Calvin Cycle (Dark Reactions)

The Calvin cycle, also known as the dark reactions, takes place in the stroma of chloroplasts. This process uses the ATP and NADPH produced during the light reactions to fix carbon dioxide and produce glucose.

The Calvin cycle consists of three main stages: carbon fixation, reduction and sugar production, and regeneration of RuBP (ribulose-1,5-bisphosphate).

In the carbon fixation stage, carbon dioxide is combined with RuBP using the enzyme rubisco. This reaction produces an unstable six-carbon molecule that quickly breaks down into two molecules of PGA (3-phosphoglycerate).

Vocabulary: Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) is the enzyme responsible for carbon fixation in the Calvin cycle.

During the reduction and sugar production stage, PGA is converted into G3P (glyceraldehyde-3-phosphate) using ATP and NADPH from the light reactions. Some G3P molecules are used to regenerate RuBP, while others are used to produce glucose and other organic molecules.

Highlight: The role of ATP and NADPH in Calvin cycle is crucial for the conversion of carbon dioxide into glucose.

The final stage involves the regeneration of RuBP, which allows the Calvin cycle to continue. ATP is used to convert some G3P molecules back into RuBP, ensuring a continuous supply of the carbon-fixing substrate.

Factors Affecting Photosynthesis

Several environmental factors can influence the rate of photosynthesis. Understanding these factors affecting the rate of photosynthesis is essential for optimizing plant growth and productivity.

Light intensity is a crucial factor in photosynthesis. Higher light intensity generally increases the rate of photosynthesis until a certain saturation point is reached. Beyond this point, further increases in light intensity do not result in higher photosynthetic rates.

Example: In a greenhouse, plants may receive supplemental lighting to increase photosynthesis rates during low-light periods.

Carbon dioxide concentration also plays a significant role in photosynthesis. Increasing CO2 levels typically leads to higher photosynthetic rates until a saturation point is reached. This relationship is particularly important in the context of rising atmospheric CO2 levels due to human activities.

Temperature is another critical factor affecting photosynthesis. The process is temperature-dependent, with an optimal range for enzyme activity. High temperatures can denature enzymes involved in photosynthesis, leading to decreased rates.

Highlight: Understanding these factors is crucial for optimizing plant growth in agriculture and horticulture.