Biochemistry: Biomolecules, Enzymes, Metabolism and Bioenergetics
Biochemistry is the chemistry of living things. It studies the molecules that make up cells, the reactions that keep an organism alive, and the energy transformations that drive those reactions. It explains how food is converted into energy, how muscles contract, how the genetic code is stored and copied, and how the body controls its own chemistry through enzymes and hormones. It stands at the meeting point of chemistry and biology.
For candidates preparing for APSC, ADRE, DHS, DME, SSC, Railways, NEET and other examinations in Assam, biochemistry is a high-yield topic because it appears in both the general science paper and the biology paper. Questions range from the classification of carbohydrates and proteins to the functions of vitamins and the steps of respiration. A clear conceptual understanding, rather than isolated memorisation, brings the best results. This article gives a complete deep-study revision of every important biochemical idea.
Basic Terms
- Carbohydrate: a polyhydroxy aldehyde or ketone, or a compound that yields one on hydrolysis; its general formula is (CH2O)n.
- Monosaccharide: the simplest sugar that cannot be hydrolysed further, such as glucose, fructose or ribose.
- Peptide bond: the amide linkage formed between the carboxyl group of one amino acid and the amino group of another, with the loss of water.
- Protein: a macromolecule made of one or more polypeptide chains, folded in a specific three-dimensional shape.
- Enzyme: a biological catalyst, almost always a protein, that speeds up a reaction by lowering its activation energy.
- Active site: the specific region of an enzyme where the substrate binds and the reaction takes place.
- Cofactor: a non-protein helper required by some enzymes for activity; an organic cofactor is called a coenzyme.
- Vitamin: an organic compound required in small amounts that the body cannot synthesise and must obtain from the diet.
- Nucleotide: the repeating unit of nucleic acids, made of a pentose sugar, a nitrogenous base and a phosphate group.
- Lipid: a biomolecule that is insoluble in water but soluble in organic solvents, such as a fat, oil or steroid.
- ATP: adenosine triphosphate, the universal energy currency of the cell.
- Denaturation: the loss of the native structure and function of a protein due to heat, acid, base or other agents.
Core Concepts and Formulas
- General formula of a carbohydrate is (CH2O)n; glucose and fructose both have the formula C6H12O6.
- Glucose is an aldohexose and fructose is a ketohexose, and both are reducing sugars.
- Sucrose is a non-reducing disaccharide of glucose and fructose, while lactose is a reducing disaccharide of glucose and galactose.
- Starch, glycogen and cellulose are polysaccharides of glucose differing in the type of glycosidic linkage and branching.
- Proteins are made of alpha-amino acids linked by peptide bonds; the number of peptide bonds in a chain of n amino acids is (n minus 1).
- The rate of an enzyme-catalysed reaction becomes constant at high substrate concentration because all active sites are saturated, as described by the Michaelis–Menten model.
- DNA contains the bases adenine, guanine, cytosine and thymine; RNA replaces thymine with uracil.
- According to Chargaff's rule, in DNA the amount of adenine equals that of thymine and the amount of guanine equals that of cytosine.
- The complete oxidation of one molecule of glucose yields about 38 molecules of ATP in prokaryotes and 36 in eukaryotes.
- Glycolysis gives a net gain of 2 ATP and 2 NADH per glucose molecule.
- The hydrolysis of one mole of ATP to ADP releases about 30.5 kilojoules of energy.
Classification of Biomolecules
Biomolecules are broadly classified into carbohydrates, proteins, lipids, nucleic acids, vitamins and hormones. Carbohydrates and lipids serve mainly as sources of energy and as structural material, proteins carry out catalysis, transport, defence and movement, and nucleic acids store and transmit genetic information. Vitamins and hormones act in small amounts to regulate metabolism. Many biomolecules are polymers built from smaller repeating units, called monomers, and the process of joining them releases water and is therefore a condensation reaction.
