The cell is built from chemical elements that are also found in non-living nature — showing the unity of living and non-living matter. But in the cell these elements form complex compounds found only in living things — biopolymers (proteins, nucleic acids, polysaccharides). Without knowing the chemistry of the cell you cannot understand metabolism, photosynthesis, protein synthesis or genetics. Exams often ask about the roles of elements, the properties of water, the structure and functions of biopolymers, and set calculation problems on DNA.
§1. Chemical elements
Many elements of the periodic table are found in cells, but in very different amounts.
Macroelements: O, C, H, N — together about 98% of cell mass (the basis of organic matter); also P, S, K, Ca, Mg, Na, Cl, Fe — each at tenths or hundredths of a per cent.
Microelements (trace elements) — present in tiny amounts but essential: Zn, Cu, I, F, Mn, Co, Mo, B and others.
Fe — in haemoglobin; Mg — at the centre of chlorophyll; I — in the thyroid hormone thyroxine; Ca — bones, teeth, blood clotting and muscle contraction; Zn — in insulin; Co — in vitamin B₁₂; P — in nucleic acids, ATP and bone; S — in some amino acids.
A shortage causes disease: lack of iodine — endemic goitre (hence iodised salt), lack of iron — anaemia, lack of calcium — brittle bones, lack of fluorine — tooth decay.
§2. Water and mineral salts
Water is the most abundant substance in the cell: about 70–80% of its mass (more in embryonic cells, less in bone and teeth). Properties and roles:
its molecule is polar, so water is a universal solvent: many substances react only when dissolved;
it has a high heat capacity, keeping cell temperature stable; evaporation absorbs a lot of heat (sweating, transpiration — cooling);
it takes part in hydrolysis (breaking down biopolymers) and is the source of hydrogen and oxygen in photosynthesis;
it transports substances (blood, cell sap, xylem) and gives cells turgor.
Substances are hydrophilic (dissolve well — salts, sugars, amino acids) or hydrophobic (do not dissolve — fats, membrane lipids).
Mineral salts are present as ions: K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl⁻, phosphate and hydrogencarbonate. They provide buffering (stable pH) and osmotic pressure; K⁺ and Na⁺ take part in nerve impulses; calcium phosphate makes bone hard.
§3. Carbohydrates
Carbohydrates contain C, H and O; their general formula is often Cₙ(H₂O)ₘ.
Monosaccharides: glucose, fructose, galactose (C₆H₁₂O₆, six-carbon); ribose and deoxyribose (five-carbon — in RNA and DNA). Very soluble, sweet.
Polysaccharides (monomer glucose): starch — the plant store; glycogen — the store of animals and fungi (liver and muscles); cellulose — plant cell walls. Chitin — fungal walls and arthropod skeletons (a nitrogen-containing polysaccharide). Almost insoluble, not sweet.
Functions: energy (the main quick energy source), storage, structural (cellulose, chitin), part of nucleic acids.
§4. Lipids
Lipids are substances insoluble in water but soluble in organic solvents (petrol, ether).
Fats — compounds of glycerol and fatty acids: energy store (they release the most energy), heat insulation (fat under the skin, whales), mechanical protection (around internal organs), a source of metabolic water — oxidising fat produces a lot of water (the fat in a camel’s hump).
Phospholipids — the basis of cell membranes.
Steroids — cholesterol, sex hormones, adrenal cortex hormones, vitamin D.
Waxes — the protective layer on leaves and fruit, beeswax.
Energy value: 1 g of carbohydrate → 17.6 kJ; 1 g of protein → 17.6 kJ; 1 g of fat → 38.9 kJ.
§5. Proteins
Proteins are the most varied biopolymers. Their monomer is the amino acid (20 kinds occur in proteins). Each amino acid has an amino group (–NH₂), a carboxyl group (–COOH) and a side chain (R group); amino acids join by peptide bonds (–CO–NH–) into a polypeptide.
