Chemistry · Rung 15 of 24

The carbon cycle

All the carbon in a tree came from the air, not from the ground, and that is what almost nobody believes the first time they hear it: photosynthesis takes CO₂ and with sunlight turns it into sugar, that is, into the plant itself. Then an animal eats the plant, breathes, and gives the CO₂ back, and another plant takes it up again. No atom is used up or made; it only changes place and company, and it has been doing so for billions of years. The natural cycle is balanced: what respiration and decomposition return, photosynthesis removes. Fossil fuels break that balance because they are forests buried some three hundred million years ago that were compressed without ever rotting: burning them returns to the air in two centuries the carbon photosynthesis took millions of years to withdraw, and nothing removes it at the same rate.

Where this sits

Comes after
Reactions and balancing

Exercises from this rung

Generated by the app, from easiest to hardest

  1. What takes CO₂ out of the air?

    • Photosynthesis
    • Decomposition
    • Respiration
    • Burning coal

    Why Photosynthesis: the plant takes CO₂ from the air and with sunlight turns it into sugar, that is, into itself. All the carbon in a tree came from the air, not from the ground, which is what almost nobody believes the first time they hear it.

    easier

  2. What puts CO₂ into the air without taking it back?

    • Burning fossil fuels
    • Rain
    • Photosynthesis
    • A forest growing

    Why Burning coal, oil or gas. The natural cycle is balanced: what respiration and decomposition return to the air, photosynthesis takes out again. Fossil fuels are carbon that had been out of the cycle for millions of years, and burning it puts it back all at once with nothing taking it out at the same rate.

    in between

  3. Put a carbon atom's journey in order.

    Answer A plant takes CO₂ from the air, An animal eats the plant, The animal breathes and releases CO₂, Another plant takes it up again

    Why And round again: the same carbon atom goes from the air into a leaf, from the leaf into an animal, from the animal back into the air, and so on for billions of years. No atom is used up or made; it only changes place and company.

    harder