Chapter 8, Problem 22A

Summary Introduction

To summarize:

The phases of photosynthesis and describe the part where each phase occurs in chloroplast

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.

Answer to Problem 22A

Photosynthesis occurs in two phases;

1. Light dependent reaction- The light dependent reaction is also called light reaction. It occurs in the thylakoids of chloroplasts. First the chlorophyll absorbs light energy and this excites the electrons in photosystem II. It splits water molecule producing an electron, a hydrogen ion and oxygen as waste product. The excited electrons move from PSII to PSI through electron- acceptor molecule. PSI then transfers electrons to ferrodoxin which in turn gives electrons to carrier molecule NADP+ forming energy storage molecule NADPH. ATP is produced through electron transport chain by the process of chemiosmosis.
2. Light independent reaction- This reaction occurs in stroma of chloroplasts.Also called the Calvin cycle, in this phase an enzyme RuBisCOhelps in fixing the carbon dioxide into glucose and other organic compounds.Energyis supplied by ATP and NADPH to carry out the cycle.

Explanation of Solution

There are two phases in photosynthesis:

1. Light reaction- Chloroplasts have two compartments; thylakoids and stroma.

Flat sac like structures called thylakoids are arranged in stacks called grana.Light reactionsoccur in the thylakoids within the chloroplasts. First step in light reaction is absorption of light by chlorophyll present in thylakoid membranes. The energy is stored in two energy storage molecules- NADPH and ATP.Thylakoid membranes have a large surface area which provides space to hold large number of electron transporting molecules and two types of protein complexes called photosystems.Light energy is absorbed by photosystem II. It is used to split water molecule. When water splits, oxygen is released from the cell, protons ( H+ ions) stay in thylakoid space and an activated electron enters the electron transport chain. As electrons move through the membrane, protons are pumped into thylakoid space. At photosystem I electrons are re-energized and NADPH is formed.

During light reactions, ATP is produced in conjunction with electron transport by the process of chemiosmosis. The H+ ions produced by splitting of water molecules accumulate in the interior of thylakoid. Due to difference in concentration of H+ ions in the interior of thylakoid and stroma, the H+ ions diffuse down the concentration gradient through ion channels. ATP synthases help in diffusing of H+ ions. ATP synthase is an enzyme used during light reaction of photosynthesis to generate ATP. As a result of this movement, ATP is formed in the stroma.

1. Calvin cycle- A fluid filled space called stroma is present outside the grana. It contains many enzymes needed for carbon fixation. Light independent reactions in phase two of photosynthesis occur in this part. This is also called Calvin cycle. The Calvin cycle occurs in stroma where 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.