The Chemistry of Life

Fertilisers

Fertilisers supply nutrients to crops and correct soil fertility where farming has removed nutrients. The main nutrients supplied by most fertilisers are nitrogen (N), phosphorus (P) and potassium (K) (often referred to as N-P-K).

• Nitrogen sources: organic forms (manure, compost, plant residues) or inorganic forms (nitrate salts, ammonia, urea).
• Phosphorus sources: organic matter or inorganic phosphates and superphosphate.
• Potassium sources: potash salts and organic sources.

Excess application of inorganic fertilisers can damage soil structure, reduce humus, and cause leaching and runoff into water bodies.

Leaching and eutrophication

Water movement through soil can leach nutrients such as nitrate and phosphate into groundwater and surface waters. Runoff containing high fertiliser concentrations can lead to eutrophication, which proceeds as follows:

• Excess nutrients stimulate rapid growth of algae and aquatic plants.
• When these plants die and decompose, microbial decomposition consumes dissolved oxygen in the water.
• Low dissolved oxygen (hypoxia) kills fish and other aquatic animals and reduces biodiversity.
• Water becomes turbid, reducing light penetration and photosynthesis, further disrupting the ecosystem.

Organic Compounds

Organic compounds are carbon-based molecules synthesised in living organisms. They supply energy, form structures and carry genetic information. Major classes include carbohydrates, lipids, proteins (including enzymes), nucleic acids and vitamins.

Carbohydrates

Carbohydrates are composed of carbon, hydrogen and oxygen (general formula (CH₂O)_n). They serve as energy stores, immediate energy sources and structural components.

• Energy storage: starch in plants; glycogen in animals.
• Energy release: glucose oxidation during cellular respiration (mitochondria).
• Structure: cellulose gives rigidity to plant cell walls.

Tests for carbohydrates

• Starch test (iodine): iodine solution gives a blue-black colour in presence of starch; a yellow-brown colour indicates no starch.
• Reducing-sugar tests:
  • Fehling's solution: a positive test gives a brick-red precipitate of copper(I) oxide; the original blue solution becomes colourless/green then red on warming with a reducing sugar.
  • Benedict's test: on heating with a reducing sugar, Benedict's solution changes from blue to green, yellow or brick red depending on the amount of reducing sugar; brick-red indicates a strong positive.

Lipids

Lipids include fats, oils and waxes. Chemically, many lipids are composed of a glycerol backbone esterified with three fatty acids (a triglyceride).

• Saturated fats have mostly single carbon-carbon bonds, are usually solid at room temperature and are common in animal fats (for example, butter, lard).
• Unsaturated fats (oils) contain one or more C=C double bonds, are usually liquid at room temperature and are common in plant oils (for example, olive oil, sunflower oil).

Functions of lipids:

• Structural components of membranes (phospholipids).
• Energy storage (high energy per gram in seeds and adipose tissue).
• Insulation and protection of organs.
• Absorption and storage of fat-soluble vitamins (A, D, E, K).
• Water source in some animals (e.g., camel humps are fat stores that can be metabolised).
• Waterproofing (cuticular waxes on plants).

Lipid tests

• Solubility test: lipids dissolve in non-polar solvents (ether, chloroform) and are immiscible with water.
• Paper test (grease spot): a translucent oil mark on paper indicates presence of lipid.

Cholesterol and blood lipids

Cholesterol is a sterol lipid produced by the body and obtained from animal-origin foods. It is important in membrane structure, in synthesis of vitamin D and steroid hormones, and in digestion (bile salts).

Blood lipids include:

• Low-density lipoprotein (LDL) - often called "bad" cholesterol because high LDL levels contribute to arterial plaque formation (atherosclerosis).
• High-density lipoprotein (HDL) - often called "good" cholesterol because HDL transports cholesterol from tissues and plaques back to the liver for excretion.
• Triglycerides - the main form of stored fat in the body; high blood levels are a risk factor for cardiovascular disease.

Excess LDL can lead to plaque build-up in arteries, narrowing them and increasing the risk of heart attack or stroke. Genetic forms of high LDL (familial hypercholesterolaemia) cause early onset of deposits and cardiovascular disease. HDL reduces plaque by removing cholesterol.

Proteins

Proteins are polymers of amino acids containing carbon, hydrogen, oxygen and nitrogen; some contain sulphur and phosphorus. There are 20 common amino acids that combine in different sequences to form proteins.

• Simple units:
  • Amino acids - single molecules (examples: glycine, alanine, tryptophan).
  • Dipeptides - two joined amino acids.
  • Tripeptides - three joined amino acids (example: glutathione).
  • Peptones - short chains (4-10 amino acids).
  • Polypeptides - longer chains (10-50 amino acids); proteins are typically >50 amino acids.
• Protein structures:
  1. Primary: linear sequence of amino acids.
  2. Secondary: local folding into alpha-helices or beta-pleated sheets stabilised by hydrogen bonds.
  3. Tertiary: three-dimensional folding of a single polypeptide chain.
  4. Quaternary: association of two or more polypeptide chains into a functional protein.

Proteins are denatured when their conformation (shape) is altered by heat, extremes of pH or high salt concentration; denaturation usually abolishes biological activity.

Functions of proteins

• Structural (collagen, keratin).
• Transport (haemoglobin, membrane transporters).
• Movement (actin, myosin).
• Storage of amino acids.
• Hormonal regulation (insulin, growth hormones).
• Immune defence (antibodies).
• Catalysis of biochemical reactions (enzymes).

Severe or prolonged protein deficiency in the diet causes clinical conditions such as kwashiorkor and marasmus.

