Chemistry of Life

From atoms to organisms

All living organisms are made of atoms that combine to form molecules. These molecules make up the basic units of life — cells — which are organised into tissues, organs, and eventually whole organisms. To understand how cells work, we first need to understand the chemistry they're built from.

Biological compounds are grouped into two broad categories:

Inorganic compounds — do not contain both carbon and hydrogen bonded together as their main structural feature. In biology, the key inorganic compound is water.
Organic compounds — built around a "backbone" of carbon (C), hydrogen (H), and oxygen (O) atoms (some also contain nitrogen, phosphorus, or sulfur). These are the molecules of life: carbohydrates, lipids, proteins, and nucleic acids.

Water — the inorganic compound of life

A water molecule (H₂O) consists of 2 hydrogen atoms bonded to 1 oxygen atom. Water makes up the majority of every living cell and is essential for life:

It's the solvent in which most chemical reactions in the cell take place.
It transports dissolved substances around the body (e.g. in blood plasma or plant xylem/phloem sap).
It helps regulate temperature — water absorbs and releases heat slowly, keeping conditions inside cells stable.
It's a reactant in many biological reactions, such as photosynthesis.

Minerals are the other main inorganic compounds in living organisms — elements like calcium, iron, and potassium play structural roles (e.g. calcium in bone) and are needed in small amounts for healthy cell function.

Carbohydrates — energy and structure

Carbohydrates are built from carbon, hydrogen, and oxygen, and come in three sizes:

Type	Meaning	Examples
Monosaccharides	Single sugar unit	Glucose, fructose
Disaccharides	Two sugar units joined	Sucrose, maltose
Polysaccharides	Many sugar units joined in a chain	Starch, cellulose, glycogen

Carbohydrates are mainly used as an energy source (glucose is broken down during respiration) and, in the case of cellulose, provide structural support in plant cell walls.

Lipids — fats and oils

A lipid molecule is made of 1 glycerol molecule bonded to 3 fatty acid chains. Fatty acids can be:

Saturated — no double bonds between carbon atoms; usually solid at room temperature (e.g. animal fats). Linked to raised cholesterol and heart disease risk when eaten in excess.
Unsaturated — contain double bonds; usually liquid at room temperature (e.g. plant oils).

Lipids store energy (more energy-dense than carbohydrates), insulate the body, protect organs, and form the cell membrane.

Proteins — the workhorses of the cell

Proteins are built from smaller units called amino acids, containing carbon, hydrogen, oxygen, and nitrogen (some also contain sulfur or phosphorus). Proteins have many roles: structural (e.g. keratin in hair), transport (e.g. haemoglobin), and — most importantly for this lesson — as enzymes.

Proteins are sensitive to temperature and pH. Extreme conditions change their shape — this is called denaturation, and it stops the protein from working.

Nucleic acids — the genetic code

DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are nucleic acids built from carbon, hydrogen, oxygen, nitrogen, and phosphorus. They store and transmit the genetic instructions that direct how a cell is built and how it functions. You'll study their detailed structure in Grade 12 — for now, it's enough to know they exist and what elements they contain.

Cells also require small amounts of vitamins (such as A, the B vitamins, C, D, and E) to function normally.

Enzymes — biological catalysts

Enzymes are proteins that speed up chemical reactions in the cell without being used up themselves. Each enzyme has a uniquely shaped active site that fits a specific substrate molecule — this is called the lock-and-key model.

The substrate binds to the enzyme's active site.
The enzyme speeds up the reaction, converting the substrate into product(s).
The product is released, and the enzyme is free to bind another substrate molecule.

Enzymes work fastest at an optimum temperature and pH. Below the optimum, reactions are slow because molecules move less. Above the optimum, the enzyme's shape starts to change (denature) — the active site no longer fits the substrate, and the enzyme stops working. Try this yourself in the Explore tab!

Answer in your exercise book.

Question 1 · (3 marks)

Distinguish between inorganic and organic compounds. Give ONE example of each as found in a living cell.

Question 2 · (4 marks)

Describe the structure of a water molecule and state TWO functions of water in living organisms.

Question 3 · (5 marks)

Name the three categories of carbohydrates based on size, and give ONE example of each. State the main function of carbohydrates in the body.

Question 4 · (3 marks)

Describe the basic structure of a lipid molecule and explain the difference between a saturated and an unsaturated fat.

Question 5 · (5 marks)

Using the lock-and-key model, explain how an enzyme works. In your answer, explain what happens to enzyme activity when the temperature rises far above the optimum, using the term "denaturation".

Question 6 · (7 marks)

The enzyme catalase breaks down hydrogen peroxide into water and oxygen gas. A learner collects the oxygen gas produced by the same amount of catalase at two different temperatures and records the following data:

Time (s)	Volume O₂ at 37°C (cm³)	Volume O₂ at 60°C (cm³)
0	0	0
20	10	3
40	18	4
60	24	4
80	24	4

(a) Calculate the average rate of oxygen production (in cm³/s) during the first 40 seconds at 37°C. Show your working.
(b) Calculate the average rate of oxygen production (in cm³/s) during the first 40 seconds at 60°C.
(c) Using your answers to (a) and (b), and the concept of denaturation, explain why the rate at 60°C is so much lower than at 37°C.
(d) Explain why the volume of O₂ stops increasing after 60 seconds at 37°C, even though the enzyme has not denatured.

From atoms to organisms

Water — the inorganic compound of life

Carbohydrates — energy and structure

Lipids — fats and oils

Proteins — the workhorses of the cell

Nucleic acids — the genetic code

Enzymes — biological catalysts

Enzyme Action Lab

Lock-and-key model