379BIOCHEMICAL CYCLESOxygen cycleOxygen plays a vital part in the re s p i r a t i o n
of animals and plants.
1 Oxygen in air
2 Oxygen breathed in by animals
3 Carbon dioxide (a carbon-oxygen compound) bre a t h e d
out by living things as waste
4 Carbon dioxide absorbed by plants and used in photo-
synthesis to make carbohydrate foods
5 Surplus oxygen released into the air by plants as waste
Carbon cycle Plant material is a valuable source of
carbon. Oxidizing carbon compounds provide energy for
animals and plants.
1 Carbon dioxide (a carbon oxygen compound) in air
2 Carbon dioxide absorbed by plants for making food
3 Plants eaten by animals
4 Carbon dioxide waste breathed out by animals and
plants
5 Dead organisms broken down by bacteria
6 These give off carbon dioxide waste
7 Remains of long-dead plants and microscopic
organisms forming hydrocarbon fossil fuels: coal, oil,
and gas
8 Carbon dioxide released back into the air by burning
fossil fuels
Nitrogen cycleAs an ingredient in proteins and nucleic
acids, nitrogen is vital to all living things.
1 Nitrogen in air
2 Atmospheric nitrogen trapped by some plants’ roots
3 Nitrogen used by plants for making proteins
4 Plant proteins eaten by animals
5 P r oteins in dead organisms and body wastes convert e d
to ammonia by bacteria and fungi
6 Ammonia converted to nitrate by other bacteria
7 Nitrate taken up by plant roots
(continues)