Introduction to Macromolecules - Carbohydrate, Protein, Lipids and Nucelic Acid

The Macromolecule is a large complex molecule, such as nucleic acid, proteins, carbohydrates, and lipids, which relatively large have larger molecular weight. 

It is a very large molecule commonly created by the polymerization of smaller subunits. The other name for macromolecule is a polymer, which is derived from the Greek word - poly means many units. It is a product of many smaller molecular units.
The large biological molecules do a wide range of jobs in an organism. Some carbohydrates store fuel for future needs, and some lipids are key structural components of the cell membrane. Likewise, the nucleic acid store and transfer hereditary information, which provide information for making proteins. The proteins provide a structural hold but many of them carry a definite job in a cell such as catalyzing metabolic reactions or accepting and transmitting signals.

Polymers and monomers:

The polymers are large biological molecules and the monomers are long chains made up of repeating molecular subunits.
Carbohydrates, proteins, and nucleic acids are often found as long polymers in nature. Because of nature and their large size, they are classified as macromolecules. Lipids are not usually polymers and are smaller than the other three i.e. carbohydrates, proteins, and nucleic acid.


Some of the carbohydrates are in the form of sugars. These provide the potato a ready fuel source for the person eating it. The potatoes carbohydrate is in the form of fiber, including cellulose polymers that give structure to the potato cell walls.
Carbohydrates are biological molecules made of carbon, hydrogen, and oxygen in a ratio of approximately one carbon atom to one water molecule. This composition gives carbohydrates their name: it is made up of carbon plus water.
Carbohydrates chains come in different lengths and biologically important carbohydrates belonging to three categories- monosaccharide, disaccharides, and polysaccharides. 

  • Monosaccharide:

  • Mono means one and sacchar means sugar called as simple sugars, with most common of which is glucose. The number of carbons ranges from three to seven. 

    If the sugar has an aldehyde group it is known as aldose and if it has a ketone group it is known as a ketose. Depending on the number of carbons in the sugar, they also may be known as trioses (3 carbons), pentoses (5 carbons) etc.
    Monosaccharide can exist as a linear chain or as ring-shaped molecules. 

    Common monosaccharide:

    Glucose is one of the common monosaccharide and an important source of energy. During cellular respiration, the energy is released from glucose and that energy is used to help make the ATP (adenosine triphosphate). 

    Plant synthesizes glucose using carbon dioxide, and water, and glucose, in turn, is used for energy requirements for the plant.
    Galactose( a milk sugar) and fructose ( found in fruit) are also one of the common monosaccharides. All have the same chemical formula but they differ structurally. Galactose and Glucose are aldoses, and fructose is a ketose.

  • Disaccharide:

  • Di means two. It is formed when two monosaccharides undergo a dehydration reaction or dehydration synthesis. During the course of action, the hydroxyl group of single monosaccharide unites with the hydrogen of an additional monosaccharide, releasing a molecule of water and forming a covalent bond. A covalent bond formed between a carbohydrate molecule and another molecule is known as glycosidic bond.

    Common disaccharide:

    Common disaccharides include lactose, maltose, and sucrose. Lactose is a disaccharide consisting of the monomers galactose and glucose. It is found naturally in milk. Maltose is formed by a dehydration reaction between two glucose molecules. The most common disaccharide is sucrose or table sugar, which is poised of the monomers glucose and fructose.

  • Polysaccharide:

  • Poly means many. A long chain of monosaccharide linked by glycosidic bonds is known as a polysaccharide. This chain may be branched or unbranched and it contains different types of monosaccharide. Examples of the polysaccharide are starch, glycogen, cellulose, and chitin.

    Plants are able to synthesize glucose, and excess glucose is stored as starch in different plant parts, including seeds and roots. Starch is the stored form of sugars in plant life and is made up of glucose monomers.
    The starch that is stored in the seeds gives food for the origin or embryo as it germinates while the starch that is consumed by humans is broken down by the enzymes into smaller molecules, such as maltose and glucose. The cells can then absorb glucose.

    Common polysaccharide:

    Glycogen is one of the common polysaccharide and it is the storage form of glucose in humans and vertebrates. Glycogen is made up of monomers of glucose. It is the animal equivalent of starch and is a highly branched molecule stored in liver and muscle cells.

