Energy in Cells

Cells need energy — as light or as chemical energy in complex molecules. Autotrophs make their own food (e.g. plants, via photosynthesis); heterotrophs must consume it.

Photosynthesis (light → chemical energy)

$$6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2$$

Aerobic respiration (chemical energy → ATP)

$$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$$

When oxygen is scarce, fermentation is the anaerobic alternative — in yeast/plants it produces ethanol + CO₂; in animals it produces lactic acid. It releases far less energy than aerobic respiration.

ATP — the energy currency

Energy is stored by adding a phosphate to make ATP, and released when ATP is broken back to ADP + Pi to power reactions.

ATP ⇄ ADP + Pi: breaking the bond releases energy; reforming it stores energy.