Carbon atoms can attach to each other and form straight or branched chains and ringed structures of organic compounds.

Today millions of different organic compounds are known.


Adenosine triphosphate

  1. 4 classes of macromolecules and their functions
  2. Isomers of biomolecules
  3. Functional groups
  4. Monomers and polymers

Structures of organic compounds form biological polymers and act as the backbones of different types of biological molecules. All 4 classes of macromolecules (biomolecules of life) are carbon based.

Some examples of important biological molecules include vitamins, enzymes, polyphenols, and plenty of others.

While the most of carbon containing molecules are organic compounds, there are a few exceptions.

Such compounds as carbides, carbonates, simple oxides of carbon (CO2), allotropes of carbon and cyanides are considered to be inorganic.

  • A carbide consists of carbon and a less electronegative element. Examples - calcium carbide(CaC2), silicon carbide (SiC), tungsten carbide (WC), and cementite (Fe3C),each used in key industrial applications.
  • A carbonate is a salt of carbonic acid (H2CO3). The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group (R-OCOO-R).
  • A cyanide is any compound that contains monovalent combining group CN (cyano group).

4 classes of macromolecules and their functions

Each of 4 major types of biomolecules is an important cell component and performs a wide variety of functions.

4 major classes of biological molecules include:

  1. Carbohydrates (monosaccharides, oligosaccharides, polysaccharides)
  2. Lipids (triglycerides, phospholipids, steroids)
  3. Proteins
  4. Nucleic Acids (DNA, RNA)

Besides their specific roles, carbohydrates, lipids, and proteins can serve as a source of energy, while nucleic acids are the most important macromolecules for the continuity of life.


Plants and algae produce millions of tons of carbohydrates each year through photosynthesis.

The major function of carbohydrates is to provide energy, particularly through glucose.

During cellular respiration, glucose is broken down and oxidized within cells. This process is used to synthesize ATP – the source of energy for cellular reactions. When the quantity of ATP are sufficient, simple carbohydrates are converted to carbohydrate polymers (glycogen or starch) or fat and stored.

Carbohydrates also have other important functions in all living organisms.

For example, they serve as building materials within the plant cells and perform cell-to-cell identification when attached to the external surfaces of the cytoplasmic membrane.


Lipids include a diverse group of biomolecules. They are insoluble in water and include mostly nonpolar carbon–carbon or carbon–hydrogen bonds.

This type of biomolecules is the most common energy-storing molecule for long-term use. Excess carbohydrates are converted into fat for later usage. 1 g of fat is equal to 38 kJ (versus 17 kJ for carbohydrates and proteins).

Lipids perform many different functions in a cell.

For example, plants and animals use fat as insulation from the environment. Lipids are an important part of all cellular membranes and many hormones.


Proteins are the most diverse group of 4 major types of biomolecules. Their macromolecular structures and functions vary greatly.

Each living cell contains thousands of proteins each performing a unique function. They can act as structural building blocks and functional molecules, involved in almost every task of the cell. All enzymes are proteins.

This class of macromolecules is all polymers of 20 amino acids.

Nucleic acids

Nucleic acids store and carry the hereditary information for the functioning of the cell.

The nucleic acids include two major classes of biological molecules, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), and consist of nucleotides.

Protein and nucleic acid enzymes catalyze biochemical reactions in both catabolism and anabolism of macromolecules.

Catabolism - the breakdown of biomolecules in living organisms.

Anabolism - the synthesis of complex biological macromolecules.

Isomers of biomolecules

One of the basic qualities of organic compounds - to possess a variety of properties, depends, in particular, on their ability to form isomers.

Isomers are macromolecules with the same molecular formula but different structures.

There are two main types of isomers:

  1. Structural isomers
  2. Stereoisomers

Structural isomers of macromolecules differ in the placement of their covalent bonds.

Examples of structural isomers is biological molecules of carbohydrates - glucose and fructose. Because of their different structures, they have different properties and are metabolized differently.

