19.1 Energy flow

The Sun is the principal source of energy input to biological systems. Plants capture light energy via photosynthesis and convert it to chemical energy. Animals (consumers) get their energy by feeding on plants or other animals. Energy flows in three forms: LIGHT (from Sun) → CHEMICAL (in organisms — glucose, fat, protein) → HEAT (eventually lost via respiration, body warmth, movement). Unlike carbon and nitrogen, energy is NOT recycled — it flows one-way through living systems and dissipates as heat.