Chemical composition of living organisms: carbohydrates, lipids, proteins, water and mineral salts

What you'll learn

This revision guide covers the chemical composition of living organisms, focusing on the five main groups of substances found in cells and tissues: carbohydrates, lipids, proteins, water and mineral salts. You will learn the structure, functions and food sources of each nutrient group, along with the biochemical tests used to identify them. This topic forms the foundation for understanding nutrition, digestion and metabolism in the CSEC Human and Social Biology syllabus.

Key terms and definitions

Macromolecule — a large biological molecule made up of many smaller repeating units joined together, such as polysaccharides, proteins and lipids.

Monomer — a single small molecule that can join with others to form a polymer; examples include monosaccharides, amino acids and fatty acids.

Polymer — a large molecule made from many monomers joined together through chemical bonds.

Dehydration synthesis — a chemical reaction that joins monomers together by removing water molecules to form polymers.

Hydrolysis — the breakdown of polymers into monomers by adding water molecules, breaking the chemical bonds between units.

Glycogen — a polysaccharide made from glucose units; the storage form of carbohydrate in animals, particularly in the liver and muscles.

Enzyme — a biological catalyst made of protein that speeds up chemical reactions in living organisms without being used up.

Emulsification — the process of breaking large fat droplets into smaller droplets to increase surface area for enzyme action.

Core concepts

Carbohydrates: structure, types and functions

Carbohydrates are organic compounds containing carbon, hydrogen and oxygen in the ratio 1:2:1 (C:H:O). They serve as the primary energy source for living organisms and provide structural support in plants.

Classification by size:

• Monosaccharides (simple sugars): Single sugar units that cannot be broken down further by hydrolysis. Examples include glucose, fructose and galactose. Glucose is the most important monosaccharide, used directly in cellular respiration to release energy.

• Disaccharides: Two monosaccharides joined together. Common examples include:

  • Sucrose (glucose + fructose) — found in sugar cane, a major Caribbean export crop
  • Maltose (glucose + glucose) — produced during starch digestion
  • Lactose (glucose + galactose) — found in milk

• Polysaccharides: Long chains of many monosaccharides joined together. Important examples include:

  • Starch — storage carbohydrate in plants (dasheen, cassava, sweet potato, yam)
  • Glycogen — storage carbohydrate in animals (liver and muscles)
  • Cellulose — structural carbohydrate in plant cell walls

Functions of carbohydrates:

• Primary source of energy (4 kcal per gram)
• Energy storage (starch in plants, glycogen in animals)
• Structural support (cellulose in plant cell walls)
• Component of DNA and RNA (ribose and deoxyribose sugars)

Food sources in Caribbean diet: Rice, bread, provision (ground provisions like yam, dasheen, cassava), plantain, breadfruit, sweet potato, cornmeal, sugar cane

Lipids: fats, oils and their importance

Lipids are organic compounds composed mainly of carbon, hydrogen and oxygen, but with proportionately less oxygen than carbohydrates. They are insoluble in water but soluble in organic solvents like alcohol and ether.

Structure of lipids:

Most dietary lipids are triglycerides, formed from one glycerol molecule and three fatty acid molecules joined by dehydration synthesis. Fatty acids may be:

• Saturated fatty acids — contain no double bonds between carbon atoms; solid at room temperature (butter, lard, coconut oil)
• Unsaturated fatty acids — contain one or more double bonds between carbon atoms; liquid at room temperature (olive oil, vegetable oils)

Functions of lipids:

• Concentrated energy source (9 kcal per gram — more than twice that of carbohydrates)
• Energy storage in adipose tissue
• Insulation to reduce heat loss from the body
• Protection of vital organs (cushioning)
• Component of cell membranes (phospholipids)
• Production of hormones (steroid hormones)
• Absorption of fat-soluble vitamins (A, D, E, K)

Food sources: Butter, margarine, cooking oils (coconut oil, palm oil), avocado, nuts, oily fish (flying fish, kingfish), fatty meat, cheese

Proteins: building blocks of life

Proteins are large, complex macromolecules made of carbon, hydrogen, oxygen, nitrogen and sometimes sulfur. They are polymers made from amino acid monomers joined by peptide bonds.

