Primary and Secondary Metabolites: Understanding metabolism is key to understanding the behaviour of all living organisms since metabolism is integral to health and proper functioning. Metabolites are intermediate products of metabolic reactions catalyzed by enzymes found within cells. All the carbon compounds that are obtained from living tissues are called biomolecules. All metabolites are biomolecules and are of two types: Primary and Secondary metabolites.

Thus, we can say that metabolites are micromolecules necessary for cellular functions like energy, stimulation, inhibition, and protection. These metabolites are divided into primary and secondary metabolites based on their role in cellular activities. In this article, we will explore these two types of metabolites, their functions and their importance in an organism’s body.

Metabolites: Definition

The metabolism is the total of all the biochemical activities carried out by an organism. Metabolites can be defined as the intermediates and end products of metabolism. These are usually confined to smaller molecules.

1. Metabolites are intermediate and also end products that are produced during or at the end of cellular metabolism.
2. Ethanol, glutamic acid, aspartic acid, acetic acid, pigments and antibiotics are a few examples of these.
3. Metabolites can also be artificially produced for industrial and pharmaceutical purposes such as developing vaccines, antibiotics, isolating chemicals for organic synthesis and getting amino acids.
4. Metabolites are produced by animals, plants and microbes.
5. Metabolites have numerous functions, such as fuel,  building blocks, signalling, stimulatory and inhibitory effects on enzymes, catalytic activity during metabolism, defense mechanism, and interactions with other organisms like microbes, plants and other higher animals.

As said earlier, metabolites are produced by animals, plants and microbes. In this article, we will learn about two different types of metabolites produced by these organisms.

Primary and Secondary Metabolites

Fig: Types of Metabolites

Plants are solar-powered biochemical and biosynthetic factories that are capable of generating a diverse group of organic compounds. There are mainly two types of metabolites.
1. Primary Metabolites
2. Secondary Metabolites

Primary Metabolites

Primary metabolites are the compounds that are directly involved in the metabolic activities of an organism essential for its growth, development, and reproduction.

Characteristics of Primary Metabolites:
1. Primary metabolites are produced as a result of the growth mechanism during the growth phase of the organism. The growth phase associated with the synthesis of primary metabolites is called trophophase.
2. Primary metabolites such as amino acids are common in all types of organisms, whereas few of these are restricted to particular cells or organisms.
3. The production of primary metabolites usually happens at a high rate in the body. This is because these metabolites are constantly required for several essential processes of the body.
4. Primary metabolites are classified into two groups, primary essential metabolites and primary metabolic end products.
5. Examples of primary metabolites involve proteins, carbohydrates, lipids, vitamins, lactic acid, enzymes, ethanol, butanol, etc.

Functions of Primary Metabolites
As we know, primary metabolites are directly involved in essential physiological processes in the body, they are necessary for many important functions. Some of these are mentioned below:

1. Primary metabolites like carbohydrates and fats provide energy to perform physical activities.
2. Proteins are important primary metabolites that help in the growth, repair and maintenance of body tissues and cells.
3. Various enzymes help to catalyze all biochemical reactions in the body.
4. Nucleic acids are other essential primary metabolites necessary for the transmission of genetic information to further generations.
5. Lipids are structural building materials of cells and organelles. It also provides twice the energy provided by carbohydrates or proteins.
6. An organism cannot survive in the absence of primary metabolites.

Secondary Metabolites

Secondary metabolites are the organic compounds produced by various organisms that are not directly involved in the growth, development, or reproduction of the organism but are essential in ecological and other activities. Secondary metabolites are also called specialized metabolites or natural products. These are mostly derived from the primary metabolites.

Characteristics of Secondary Metabolites
1. As secondary metabolites are not involved in the growth and development of an organism, the lack of these compounds has little or no effects on the survival of the organism. However, some mild effects might be observed if these are not available for a long duration.
2. Secondary metabolites are formed during the stationary phase of growth in most organisms, this phase of growth is termed idiophase.
3. Some secondary metabolites are specific to a species and are only found in them, though the horizontal transfer of these metabolites across species has played an important role in the evolution of certain organisms.
4. Secondary metabolites are found in plant cells and are absent in animal cells.
5. Since, secondary metabolites are specific to species different secondary metabolites are required in various procedures.
6. Most secondary metabolites act as a defence mechanism against various harmful foreign molecules or agents.
7. Steroids, essential oils, phenolics, alkaloids, pigments, antibiotics, etc., are some examples of secondary metabolites.

