380 Appendix VI
BIOCHEMICAL CYCLES
(continued)Sulfur cycleSulfur is in two of the 20 amino acids which
are used by the body to make proteins.
1 Sulfates (sulfur–oxygen compounds) absorbed by plant
roots
2 The oxygen in the sulfate is replaced by hydrogen in a
plant process that produces certain amino acids
3 Plants eaten by animals
4 S u l f u r-containing amino acids of dead plants and
animals broken down to hydrogen sulfide (which gives
o ff a rotten egg odor) by decomposer micro o rg a n i s m s
5 Sulfur extracted from sulfides by bacteria
6 Other bacteria combine sulfur with oxygen, producing
sulfates
Phosphorus cycle Phosphorus is a vital ingredient of
proteins, nucleic acids, and some other compounds
found in living things.
1 Phosphates (compounds of phosphorus, hydrogen, and
oxygen) absorbed by plant roots
2 Phosphates used by plants in making organic phos-
phorus compounds
3 Plants eaten by animals
4 Compounds in dead plants and animals broken down
to phosphates by microorganisms
Krebs cycleThe Krebs or citric acid cycle is the second
stage of aerobic respiration in which living things
produce energy from foods. It requires oxygen; enzymes
(proteins that promote but are not used up in chemical
changes) create successive compounds, thus transforming
pyruvate to carbon dioxide and water and releasing
energy.
1 Acetic acid combines with...
2 Oxaloacetic acid to form...
3 Citric acid. Later changes produce...
4 Aconitic acid
5 Isocitric acid
6 Ketoglutaric acid
7 Succinic acid, carbon dioxide, and energy-rich ATP
(adenosine triphosphate)
8 Fumaric acid
9 Malic acid