Form 4 Biology Bab 4: Chemical Composition in a Cell
4.1 Water
Water is polar because shared electrons are drawn towards oxygen; this produces hydrogen bonds and makes water a universal solvent. (p.72)
Cohesive force (between water molecules) and adhesive force (with other surfaces) produce capillary action, e.g. in the xylem. (p.72)
Specific heat capacity of water is 4.2 kJ kg-1 °C-1, so it absorbs much heat with a small temperature rise, helping maintain body temperature. (p.72)
4.2 Carbohydrates
Carbohydrates contain C, H and O in the ratio 1:2:1, formula (CH2O)n; they are an energy source and structural material. (p.73)
Monosaccharides (glucose, fructose, galactose) are the monomers; they are sweet, crystalline, soluble and reducing sugars (brick-red precipitate with Benedict's solution). (p.73)
Disaccharides form by condensation (release one water molecule) and break down by hydrolysis (add one water molecule). (p.74)
Lactose and maltose are reducing sugars; sucrose is non-reducing. (p.74)
Polysaccharides (starch, glycogen, cellulose) form by condensation of many monosaccharides; they are insoluble, not sweet and do not crystallise. (p.75)
Importance: energy source (glucose), food reserve (glycogen in animals, starch in plants), support structure (cellulose in cell walls). (p.75)
4.3 Proteins
Proteins contain C, H, O and N; most also contain sulphur and phosphorus. (p.76)
Amino acids are the monomers; peptide bonds form by condensation; a dipeptide has two amino acids, a polypeptide can have fifty to thousands. (p.76)
About 20 natural amino acids; proteins differ in amino acid sequence. (p.76)
Importance: build new cells, repair tissue, form enzymes, hormones, antibodies, haemoglobin, keratin, collagen and myosin. (p.76)
4.4 Lipids
Lipids contain C, H and O (much higher H:O ratio than carbohydrates); insoluble in water, soluble in alcohol, ether and chloroform. (p.77)
Triglyceride = one glycerol + three fatty acids (condensation, releasing 3 H2O); hydrolysis reverses this. (p.77)
Ethanol emulsion test: white emulsion shows lipids present. (p.77)
Types: fats, waxes, phospholipids and steroids. (p.77)
Saturated fats: only single C–C bonds, solid at room temperature (butter, animal fat). (p.78)
Unsaturated fats: at least one C=C double bond, liquid at room temperature (olive and fish oil). (p.78)
Wax: alcohol + fatty acid, waterproof; phospholipid: glycerol + two fatty acids + phosphate group; steroids have no fatty acids (cholesterol, testosterone, oestrogen, progesterone). (p.78)
Importance: reserve energy, insulation, organ protection; wax in leaf cuticle and sebum; cholesterol for steroid hormones. (p.78)
4.5 Nucleic Acids
Nucleic acids contain C, H, O, N and P; a nucleotide = pentose sugar + nitrogenous base + phosphate group. (p.79)
DNA: two polynucleotide chains forming a double helix; bases A, G, T, C; A pairs with T and G pairs with C by hydrogen bonds; contains deoxyribose. (p.79)
RNA: single shorter polynucleotide chain with ribose; thymine replaced by uracil; types mRNA, rRNA, tRNA. (p.80)
DNA carries hereditary information; a three-base codon codes for an amino acid (e.g. AUG codes for methionine). (p.80)
DNA is transcribed into mRNA, which is translated into polypeptides. (p.80)
Chromosome formation: DNA wound around histone proteins forms nucleosomes, which coil into chromosomes. (p.80)