Topic 5A p5

Question 1

Carbon Fixation

Answer 1

Carbon fixation is the conversion of inorganic carbon dioxide (CO₂) into organic compounds, typically sugars, within living organisms. In photosynthesis, this process occurs in the Calvin cycle where CO₂ is fixed by the enzyme RuBisCO to form stable 3-carbon molecules. It is a key step in building biomass from atmospheric carbon.

Question 2

Nitrogen Fixation

Answer 2

Nitrogen fixation is the conversion of atmospheric nitrogen gas (N₂) into ammonia (NH₃) or related compounds by certain prokaryotes such as nitrogen-fixing bacteria. This process makes nitrogen biologically available for the synthesis of amino acids and nucleotides in plants and animals. It can occur abiotically (lightning) or biotically (by bacteria in root nodules of legumes).

Question 3

Photolysis

Answer 3

Photolysis is the splitting of water molecules using light energy during the light-dependent reactions of photosynthesis. It takes place in Photosystem II, producing oxygen (O₂), protons (H⁺), and electrons (e⁻). The electrons replace those lost from chlorophyll, while the protons contribute to ATP formation.

Question 4

Calvin Cycle

Answer 4

The Calvin cycle is the light-independent stage of photosynthesis, taking place in the stroma of chloroplasts. It uses ATP and NADPH from the light-dependent reactions to fix carbon dioxide into glucose. It involves three main phases: carbon fixation, reduction, and regeneration of RuBP.

Question 5

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase)

Answer 5

RuBisCO is a crucial enzyme that catalyses the first step of carbon fixation in the Calvin cycle. It binds carbon dioxide to RuBP, forming an unstable 6-carbon compound that splits into two molecules of GP. Although highly abundant, RuBisCO is relatively inefficient and can mistakenly bind oxygen, reducing photosynthetic efficiency.