Structure of proteins:

• There are approximately 20 different amino acids used to build proteins
• Amino acids join together through dehydration synthesis, forming peptide bonds
• The sequence and number of amino acids determines the protein's specific structure and function
• Proteins can be denatured (permanently changed in shape) by heat or extreme pH, losing their function

Functions of proteins:

• Growth and repair of body tissues
• Production of enzymes (biological catalysts)
• Production of antibodies for immune defense
• Production of hormones (insulin, growth hormone)
• Transport of substances (hemoglobin carries oxygen)
• Structural components (collagen in skin, keratin in hair)
• Source of energy when carbohydrates and fats are insufficient (4 kcal per gram)

Food sources: Meat (chicken, beef, pork), fish (saltfish, tilapia, snapper), eggs, milk, cheese, legumes (red beans, pigeon peas, lentils), nuts, soy products

Complete vs incomplete proteins:

• Complete proteins contain all essential amino acids (amino acids the body cannot make); found in animal sources
• Incomplete proteins lack one or more essential amino acids; found in plant sources
• Combining different plant proteins (rice and peas) provides all essential amino acids

Water: the universal solvent

Water makes up 60-70% of body mass and is essential for all life processes. Although it contains no energy, water is crucial for survival.

Properties of water:

• Excellent solvent for many substances
• High specific heat capacity (helps regulate body temperature)
• Provides medium for chemical reactions
• Transparent, allowing light to pass through

Functions of water in the body:

• Solvent — dissolves nutrients, minerals and waste products for transport
• Transport medium — carries dissolved substances in blood and lymph
• Temperature regulation — cooling through sweat evaporation; absorbs excess heat
• Chemical reactions — provides medium for metabolic reactions; participates in hydrolysis
• Lubrication — reduces friction in joints (synovial fluid), between organs
• Support — maintains cell turgidity; provides cushioning (amniotic fluid, cerebrospinal fluid)
• Waste removal — dissolves and eliminates waste products through urine and sweat

Water balance:

The body gains water through drinking fluids, eating food (especially fruits and vegetables) and metabolic water (produced during cellular respiration). Water is lost through urine, sweat, breathing (water vapor) and feces.

Mineral salts: essential micronutrients

Mineral salts are inorganic substances required in small amounts for proper body functioning. Unlike organic nutrients, they contain no carbon-hydrogen bonds.

Calcium (Ca):

• Functions: Formation and maintenance of bones and teeth; blood clotting; muscle contraction; nerve impulse transmission
• Sources: Milk, cheese, small fish with bones (sardines), green leafy vegetables
• Deficiency: Rickets in children (soft, deformed bones); osteoporosis in adults (brittle bones); poor blood clotting; tooth decay

Iron (Fe):

• Functions: Component of hemoglobin in red blood cells (oxygen transport); component of myoglobin in muscles; component of some enzymes
• Sources: Red meat, liver, green leafy vegetables (callaloo), legumes, fortified cereals
• Deficiency: Anemia (reduced oxygen-carrying capacity), fatigue, weakness, pale skin

Phosphorus (P):

• Functions: Formation of bones and teeth (with calcium); component of cell membranes; component of ATP (energy transfer); component of DNA and RNA
• Sources: Milk, cheese, meat, fish, eggs, nuts
• Deficiency: Weak bones and teeth (rare as phosphorus is widespread in foods)

Sodium (Na):

• Functions: Maintenance of water balance; nerve impulse transmission; muscle contraction
• Sources: Table salt, processed foods, salted fish, cheese
• Deficiency: Muscle cramps, weakness (rare); Excess: High blood pressure, water retention

Iodine (I):

• Functions: Component of thyroid hormones (thyroxine) which regulate metabolic rate and growth
• Sources: Iodized salt, seafood, sea vegetables
• Deficiency: Goiter (enlarged thyroid gland); cretinism in infants (mental and physical retardation)

Biochemical tests for nutrients

The CSEC syllabus requires knowledge of food tests to identify the presence of specific nutrients.

Test for reducing sugars (glucose, fructose):

• Reagent: Benedict's solution (blue)
• Method: Add Benedict's solution to food sample and heat in water bath
• Positive result: Color change from blue → green → yellow → orange → brick red precipitate

Test for starch:

• Reagent: Iodine solution (brown/yellow)
• Method: Add iodine solution to food sample at room temperature
• Positive result: Blue-black color

Test for protein (Biuret test):

• Reagent: Sodium hydroxide solution, then copper sulfate solution
• Method: Add sodium hydroxide, then a few drops of dilute copper sulfate to food sample
• Positive result: Purple/violet color

Test for lipids (grease spot test):

• Method: Rub food sample on brown paper and hold up to light
• Positive result: Translucent grease spot that does not disappear when paper dries

Test for lipids (emulsion test):

• Reagent: Ethanol
• Method: Shake food sample with ethanol, then pour into water
• Positive result: White cloudy emulsion forms

Worked examples

Example 1: A student tested four food samples (A, B, C, D) using different reagents. The results are shown in the table below.

