1. Carbohydrates

In brief:Carbohydrates are made of carbon, hydrogen, and oxygen. They range from simple sugars (monosaccharides) to complex chains (polysaccharides) and serve structural and energy roles.

Carbohydrates contain the elements C, H, and O in the ratio Cₙ(H₂O)ₙ.

Three types:

Monosaccharides (1 sugar unit) - glucose, fructose, galactose. Quick energy source.
Disaccharides (2 sugar units joined by condensation) - sucrose (glucose + fructose), maltose (glucose + glucose), lactose (glucose + galactose).
Polysaccharides (many sugar units) - starch (energy storage in plants), glycogen (energy storage in animals), cellulose (structural - plant cell walls).

Roles of carbohydrates:

Structural - cellulose forms rigid plant cell walls; chitin in insect exoskeletons
Metabolic/Energy - glucose is broken down in respiration to release ATP energy

Food test: Benedict's test for reducing sugars (turns brick-red when heated). Iodine test for starch (turns blue-black).

Glucose (monosaccharide) — Glucose — a monosaccharide carbohydrate
Wikimedia Commons (public domain / CC)

Sucrose (disaccharide) — Sucrose — glucose + fructose (disaccharide)
Wikimedia Commons (public domain / CC)

Amylose (starch) — Amylose — a polysaccharide (starch)
Wikimedia Commons (public domain / CC)

table

Types of Carbohydrates

Type	Sugar Units	Examples	Role
Monosaccharide	1	Glucose, Fructose	Quick energy
Disaccharide	2	Sucrose, Maltose, Lactose	Transport sugar
Polysaccharide	Many	Starch, Glycogen, Cellulose	Storage/Structure

Key Points

1Carbohydrates contain C, H, O in the ratio Cₙ(H₂O)ₙ.
2Monosaccharides (glucose) → Disaccharides (sucrose) → Polysaccharides (starch).
3Structural role: cellulose in plant cell walls. Energy role: glucose in respiration.
4Benedict's test detects reducing sugars; iodine test detects starch.
5Condensation joins sugars; hydrolysis breaks them apart.

Learning Outcomes

Outline the structures and metabolic roles of carbohydrate, lipid and protein, and identify nutritional sources of each
Investigate qualitatively the presence of nutrients in a range of food samples, use primary data to support conclusions
Investigate quantitatively the level of reducing sugars in a range of food samples, use primary data to support conclusions

2. Lipids

In brief:Lipids include fats, oils, and waxes. They are made from glycerol and fatty acids and serve as energy stores, insulation, and cell membrane components.

Lipids contain C, H, and O but have a much higher proportion of H than carbohydrates, meaning they store more energy per gram.

Structure of a triglyceride: One glycerol molecule bonded to three fatty acid chains by ester bonds (condensation reaction).

Types of fatty acids:

Saturated - no double bonds between carbons. Solid at room temperature (animal fats: butter, lard). Too much increases cholesterol and heart disease risk.
Unsaturated - one or more double bonds. Liquid at room temperature (plant oils: olive oil, sunflower oil). Healthier choice.
Trans fats - artificially hydrogenated unsaturated fats. Found in processed foods. Increase heart disease risk.

Phospholipids: Glycerol + 2 fatty acids + 1 phosphate group. Amphipathic (hydrophilic head, hydrophobic tail). Form the phospholipid bilayer of cell membranes.

Roles of lipids:

Energy storage - adipose tissue stores fat; lipids provide 2× more energy than carbs
Insulation - subcutaneous fat retains body heat
Protection - fat cushions organs (kidneys, heart)
Structural - phospholipids in cell membranes; waxes waterproof leaves

Food test: Emulsion test - shake with ethanol, pour into water. A cloudy white emulsion = lipid present.

Triglyceride — glycerol + 3 fatty acids
Wikimedia Commons (public domain / CC)

Key Points

1Triglyceride = glycerol + 3 fatty acids joined by ester bonds.
2Saturated fats (no double bonds) increase cholesterol; unsaturated fats (double bonds) are healthier.
3Phospholipids form the cell membrane bilayer (hydrophilic head, hydrophobic tail).
4Lipids store 2× more energy per gram than carbohydrates.
5Roles: energy storage, insulation, protection, cell membrane structure.
6Emulsion test: ethanol + water → cloudy white = lipid present.

Learning Outcomes

Outline the structures and metabolic roles of carbohydrate, lipid and protein, and identify nutritional sources of each
Investigate qualitatively the presence of nutrients in a range of food samples, use primary data to support conclusions

3. Proteins, Minerals & ATP

In brief:Proteins are polymers of amino acids with diverse structural and metabolic roles. Minerals like calcium are essential for health. ATP is the energy currency of cells.

Proteins are made from amino acids joined by peptide bonds (condensation reaction). There are 20 different amino acids.

Protein roles:

Structural - keratin (hair, nails), collagen (skin, tendons), myosin (muscle)
Metabolic - enzymes (biological catalysts), antibodies (immune defence), haemoglobin (oxygen transport)

Food test: Biuret test - add NaOH then CuSO₄. Purple/lilac colour = protein present.

Minerals:

Calcium - essential for strong bones and teeth. Deficiency causes osteoporosis (porous, fragile bones). Sources: dairy, green vegetables, fortified foods.
Iron - needed for haemoglobin in red blood cells. Deficiency causes anaemia.

