Carbohydrates
Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give these on hydrolysis. Many have the formula Cₓ(H₂O)ᵧ, which is why they were once called "hydrates of carbon".
Class | Hydrolysis gives | Examples |
|---|---|---|
Monosaccharides | cannot be hydrolysed further | glucose, fructose, ribose |
Oligosaccharides | 2 to 10 monosaccharides | disaccharides: sucrose, maltose, lactose |
Polysaccharides | many monosaccharides | starch, cellulose, glycogen |
Monosaccharides with an aldehyde group are aldoses (glucose is an aldohexose); those with a ketone group are ketoses (fructose is a ketohexose).
Reducing sugars reduce Tollens' reagent and Fehling's solution: all monosaccharides and maltose and lactose. Sucrose is non-reducing, because both its reducing groups are used in the bond joining its two units.
Glucose
Glucose, C₆H₁₂O₆, is made commercially by hydrolysing starch with dilute acid.
Evidence for its open-chain structure:
With HI it gives n-hexane: the six carbons are in a straight chain.
With hydroxylamine it forms an oxime: a carbonyl group is present.
With bromine water it gives gluconic acid: the carbonyl is an aldehyde.
Acetylation gives a pentaacetate: there are five –OH groups.
Nitric acid oxidises it to saccharic acid: there is a primary alcohol (–CH₂OH) group.
Glucose is D-(+)-glucose. "D" refers to the configuration at the carbon farthest from the aldehyde (related to D-glyceraldehyde); "(+)" means it rotates plane-polarised light to the right.
Cyclic structure. The open-chain structure couldn't explain some facts: glucose doesn't give Schiff's test or form a bisulphite product, and its pentaacetate doesn't react with hydroxylamine. In fact, the –OH at C5 adds to the aldehyde group, forming a six-membered ring (a pyranose ring). Two forms result, α and β, which differ only at C1, called anomers.
Disaccharides and polysaccharides
Monosaccharide units join through a glycosidic linkage, an oxygen bridge formed with the loss of water.
Sugar | Made of | Notes |
|---|---|---|
Sucrose | α-D-glucose + β-D-fructose | non-reducing; hydrolysis gives an equal mixture of glucose and fructose called invert sugar, because the rotation changes from dextro to laevo |
Maltose | two α-D-glucose units (C1–C4) | reducing |
Lactose (milk sugar) | β-D-galactose + β-D-glucose | reducing |
Starch | α-glucose; amylose (unbranched, water-soluble, 15–20%) and amylopectin (branched, insoluble, 80–85%) | main stored food in plants |
Cellulose | β-glucose, unbranched chains | main structural material of plant cell walls; humans can't digest it |
Glycogen | α-glucose, highly branched | stored in animal liver and muscles (called "animal starch") |
Amino acids and proteins
Amino acids contain an amino group and a carboxyl group. Those in proteins are α-amino acids: R–CH(NH₂)–COOH.
Essential amino acids can't be made by the body and must come from food (valine, leucine, lysine and others). Non-essential ones can be made in the body.
In solution, amino acids exist as zwitterions: H₃N⁺–CHR–COO⁻, with both positive and negative charges. So they behave like salts, with high melting points and water solubility, and are amphoteric.
All naturally occurring α-amino acids except glycine are optically active, with the L-configuration.
Peptide bond: the –CO–NH– amide link formed between the –COOH of one amino acid and the –NH₂ of the next, with the loss of water. Two amino acids form a dipeptide; many form a polypeptide. A polypeptide with more than about a hundred amino acids is generally a protein.
Fibrous proteins have parallel chains held by hydrogen and disulphide bonds; they are insoluble in water (keratin in hair, myosin in muscle). Globular proteins are folded into spheres and are water-soluble (insulin, albumins).
Levels of structure:
Level | What it describes |
|---|---|
Primary | the sequence of amino acids. Changing even one amino acid can change the function (as in sickle-cell anaemia) |
Secondary | regular folding: α-helix or β-pleated sheet, held by hydrogen bonds between –CO– and –NH– groups |
Tertiary | overall folding of the chain, held by hydrogen bonds, disulphide links, van der Waals and electrostatic forces |
Quaternary | arrangement of two or more polypeptide subunits |
Denaturation. Heat or changes in pH disrupt the hydrogen bonds, the protein unfolds and loses its biological activity, while the primary structure stays intact. Boiling an egg coagulates the albumin; curdling milk is caused by lactic acid from bacteria.
Enzymes
Enzymes are biological catalysts, almost all of them proteins. They are highly specific, each catalysing a particular reaction. They work under mild conditions (body temperature and near-neutral pH) and lower the activation energy. Enzymes are named after the reaction or substrate, usually ending in -ase: maltase hydrolyses maltose.
Vitamins
Vitamins are organic compounds needed in small amounts in the diet for normal health.
Vitamin | Solubility | Deficiency disease |
|---|---|---|
A | fat-soluble | night blindness, xerophthalmia |
B₁ (thiamine) | water-soluble | beriberi |
B₂ (riboflavin) | water-soluble | cheilosis (cracked lips) |
B₆ (pyridoxine) | water-soluble | convulsions |
B₁₂ | water-soluble | pernicious anaemia |
C (ascorbic acid) | water-soluble | scurvy (bleeding gums) |
D | fat-soluble | rickets (children), osteomalacia (adults) |
E | fat-soluble | increased fragility of red blood cells, muscle weakness |
K | fat-soluble | increased blood clotting time |
Fat-soluble vitamins (A, D, E, K) are stored in the liver and fatty tissue. Water-soluble vitamins (B group and C) are excreted in urine and must be supplied regularly. Vitamin B₁₂ is the exception: it is stored in the liver.
Nucleic acids
DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) carry hereditary information. They are polymers of nucleotides; each nucleotide has a base, a pentose sugar and a phosphate. Nucleotides are joined by phosphodiester linkages between the sugar of one and the phosphate of the next.
DNA | RNA | |
|---|---|---|
Sugar | 2-deoxyribose | ribose |
Bases | A, G, C, T (thymine) | A, G, C, U (uracil) |
Structure | double helix | usually single-stranded |
Role | stores genetic information; replicates | protein synthesis (mRNA, tRNA, rRNA) |
In the DNA double helix (Watson and Crick, 1953), the two strands are complementary: A pairs with T and G pairs with C, held by hydrogen bonds.
DNA fingerprinting, based on the unique sequence of bases in each individual, is used in forensics and to establish parentage.
Summary
Carbohydrates: monosaccharides, oligosaccharides, polysaccharides; sucrose is non-reducing.
Glucose is an aldohexose with a cyclic pyranose structure (α and β anomers).
Amino acids exist as zwitterions; peptide bonds join them; denaturation destroys secondary and tertiary structure.
Vitamins A, D, E, K are fat-soluble; B and C are water-soluble.
DNA has deoxyribose and thymine and is a double helix; RNA has ribose and uracil.