Cellular Respiration, Photosynthesis, and the Carbon Cycle

Cellular Respiration and Fermentation

• Chemiosmotic phosphorylation occurs across the mitochondrial membrane.
• Protons ($H^+$) move through ATP synthase to enable ATP synthesis.
• Sugars are broken down during glycolysis, pyruvate conversion, and fermentation.
• Electron carriers (NADH and FADH2) transport electrons to the electron transport chain, where oxygen ($O_2$) serves as the final electron acceptor.
• In cellular respiration, glucose is oxidized by losing electrons and hydrogen atoms, while oxygen ($O_2$) is reduced to form water ($H_2O$).
• Fermentation does not require oxygen ($O_2$) and shares glycolysis as a common stage with cellular respiration.
• Lactic acid fermentation occurs in muscle cells.

Photosynthesis

• Photosynthesis takes place inside the chloroplasts of eukaryotic cells.
• The general equation for photosynthesis is $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$.
• Photosynthesis occurs in two main stages:
  • Light-dependent reactions: Located in the thylakoid membrane, where chlorophyll absorbs light energy and water ($H_2O$) is split, releasing oxygen ($O_2$).
  • Calvin cycle: Utilizes energy and carbon dioxide ($CO_2$) to build glucose ($C_6H_{12}O_6$).
• In photosynthesis, water ($H_2O$) is oxidized (loses electrons) and carbon dioxide ($CO_2$) is reduced.

Connection to the Carbon Cycle

• Photosynthesis removes carbon dioxide ($CO_2$) from the atmosphere and stores it in glucose ($C_6H_{12}O_6$).
• Cellular respiration releases carbon dioxide ($CO_2$) back into the atmosphere.
• This continuous exchange of carbon is a core mechanism of the global carbon cycle.