B2 - The Carbon and Nitrogen Cycle

Question 1

Describe the 5 main stages of the carbon cycle

Answer 1

1. Carbon dioxide is absorbed by plants during photosynthesis, and built into carbohydrate molecules such as sugars.
2. Plants are eaten by animals and so these carbon
   compounds can pass into animals and become part of
   their bodies.
3. Both plants and animals respire. This returns some
   carbon dioxide back to the atmosphere.
4. When plants and animals die, soil bacteria and fungi known as decomposers digest their bodies in the process of decay, and carbon dioxide is released back into the atmosphere through respiration.
5. Some plants and animals are buried under layers of silt and over millions of years begin to forms fossil fuels which humans extract and burn to release energy. This releases carbon dioxide.

Question 2

In terms of the carbon cycle, describe a stable environment.

Answer 2

In a stable environment the amount of carbon dioxide released should approximately equal the amount absorbed.

Question 3

What are the conditions microbes need to survie and respire?

Answer 3

• food (dead plants or fallen leaves)
• oxygen
• a suitable temperature
• moisture
• suitable pH

Question 4

Why are oceans called carbon sinks?

Answer 4

In the oceans there are many invertebrate animals with shells made of compounds such as calcium carbonate. When they die, their shells are difficult to decay, and they sink to the bottom of the ocean. The fragments are compressed together to form rocks such as limestone and chalk. Because they store carbon in this way, the oceans are called carbon sinks.

Question 5

How is carbon dioxide released from the ocean?

Answer 5

Some limestone rocks become exposed above the sea. Gradually, the limestone and chalk are eroded by weathering, and the carbon is released as carbon dioxide into the air.

Question 6

Describe briefly the 3 main stages of the nitrogen cycle

Answer 6

1. Nitrogen exists as nitrates in the soil, dissolved in water which are absorbed by plant roots to build compounds such as protein.
2. These nitrogen-containing compounds pass through food
   chains to animals.
3. Nitrogen compounds in dead plants and animals are broken down into nitrates for absorption by plants.

Question 7

What are the two ways decomposers return the nitrates in plants and animals to the soil ? What do nitrifying and denitrifying bacteria do?

Answer 7

• animal wastes, urea and faeces, contain nitrogen, and are converted to ammonia
• when plants and animals die and decay, their proteins break down, releasing ammonia.

Nitrifying (in soil) : convert the ammonia to nitrates to complete the cycle.
Denitrifying ( in soil) : break down nitrates, releasing nitrogen into the air.

Question 8

How can nitrogen gas get back into soil?

Answer 8

1. Lightning releases enough energy to cause nitrogen in the air to react with oxygen to form nitrates, which wash into the soil.
2. There are nitrogen-fixing bacteria in the roots of leguminous plants such as peas, beans, and clover. The bacteria combine nitrogen in the air with oxygen to make nitrates, which the plants can use.
3. Chemists are able to make ammonia industrially, using the Haber process.