Biomolecules
Chemical Composition, Carbohydrates and Lipids
All living organisms are made of the same kinds of chemical substances called biomolecules. By weight, the most abundant compound in living cells is water (70–90%), which acts as a solvent and a medium for reactions. The other biomolecules are carbohydrates, proteins, lipids and nucleic acids, plus minerals.
Substances are also classed as primary metabolites (directly involved in growth and metabolism — amino acids, sugars, etc.) and secondary metabolites (not directly essential, e.g. alkaloids, pigments, rubber, drugs). Small biomolecules (under 1000 Da) are micromolecules; large ones (polymers) are macromolecules (polysaccharides, proteins, nucleic acids).
Carbohydrates are made of carbon, hydrogen and oxygen and are the chief source of energy. They are: monosaccharides (simple sugars — glucose, fructose), disaccharides (two units — sucrose, lactose, maltose) and polysaccharides (many units — starch and glycogen for storage, cellulose for structure in plant cell walls).
Lipids (fats and oils) are made of fatty acids and glycerol. They are insoluble in water but soluble in organic solvents. They store energy (more than carbohydrates per gram), form cell membranes (phospholipids), insulate the body and act as some hormones. A fat that is solid at room temperature is generally a saturated fat; a liquid (oil) is usually unsaturated.
One compound dominates by weight.
- Water is the most abundant (70–90%).
- It acts as a solvent and medium for biochemical reactions.
Sort by the number of sugar units.
- Glucose — monosaccharide.
- Sucrose — disaccharide.
- Starch — polysaccharide.
Lipids do several jobs.
- They store energy (long-term).
- They form cell membranes (phospholipids) and insulate the body.
Key Points
- Water is the most abundant biomolecule (70–90%).
- Primary vs secondary metabolites; large polymers = macromolecules.
- Carbohydrates (C,H,O): monosaccharides (glucose), disaccharides (sucrose), polysaccharides (starch/storage, cellulose/structure).
- Lipids = fatty acids + glycerol; energy store, membranes, insulation.
Proteins and Nucleic Acids
Proteins are the most abundant biomolecules of the cell's dry mass and are polymers of amino acids joined by peptide bonds. There are 20 types of amino acids. Proteins do an enormous variety of jobs: structural (collagen, keratin), enzymatic (almost all enzymes are proteins), transport (haemoglobin carries oxygen), defence (antibodies), hormonal (insulin) and contractile (actin, myosin in muscle).
A protein has up to four levels of structure: primary (the sequence of amino acids), secondary (coiling/folding into helices or sheets), tertiary (the overall 3D shape) and quaternary (assembly of more than one polypeptide chain, e.g. haemoglobin). The shape determines the function; heat or extreme pH can denature a protein (unfold it) and destroy its function.
Nucleic acids are the genetic macromolecules, polymers of units called nucleotides. Each nucleotide has a nitrogenous base, a pentose sugar and a phosphate group. There are two kinds:
- DNA (deoxyribonucleic acid) — the hereditary material; a double helix (Watson and Crick). Its sugar is deoxyribose; bases are A, T, G, C (A pairs with T, G with C).
- RNA (ribonucleic acid) — usually single-stranded; involved in protein synthesis. Its sugar is ribose; uracil (U) replaces thymine (T).
Proteins are polymers of small units.
- Building blocks: amino acids (20 types).
- They are joined by peptide bonds.
Denaturation affects protein shape.
- It is the loss of the protein's specific 3D shape (unfolding).
- It is caused by heat or extreme pH, and destroys the protein's function.
Compare sugar and bases.
- DNA has deoxyribose sugar and the base thymine; it is double-stranded.
- RNA has ribose sugar and uracil (instead of thymine); it is usually single-stranded.
Key Points
- Proteins = polymers of amino acids (peptide bonds); functions: structural, enzymatic, transport, defence, hormonal, contractile.
- Four structure levels (primary–quaternary); shape = function; heat/pH cause denaturation.
- Nucleic acids = polymers of nucleotides (base + sugar + phosphate).
- DNA (deoxyribose, A-T-G-C, double helix) vs RNA (ribose, uracil, single-stranded).
Enzymes and Vitamins
Enzymes are biological catalysts — substances (almost all are proteins) that speed up the chemical reactions of the body without being used up. They are essential because, without them, metabolic reactions would be far too slow to support life. The substance on which an enzyme acts is the substrate, and the substance formed is the product.
The enzyme has a specific pocket called the active site into which only its particular substrate fits — this gives enzymes their high specificity. The lock-and-key model pictures the substrate (key) fitting the active site (lock). The enzyme lowers the activation energy needed for the reaction. An enzyme works in steps: the substrate binds to the active site forming an enzyme–substrate complex, the reaction occurs, and the product is released, freeing the enzyme to act again.
Enzyme activity is affected by several factors: temperature (each enzyme has an optimum; high heat denatures it), pH (each has an optimum pH, e.g. pepsin works in acid, trypsin in alkaline), substrate concentration (rate rises then levels off when all active sites are busy), and the presence of inhibitors.
Vitamins are organic substances needed in tiny amounts for normal functioning. They are fat-soluble (A, D, E, K) or water-soluble (B-complex and C). Each plays a special role and its deficiency causes a specific disease (e.g. vitamin C deficiency → scurvy; vitamin D deficiency → rickets). Many vitamins act as coenzymes that help enzymes work.
Enzymes are catalysts with a specific binding region.
- An enzyme is a biological catalyst that speeds up reactions without being used up.
- The active site is the specific pocket where the substrate binds.
Several conditions influence enzymes.
- Temperature (optimum, then denaturation).
- pH (each enzyme has an optimum pH). (Also substrate concentration.)
Vitamins are fat- or water-soluble.
- Fat-soluble: vitamin D — deficiency causes rickets.
- Water-soluble: vitamin C — deficiency causes scurvy.
Key Points
- Enzymes = biological catalysts (mostly proteins) that speed reactions without being used up; they lower the activation energy.
- High specificity via the active site (lock-and-key); form an enzyme–substrate complex.
- Affected by temperature, pH, substrate concentration and inhibitors.
- Vitamins: fat-soluble (A, D, E, K) and water-soluble (B, C); deficiencies cause specific diseases; many act as coenzymes.