Higher Metabolism: Linking Photosynthesis and Respiration

Higher Metabolism: Linking Photosynthesis and Respiration

Metabolism refers to the sum of all chemical reactions that occur in a living organism. It has two complementary components:

• Anabolism: Building up complex molecules from simpler ones. These reactions require energy. Example: synthesising proteins from amino acids, or starch from glucose.
• Catabolism: Breaking down complex molecules into simpler ones. These reactions release energy. Example: breaking down glucose in respiration, or glycogen into glucose.

Photosynthesis and respiration form the central metabolic axis in plants:

• Photosynthesis (anabolic): CO2 + H2O + light → glucose (stores energy)
• Respiration (catabolic): glucose + O2 → CO2 + H2O (releases energy)

The glucose produced by photosynthesis does not go directly to respiration — much of it is converted to other substances first. In plants, glucose is converted to starch (storage), cellulose (cell walls), lipids (seed oils), and amino acids (proteins, requiring nitrate ions). All of these intermediates can ultimately be broken down by respiration or other catabolic pathways when the organism needs energy.

Metabolic rate (the overall rate of metabolism) varies between organisms and can be measured by heat output or oxygen consumption. Organisms with high activity levels (e.g., hummingbirds, athletes) have high metabolic rates and consume glucose rapidly. This concept connects to homeostasis (Unit 6) — regulating blood glucose is partly about matching glucose supply to metabolic demand.