Question

Explain the major functions of the dark and light reaction phases in photosynthesis.

Short answer

Light reactions produce ATP and NADPH and release oxygen, while dark reactions use ATP and NADPH to synthesize glucose.

Step-by-step solution

- Understand Photosynthesis

Photosynthesis is the process through which plants convert light energy into chemical energy, producing oxygen and glucose from carbon dioxide and water.

- Identify the Two Phases

The two main phases of photosynthesis are the light reactions and the dark reactions (Calvin cycle). Each has distinct roles and occurs in different parts of the chloroplast.

- Light Reactions Function

The light reactions occur in the thylakoid membranes. Their main functions are to capture light energy and convert it into chemical energy in the form of ATP and NADPH. Oxygen is also produced as a byproduct from the splitting of water molecules.

- Dark Reactions Function

The dark reactions take place in the stroma of the chloroplast and do not require light to occur. These reactions use the ATP and NADPH produced in the light reactions to fix carbon dioxide into organic molecules, ultimately synthesizing glucose through the Calvin cycle.

Key concepts

Light Reactions

Photosynthesis begins with the light reactions. These occur in the thylakoid membranes of the chloroplasts. Here, plants capture energy from sunlight. This energy is then used to produce ATP and NADPH, which are energy-rich molecules. This process is also known for producing oxygen (O₂) by splitting water (H₂O) molecules. Light reactions can be broken down into smaller steps:

• **Photon Absorption**: Chlorophyll and other pigments absorb light energy.
• **Water Splitting**: Enzymes break down water into oxygen, protons, and electrons.
• **Electron Transport Chain (ETC)**: Electrons energize as they pass through a series of proteins, this movement helps produce ATP.
• **Reduction of NADP⁺**: Electrons reduce NADP⁺ to form NADPH.

These steps ensure that light energy is effectively converted into chemical energy, ready for use in the Calvin cycle.

Dark Reactions

Unlike the light reactions, dark reactions do not require light and occur in the stroma of the chloroplasts. These reactions utilize the ATP and NADPH produced during the light reactions to synthesize glucose. This series of steps is often referred to as the Calvin Cycle. Here is how the process works:

• **Carbon Fixation**: CO₂ combines with a 5-carbon compound called ribulose-1,5-bisphosphate (RuBP).
• **Reduction Phase**: ATP and NADPH are used to convert the fixed carbon into G3P (glyceraldehyde-3-phosphate), a three-carbon sugar.
• **Regeneration of RuBP**: Some G3P molecules are used to regenerate RuBP, allowing the cycle to continue.

The ultimate goal of the dark reactions is to produce glucose, which plants use as an energy source and for growth.

Calvin Cycle

The Calvin cycle is the main process within the dark reactions of photosynthesis. It consists of three main stages:

• **Carbon Fixation**: CO₂ from the atmosphere is attached to RuBP, catalyzed by the enzyme RuBisCO, forming an unstable 6-carbon compound that splits into two 3-carbon molecules.
• **Reduction Phase**: ATP and NADPH are used to transform the 3-carbon molecules into G3P. Most G3P serves as a building block for glucose, while some enter the regeneration phase.
• **Regeneration of RuBP**: Use of G3P along with ATP to regenerate RuBP, ensuring that the cycle can continue.

One complete cycle must happen multiple times to produce a single glucose molecule. The Calvin cycle is crucial because it converts inorganic carbon (CO₂) into organic compounds that make up the plant's structure, providing the energy necessary for growth and maintenance.