Cycles of Life
Carbon and water never run out — they go round and round. Trace the atoms as photosynthesis, respiration, combustion and rain pass them from air to living things and back again.
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Cycles of Life 🔁
The carbon atoms in your breath were once in a dinosaur, a tree, the sea. Materials on Earth are **finite** — they never leave, they just get **recycled** through living things and back into the environment, over and over. This module follows two of those loops: the **carbon cycle** and the **water cycle**.
Carbon goes in 🌿
Carbon starts as **carbon dioxide** in the air. **Photosynthesis** in green plants and algae **removes** that CO₂ and locks the carbon into **carbohydrates, proteins and fats**. That carbon then passes **along food chains** as animals eat the plants — building the carbon into their own bodies.
Taking carbon out
Which process **removes** carbon dioxide from the atmosphere and locks the carbon into living things?
- Photosynthesis
- Respiration
- Combustion
- Decomposition
Putting carbon back
Carbon dioxide is **returned** to the air by several processes. Select **all three** that release CO₂ back into the atmosphere.
- Respiration (plants, animals and microorganisms)
- Combustion (burning fuels)
- Decomposition (decay by microorganisms)
- Photosynthesis
What each process does
- Photosynthesis
- Respiration
- Combustion
- Decomposition
- Removes CO₂ from the air into carbon compounds
- Releases CO₂ as living things break down glucose for energy
- Releases CO₂ by burning wood or fossil fuels
- Releases CO₂ as microbes break down dead matter
One carbon atom's journey
An interactive activity.
The long detour 🔥
Sometimes carbon takes a **very** long detour. When plants and animals died millions of years ago and did **not** fully decay, their carbon became locked in **fossil fuels** — coal, oil and gas. Burning them — **combustion** — releases that ancient carbon back into the air as CO₂ all at once, which is why fossil fuels matter for the carbon cycle.
The water cycle 💧
Water cycles too — driven by the Sun. The energy from the Sun makes water **evaporate** from seas, rivers and land into water vapour. That vapour rises, cools and **condenses** into clouds, then falls as **precipitation** (rain, snow, hail). The water runs back over the land into rivers and the sea — providing **fresh water** for life on the way.
Round the water cycle
An interactive activity.
Why it matters
Why is the water cycle so important for organisms on land?
- It provides a constant supply of fresh water for living things
- It makes the oxygen that animals breathe
- It removes carbon dioxide from the air
- It adds salt to seawater
In the exam 🎓
Both cycles mapped. Grade-9 habits: • **Carbon in:** photosynthesis is the **only** process that removes CO₂. • **Carbon out:** name **all three** returners — **respiration, combustion and decomposition** — not just one. • **Water:** evaporation → condensation → precipitation → runoff, driven by the Sun, supplying **fresh water**.