Levels of structure:
primary — the sequence of amino acids in the chain (peptide bonds); it is set by DNA;
secondary — a helix (or pleated sheet), held by hydrogen bonds;
tertiary — the helix folded into a three-dimensional globule;
quaternary — several polypeptide chains joined together (haemoglobin has 4 chains).
Denaturation is the loss of a protein’s natural shape caused by heat, acids, alkalis or heavy-metal salts (egg white setting); the primary structure remains, and sometimes the process is reversible — renaturation.
Functions: structural (collagen, keratin), catalytic — all enzymes are proteins; transport (haemoglobin, membrane carriers), protective (antibodies, fibrinogen), movement (actin, myosin), signalling and control (insulin, receptors), energy (used last). Enzymes are biological catalysts: they speed reactions up thousands of times, are specific (fit their substrate like a key in a lock) and work at a particular temperature and pH.
§6. Nucleic acids and ATP
Nucleic acids are biopolymers that store and pass on hereditary information. Their monomer is the nucleotide: a nitrogenous base + a five-carbon sugar + a phosphate group.
DNA — sugar deoxyribose; bases A, T, G, C; a two-stranded double helix (J. Watson and F. Crick, 1953). Found in the nucleus (chromosomes) and in mitochondria and chloroplasts. Role: storing hereditary information and copying itself (replication).
RNA — sugar ribose; bases A, U, G, C; single-stranded. Types: mRNA (carries information to the ribosome), tRNA (carries amino acids), rRNA (part of ribosomes).
Complementarity: A = T (2 hydrogen bonds), G ≡ C (3 hydrogen bonds).
Chargaff’s rule: A = T, G = C, A + G = T + C; one base pair spans 0.34 nm along the helix.
Purines — A, G (two rings); pyrimidines — T, C, U (one ring).
ATP (adenosine triphosphate) is the cell’s universal energy carrier: adenine + ribose + 3 phosphate groups. The two bonds between phosphates are high-energy bonds:
ATP + H₂O → ADP + P + 40 kJ
ATP is made in mitochondria (and chloroplasts) and used in every energy-requiring process.
Worked example
A double-stranded DNA fragment has 600 nucleotides, of which adenine is 20% (120). Find the number of hydrogen bonds and the length of the fragment.
A = T = 120; G + C = 600 − 240 = 360, so G = C = 180.
120 A–T pairs with 2 bonds each: 120 × 2 = 240.
180 G–C pairs with 3 bonds each: 180 × 3 = 540.
Total: 240 + 540 = 780 hydrogen bonds.
Length: 300 pairs × 0.34 = 102 nm.
Answer: 780 hydrogen bonds, 102 nm.
Worked example
The body completely oxidised 10 g of fat and 20 g of glucose. How much energy was released?
From fat: 10 × 38.9 = 389 kJ.
From carbohydrate: 20 × 17.6 = 352 kJ.
Total: 389 + 352 = 741 kJ.
Answer: 741 kJ.
Worked example
A protein molecule has 300 amino acids. How many peptide bonds does it contain, and what is its approximate mass (average amino acid mass 110)?
A linear chain has one fewer peptide bond than amino acids: 300 − 1 = 299.
Mass: 300 × 110 = 33 000.
Answer: 299 peptide bonds, mass about 33 000.
Check yourself
Which monosaccharides make up lactose?
What is the primary structure of a protein?
If DNA contains 30% guanine, what percentage is thymine?
Which base in RNA is absent from DNA?
Which metal is in chlorophyll?
Why is the fat in a camel’s hump a source of water?
How much energy is released by completely oxidising 5 g of fat?
What is the storage polysaccharide of animals?
Common mistakes
Writing thymine in RNA — RNA has uracil.
Treating the energy value of carbohydrate and fat as equal — fat gives more than twice as much.
Giving A–T 3 hydrogen bonds and G–C 2 — it is the other way round.
Calling glycogen a plant store — plants store starch.
Writing that denaturation destroys the primary structure — peptide bonds remain.
Calling iodine a macroelement — it is a trace element.