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. 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 it 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
- Combustion
- Decomposition
- 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
Follow a single carbon atom around the cycle. Put its journey into order, starting in the air.
- Carbon is CO₂ in the atmosphere
- Photosynthesis fixes it into a carbon compound in a plant
- An animal eats the plant and gains the carbon
- The animal dies and its body is broken down
- Decomposers respire and release the carbon as CO₂ again
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. Four words carry every mark in this half of the topic:
Label the water cycle
The water cycle with its labels removed. Put each stage where it happens: the arrows rising from the sea, the cloud, the arrows falling on the hills, and the river running back.
Round the water cycle
Put the stages of the water cycle into order, starting at the sea.
- The Sun's energy evaporates water into vapour
- The vapour rises, cools and condenses into clouds
- Water falls as precipitation (rain, snow, hail)
- Water runs back over land into rivers and the sea
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**.