Biomolecules Class 11 Chapter Notes

What are Biomolecules? Analysis and Classification

Biomolecules are all the carbon-based organic compounds produced by living organisms. They range from small molecules (micromolecules) like amino acids and sugars to large polymers (macromolecules) like proteins and DNA. To analyze the chemical composition of living tissue, scientists use a method involving trichloroacetic acid (Cl₃CCOOH). When a living tissue (like a piece of liver or a vegetable) is ground with this acid, it forms a thick slurry. This slurry is then filtered. The filtrate, or the part that passes through the filter, is called the acid-soluble pool. It contains thousands of organic compounds, generally with molecular weights ranging from 18 to 800 daltons. The retentate, or the material left on the filter, is called the acid-insoluble fraction. This fraction contains the macromolecules: proteins, nucleic acids, polysaccharides, and lipids. Although lipids have a molecular weight not exceeding 800 Da, they are found in the acid-insoluble fraction because they are components of cell membranes, which form insoluble vesicles upon cell disruption.

Key Terms in Biomolecules

Biomolecules

Organic molecules present in living organisms, such as carbohydrates, proteins, lipids, and nucleic acids.

Amino Acids

Organic compounds containing both an amino group (-NH₂) and a carboxyl group (-COOH), which are the building blocks (monomers) of proteins.

Zwitterion

A molecule (like an amino acid) that has both a positive and a negative charge, but a net charge of zero. This occurs at a specific pH called the isoelectric point.

Peptide Bond

A covalent chemical bond (-CO-NH-) formed between two amino acid molecules when the carboxyl group of one reacts with the amino group of the other, releasing a molecule of water.

Nucleotide

The monomer of nucleic acids (DNA and RNA), consisting of a pentose sugar, a phosphate group, and a nitrogenous base.

Nucleoside

A component of a nucleotide, consisting of a pentose sugar and a nitrogenous base (without the phosphate group).

Enzyme

A biological catalyst, almost always a protein, that speeds up the rate of a specific biochemical reaction without being consumed in the process.

Active Site

A specific region on an enzyme's surface where the substrate binds and the chemical reaction occurs.

Metabolism

The sum of all chemical reactions occurring within a living organism. It comprises anabolism (synthesis) and catabolism (breakdown).

Cofactor

A non-protein chemical compound or metallic ion required for an enzyme's catalytic activity. It can be a prosthetic group, co-enzyme, or metal ion.

Must-Remember Concepts

• Primary vs. Secondary Metabolites: Primary metabolites (e.g., amino acids, sugars) have identifiable functions and are essential for growth. Secondary metabolites (e.g., alkaloids, toxins, pigments) are not directly involved in growth but have important ecological roles.
• Protein Structure: Proteins have four levels of structure: Primary (sequence of amino acids), Secondary (α-helix, β-pleated sheet), Tertiary (3D folding of the polypeptide), and Quaternary (assembly of multiple polypeptide subunits).
• Glycosidic Bond: This is the covalent bond that joins a carbohydrate (sugar) molecule to another group. It is formed between the hemiacetal or hemiketal group of a saccharide.
• Ester Bond: In lipids, this bond connects fatty acids to a glycerol molecule. In nucleic acids, a phosphodiester bond links nucleotides together.
• Denaturation: The loss of the tertiary and secondary structure of a protein or nucleic acid due to external stress like heat or extreme pH. This results in the loss of biological activity.
• Watson-Crick Model of DNA: Key features are: two polynucleotide chains in a right-handed double helix, antiparallel strands (5'→3' and 3'→5'), and specific base pairing (A with T via 2 H-bonds, G with C via 3 H-bonds).
• Enzyme Inhibition: Competitive inhibitors resemble the substrate and compete for the active site. Non-competitive inhibitors bind to a different site (allosteric site) and change the enzyme's shape.
• Turnover Number: The number of substrate molecules converted to product by one molecule of an enzyme per unit time when the enzyme is fully saturated with substrate.

Comparison of Key Biomolecules

Mechanism of Enzyme Action

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Examples of Biomolecules and their Functions

• {"item":"Carbohydrate: Glycogen","description":"A branched homopolysaccharide of glucose. It serves as the main form of energy storage in animals and fungi. Stored primarily in the liver and muscle cells."}
• {"item":"Protein: Collagen","description":"The most abundant protein in the animal kingdom. It is a fibrous, structural protein that provides strength and support to connective tissues like skin, bones, tendons, and cartilage."}
• {"item":"Lipid: Cholesterol","description":"A sterol that is a vital component of animal cell membranes, regulating membrane fluidity. It is also a precursor for steroid hormones (like testosterone, estrogen) and bile acids."}

Exam Traps & Scoring Tips

Examiners often test your understanding of nuances.

• Structures are Key: Practice drawing the basic structure of an amino acid (with R-group), α-D-glucose, and a nucleotide. Even simplified diagrams can fetch marks.
• Graphs of Enzyme Activity: Be precise when labeling axes (e.g., 'Reaction Rate' on y-axis, 'Temperature' or 'pH' on x-axis). Remember to draw the bell-shaped curve and mark the 'Optimum' point.
• Nucleoside vs. Nucleotide: This is a classic confusion point. Remember Tide = Triple (Sugar + Base + PhosphaTe). Side = Simpler (Sugar + Base).
• Why Lipids are Macromolecules (in the acid-insoluble fraction): This is a frequently asked conceptual question. Explain that their hydrophobic nature causes them to form vesicles with membrane fragments, which are large and get trapped in the filter, not because of their individual molecular weight.

Practice Questions with Solutions

• What type of bond is formed between two monosaccharides to form a disaccharide? A glycosidic bond.
• Name the most abundant protein in the animal world and the most abundant protein in the whole biosphere. Animal world: Collagen. Whole biosphere: RuBisCO (Ribulose-1,5-bisphosphate carboxylase oxygenase).
• What is the difference between a prosthetic group and a co-enzyme? Both are cofactors. A prosthetic group is an organic molecule tightly bound to the enzyme (e.g., heme in peroxidase). A co-enzyme is also organic but is only transiently bound during catalysis (e.g., NAD, FAD).
• What makes a fatty acid 'unsaturated'? The presence of one or more double bonds (C=C) in its hydrocarbon chain.

Frequently Asked Questions on Biomolecules

Frequently Asked Questions