Protein tests

• Biuret test: addition of sodium hydroxide (NaOH) and copper(II) sulphate (CuSO₄) to a protein solution produces a violet or purple colour if peptide bonds are present; blue indicates a negative result.
• Millon's reagent: gives a red or pink colour in presence of phenolic groups of certain amino acids (for example, tyrosine).

Factors affecting protein structure

Proteins are sensitive to environmental conditions. Factors that denature proteins and affect their function include:

• High temperature.
• Extreme pH (strong acidity or alkalinity).
• High salt concentrations or other denaturing agents.

Enzymes

Enzymes are biological catalysts, usually globular proteins, that accelerate metabolic reactions without being consumed. Some enzymes contain non-protein cofactors or prosthetic groups.

Key properties of enzymes:

• Each enzyme has a specific three-dimensional shape with an active site that binds a particular substrate.
• Enzymes may change shape slightly during a reaction (induced fit) but are not permanently altered.
• Enzyme activity depends on environmental conditions and substrate concentration.
• Enzymes lower the activation energy required for a reaction, increasing its rate.

Example: catalase

Catalase (also a peroxidase) catalyses the decomposition of hydrogen peroxide, a harmful by-product of metabolism:

2H₂O₂ → 2H₂O + O₂

This reaction rapidly converts hydrogen peroxide into harmless water and oxygen (bubbles).

Factors that affect enzyme action

Enzyme activity is influenced by:

• Temperature: low temperatures reduce molecular motion and enzyme activity; an optimum temperature gives maximum activity; very high temperatures denature the protein and stop activity.
• pH: each enzyme has an optimum pH; deviations cause denaturation and loss of activity.
• Substrate concentration: increasing substrate increases rate up to a saturation point (Vmax) where all active sites are occupied.
• Enzyme concentration: more enzyme molecules generally increase reaction rate proportionally (when substrate is in excess).

Enzymes in everyday life

Enzymes are used in many industrial and household processes. For example, washing powders often contain enzymes to break down stains:

• Proteases - break down protein stains (blood, egg, gravy).
• Amylases - break down starch-based stains.
• Lipases - break down fat and grease stains.

Nucleic Acids

Nucleic acids are large biomolecules composed of carbon, hydrogen, oxygen, nitrogen and phosphorus. The two main types are:

• Deoxyribonucleic acid (DNA) - found mainly in the nucleus; stores genetic information used to build proteins and to control heredity.
• Ribonucleic acid (RNA) - found in the nucleus and cytoplasm (including ribosomes); helps translate genetic information into proteins (messenger RNA, transfer RNA, ribosomal RNA).

Vitamins

Vitamins are organic compounds required in small amounts for normal metabolism. They act as cofactors or precursors for enzymes and regulatory molecules. Deficiency of vitamins causes characteristic deficiency diseases.

Recommended Dietary Allowance and Nutrition

Recommended Dietary Allowance (RDA)

The Recommended Dietary Allowance (RDA) indicates the average daily intake level of nutrients considered sufficient to meet the requirements of most healthy people. Nutritionists use RDA values to plan diets and assess nutrient intake.

Nutrient labelling and food packaging

Food packaging commonly lists the nutrient content, including amounts of energy, protein, carbohydrate, fat, fibre, vitamins and minerals, and any additives or preservatives. This information helps consumers choose foods that meet their dietary needs.

Energy value of foods

Food energy is measured in kilocalories (kcal, often shortened to 'calories' in diet language) and kilojoules (kJ). A calorie (cal) is defined as the amount of energy needed to raise the temperature of 1 g (equivalently 1 mL) of water by 1 °C; the standard reference is often at 15 °C. In practice, kilocalories (1 kcal = 1 000 cal) and kilojoules are used for food energy.

To convert kilocalories to kilojoules multiply by 4.2:

1 kcal = 4.2 kJ

Energy supplied by macronutrients

The energy provided by macronutrients is:

• 1 g protein → 4 kcal
• 1 g carbohydrate → 4 kcal (dietary fibre contributes little metabolised energy)
• 1 g fat → 9 kcal

Example calculation

Calculate the energy of a hard-boiled egg (50 g) containing 6 g protein, 1 g carbohydrate and 6 g fat.

Energy (kcal) = (6 g protein × 4 kcal/g) + (1 g carbohydrate × 4 kcal/g) + (6 g fat × 9 kcal/g)

Energy (kcal) = 24 + 4 + 54 = 82 kcal

Energy (kJ) = 82 kcal × 4.2 = 344.4 kJ

Balanced diets and malnutrition

A balanced diet provides the right amounts of energy and nutrients to sustain health, growth and activity. Malnutrition refers to poor nutritional status caused by undernutrition (insufficient energy or nutrients) or overnutrition (excess energy leading to obesity).

Causes of malnutrition include:

• Inadequate diet (missing food groups or insufficient quantity).
• Excessive intake of energy-dense, nutrient-poor foods.
• Diseases or genetic disorders that affect nutrient absorption or metabolism.
• Social and environmental factors: poverty, food availability, pollution and illness.

Common nutritional disorders

• Kwashiorkor - severe protein deficiency leading to oedema, fatty liver and growth failure.
• Marasmus - severe deficiency of calories and protein causing extreme wasting.
• Obesity - excess energy intake relative to expenditure increases risk of metabolic disease.
• Deficiency diseases - for example, scurvy (vitamin C), rickets (vitamin D), beri-beri (vitamin B₁), anaemia (iron deficiency).

Guidelines to maintain a healthy diet

• Balance energy intake with physical activity.
• Eat a variety of foods from all major food groups (grains, vegetables, fruits, proteins, dairy or alternatives, oils in moderation).
• Prefer whole grains, legumes, vegetables and fruits for fibre and micronutrients.
• Limit intake of sugars, salt and alcohol.
• Limit saturated fats and dietary cholesterol; prefer unsaturated fats from plant oils and fish.