Question

Compare and contrast the light reactions of Photosynthesis to electron transport chain and chemiosmosis of Cellular Respiration.  

Answer 1

In many respects, photosynthesis and cell respiration are complementary reactions within the environment. The products of photosynthesis function as the inputs of cell respiration (oxygen and glucose) whereas the products of cell respiration function as the inputs of photosynthesis (carbon dioxide and water). Most producers (i.e. photoautotrophs) undertake both photosynthesis and cell respiration to survive. Consumers (i.e. heterotrophs) only undertake cell respiration, but will ingest or absorb photosynthetic products.

Interrelationship Between Processes

Photosynthesis and cell respiration both involve the production of chemical energy (ATP). In photosynthesis, ATP is produced via light energy (photophosphorylation) and used to make organic molecules. In cell respiration, ATP is produced by breaking down organic molecules (oxidative phosphorylation).

In both cases, the production of ATP involves an electron transport chain and chemiosmosis. In photosynthesis, electrons are donated by chlorophyll and protons accumulate within the lumen of the thylakoid. In cell respiration, electrons are donated by hydrogen carriers and protons accumulate in the intermembrane space.

Comparison of ATP Production

Photosynthesis and cell respiration are in many respects the reverse of one another. Photosynthesis is an anabolic process, whereas cell respiration is a catabolic process.

Photosynthesis:

• Water is broken down to oxygen to release electrons for an electron transport chain
• Electrons from the transport chain are taken up by hydrogen carriers (NADPH specifically)
• Uses the Calvin cycle to synthesise glucose (requires hydrogen carriers and carbon dioxide)

Cell Respiration:

• Uses the Krebs cycle to break down glucose (releases hydrogen carriers and carbon dioxide
• Hydrogen carriers release electrons for an electron transport chain (NADH and FADH₂specifically)
• Electrons from the transport chain are taken up by oxygen (to form water).

Differences Between Photosynthesis and Cell Respiration