The Carbon Cycle

Key Idea

• The carbon cycle starts with carbon dioxide (CO₂) in the atmosphere.

• Photosynthesis is the only way carbon enters the cycle.

• Respiration is the main way carbon returns to the atmosphere.

Photosynthesis (Entry of Carbon)

• Plants and algae take in CO₂ + water → glucose (using light energy).

• Glucose used to make carbohydrates, fats, proteins in cells.

• This is the only process that removes CO₂ from the air.

Respiration (Return of Carbon)

• All living organisms (plants, animals, decomposers) respire.

• Respiration releases CO₂ back into the atmosphere.

• ⚠ Respiration ≠ breathing → do not confuse the two.

Feeding and Transfer of Carbon

• Plants and algae are eaten by animals → carbon compounds passed into animals.

• Animals can be eaten by other animals → carbon transferred through the food chain.

• Plants and animals both respire, releasing CO₂.

Decomposition

• Animals produce waste products (e.g. faeces).

• Plants and animals eventually die.

• Decomposers (bacteria, fungi):

  • Break down waste and dead remains.

  • Respire → return CO₂ to atmosphere.

  • Release mineral ions into soil.

• Decomposers are vital for cycling materials in ecosystems.

Fossil Fuels

• If decomposers cannot act (e.g. low oxygen), carbon in dead material can form fossil fuels over millions of years.

• In the last 200 years, humans have been burning fossil fuels (coal, oil, gas).

This releases large amounts of CO₂ back into the atmosphere.

✅ Key Points / Summary

• Photosynthesis: the only entry point for carbon.

• Respiration: returns CO₂ to the air (all organisms respire).

• Decomposers: recycle materials and release mineral ions.

• Fossil fuels: store carbon; burning them releases it back.