energy. On the other hand, the opposite reaction is exothermic, meaning it produces heat.


The following energy types are helpful to have a better understanding of Metabolism in Biology.

Potential energy or PE

Potential energy is stored energy that depends on the relative position of various parts of a system. For example, spring has more potential energy when compressed or stretched. The type of potential energy that exists within chemical bonds and is released when those bonds are broken is called chemical energy in the context of metabolism.

Kinetic energy or KE

Kinetic energy is the energy of motion, which is related to the movement of an object, a particle, or a group of particles. It is used by any moving item, such as a person walking, a baseball being thrown, a crumb falling off a table, or a charged particle in an electric field.

Gibbs free energy

The Gibbs free energy is a thermodynamic potential in thermodynamics that can be used to calculate the maximum amount of work a thermodynamically closed system can accomplish at constant temperature and pressure. It also creates a necessary environment for processes like chemical reactions to take place.

Activation energy

The lowest amount of extra energy required by a reactive molecule to turn into a product is known as activation energy. It’s also known as the smallest amount of energy required to activate or energize molecules or atoms for them to conduct a chemical reaction or transformation. Clearly, it is an important concept to understand if we want to understand about Metabolism in Biology.

Endergonic reactions

Free energy is required for an endergonic reaction to take place. Photosynthesis is an example of a biologically significant endergonic reaction. The redox reaction of oxidation of water to oxygen and carbon dioxide to glucose is carried out by photosynthetic organisms using sun photons.

Exergonic reactions

A reaction that releases free energy is known as an exergonic reaction. Because this reaction produces energy rather than consuming it, it can happen on its own, without the intervention of other forces. Exergonic reactions in chemistry are those in which the change in free energy is negative. The total quantity of energy accessible in a system is measured by free energy; negative changes indicate that energy has been released, while positive changes indicate that energy has been stored.

Special note: Metabolism of carbohydrates

Carbohydrate metabolism refers to all biochemical processes in living organisms to produce, break down, and interconvert carbohydrates. The metabolic pathways of carbohydrates include Glycolysis, Gluconeogenesis, Glycogenolysis, Glycogenesis, Pentose phosphate pathway, Fructose metabolism, and Galactose metabolism. 


10 Interesting facts about Metabolism in Biology

1. Estimated 10 million molecules of ATP are made from ADP every second in every cell.

2. Adult human at rest uses about 45 kg of ATP each day.

3. ATP present in the body is less than 1g

4. Dehydration can have a negative impact on your mood and energy levels

5. It is scientifically proven that men have a higher BMR than women. Lean muscle mass determines how efficiently the body burns energy, causing men to burn calories much faster than women.

6. Our metabolism is linked to the thyroid gland – the thyroid gland, located in the frontal portion of the neck, serves as a control panel for a variety of processes, including hormone regulation, protein production, and so on. It has complete control over metabolism because it monitors the rate at which food is converted into energy.

7. Ageing process reduces metabolism – according to research, our metabolic rate starts to decline in our mid-20s and drops by about 2% every ten years. As we get older, we lose muscle tissue and do less physical activity, both of which contribute to a sluggish BMR.

8. Body weight is determined by catabolism minus anabolism, or the amount of energy released into our bodies minus the amount of energy used up by our bodies.

9. Each of the 37 trillion cells that make up the human body is involved in metabolic reactions, and they all work together to keep our systems running smoothly.

10. 1 pound of body fat contains enough energy to power a 40-watt light bulb for over six minutes.