1. B3 L IFE ON E ARTH Lesson 4: Recycling Nutrients

2. O BJECTIVES MUST describe the carbon cycle SHOULD describe the nitrogen cycle

3. K EY W ORDS You need to be able to define the following: Carbon Carbon cycle Carbon dioxide Combustion Respiration Decomposition Nitrogen cycle Nitrates Proteins Nitrogen-fixing bacteria (Higher tier only) Denitrifying bacteria (Higher tier only)

4. T EXTBOOK A NSWERS 1)a) Through combustion (burning); decomposition and respiration. b) By photosynthesis. 2)Increased combustion of fossil fuels; the reduction of forest cover on Earth. 3)In the form of proteins that plants have made; in food as proteins from other animals (or animal products). 4)By decomposition.

5. T EXTBOOK A NSWERS 5)Some bacteria and fungi decompose dead matter, releasing nitrates into the soil; denitrifying bacteria turn these nitrates back into nitrogen; nitrogen-fixing bacteria in the roots of beans and peas turn nitrogen gas from the air into nitrates that plants can use to make proteins. 6)Peas and beans contain nitrogen-fixing bacteria in their roots that can ‘fix’ atmospheric nitrogen into nitrates; this means these plants are able to access more nitrates than most other plants; these nitrates are used to make proteins.

6. W ORKSHEET A NSWERS Activity 1 (Low demand) a) Photosynthesis; b) combustion; c) respiration; d) decomposition

7. W ORKSHEET A NSWERS Activity 2 (Standard demand) Plants absorb nutrients from the soil for growth nitrates  proteins in plants Plants and animals die and decay proteins in plants and animals  nitrates in the soil Animals excrete waste products from breakdown of proteins  nitrates in soil Animals eat plants protein in plants  proteins in animals

8. W ORKSHEET A NSWERS Activity 3 (High demand) 1)Lightning; nitrogen-fixing bacteria in the roots of beans and peas; nitrogen-fixing bacteria in the soil; decomposition by bacteria and fungi. 2)By the decay of nitrogen-containing plants and animals and their urine and faeces; and by denitrifying bacteria. 3)Fertilisers contain nitrates; plants need nitrates to create proteins, which help them grow; fertilisers are added to help plants grow.