Stereoisomers have similar placements of their covalent bonds but differ in how these bonds are made to the surrounding atoms. Stereoisomers can be geometrical or optical.

Geometrical isomers can have different physical, but similar chemical properties.

Examples of geometrical isomers are glucose and galactose.

Optical isomers (enantiomers) usually have similar chemical and physical properties, but enzymes can distinguish one biomolecule from another.

Typically, one optical isomer is biologically active, and the other is inactive.

Isomerism of carbon based macromolecules

Functional groups of four types of biomolecules: carbohydrates, lipids, proteins, and nucleic acids

Functional groups of different types of biomolecules: carbs, lipids, proteins and nucleic acidsWhen one biological molecules react with other biomolecules, generally just the functional groups are involved. Therefore, each functional group of biomolecule has a specific role in cell metabolism.

Functional groups of different types of biomolecules are specific groups (moieties) of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.

These functional groups include such groups as hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, and phosphate groups.

Many biomolecules have more than one functional group.

Each functional group is able to modify the chemical properties of the macromolecules to which it bonds.

Hydroxyl group

Hydroxyl group is the functional group of alcohols. It adds polarity to biological molecules.

One example of alcohols is glycerol, also commonly known as glycerine. Glycerol is a polyalcohol and an important part of triglycerides and phospholipids.

Carbonyl group

Carbonyl groups of aldehydes and ketones generally also increase polarity and reactivity of biological molecules.

Biomolecules containing carbonyls tend to be volatile and stimulate senses with both pleasant and unpleasant odors.

Carboxyl group

A carboxyl group of carboxylic acids contains both a carbonyl group and a hydroxyl group, bonded to the same carbon atom.

Biological macromolecules containing carboxyl groups are often highly polar and reactive. Common biomolecules, containing the carboxyl functional groups, are fatty acids and amino acids.

Amino group

Amino groups also increase polarity and reactivity of a biological macromolecule. They readily form hydrogen bonds with other polar molecules and water. Amines are weakly basic.

Amino and carboxyl groups of amino acids react to each other to form peptide bonds of proteins.

Phosphate group

Phosphate groups are highly acidic and reactive. Phosphates are essential to the metabolic processes of photosynthesis and cellular respiration.

A transfer of a phosphate group from one molecule to another delivers energy to chemical reactions.

Sulfhydryl group

The sulfhydryl (–SH) group is essential to protein stabilization.  

Amino acids with –SH groups form bonds called disulfide bridges (S–S bonds) that help protein molecules to take on and maintain a specific shape.

Monomers and polymers of 4 types of biomolecules

Most biological macromolecules are made from single subunits, or building blocks, called monomers.

The monomers combine with each other using covalent bonds to form larger macromolecules known as polymers.

Polymers can be divided into two groups:

  1. natural polymers (different types of biomolecules),
  2. synthetic polymers.

Two main type of reactions involved in synthesis and degradation of biological molecules are hydrolysis and dehydration.

Polymers are broken down into monomers in a process known as hydrolysis, which means “to split water,” a reaction in which a water molecule is used during the breakdown.

Dehydration reactions involve the formation of new bonds, requiring energy, while hydrolysis reactions break bonds and release energy.

In a dehydration reaction, the hydrogen of one monomer combines with the hydroxyl group of another monomer, releasing a molecule of water and forming a polymer.

In a process of hydrolysis, a water molecule is used to brake down a macromolecule into monomers.

Dehydration and hydrolysis reactions are catalyzed by specific enzymes made up of proteins.

4 types of biomolecules (natural polymers) are formed from smaller building blocks called monomers.

4 biological molecules and their monomers

 Natural Polymer



Carbohydrates Monosaccharides energy storage, component of plant cell walls, outer skeleton of insects and related groups
Proteins Amino acids catalysis, support and structure
Nucleic acids  (DNA, RNA) Nucleotides encoding of hereditary information



energy storage, component of cell membranes, message transmission (hormones), pigments in photosynthesis


References & further readings:

Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. The Molecular Composition of Cells.