By Jacob Stutsman — Updated Mar 24, 2022

Photosynthesis and cellular respiration are chemical mirror images. In early Earth, photosynthetic organisms used CO₂ and released O₂. Today, plants, algae, and cyanobacteria still perform this process, while animals and other eukaryotes rely on cellular respiration.

Cellular Organelles Involved

Plants and animals are eukaryotes and contain specialized organelles. In photosynthesis, the thylakoid membranes of chloroplasts house the electron transport chain that generates glucose. In cellular respiration, mitochondria—often called the cell’s power plant—run the electron transport chain that produces ATP.

Prokaryotes lack complex organelles; some use simplified pathways or rely on the same core electron‑transport chain. This discussion focuses on eukaryotic cells, where the organelles are well defined.

Electron Transport Chain: Timing and Purpose

In photosynthesis, the electron transport chain initiates the process, using light energy to excite chlorophyll and liberate electrons. In cellular respiration, the chain operates after glucose has been broken down, receiving electrons from NADPH and FADH₂.

Both systems harness electron flow to pump protons across membranes, creating a proton gradient that drives ATP synthesis. The main products differ: photosynthesis yields glucose, whereas respiration produces ATP.

Key Steps of Each Process

Photosynthesis:

• Light energy excites electrons in chlorophyll.
• Water is split, providing electrons, hydrogen ions, and releasing O₂.
• Electrons travel through the thylakoid chain, pumping protons into the thylakoid lumen.
• Proton gradient powers ATP synthase, producing ATP; NADP⁺ is reduced to NADPH.
• ATP and NADPH drive the Calvin cycle to synthesize glucose.

Cellular Respiration:

• Glucose is metabolized to produce NADPH and FADH₂.
• These carriers donate electrons to the mitochondrial electron transport chain.
• Proton pumping creates a gradient across the inner mitochondrial membrane.
• ATP synthase uses the gradient to generate ATP.
• Electrons ultimately reduce O₂, forming water.

Interrelation: Reverse of the Process

The final step of respiration—oxygen reduction—mirrors the initial water-splitting step of photosynthesis. Understanding this symmetry helps explain why plants release oxygen and why cells convert oxygen into water.

In summary, photosynthesis and cellular respiration are complementary processes that together sustain life on Earth by converting light energy into chemical energy and vice versa.