Question

Explain why plants are dependent on animals (or other organisms performing respiration), and why animals are dependent on plants. Refer to specific gases in your answer.

Answer 1

Plants and animals (and other organisms) are linked by the exchange of two gases: oxygen (O2) and carbon dioxide (CO2), through the complementary processes of photosynthesis and cellular respiration.

- Photosynthesis (by plants, algae, cyanobacteria) uses carbon dioxide and water plus light energy to make sugars and releases oxygen:
6 CO2 + 6 H2O + light → C6H12O6 + 6 O2
Thus plants remove CO2 from the air and produce O2.

- Cellular respiration (by animals, fungi, many microbes — and by plants too) uses oxygen to break down sugars and releases carbon dioxide:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (ATP)
Thus animals (and other respiring organisms) consume O2 and produce CO2.

Why plants depend on animals/other respirers
- Animals and other nonphotosynthetic organisms produce CO2 as a waste product of respiration. Plants need CO2 as their carbon source to build sugars and grow, so they depend on that CO2 supply (plus CO2 from decomposition and the atmosphere).

Why animals depend on plants
- Animals require O2 for aerobic respiration to generate ATP. Plants (and other photosynthesizers) replenish atmospheric O2 as a byproduct of photosynthesis, so animals depend on them for a continuous supply of oxygen. Animals also depend on plants as the primary producers of the organic carbon in their food.

Because photosynthesis and respiration are inverse chemical processes, together they keep atmospheric O2 and CO2 in balance and sustain life on Earth.