Osteoporosis: A disease where bones become porous and fragile. Affects 300,000 people in Ireland. 1 in 2 women and 1 in 4 men over 50 will develop a fracture. Prevention: calcium-rich diet, vitamin D, weight-bearing exercise.

ATP (Adenosine Triphosphate):

The universal energy currency of all living cells
Structure: adenine base + ribose sugar + 3 phosphate groups
Energy is stored in the bonds between the phosphate groups
ATP → ADP + Pᵢ + Energy (hydrolysis releases energy for cellular work)
ADP + Pᵢ + Energy → ATP (formed during respiration)

Amino acid structure — Amino acid — building block of proteins
Wikimedia Commons (public domain / CC)

Protein structure levels — Protein structure — primary, secondary, tertiary, quaternary
Wikimedia Commons (public domain / CC)

equation

ATP Energy Release

ATP → ADP + Pᵢ + Energy
Hydrolysis breaks a phosphate bond, releasing energy for cell work

ADP + Pᵢ + Energy → ATP
Formed during respiration (phosphorylation)

Key Points

1Proteins = amino acids joined by peptide bonds. 20 different amino acids exist.
2Structural proteins: keratin, collagen, myosin. Metabolic: enzymes, antibodies.
3Biuret test: NaOH + CuSO₄ → purple = protein present.
4Calcium deficiency causes osteoporosis (porous bones). Affects 300,000 in Ireland.
5ATP = adenine + ribose + 3 phosphate groups. Energy stored in phosphate bonds.
6ATP → ADP + Pᵢ releases energy; respiration regenerates ATP.

Learning Outcomes

Outline the structures and metabolic roles of carbohydrate, lipid and protein, and identify nutritional sources of each
Recognise the roles of minerals in biological processes
Outline the role of ATP, NAD+ and NADP+ in metabolic pathways

4. ATP - The Energy Currency of the Cell

In brief:ATP (adenosine triphosphate) is the molecule cells use to store and release energy for every biological process, from muscle contraction to protein synthesis.

Every cell needs a way to move energy from where it is made (respiration, photosynthesis) to where it is used (movement, biosynthesis, active transport). ATP is that portable energy carrier.

Structure of ATP:

Adenine - a nitrogenous base
Ribose - a 5-carbon sugar (adenine + ribose = adenosine)
Three phosphate groups - joined in a chain by high-energy bonds

How ATP releases energy: When a cell needs energy, the terminal phosphate is removed by hydrolysis, forming ADP (adenosine diphosphate) plus a free phosphate. This reaction releases a usable burst of energy. A second phosphate can also be removed to form AMP, releasing more energy.

ATP → ADP + Pi + energy

Recharging ATP: Energy released during respiration (from glucose) or captured during photosynthesis (from sunlight) is used to reattach a phosphate to ADP, re-forming ATP. A typical cell recycles its own body weight of ATP many times each day.

Why ATP and not glucose directly? Glucose stores far more energy per molecule, but ATP releases small, manageable packets on demand - just enough for one enzyme reaction, one muscle twitch, one ion pumped across a membrane.

ATP molecule — The structure of adenosine triphosphate (ATP)
Wikimedia Commons (CC)

svg

The ATP-ADP Cycle

Key Points

1ATP = adenine + ribose + 3 phosphate groups.
2Energy is stored in the bonds between the phosphate groups.
3ATP → ADP + Pi releases a small, usable packet of energy.
4ADP is recharged to ATP using energy from respiration or photosynthesis.
5ATP powers muscle contraction, active transport, and biosynthesis.

5. Food Tests - Practical Chemistry of Biomolecules

In brief:Three standard qualitative tests identify reducing sugars, starch and protein in food samples using colour changes.

To identify which biomolecules a food contains, biologists use characteristic colour-change tests. Each test detects a specific chemical group.

Reducing sugars (e.g. glucose, fructose) - Benedict's test

Grind the food and mix with a small volume of deionised water in a test tube.
Set up a control tube with water only.
Add ~2 ml of Benedict's reagent to each tube and swirl gently.
Heat in a water bath at ~80 °C for around five minutes.

Positive result: the blue solution turns green → yellow → orange → brick-red, depending on how much sugar is present. The control stays blue.

Starch - Iodine test

Prepare the food solution as above.
Add a few drops of iodine solution.

Positive result: a blue-black colour develops if starch is present. Iodine stays orange-brown if not.

Protein - Biuret test

Prepare the food solution and a water control.
Add an equal volume of Biuret reagent (sodium hydroxide + copper sulfate) and swirl.

Positive result: the blue reagent turns violet/purple when peptide bonds are present. No heating is needed. The control stays blue.

Quantitative extension: after the Benedict's or Biuret test, sample colour intensity can be measured in a colorimeter. Comparing the absorbance to a standard curve gives the actual concentration of sugar or protein in the food.

table

Food Test Summary

Biomolecule	Reagent	Conditions	Positive result
Reducing sugar	Benedict's	Heat ~80 °C, 5 min	Blue → brick-red
Starch	Iodine	Room temperature	Orange-brown → blue-black
Protein	Biuret	Room temperature	Blue → violet/purple

Key Points

1Benedict's detects reducing sugars; needs heat; colour goes blue → brick-red.
2Iodine detects starch at room temperature; positive result is blue-black.
3Biuret detects peptide bonds in proteins; blue → violet at room temperature.
4Always run a water control to compare colour changes.
5Colorimeter + standard curve turns any of these into a quantitative test.