1. Key Biological Molecules
| Molecule | Monomers / subunits | Functions | Examples |
|---|---|---|---|
| Carbohydrates | Monosaccharides (e.g. glucose) | Energy source; structural (cellulose); energy store (starch, glycogen) | Glucose, starch, glycogen, cellulose, sucrose |
| Proteins | Amino acids (20 types) | Enzymes, hormones, structural (muscle), antibodies, transport (haemoglobin) | Insulin, haemoglobin, collagen, amylase |
| Lipids (fats/oils) | Fatty acids + glycerol | Long-term energy store; thermal insulation; cell membrane structure; fat-soluble vitamins | Triglycerides, phospholipids |
Monosaccharides (single sugar) → Disaccharides (two sugars joined) → Polysaccharides (many sugars). Starch and glycogen are energy stores. Cellulose forms plant cell walls. All are made of glucose monomers but joined in different ways.
2. Food Tests
| Food substance | Reagent | Positive result | Negative result |
|---|---|---|---|
| Reducing sugars (e.g. glucose) | Benedict's solution + heat | Brick-red/orange precipitate | Stays blue |
| Starch | Iodine solution | Blue-black colour | Stays yellow-brown |
| Protein | Biuret reagent (NaOH + CuSO₄) | Purple/violet colour | Stays blue |
| Lipids (emulsion test) | Ethanol then water | White emulsion (cloudy) | Stays clear |
Benedict's tests for reducing sugars — this includes glucose, fructose, maltose but NOT sucrose (a non-reducing sugar). To test for sucrose: hydrolyse with dilute acid or sucrase enzyme first, then test with Benedict's — it will then give a positive result.
3. Digestion
Digestion breaks large, insoluble food molecules into small, soluble molecules that can be absorbed into the blood.
| Food substance | Enzyme | Products | Site |
|---|---|---|---|
| Starch | Amylase | Maltose | Mouth, small intestine |
| Maltose | Maltase | Glucose | Small intestine |
| Proteins | Protease (e.g. pepsin, trypsin) | Amino acids | Stomach, small intestine |
| Fats/lipids | Lipase | Fatty acids + glycerol | Small intestine |
Role of bile
Bile is produced by the liver, stored in the gall bladder and released into the duodenum. It emulsifies fats — breaking large fat droplets into smaller ones. This increases the surface area for lipase to act on. Bile is not an enzyme — it does not chemically digest fats.
Glucose and amino acids are absorbed by active transport and diffusion into blood capillaries in the villi. Fatty acids and glycerol recombine to form fat droplets that enter lacteals (lymph vessels) in the villi.
- Food tests: Benedict's (reducing sugars: blue to brick-red), Iodine (starch: orange to blue-black), Biuret (proteins: blue to purple), Emulsion (lipids: cloudy white)
- Digestive enzymes: Amylase (starch to maltose), Protease (protein to amino acids), Lipase (lipid to glycerol + fatty acids)
- Monomers join by condensation (water released); polymers broken by hydrolysis (water added)
- Starch: plants, insoluble, branched. Glycogen: animals, insoluble, highly branched. Cellulose: plants, insoluble, structural.
4. Common Exam Traps
Bile emulsifies fat (physical process — increases surface area) but does not chemically digest it. Lipase digests fat. This is a 1-mark distinction that frequently appears.
The purpose of digestion is not just to break food down — it is specifically to produce molecules small enough to be absorbed through the gut wall into the blood. State both: small AND soluble.
A positive Biuret result turns purple/violet. The reagent itself is blue (copper sulfate). "The solution turns blue" is a negative result, not positive.
Key Terms — Flashcard Review
Tap each card to reveal the definition.
🎯 Practice Quiz — Test Yourself
8 O Level-style questions on this topic. Select an answer to see instant feedback.
Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.