Class XI Biology - Chapter: Plant Physiology - Topic: Photosynthesis Numerical Problems

Problem 1: ATP and NADPH Production in Light Reaction

If 12 molecules of water are consumed during the light-dependent reactions of photosynthesis, how many molecules of ATP and NADPH are theoretically produced?

Solution:

For every 2 molecules of water consumed in the light reactions, 2 molecules of NADPH and 3 molecules of ATP are produced (non-cyclic photophosphorylation).

Given: 12 molecules of water consumed.

• NADPH production: (12 molecules H2O / 2 molecules H2O) * 2 molecules NADPH = 12 molecules NADPH
• ATP production: (12 molecules H2O / 2 molecules H2O) * 3 molecules ATP = 18 molecules ATP

Therefore, 12 molecules of NADPH and 18 molecules of ATP are theoretically produced.

Problem 2: Carbon Dioxide Fixation in Calvin Cycle

How many turns of the Calvin cycle are required to produce one molecule of glucose (C6H12O6)? How many molecules of ATP and NADPH are consumed in this process?

Solution:

To produce one molecule of glucose (a 6-carbon sugar), the Calvin cycle needs to fix 6 molecules of carbon dioxide.

• Each turn of the Calvin cycle fixes one molecule of CO2.
• Therefore, 6 turns of the Calvin cycle are required to produce one molecule of glucose.

For each molecule of CO2 fixed, 3 ATP and 2 NADPH are consumed.

• ATP consumed: 6 turns * 3 ATP/turn = 18 molecules of ATP
• NADPH consumed: 6 turns * 2 NADPH/turn = 12 molecules of NADPH

Therefore, 6 turns of the Calvin cycle are required, consuming 18 molecules of ATP and 12 molecules of NADPH.