Biochemistry: Class 10 Chemistry

Key points

Biochemistry is the study of the chemical compounds and reactions in living organisms. The main biomolecules are carbohydrates, proteins, lipids, nucleic acids, vitamins, enzymes and minerals. All are organic except minerals and water. They supply energy, build body tissue and control body reactions.

Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that give them on hydrolysis. Most have the general formula Cₓ(H₂O)ᵧ, for example glucose C₆H₁₂O₆. They are the main source of energy, giving about 17 kJ per gram. Cellulose gives structure in plants.

Classification of carbohydrates:

• Monosaccharides: simple sugars that cannot be hydrolysed, such as glucose, fructose and galactose (C₆H₁₂O₆) and ribose (C₅H₁₀O₅).
• Disaccharides: two monosaccharides joined by a glycosidic bond with loss of water. Sucrose = glucose + fructose, maltose = glucose + glucose, lactose = glucose + galactose. Formula C₁₂H₂₂O₁₁.
• Polysaccharides: long chains of many glucose units, such as starch (plant store), glycogen (animal store) and cellulose (plant wall). Formula (C₆H₁₀O₅)ₙ.

Glucose is an aldohexose and fructose is a ketohexose. Glucose is the sugar in blood and is the fuel for cells. Glucose and maltose are reducing sugars: they reduce Benedict's or Fehling's solution to a brick red precipitate. Sucrose is a non-reducing sugar until it is hydrolysed by boiling with dilute acid. Starch gives a blue black colour with iodine solution.

Proteins are polymers of amino acids. An amino acid has an amino group (−NH₂) and a carboxyl group (−COOH) on the same carbon, with a side group R: H₂N−CHR−COOH. About twenty amino acids occur in proteins. Eight or nine of them are essential, meaning that the body cannot make them and they must come in food.

Amino acids join by peptide bonds, −CO−NH−, formed when the −COOH of one amino acid reacts with the −NH₂ of the next with loss of water. Two amino acids make a dipeptide, and many make a polypeptide. A protein of n amino acids in one chain has n − 1 peptide bonds.

Structure and function of proteins:

• Primary structure: the order of amino acids in the chain.
• Secondary structure: coiling or folding held by hydrogen bonds, such as the alpha helix and beta sheet.
• Tertiary structure: the full three dimensional fold of one chain.
• Quaternary structure: several chains together, as in haemoglobin.
• Functions: growth and repair, enzymes, hormones such as insulin, antibodies, and transport of oxygen.

Denaturation is the loss of the natural shape of a protein by heat, strong acid or base, or heavy metal salts. The protein's activity is lost. Cooking an egg is an example: the white turns solid. The biuret test (copper sulphate with NaOH, giving purple) detects the peptide bond. Fibrous proteins such as keratin are insoluble and globular proteins such as albumin dissolve.

Lipids are organic compounds that dissolve in organic solvents and not in water. Fats and oils are esters of glycerol with three fatty acids (triglycerides). Fats are solid at room temperature and mostly from animals; oils are liquid and mostly from plants. Saturated fatty acids have no double bond and unsaturated fatty acids have one or more. Fats give about 38 kJ per gram, more than twice the energy of carbohydrates.

Lipids store energy, insulate the body, protect organs, and form cell membranes (phospholipids). Steroids such as cholesterol are also lipids. Hydrolysis of a fat with NaOH gives soap and glycerol. Fats go rancid when oxidized by air.

Enzymes are protein catalysts of living cells. They speed up reactions without being used up. Each enzyme acts on a specific substrate at its active site (lock and key model). Their activity depends on temperature (best near 37 °C in humans), pH, and substrate concentration. High temperature denatures them. Examples: amylase breaks starch to maltose, pepsin acts on protein in the stomach, and lipase acts on fats.

Nucleic acids store and pass on genetic information. They are polymers of nucleotides, each with a sugar, a phosphate group and a nitrogen base. DNA has deoxyribose and the bases A, T, G and C, forming a double helix with pairing A with T and G with C. RNA has ribose, single strand, and uracil (U) in place of T. DNA carries the genetic code and RNA helps in making proteins.

Vitamins are organic compounds needed in small amounts. Fat soluble ones are A, D, E and K; water soluble ones are the B group and C. Vitamin A deficiency causes night blindness, D causes rickets, C causes scurvy and B₁ causes beriberi. Minerals such as calcium, iron, iodine and sodium are needed for bones, blood and nerves. Iodine deficiency causes goitre.

Study tip: Make a table of the three food groups with building units, bonds, tests and functions. Learn the tests: Benedict's for reducing sugar, iodine for starch and biuret for protein. Know the deficiency diseases of each vitamin, which are easy marks.

Definitions and flashcards

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Formula sheet

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Short questions with model answers

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Numericals with worked steps

Common mistakes

• Thinking sucrose is a reducing sugar. It becomes so only after hydrolysis.
• Writing the formula of a disaccharide as the sum of two monosaccharides without removing H₂O.
• Confusing denaturation with hydrolysis. Denaturation only changes the shape and does not break peptide bonds.
• Saying DNA has uracil. Uracil is in RNA and thymine is in DNA.
• Mixing up fat soluble vitamins (A, D, E, K) with water soluble ones (B and C).

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