(a) Which nutrient(s) are present in Sample A? (2 marks)

Answer: Starch and protein are present. The blue-black color with iodine indicates starch (1 mark). The purple color with Biuret reagent indicates protein (1 mark).

(b) Suggest one Caribbean food that could be Sample C. (1 mark)

Answer: Provision such as dasheen/yam/sweet potato OR breadfruit (any suitable starchy food containing protein and reducing sugar). (1 mark)

Example 2: Explain why the body needs both iron and calcium. (4 marks)

Answer: Iron is needed to form hemoglobin in red blood cells (1 mark), which transports oxygen from the lungs to the body tissues (1 mark).

Calcium is needed for the formation and maintenance of strong bones and teeth (1 mark). Calcium is also required for blood clotting / muscle contraction / nerve impulse transmission (1 mark for any one additional function).

Example 3: A person's diet consists mainly of white rice, fried chicken and soft drinks.

(a) State TWO nutrients that are likely to be deficient in this diet. (2 marks)

Answer: Any TWO from: Calcium (1 mark); Iron (1 mark); Vitamin C (1 mark); Dietary fiber/cellulose (1 mark); Water (1 mark)

(b) Suggest ONE health problem that may develop due to this diet. (1 mark)

Answer: Obesity / heart disease / high cholesterol / type 2 diabetes / constipation / anemia / weak bones (any ONE suitable condition) (1 mark)

Common mistakes and how to avoid them

• Confusing the chemical elements in nutrients: Remember that proteins contain nitrogen (and sometimes sulfur), which carbohydrates and lipids do NOT contain. Use the mnemonic CHO for carbohydrates and lipids, CHON for proteins.

• Mixing up monosaccharides and disaccharides: Glucose, fructose and galactose are monosaccharides (single units). Sucrose, maltose and lactose are disaccharides (two units joined). Learn the combinations: sucrose = glucose + fructose.

• Stating functions too vaguely: Don't write "calcium is good for bones." Be specific: "Calcium is needed for the formation and maintenance of bones and teeth" or "Calcium combines with phosphorus to form calcium phosphate in bones."

• Forgetting Benedict's test requires heating: A common error is saying Benedict's test works at room temperature. You must heat the mixture in a water bath for the color change to occur. The iodine test for starch works at room temperature.

• Confusing deficiency diseases: Iron deficiency causes anemia (not rickets). Calcium deficiency causes rickets in children and osteoporosis in adults. Iodine deficiency causes goiter.

• Not using correct color descriptions for food tests: For Benedict's test, describe the full range (blue → green → yellow → orange → brick red), not just "red." For the Biuret test, the positive result is purple/violet, not just "color change."

Exam technique for chemical composition questions

• Command word awareness: "State" requires a simple fact (1 mark each). "Explain" requires a reason or mechanism (usually 2 marks — statement + elaboration). "Describe" requires multiple linked points about a process.

• Food test questions: Always include both the reagent name AND the positive result with specific colors. For Benedict's test, mention heating. Award yourself 1 mark for reagent, 1 mark for correct positive result.

• Function questions: Give specific functions with biological detail. "Energy" alone is insufficient — write "provides energy for cellular respiration/metabolism/physical activity." Link structure to function where possible.

• Drawing from Caribbean context: When asked for food sources or examples, use familiar Caribbean foods (saltfish, callaloo, provision, coconut oil) to demonstrate applied knowledge and gain full marks.

Quick revision summary

Living organisms contain five main chemical groups. Carbohydrates (C:H:O in 1:2:1 ratio) include monosaccharides, disaccharides and polysaccharides; they provide energy and structure. Lipids are fats and oils that store concentrated energy and insulate organs. Proteins are made from amino acids and function in growth, repair, enzymes and antibodies. Water acts as a solvent, transport medium and temperature regulator. Mineral salts like calcium, iron and iodine are essential for bones, blood and metabolism. Food tests identify these nutrients using specific reagents and color changes.