# 1.0–1.2 Molecules of Life

F.3 Biology · Topic I · Lesson 1

What organisms are made of, the jobs water does for them, and what each inorganic ion is needed for.

## Key points

- Organisms are made of **inorganic substances** (water, minerals) and **organic substances** — carbohydrates, lipids, proteins and nucleic acids, the **biomolecules ／ 生物分子**, which all contain **carbon**.
- Water is the largest single component of the human body, about **65 %** by mass; the rest is mostly proteins and lipids.
- Water is a good **solvent** and **transport medium**: dissolved reactants can move freely and meet, and blood carries nutrients, gases and wastes around the body.
- Water is a **temperature buffer**: its high **specific heat capacity** ($c = 4200$ J kg^−1^ °C^−1^) keeps body temperature fairly stable and makes oceans and lakes a stable habitat.
- Water is a **cooling agent**: its high **latent heat of vaporization** ($l_v = 2.26\times10^6$ J kg^−1^) means evaporating sweat — and **transpiration ／ 蒸騰作用** in plants — takes heat away.
- Water is a **reactant** in digestion and in photosynthesis: $\text{carbon dioxide}+\text{water}\xrightarrow{\ \text{light energy, chlorophyll}\ }\text{glucose}+\text{oxygen}$.
- Water **supports** and **lubricates**: turgid plant cells, the **hydrostatic skeleton ／ 靜水骨骼** of earthworms, buoyancy, and synovial fluid in joints. It is densest at **4 °C** and ice is less dense, so ice floats and insulates the water below.

## Must know

**Inorganic ions ／ 無機離子**

| Element | Inorganic ion | Needed for |
|---|---|---|
| Magnesium | Mg^2+^ | forming **chlorophyll** in plants |
| Calcium | Ca^2+^ | muscle contraction · blood clotting · bones and teeth |
| Iron | Fe^2+^ | forming **haemoglobin ／ 血紅素** in red blood cells, to carry oxygen |
| Nitrogen | NO~3~^−^ / NH~4~^+^ | amino acids and proteins · nucleic acids · chlorophyll |
| Phosphorus | PO~4~^3−^ | bones and teeth · nucleic acids · cell membranes |

## Common mistakes

- ✗ "Sweating cools the body." ✓ Sweat reaches the skin at body temperature. The cooling comes from its **evaporation**, which takes a lot of heat from the skin.
- ✗ "Sweating cools you because water has a high specific heat capacity." ✓ Cooling by sweat is the **latent heat of vaporization**. High specific heat capacity is why temperature stays **stable**.
- ✗ "Magnesium is needed for blood clotting." ✓ **Magnesium → chlorophyll.** **Calcium → muscle contraction, blood clotting, bones and teeth.**
- ✗ "Ice floats because ice keeps the cold out." ✓ Water is densest at **4 °C** and ice is less dense than water, so ice floats. The floating layer then **insulates** the water below.
- ✗ "Water is a biomolecule." ✓ Biomolecules are **organic** — they contain carbon. Water is an **inorganic** substance.

## Quick check

1. Name the property of water behind each: a lake is a stable habitat; sweating cools you; ice floats.
2. Which ion does a plant need to make chlorophyll, and which one do you need for blood clotting?
3. Why is water not counted as a biomolecule?

## Answers

1. Stable habitat — high specific heat capacity. Cooling — high latent heat of vaporization. Ice floats — water is densest at 4 °C, so ice is less dense.
2. Chlorophyll: magnesium ion (Mg^2+^), with nitrate or ammonium ions. Blood clotting: calcium ion (Ca^2+^).
3. Biomolecules are organic and contain carbon; water is inorganic and contains no carbon.

## Did you know?

An adult body holds about 1 kg of calcium, almost all of it in bones and teeth, but only about 4 g of iron — most of that inside haemoglobin.