    Cellulose is also a polysaccharide and is the most abundant natural biopolymer. The cell wall of plants is made up of cellulose and it provides the structural support to the cell. 


    Lipid molecules:

    Oils and fats, which may be saturated or unsaturated can be healthy but also serve important functions for plants and animals. Lipids do not form polymers. The unifying characteristic of lipids is that they all have small or no likeness for water because it consists of mostly hydrocarbons. Lipids are highly diverse in function and form. Fats store more amount of energy.

    Though the fats are not strictly polymers they are large molecules assembled from a smaller molecule by dehydration process. The fat is constructed from two smaller molecules namely- glycerol and fatty acids.

    Glycerol is three-carbon alcohol with a hydroxyl group close to each carbon. Likewise, fatty acid consists of a carboxyl group attached to a long skeleton of carbon. 

    In a fat, three fatty acids are joined to glycerol by an ester linkage, creating triacylglycerol, or triglyceride. The three fatty acids may be the same or different. The fatty acids may vary in length, number, and locations of the double bonds.

    Unsaturated fatty acids are that the fatty acids have one or more carbon double bonds formed by the removal of hydrogen atoms from the carbon skeleton.

    The saturated fatty acid is a straight chain, but unsaturated acid has a kink wherever there is a double bond. The saturated fatty acids are solid at room temperature and most animal fats are saturated.

    Plants and fish fats are liquid at room temperature and they are known as oils. The kink caused by the double bonds prevents the molecules from packing tightly enough to solidify at room temperature.


    Proteins report for more than 50% of the dry mass for the most part cells. Protein functions include structural support, transport, storage, cellular signaling, movement, and defense against foreign substances.

    The protein enzymes function as a catalyst in cells, adaptable metabolism by selectively accelerating chemical reactions without being consumed.

    Humans have thousands of different proteins, each with specific function and structure. Proteins are the most structurally complex molecules known. Each type of protein has a complex 3- dimensional shapes. 

    All protein polymers are formed from the same set of 20 amino acid monomers. Amino acids are the monomers from which proteins are formed. Polymers of proteins are called polypeptides. A protein consists of one or more polypeptides folded and coiled into specific three-dimensional shapes.
    Amino acids are organic molecules with both amino and carboxyl groups. At the center of the amino acid is an asymmetric carbon atom called the alpha carbon. Four components are attached to the alpha carbon namely- a hydrogen atom, a carboxyl group, an amino acid, and side chain i.e. a variable R group. 

    Nucleic acids:

    The amino acid sequence of a polypeptide is intended by a unit of inheritance known as a gene. A gene subsists of DNA, a polymer known as a nucleic acid. There are two types of nucleic acid namely RNA and DNA.

    RNA and DNA are the molecules that allow living organisms to reproduce their complex component from generation to generation.

    DNA provides direction for its own replication. It also directs the RNA synthesis and through RNA it controls the protein synthesis. The organisms inherit DNA from their parents.

    Each DNA molecule is very long and it consists of hundreds to thousands of genes. The DNA is copied before the cell reproduces itself by dividing. DNA encodes the information that programs all the cells activities. Proteins are only responsible for implementing the information contained in the DNA as it is not directly involved in the day-to-day operations of the cell.
    The Nucleic acid is polymers made of nucleotide monomers. Each nucleotide consists of three parts namely- nitrogenous base, a pentose sugar, and the phosphate group.

    The nitrogen base is rings of carbon and nitrogen that comes in two types- purines and pyrimidines. Pyrimidines have the six-membered ring. They are further divided into three different types they are- cytosine, thymine, and uracil.
    Purines contain a six-membered ring connected to a five-membered ring. The two purines are adenine and guanine.
    An RNA molecule is a single polynucleotide chain. DNA has two polynucleotide strands that spiral around an imaginary axis to form a double helix.

    Double helix was first proposed by James Watson and Francis Crick as the structure of DNA.
    Genes i.e. DNA and their proteins document or function the genetic background of an organism, as DNA molecule is passed from parent to offspring/ siblings have greater similarity in their DNA.