Amino acid: organic compound with an amine group (-NH₂) and a carboxylic group (-COOH) on the same carbon, plus a variable side chain R. General structure H₂N-CH(R)-COOH. Label the amino group and the carboxyl group.
Protein: natural polyamide (polymer of amino acids) joined by peptide bonds (-CO-NH-). Two amino acids make a dipeptide, with loss of one water: the -NH₂ of one reacts with the -COOH of the other.
Dipeptide general structure: H₂N-CH(R₁)-CO-NH-CH(R₂)-COOH, with the peptide bond -CO-NH- in the middle.
Lipids: insoluble in water, soluble in organic solvents (ether, hexane); simple lipids (fats and oils) are esters of fatty acids with glycerol (glycerides). Main function: long-term energy storage, plus insulation, cell membranes and absorbing fat-soluble vitamins. Triglyceride = glycerol + 3 fatty acids + loss of 3 water.
Carbohydrates: general formula Cₓ(H₂O)ᵧ, made by plants in photosynthesis. Glucose is a monosaccharide, starch and cellulose are polymers of glucose. Main function: supply energy.
Nucleic acids (DNA and RNA): long-chain molecules that store and pass on genetic information from one generation to the next and direct the making of proteins.
Vitamins: organic compounds needed in small amounts for metabolism, growth and immunity, which must come from food. Fat-soluble: A, D, E, K (stored in fat and liver). Water-soluble: B complex and C (not stored, lost in urine, so needed regularly).
Vitamins: sources and functions (high value for 1x3 answers)
Vitamin B complex: whole grains, meat, fish, eggs, milk, legumes, leafy greens. Function: energy production, red blood cells, nervous system.
Vitamin C: oranges, strawberries, kiwi, bell peppers, broccoli, tomatoes. Function: immune system, antioxidant, collagen formation. Deficiency disease: scurvy (this name is not in the book text, it is general knowledge, so state it with confidence only if your teacher has covered it).
Vitamin D: sunlight on skin, fatty fish (salmon, mackerel), fortified milk, egg yolk. Function: calcium absorption and healthy bones. Deficiency: rickets (general knowledge, not in the book text).
Vitamin E: nuts, seeds, spinach, vegetable oils. Function: antioxidant. Vitamin K: leafy greens, broccoli. Function: blood clotting and bone health.
Applications and genetics
Blood tests: measure ions, enzymes, sugars, lipids and proteins to check kidney, liver and other organ function. Pregnancy test: detects the hormone hCG (human chorionic gonadotropin) in urine or blood. Cancer screening: finds early signs in people without symptoms. Parental genetic screening: finds the chance of a child having a serious genetic disorder; also used to determine parentage.
Gene: unit of DNA on a chromosome that controls a trait and is passed from parent to child.
Genetic engineering: modifying an organism's DNA in the lab by adding, removing or repairing genes (vaccines, drugs). Gene therapy: inserting a healthy copy of a defective gene into a patient's cells to treat or prevent disease; it targets faulty cells, not all DNA.
Cloning: producing a genetically identical copy. Three types: reproductive (identical organisms), therapeutic (embryonic stem cells for medical research and treating spinal injury, diabetes, Parkinson's), gene cloning (copying specific genes or DNA segments).
Past-paper questions with model answers
Two sources of vitamin A (B), vitamin D (B, M), vitamin C (M) and state one function or deficiency disease (1x3): use the list above.
How do amino acids join to form a protein, with an equation (B, 3 marks): amino group of one reacts with the carboxyl group of another, losing water to form a peptide bond (-CO-NH-); H₂N-CHR-COOH + H₂N-CHR′-COOH → H₂N-CHR-CO-NH-CHR′-COOH + H₂O; thousands of units give a protein.
Draw the general structure of an amino acid and label the main functional groups (B, D); draw the general structure of a dipeptide from two amino acids (M).
2025 paper (old syllabus): the bonds holding amino acids together in proteins are peptide bonds; a tripeptide with its two peptide bonds marked.