cycles

Question 1

Describe the carbon cycle

Answer 1

• Plants fix carbon dioxide into organic molecules during photosynthesis.
• The organic carbon-containing molecules are passed onto organisms that eat the plants.
• Carbon dioxide is released back into the atmosphere by respiration from animals and plants.
• Burning fossil fuels also releases carbon dioxide into the atmosphere.

Question 2

Why is the carbon cycle important?

Answer 2

Carbon-containing molecules such as

glucose are important for living organisms to grow and provide energy for vital functions within cells

Question 3

Describe the water cycle

Answer 3

• Water from lakes and oceans evaporates.
• The evaporated water condenses into clouds and returns to earth as precipitation.
• The water from precipitation is useful for life on land.
• The water then returns to rivers and oceans through surface runoff.

Question 4

Why is the water cycle important?

Answer 4

Living organisms require water and the
water cycle provides organisms on land
with a continuous supply of water.

Question 5

Why are microorganisms important for
the cycling of materials through an
ecosystem?

Answer 5

Microorganisms (bacteria and fungi) return carbon to the environment by releasing carbon dioxide through respiration while they decompose dead matter. The
decomposition of dead matter in soil returns mineral ions to the environment for other organisms to use e.g. plants use mineral ions for growth.

Question 6

What is meant by decomposition?

Answer 6

The breakdown of dead materials into

simpler organic matter

Question 7

How do decomposers break down dead

matter?

Answer 7

Decomposers release enzymes which
catalyse the breakdown of dead material
into smaller molecules.

Question 8

What are the two types of

decomposition?

Answer 8

Aerobic decomposition (with oxygen)
Anaerobic decomposition (without oxygen)

Question 9

What factors affect the rate of

decomposition?

Answer 9

● Oxygen availability
● Temperature
● Water content

Question 10

Why is oxygen required for

decomposition?

Answer 10

Most decomposers require oxygen for

aerobic respiration.

Question 11

How does the availability of oxygen

affect the rate of decomposition?

Answer 11

● As oxygen levels increase, the rate of
decomposition increases.
● As oxygen levels decrease, the rate of
decomposition decreases.

Question 12

Why can decomposition still occur in the

absence of oxygen?

Answer 12

Some decomposers respire anaerobically.*

*However, the rate of decomposition is slower as anaerobic respiration produces less energy.

Question 13

How does soil water content affect the

rate of decomposition?

Answer 13

Decomposers require water to survive:
● In moist conditions the rate of decomposition is high.
● In waterlogged soils there is little oxygen for respiration so the rate of decomposition decreases.

Question 14

Why does decomposition require water?

Answer 14

Water is required for the secretion of
enzymes and absorption of dissolved
molecules.

Question 15

How does temperature affect the rate of

decomposition?

Answer 15

Decomposers release enzymes:
● Rate highest at 50°C (optimum temperature for enzymes).
● Lower temperatures, enzymes work too slowly, rate decreases.
● High temperatures, enzymes denature, decomposition stops.

Question 16

How is the rate of change calculated
when considering the decay of biological
material?

Answer 16

Rate of change = Change in value /
Change in time
Where value is a measurable variable associated with the decay of the material