Dark reactions

Definition

Dark reactions, also known as the Calvin cycle, are a series of biochemical processes that occur in the stroma of chloroplasts, where carbon dioxide is converted into glucose without the direct use of light. These reactions are driven by the products of light-dependent reactions, specifically ATP and NADPH, and are essential for the synthesis of organic compounds that plants use for energy and growth.

5 Must Know Facts For Your Next Test

1. Dark reactions do not require light to occur, but they depend on the products of light reactions, which provide the necessary energy and electrons.
2. The primary enzyme involved in fixing carbon dioxide in the dark reactions is RuBisCO, which catalyzes the first step of the Calvin cycle.
3. Dark reactions take place in the stroma of chloroplasts, where enzymes and substrates for carbohydrate synthesis are present.
4. The end product of dark reactions is a three-carbon sugar called glyceraldehyde-3-phosphate (G3P), which can be further converted into glucose and other carbohydrates.
5. The overall process of dark reactions contributes significantly to the carbon cycle by converting atmospheric CO2 into organic compounds that can be utilized by living organisms.

Review Questions

• How do dark reactions depend on light-dependent reactions in photosynthesis?
  • Dark reactions rely heavily on the products generated during light-dependent reactions. Specifically, ATP and NADPH produced during these light-driven processes provide the energy and electrons needed for converting carbon dioxide into organic molecules. Without these products from light reactions, dark reactions would not be able to proceed effectively, highlighting the interconnectedness of these two stages of photosynthesis.
• Discuss the role of RuBisCO in dark reactions and its importance to plant metabolism.
  • RuBisCO is a crucial enzyme in dark reactions, responsible for catalyzing the first step of carbon fixation in the Calvin cycle. It facilitates the reaction between carbon dioxide and ribulose bisphosphate (RuBP), ultimately leading to the production of glyceraldehyde-3-phosphate (G3P). The efficiency of RuBisCO directly affects a plant's ability to synthesize carbohydrates, making it vital for overall plant metabolism and growth.
• Evaluate the impact of factors like temperature and CO2 concentration on the efficiency of dark reactions in photosynthesis.
  • The efficiency of dark reactions can be significantly impacted by environmental factors such as temperature and CO2 concentration. Higher temperatures can increase enzyme activity up to a certain point, but extreme heat may denature RuBisCO and hinder carbon fixation. Similarly, increased CO2 concentration generally enhances the rate of photosynthesis as more substrate is available for dark reactions. However, if temperatures become too high or other limiting factors come into play, it can lead to decreased efficiency in these critical processes.

Related terms

Calvin cycle:

The specific set of dark reactions in photosynthesis where carbon fixation occurs to produce glucose.

Photosynthesis: The process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose.

NADPH:

A reduced coenzyme that acts as an electron carrier in the light-dependent reactions and is used in the dark reactions to help convert carbon dioxide into glucose.