Living cells contain water, inorganic substances and many organic compounds. The organic molecules found in living organisms are called biomolecules. The major groups studied in this chapter are carbohydrates, proteins, lipids and nucleic acids. We will also study enzymes, which speed up reactions in living cells.
Table of Contents
What Are Biomolecules?
Biomolecules differ in their structure and function. Some are small, such as glucose and amino acids. Others are large, such as proteins, polysaccharides and nucleic acids.
| Biomolecule group | Main role | Examples |
|---|---|---|
| Carbohydrates | Energy supply, storage and support | Glucose, starch, glycogen, cellulose |
| Proteins | Structure, transport and catalysis | Collagen, haemoglobin, enzymes |
| Lipids | Energy storage and cell membranes | Fats, oils, phospholipids |
| Nucleic acids | Storage and use of genetic information | DNA, RNA |
Carbohydrates
Carbohydrates include simple sugars and longer chains of sugars.
- Monosaccharides are single sugar units, such as glucose and fructose.
- Disaccharides contain two sugar units, such as sucrose.
- Polysaccharides are long chains of sugar units.
Three important polysaccharides are:
- Starch: Stores food in plants.
- Glycogen: Stores glucose in animals.
- Cellulose: Provides structural support in plant cell walls.
Chitin is another structural polysaccharide. It occurs in the exoskeleton of arthropods.
Proteins and Amino Acids
Proteins are chains of amino acids joined together. The bond joining two amino acids is called a peptide bond. The sequence and arrangement of amino acids help determine what a protein does.
Proteins have many functions. For example, collagen provides structural support, haemoglobin helps transport oxygen, and many enzymes catalyse reactions.
Levels of Protein Structure
- Primary structure: The order of amino acids in a protein chain.
- Secondary structure: Local folding of parts of the chain, such as a helix or sheet.
- Tertiary structure: The overall three-dimensional folding of one polypeptide chain.
- Quaternary structure: The arrangement of two or more polypeptide chains in one protein.
A protein’s shape is closely linked to its function. A major change in that shape can reduce or stop its activity.
Lipids
Lipids are generally insoluble in water. Many fats and oils contain glycerol and fatty acids. They are useful for storing energy.
Phospholipids are important components of cell membranes. Their arrangement helps form the membrane boundary around a cell and its organelles.
Fatty acids may be saturated, with no carbon–carbon double bond, or unsaturated, with one or more such double bonds.
Important distinction: Lipids are studied alongside biomacromolecules, but many lipids are not long-chain polymers in the way proteins, polysaccharides and nucleic acids are.
Nucleic Acids
DNA and RNA are nucleic acids. They are chains of units called nucleotides.
Each nucleotide contains:
- A nitrogenous base.
- A pentose sugar.
- A phosphate group.
DNA contains deoxyribose sugar and stores genetic information. RNA contains ribose sugar and has important roles in using that information to make proteins.
Enzymes
An enzyme is a biological catalyst: it increases the rate of a reaction without being used up in the process. Most enzymes are proteins, although some RNA molecules called ribozymes can also act as catalysts.
The substance on which an enzyme acts is its substrate. The substrate binds at a particular region of the enzyme called the active site.
Simple sequence of enzyme action:
- The substrate binds to the enzyme’s active site.
- An enzyme–substrate complex forms.
- The reaction takes place.
- The products are released, and the enzyme can act again.
Enzymes make reactions easier by lowering the activation energy needed to begin them.
Factors Affecting Enzyme Activity
- Temperature: Activity rises towards an optimum temperature. Excessive heat can change the enzyme’s shape and reduce its activity.
- pH: Each enzyme works best within a particular pH range.
- Substrate concentration: Increasing substrate may increase the reaction rate until the available enzyme molecules are occupied.
- Inhibitors: Certain substances reduce enzyme activity. A competitive inhibitor competes with the substrate for the active site.
Some enzymes also need a non-protein component called a cofactor to function. Cofactors may be metal ions or organic molecules such as coenzymes.
Quick Revision Table
| Term | Meaning |
|---|---|
| Amino acid | Building block of a protein |
| Peptide bond | Bond joining amino acids |
| Nucleotide | Building block of DNA and RNA |
| Polysaccharide | Long chain of sugar units |
| Active site | Region of an enzyme where its substrate binds |
| Activation energy | Energy barrier that must be overcome for a reaction to proceed |
| Cofactor | Non-protein component needed by some enzymes |
Check Your Understanding
- Which polysaccharide stores food in plants?
- What are the building blocks of proteins?
- Name the three components of a nucleotide.
- What is an enzyme’s active site?
- Why can very high temperature stop an enzyme from working?
Answers: (1) Starch. (2) Amino acids. (3) Nitrogenous base, pentose sugar and phosphate group. (4) The region where the substrate binds. (5) Heat can alter the enzyme’s shape, including its active site.
For all chapters, visit the CBSE Class 11 Biology – Complete Study Guide.
Frequently Asked Questions
What are biomolecules?
They are chemical compounds found in living organisms, including carbohydrates, proteins, lipids and nucleic acids.
What are primary metabolites?
They are molecules directly involved in essential growth and metabolism, such as sugars and amino acids.
What is a monosaccharide?
It is a simple sugar unit, such as glucose.
What is a polysaccharide?
It is a large carbohydrate made of many sugar units, such as starch or cellulose.
What is the main role of starch in plants?
Starch stores carbohydrate for later use.
What are proteins made of?
Proteins are chains of amino acids folded into functional shapes.
What is a peptide bond?
It is the covalent bond joining two amino acids in a protein chain.
What are lipids?
Lipids are diverse water-insoluble or poorly water-soluble biomolecules, including fats and phospholipids.
Why are phospholipids important?
They are major components of cell membranes.
What is a nucleotide?
It is a unit of nucleic acid containing a nitrogenous base, pentose sugar and phosphate group.
What is the difference between DNA and RNA?
DNA generally contains deoxyribose and thymine; RNA contains ribose and uracil.
What are enzymes?
They are biological catalysts that speed up reactions without being used up in the overall process.
What is an enzyme’s active site?
It is the region where the substrate binds and the reaction is catalysed.
What factors affect enzyme activity?
Temperature, pH, substrate concentration and inhibitors can affect activity.
What is enzyme specificity?
It means an enzyme acts on a particular substrate or a limited group of related substrates.