Sugars: glucose, sucrose, starch and cellulose
Sugars (carbohydrates) are made of carbon, hydrogen and oxygen. The smallest unit is a monosaccharide such as glucose (CβHββOβ), the fuel our cells burn for energy.
- Glucose: one ring, sweet, found in fruits and in our blood.
- Sucrose: table sugar, made of glucose + fructose joined together (two rings). It is a disaccharide.
- Starch: a long chain of many glucose units. Plants store food as starch (rice, wheat, potato). It is a polysaccharide.
- Cellulose: another long chain of glucose, but joined differently. It makes plant cell walls and cotton. Humans cannot digest it, but it is fibre that helps the gut.
Fats and proteins
Fats are esters of glycerol with three fatty acids. They store more than twice as much energy as sugars per gram, protect organs and help us absorb some vitamins. Oils are liquid fats. Too much fat can harm the heart.
Proteins are long chains of amino acids. There are about 20 kinds of amino acids, and different orders make different proteins. The chain folds into a special shape. Proteins build muscles, skin and hair, and enzymes (which speed up reactions) and antibodies are proteins. Heat or acid can unfold a protein, like when an egg white turns solid on boiling.
Medicines and how they work
A medicine (drug) is a chemical that acts on the body to treat, cure or prevent a disease. Most medicines work like a key in a lock: the drug molecule fits a receptor or enzyme in the body and changes what it does.
- Analgesics: reduce pain (paracetamol, aspirin).
- Antipyretics: reduce fever.
- Antibiotics: kill or stop bacteria (not viruses).
- Antacids: reduce excess stomach acid.
- Antiseptics and disinfectants: kill germs on skin or on surfaces.
Safety: take medicine only when a doctor or pharmacist advises. The wrong dose or the wrong medicine can harm.
Vitamins and deficiency diseases
Vitamins are organic compounds we need in tiny amounts. The body cannot make most of them, so food must supply them. A lack of a vitamin causes a deficiency disease.
| Vitamin | Good sources | Lack causes |
|---|---|---|
| A | Carrot, milk, green leaves | Night blindness |
| B1 | Whole grains, pulses | Beriberi |
| C | Citrus fruits, amla | Scurvy |
| D | Sunlight, fish, eggs | Rickets |
Vitamins A and D dissolve in fat; B and C dissolve in water. A balanced diet usually gives enough.
Try it
1. In the 3D, watch the sugar step. How many rings make starch, and how many make sucrose?
2. At home: chew a piece of plain rice or bread for two minutes. It starts to taste sweet, because enzymes in saliva break starch into smaller sugars. Write what you notice.
Key formulas and definitions
- Glucose CβHββOβ; sucrose = glucose + fructose
- Starch and cellulose = many glucose units joined
- Fat = glycerol + 3 fatty acids
- Protein = chain of amino acids
- Vitamin lack β deficiency disease (A: night blindness, B1: beriberi, C: scurvy, D: rickets)
Worked examples
1. Which vitamin lack causes bleeding gums and which food helps?
Vitamin C lack causes scurvy. Eating citrus fruits or amla helps.
2. A starch chain has 1000 glucose units and each joint loses one water molecule. How many joints and water molecules?
1000 units in one chain have 999 joints, so 999 water molecules are lost.
3. A child has weak, bent bones. Which vitamin and what simple help?
Vitamin D lack (rickets). Sunlight, eggs, fish and milk help, and see a doctor.
Common mistakes
- Thinking all sugars are the same. Glucose, sucrose, starch and cellulose differ in size and job.
- Thinking humans digest cellulose. We cannot, but it is useful fibre.
- Thinking antibiotics work on viruses like flu. They work on bacteria only.
- Taking vitamins or medicines in large amounts without advice.