4.11B describe the stages in the nitrogen cycle, including the roles of nitrogen fixing bacteria, decomposers, nitrifying bacteria and denitrifying bacteria (specific names of bacteria are not required)

Energy in ecosystems moves in a linear flow; that is, there is no "recycling" of energy in an ecosystem. There is no possible way that energy is recycled. Matter, on the other hand, is what's being recycled. For example, all atoms in your body have been in other organisms and will be again in the future.

- The atmosphere contains around 78% nitrogen gas. This is very unreactive so it can't be used directly by plants or animals.
- Nitrogen is needed for making proteins for growth. This is because amino acids are part of the -NH2 group, and so nitrogen is found in proteins. While animals can only get proteins by eating plants (or each other), plants can only get proteins by turning nitrogen in the air into nitrogen compounds (such as nitrates) before plants can use it.

Nitrogen Fixation: the process of turning nitrogen gas from the air into nitrogen compounds in the soil which plants can use. There are two ways of nitrogen fixation:
1. Lightning: energy in lightning allows nitrogen to react with oxygen in the air to give nitrates.
2. Nitrogen-fixing bacteria: turns atmospheric nitrogen gas to nitrogen compounds in soil that plants can use.

There are 4 different types of bacteria involved in the nitrogen cycle:
1. Decomposers: break down proteins and urea, and turns them into ammonia (nitrogen compound). This forms ammonium ions in the soil.
2. Nitrifying bacteria: turn ammonium ions in decaying matter to nitrates.
3. Nitrogen-fixing bacteria: turns atmospheric nitrogen gas to nitrogen compounds in soil for plants
4. De-nitrifying bacteria: turns nitrates back to nitrogen gas.