Functions of Secondary Metabolites
1. These molecules act as competitive weapons against other harmful organisms including animals, plants, insects, and microorganisms.
2. There are certain secondary metabolites that help plants to create mechanisms of aluminum resistance as it is highly toxic to them.
3. Secondary metabolites like nicotine or caffeine are used as stimulants.
4. Some alkaloids have medicinal properties and are used as painkillers or effective remedies for many disorders.

Important Primary and Secondary Metabolites

We have learned some examples of primary and secondary metabolites in this article. Let us discuss a few of these in further detail.

Examples of Primary Metabolites

1. Carbohydrates: A carbohydrate is a metabolite composed of carbon, hydrogen, and oxygen. These give structure to organisms like cellulose in plants and peptidoglycan in bacteria. Furthermore, these are also compounds that are oxidized to get energy for the growth and functioning of the organism.
2. Proteins: Proteins are called bodybuilding nutrients as these are needed by the body for growth and maintenance. It helps repair and build the body tissues, it permits metabolic activities to take place and coordinates other bodily functions.
3. Hormones: Hormones are primary metabolites produced by the endocrine system. These molecules transmit messages to various parts of the body and help regulate all processes in the body such as hunger, blood pressure, sexual desire, etc. These are fundamental to all the systems of the body.
4. Nucleic acids: Nucleic acids are found as DNA or RNA in cells. These play an important role in transmission of genetic information from one generation to the next. They are also responsible for protein synthesis in a cell.
5. Lipids: Lipids function as energy reservoirs, regulate hormones, transmission of nerve impulses, cushion important organs, and also transport fat-soluble nutrients.
6. Enzymes: Enzymes are proteins that are formed in the body of different organisms as primary metabolites. These are involved in almost all types of metabolic cycles from cellular respiration to internal digestion, absorption, etc. A few examples of enzymes are lipases, amylases, proteases, etc.

Fig: Examples of Primary metabolites

Examples of Secondary Metabolites

Alkaloids: Alkaloids are a large group of nitrogen-containing compounds. These are derived from primary metabolites like amino acids. Alkaloids are stored in plant organs like leaves or fruits and are transferred to animals when they consume those plant parts.
1. Many alkaloids are toxic, mainly to mammals, and act as strong nerve poisons, enzyme inhibitors, or membrane transport inhibitors.
2. Alkaloids are also bitter or bad-tasting helps animals to avoid eating such plants.
3. Morphine, codeine and cocaine are alkaloids that work on the nervous system and are used as painkillers. Atropine is another example of medicinal alkaloids used in anaesthesia and ophthalmology.
4. Alkaloids like vincristine and vinblastine are inhibitors of cell division and are used to treat cancers of the blood and lymphatic systems.
5. Quinine is a poisonous alkaloid used to kill the Plasmodium parasite that causes malaria.
6. Other alkaloids like caffeine and nicotine are used as stimulants. Nicotine preparations are also used as an aid in smoking cessation and as insecticide. Though the use of nicotine as an insecticide is not advised due to its toxicity to humans.

Terpenoids: Plant terpenoids are used for their aromatic qualities. These provide a characteristic smell to plants and play an important role in herbal remedies.
1. Monoterpenes like menthol present in the leaves give characteristic taste and fragrance and their aromatic oils also have insect-repellent properties.
2. Taxol is a diterpene that is a strong inhibitor of cell division in animals and used as powerful chemotherapeutic treatment for people with solid tumours like ovarian cancer patients.
3. Triterpenoids contain plant steroids or act as plant hormones to protect plants from insect attack.
4. Tetraterpenoids include important pigments such as beta-carotene, which is a precursor of vitamin A, and lycopene that imparts red colour in tomatoes. It aids in seed dispersal.
5. The polyterpenes like rubber present in the latex of rubber trees probably aids in wound healing in the plant, and is used in the manufacture of tires and other products.

