Carbon cycles through photosynthesis, respiration, the ocean and fossil fuels; nitrogen through fixation, nitrification and denitrification by bacteria; phosphorus slowly through rock, soil and water, with no gas phase.
Energy flows through an ecosystem and out as heat, but matter is recycled. The atoms in your body have been in rocks, air and other living things. Photosynthesis, Respiration and the Carbon Cycle and The Nitrogen Cycle started the story; here are three cycles side by side.
🔑 Key idea
Carbon moves from the air into living things by photosynthesis and back by respiration, decomposition and burning; the ocean and fossil fuels are large stores. Nitrogen is cycled mostly by bacteria: fixation (N₂ → ammonia), nitrification (ammonia → nitrate), and denitrification (nitrate → N₂). Phosphorus cycles slowly through rock, soil, water and living things, with no gas phase.
The carbon cycle. Tap a process to see where carbon goes.
💡 Why it works
Why do living things need these elements? Carbon is the backbone of every biological molecule. Nitrogen is in every amino acid and every base of DNA. Phosphorus is in DNA's backbone, in ATP and in membranes. Since the planet gets almost no new atoms, life must reuse the ones it has.
The nitrogen cycle. Air is about 78% nitrogen gas, but plants cannot use N₂ until bacteria (or lightning) fix it.
✏️ Worked example: Following a nitrogen atom
Trace a nitrogen atom from the air into a rabbit.
Bacteria in the soil, or in the root nodules of plants such as beans, fix N₂ into ammonia.
Other bacteria turn ammonia into nitrate.
A plant's roots take up the nitrate and use it to build amino acids.
A rabbit eats the plant.
Answer: Air → fixation → nitrification → roots → plant protein → rabbit.
The phosphorus cycle: weathering releases phosphate from rock; it passes through soil, water and living things, and settles back into sediment.
People speed up some steps. Burning fossil fuels moves carbon from long-term stores into the air faster than photosynthesis takes it back. Nitrogen and phosphorus spread on farm fields can wash into lakes and feed blooms of algae. How best to manage these changes is debated, and the trade-offs involve farming, energy and cost; the cycles themselves are well understood.
⚠️ Common mistake
It is natural to think every element cycles through the air the way carbon and nitrogen do. Phosphorus does not: it has no significant gas form. It moves through rocks, soil, water and organisms, and its slowest step, rock forming and then weathering, takes millions of years.
Practice
Pass with 3 right answers, the last 2 in a row. Each right answer earns 2 XP.