Biological Molecules

Carbohydrates. Made of carbon, hydrogen and oxygen (C, H, O).

• Monosaccharides (single sugar units): glucose, fructose, galactose. Used directly for energy.
• Disaccharides (two units): maltose, sucrose, lactose.
• Polysaccharides (many units): starch (plant energy store), glycogen (animal energy store), cellulose (plant cell walls).

Made by joining glucose units together. Broken down to release glucose for respiration.

Proteins. Made of carbon, hydrogen, oxygen, NITROGEN (and sometimes sulfur).

• Amino acids are the building blocks (~20 types).
• Polypeptides: chains of amino acids.
• Proteins fold into specific 3D shapes.
• Functions: growth and repair (muscle, skin), enzymes, hormones, antibodies, transport (haemoglobin).

Fats (lipids / triglycerides). Made of C, H, O.

• One glycerol + three fatty acids = a triglyceride.
• Energy store (more energy per gram than carbs).
• Insulation (under skin).
• Make membranes (phospholipids).

DNA (deoxyribonucleic acid). Genetic material.

• Made of nucleotides (sugar + phosphate + base).
• Double helix structure.
• Stores genetic information that codes for proteins.

Worked qualitative. Why do athletes carbohydrate-load before a marathon?

• Carbohydrates → digested to glucose → glycogen stored in muscles + liver.
• During the race, glycogen is broken down for energy.
• More glycogen = more available energy = better endurance.

1. Test for STARCH.

• Reagent: IODINE solution (yellow-brown).
• Method: Add a few drops to the food.
• Positive: iodine turns BLUE-BLACK.
• Negative: stays yellow-brown.

2. Test for REDUCING SUGAR (e.g. glucose).

• Reagent: BENEDICT'S solution (blue).
• Method: Add equal volume of Benedict's reagent. WARM in a hot water bath ($\sim 80\degree\text{C}$).
• Positive: blue → green → yellow → orange → BRICK-RED (depends on amount of sugar).
• Negative: stays blue.

3. Test for PROTEIN.

• Reagent: BIURET reagent (or NaOH + dilute CuSO₄).
• Method: Add to the food. NO heat needed.
• Positive: blue → PURPLE (lilac).
• Negative: stays blue.

4. Test for FAT (lipid).

• Reagent: ETHANOL.
• Method: Add ethanol, shake, then add water.
• Positive: white CLOUDY EMULSION forms.
• Negative: stays clear.

Order matters. For fat test: ethanol FIRST (dissolves fat), then water (forms emulsion). Wrong order = no result.

Cambridge tip. Always state both the reagent AND the colour change FROM → TO. "Iodine turns blue-black" loses a mark; "iodine turns from brown to blue-black" earns it.

Roles of water in living organisms.

1. Solvent.

• Most chemical reactions in cells take place in water (cytoplasm is mostly water).
• Dissolves nutrients, salts, gases (O₂, CO₂), and waste products for transport.

2. Transport medium.

• Blood plasma is ~90% water — carries dissolved nutrients, hormones, urea, and CO₂.
• Plant sap (in xylem and phloem) is mostly water — carries water + minerals UP, sugars DOWN.

3. Reactant.

• Photosynthesis: water is split for hydrogen, oxygen released.
• Hydrolysis (digestion): water added to break down polymers (e.g. starch → glucose, protein → amino acids).

4. Temperature regulation.

• Sweating: water evaporates → high latent heat of vaporisation absorbs lots of energy → cools the body.
• Plants transpire to cool leaves.

5. Other.

• Lubricant (synovial fluid in joints, mucus in gut).
• Support (turgor pressure in plant cells; amniotic fluid around foetus).

Cambridge tip. When asked "give one role of water in plants AND one in animals", be specific:

• Plants: maintains turgor pressure (water in vacuoles); transport in xylem; reactant in photosynthesis.
• Animals: transport in blood plasma; cooling by sweating; medium for chemical reactions.

Worked qualitative. Why do dehydrated plants wilt? Cells lose water from their vacuoles → less turgor pressure → flaccid cells → leaves and stems collapse. Watering reverses this.