The four main classes of macromolecules are polysaccharides, proteins, nucleic acids and lipids, although lipids are not true polymers. The relationship between structure and function is the central theme: a change in the shape of a protein changes its function, and the exact sequence of bases in DNA determines the sequence of amino acids in a protein. This principle explains genetic disorders and many diseases.
Carbohydrates
Carbohydrates are classified as monosaccharides, disaccharides and polysaccharides. Monosaccharides are the simplest units and cannot be hydrolysed; glucose, the main fuel of the body, and fructose, the sugar of fruits, are the most familiar examples. Disaccharides are formed by the condensation of two monosaccharides; sucrose is common sugar, maltose comes from starch digestion, and lactose is milk sugar. Polysaccharides are long chains of monosaccharides and include starch and glycogen as storage forms and cellulose as a structural material in plants.
The presence of a free aldehyde or ketone group makes a sugar reducing, which is detected by Fehling's solution and Tollens' reagent. Starch gives a blue-black colour with iodine, which is a standard test. Cellulose cannot be digested by humans because we lack the enzyme to break its beta-linkage, though it is an important source of dietary fibre. Carbohydrates are the body's first choice for quick energy.
Proteins, Amino Acids and Enzymes
Proteins are polymers of alpha-amino acids. An amino acid contains both an amino group and a carboxyl group on the same carbon, and in solution it exists as a dipolar ion called a zwitterion, which gives it amphoteric behaviour. There are twenty standard amino acids, and the sequence in which they are linked decides the primary structure of the protein. The chain then folds into secondary structures such as the alpha helix and the beta sheet, and further into tertiary and quaternary structures held together by hydrogen bonds, disulphide bonds, hydrophobic interactions and ionic attractions.
Enzymes are protein catalysts that are highly specific, acting on a particular substrate as a lock fits a key. They work by lowering the activation energy and are sensitive to temperature and pH, which is why high fever or extreme acidity can denature them. Many enzymes require cofactors, coenzymes or metal ions, and vitamins often form part of coenzymes. Enzymes are widely used in industry, in detergents, in food processing and in medicine.
Nucleic Acids
Nucleic acids are the molecules of heredity. A nucleotide consists of a pentose sugar, a nitrogenous base and a phosphate, and a nucleoside is the same unit without the phosphate. DNA is a double-stranded helix in which the two strands are held by hydrogen bonds between complementary base pairs, adenine pairing with thymine and guanine with cytosine. RNA is usually single-stranded and exists as messenger, transfer and ribosomal RNA.
DNA stores the genetic information, RNA carries and translates it into proteins, and this flow of information from DNA to RNA to protein is called the central dogma of molecular biology. The double-helical model was proposed by Watson and Crick, and it explains both the stability of the genetic material and its accurate replication. Charges, hydrogen bonding and base stacking all contribute to the structure and function of nucleic acids.
Lipids and Vitamins
Lipids are a diverse group that includes fats, oils, waxes, phospholipids, steroids and cholesterol. Fats and oils are triglycerides formed from glycerol and fatty acids; those with saturated fatty acids are solid at room temperature, while those with unsaturated fatty acids are liquid. The hydrolysis of a fat with an alkali is called saponification, and it produces soap and glycerol. Phospholipids form the bilayer of cell membranes, and cholesterol is the starting material for the synthesis of steroid hormones.
Vitamins are classified as fat-soluble (A, D, E and K) and water-soluble (B complex and C). Fat-soluble vitamins are stored in the body and can accumulate to toxic levels, whereas water-soluble vitamins are excreted and must be supplied regularly. Their deficiency causes characteristic diseases, such as night blindness for vitamin A, scurvy for vitamin C, rickets for vitamin D and beriberi for vitamin B1. Vitamins generally act as coenzymes in metabolism.
Metabolism and Bioenergetics
Metabolism is the sum of all the chemical reactions in a living organism, divided into catabolism, which breaks down molecules to release energy, and anabolism, which builds molecules using energy. Cellular respiration is the central catabolic pathway. It begins with glycolysis in the cytoplasm, where glucose is split into pyruvate with a small yield of ATP. In the presence of oxygen, pyruvate enters the mitochondrion, is converted to acetyl coenzyme A, and enters the Krebs cycle, which produces reduced coenzymes and a little ATP.