Phenolic Compounds
Phenolic compounds are characterized by the presence of one or more aromatic rings having a hydroxyl functional group. Many of these are synthesized from the amino acid phenylalanine.
1. Simple phenolic compounds like salicylic acid are valuable in defence against fungal pathogens. Aspirin which is a derivative of salicylic acid is used to reduce inflammation, pain, and fever in humans.
2. Other phenolic compounds, named isoflavones, are rapidly produced in plants of the legume family when they are attacked by bacterial or fungal pathogens, and they have strong antimicrobial activity.
3. Lignin, a complex phenolic macromolecule, is a very significant structural molecule in all woody plants, enabling them to achieve height, girth, and longevity. It is also valuable for plant defense.
4. Anthocyanins and anthocyanidins are phenolic pigments that function as attractants, they give pink and purple colours to flowers and fruits. This pigmentation helps to attract insects and other animals and perform pollination and fruit dispersal.

Differences Between Primary and Secondary Metabolites

Primary Metabolites	Secondary Metabolites
Primary metabolites perform essential physiological functions in the body. These are directly involved in growth, development, and reproduction.	Secondary metabolites are derivatives of primary metabolites. These play a role in ecological functions.
They are generated during the organism’s growth phase called trophophase.	They are generated during the stationary phase, this phase is called idiophase.
They are produced in large quantities and can be easily extracted.	They are produced in small quantities and difficult to extract.
These are non-poisonous in nature.	Some of these are poisonous in nature.
Generally these are inexpensive to produce. Example: Starch from potato	These are expensive to produce. Example: Quinine from cinchona bark.
These are part of the basic molecular structure of an organism.	These are not a part of the basic molecular structure of an organism.
Primary metabolites do not directly participate in defense reactions.	Secondary metabolites participate in defense reactions.

Summary

Metabolism causes all the chemical transformations happening in the cells of living organisms, and these transformations are necessary for an organism to stay alive, develop and reproduce. End products of these metabolic processes and intermediates produced during them are called metabolites. Metabolites that have identifiable functions are called primary metabolites. They play known key roles in normal physiological processes.

There are certain metabolites that are not directly involved in physiological processes but play important roles in other functions in certain living organisms. Such metabolites are called secondary metabolites. All the primary metabolites are found in animal cells, whereas secondary metabolites are not found in animal cells.

Frequently Asked Questions (FAQs) on Primary and Secondary Metabolites

Let’s look at some of the commonly asked questions about primary and secondary metabolites:

Q.1. What are the main differences between primary and secondary metabolites?
Ans: The main difference between primary and secondary metabolites is that primary metabolites are directly involved in the physiological processes of an organism, whereas secondary metabolites are not. Primary metabolites are called central metabolites and are generated during trophophase. And secondary metabolites are formed during idiophase and are also called specialized metabolites.

Q.2. Is rubber a secondary metabolite?
Ans: Yes, rubber is an example of a secondary metabolite. Secondary metabolites are the ones that do not have any identifiable function in the host organism. Rubber does not have any known function in plants and thus can be called a secondary metabolite.

Q.3. What are the examples of primary metabolites?
Ans: Primary metabolites are biomolecules that are directly involved in the metabolic activities of an organism essential for its growth, development, and reproduction. Some examples of primary metabolites include carbohydrates, proteins, lipids, hormones, nucleic acids, etc.

Q.4. What are secondary metabolites? Explain with examples?
Ans: Plants, fungi and some other microorganisms produce certain biomolecules during metabolism that are not directly involved in physiological processes but are important for the organisms. These metabolites are called secondary metabolites. Examples of secondary metabolites are pigments, antibiotics, steroids, essential oils, phenolics, alkaloids, etc.

Q.5. What is the major function of primary and secondary metabolites?
Ans: Primary metabolites are the organic compounds produced by various organisms that are directly involved in the growth, development, or reproduction of the organism. An organism cannot live in the absence of any one of primary metabolites. Secondary metabolites are essential in ecological and other activities though these are not required for living. Secondary metabolites act as a defence mechanism against various harmful foreign molecules or agents.

Q.6. What are the examples of Primary and Secondary Metabolites? 
Ans: Examples of primary metabolites are Carbohydrates, Proteins, Hormones, Nucleic acids, Lipids, Enzymes etc. Examples of secondary metabolites are Alkaloids, Terpenoids, Phenolic Compounds etc. 

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