Cycles of Life

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

The carbon cycle

Carbon moves between the air and living things in a cycle. Photosynthesis: plants take in carbon dioxide from the air. Feeding: animals eat plants, so carbon passes along the food chain. Respiration: plants, animals and decomposers all release carbon dioxide back into the air. Death and waste: dead organisms and waste pass their carbon to decomposers, which break them down. Combustion: burning fossil fuels releases carbon that was locked away for millions of years.

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

Label the carbon cycle arrows

Tap a process, then tap the pin beside the arrow it describes.

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

The arrow that cleans the air

Tap the arrow for the process that removes carbon dioxide from the air.

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.