The reduced coenzymes are then oxidised in the electron transport chain, where the bulk of the ATP is produced by oxidative phosphorylation. Fermentation is the anaerobic alternative, giving lactic acid in muscles and ethanol in yeast, with a much smaller energy yield. ATP is the universal energy currency, and bioenergetics studies how this molecule is generated and spent. Disorders of metabolism, such as diabetes and obesity, arise when these pathways are disturbed.
How Numerical Problems Are Solved
Biochemistry numericals are usually simple and conceptual. To find the number of peptide bonds in a protein, subtract one from the number of amino acids. For energy problems, multiply the number of moles of glucose by the number of ATP molecules produced and then by the energy released per mole of ATP hydrolysis. In genetics problems, apply Chargaff's rule, remembering that in double-stranded DNA the total purines equal the total pyrimidines. For enzyme problems, note that the rate becomes independent of substrate concentration once saturation is reached. In all cases, keep the units consistent and check that the answer makes biological sense.
Examination Trends
In APSC, ADRE and the Assam health and police examinations, biochemistry questions are largely factual: the classification of biomolecules, the functions of vitamins, the tests for carbohydrates and proteins, and the overall steps of respiration. In NEET, questions are more analytical and often ask about enzyme action, the structure of DNA and RNA, the products of the Krebs cycle, or the number of ATP molecules produced. Matching, assertion-reason and statement-based questions are common, and diagram-based questions on the cell and its organelles are frequent.
Common Mistakes to Avoid
- Confusing a nucleoside with a nucleotide, when the difference is the phosphate group.
- Mixing up the sugar and the base present in DNA and RNA.
- Forgetting that sucrose is a non-reducing sugar.
- Assuming that all proteins are enzymes, when many are structural, transport or defence proteins.
- Reversing the functions of fat-soluble and water-soluble vitamins.
- Forgetting that glycolysis occurs in the cytoplasm while the Krebs cycle occurs in the mitochondrion.
- Mixing up the end products of aerobic respiration and fermentation.
- Ignoring the role of coenzymes and metal ion activators in enzyme action.
How to Prepare This Topic
- Draw the classification chart of biomolecules and revise it until it is fixed in memory.
- Learn the structure of a nucleotide, an amino acid and a triglyceride, since many questions are based on these diagrams.
- Understand enzymes thoroughly, including specificity, the effect of temperature and pH, and the meaning of cofactors and coenzymes.
- Write out the steps of cellular respiration with their products and energy yield, and compare it with fermentation.
- Make a table of vitamins, their deficiency diseases and their functions, and test yourself on it regularly.
Frequently Asked Questions
What is a peptide bond and how is it formed?
A peptide bond is an amide linkage formed between the carboxyl group of one amino acid and the amino group of another, with the elimination of a molecule of water. A chain of n amino acids contains n minus one peptide bonds.
Why are enzymes called biological catalysts?
Because they speed up biochemical reactions by lowering the activation energy without being consumed in the process. They are highly specific, being proteins whose active site fits a particular substrate, and their activity depends on temperature and pH.
What is the difference between DNA and RNA?
DNA is usually double-stranded and contains the sugar deoxyribose and the base thymine; RNA is usually single-stranded and contains ribose and uracil instead of thymine. DNA stores genetic information, while RNA carries and translates it into proteins.
What is ATP and why is it important?
ATP, or adenosine triphosphate, is the universal energy currency of the cell. Hydrolysis of one mole of ATP to ADP and phosphate releases about 30.5 kilojoules of energy, which drives energy-requiring processes such as muscle contraction and active transport.
What happens during denaturation of a protein?
The three-dimensional shape of the protein is lost because the hydrogen bonds and other weak forces holding it together are disrupted by heat, acid, base or heavy metals. Since function depends on shape, the protein loses its biological activity, as happens when an egg is boiled.