Chapter 8, Problem 24A

Summary Introduction

To explain:

The reason calvin cycle depends on light reactions

Introduction:

Photosynthesis is an anabolic pathway in which light energy from the Sun is converted to chemical energy for use by the cell. Light energy is trapped by pigments called chlorophyll present in the chloroplasts and is converted to chemical energy during the process of photosynthesis. It occurs in two phases; Light dependent reaction and Light independent reaction.

Answer to Problem 24A

Light independent reactionoccurs in stroma of chloroplasts. Also called the Calvin cycle, in this phase an enzyme RuBisCO helps in fixing the carbon dioxide into glucose and other organic compounds. Energy is supplied by ATP and NADPH to carry out the cycle. These energy storage molecules are produced in light reactions. Hence Calvin cycle depends on light reactions.

Explanation of Solution

There are two phases in photosynthesis:Light reaction and Calvin cycle

Light reactions occur in the thylakoids within the chloroplasts. During light reaction light energy is absorbed by chlorophyll present in thylakoid membranes. The energy is stored in two energy storage molecules- NADPH and ATP. ATP is produced in conjunction with electron transport by the process of chemiosmosis. ATP is formed in the stroma.

Light independent reactions in phase two of photosynthesis occur in stroma. This is also called Calvin cycle. During this cycle enzyme RuBisCO fixes the carbon dioxide into 3- carbon molecules called 3- phosphoglycerate (3-PGA). In the next step energy is transferred from ATP and NADPH to 3- phosphoglycerate (3-PGA) to form glyceraldehyde 3- phosphates (G3P). Next two G3P molecules leave the cycle to be used for the production of glucose and other organic compounds. In the final step of the Calvin cycle, Rubisco, an enzyme converts ten G3P(glyceraldehyde 3-phosphate) molecules into 5- carbon molecules called ribulose 1,5- bisphosphates (RuBP). These molecules combine with new carbon dioxide molecules to continue the Calvin cycle.

Hence Calvin cycle is dependent on light reaction for supply of ATP and NADPH.