Biological molecules: Class 11 Biology

Key points

Living things are made of a small number of elements, mainly carbon, hydrogen, oxygen and nitrogen. These form biological molecules, which are grouped as carbohydrates, lipids, proteins and nucleic acids. Most are polymers, large molecules made by joining many small units called monomers. Monomers join by condensation reactions, which release water, and polymers are broken down by hydrolysis, which uses water.

Water is a polar molecule. The oxygen end carries a slight negative charge and the hydrogen ends a slight positive charge, so water molecules form hydrogen bonds with each other. This gives water a high specific heat capacity, so it resists changes in temperature, and a high latent heat of vaporisation, so evaporation cools the body. Water is also a good solvent for ionic and polar substances, and its cohesion lets it move up the xylem in plants.

Carbohydrates contain carbon, hydrogen and oxygen, usually in the ratio 1 : 2 : 1. Monosaccharides are single sugars such as glucose, fructose and galactose. Two monosaccharides join by a glycosidic bond to form a disaccharide: glucose + glucose gives maltose, glucose + fructose gives sucrose and glucose + galactose gives lactose.

Polysaccharides are polymers of glucose with different jobs:

• Starch is the energy store in plants. It is a mixture of amylose (an unbranched helix) and amylopectin (branched).
• Glycogen is the energy store in animals and fungi. It is more branched than amylopectin, so enzymes can release glucose quickly.
• Cellulose forms plant cell walls. Its glucose units are joined in a way that makes straight chains, and hydrogen bonds between chains give microfibrils with great strength.

Lipids are made of carbon, hydrogen and oxygen, with much less oxygen than carbohydrates. A triglyceride is one glycerol molecule joined to three fatty acids by ester bonds. In a saturated fatty acid there are no double bonds between carbon atoms. An unsaturated fatty acid has one or more double bonds, which puts a bend in the chain, so unsaturated fats are usually liquid at room temperature.

Lipids store energy and give twice as much energy per gram as carbohydrates. They also insulate the body, protect organs, and form waterproof layers. A phospholipid has a glycerol, two fatty acids and a phosphate head. The head is hydrophilic and the tails are hydrophobic, so phospholipids form the bilayer of cell membranes. Cholesterol keeps membranes stable.

Proteins are polymers of amino acids. Every amino acid has an amino group, a carboxyl group, a hydrogen atom and a variable R group attached to a central carbon. Amino acids join by peptide bonds in condensation reactions to form a polypeptide.

Protein structure has four levels:

• The primary structure is the sequence of amino acids.
• The secondary structure is the coiling into an α-helix or folding into a β-pleated sheet, held by hydrogen bonds.
• The tertiary structure is the overall three-dimensional folding, held by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions.
• The quaternary structure is the arrangement of two or more polypeptide chains, as in haemoglobin, which has four.

Fibrous proteins such as collagen and keratin are long and insoluble, and they give strength. Globular proteins such as enzymes and haemoglobin are rounded and soluble, and they carry out chemical and transport tasks. A change in temperature or pH can break the weak bonds and change the shape. This is denaturation, and the protein stops working.

Nucleic acids are polymers of nucleotides. A nucleotide has a phosphate group, a pentose sugar and a nitrogenous base. DNA has deoxyribose and the bases A, T, G and C, and it forms a double helix with complementary base pairing: A with T by two hydrogen bonds, and G with C by three. RNA has ribose and uses uracil instead of thymine, and it is usually single stranded.

Food tests:

• Reducing sugars: heat with Benedict's solution. The colour changes from blue to green, yellow, orange or brick red.
• Starch: add iodine solution. A blue black colour shows starch.
• Protein: add Biuret reagent. A purple colour shows protein.
• Lipids: the emulsion test. Shake the sample with ethanol and then pour into water. A white cloudy emulsion shows lipid.

Study tip: For a food test answer, always give the reagent, the condition (heat or not), the positive result and the negative result.

Definitions and flashcards

Say the meaning first, then open the card to check.

Short questions with model answers

These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.

Common mistakes

• Saying that sucrose is a reducing sugar. It is not, so it gives no colour change with Benedict's solution until it is boiled with acid.
• Mixing up condensation and hydrolysis.
• Calling the sequence of amino acids the secondary structure. That is the primary structure.
• Writing that a protein is killed when it denatures. Proteins are not alive. They lose their shape and function.
• Stating that glycogen is found in plants.

Practise this chapter

Practice questions and timed test Printable worksheet