The spelling of the term "dark reaction" is often confused due to its pronunciation. In IPA phonetic transcription, it is spelled /dɑːrk rɪˈækʃən/. The initial "d" is pronounced as a clear voiced dental plosive. The "a" sound in "dark" is long, while the "r" sound is pronounced with a slight retroflexion. The "k" sound in "reaction" is a voiceless velar stop, followed by a short "a" sound and a fricative "sh" sound. The final syllable "tion" is pronounced with a nasal "n" sound.
The term "dark reaction" refers to a series of biochemical reactions that occur during photosynthesis in the absence of light. Also known as the Calvin cycle or the light-independent reactions, it takes place in the stroma of chloroplasts and is the second stage of photosynthesis, following the light-dependent reactions.
During the dark reaction, carbon dioxide (CO2) is utilized to produce glucose and other organic compounds. The process involves a complex series of enzymatic reactions that are essential for the synthesis of carbohydrate molecules. The primary enzyme involved in this cycle is called RuBisCO (Ribulose Bisphosphate Carboxylase/Oxygenase), which catalyzes the reaction between CO2 and a five-carbon sugar called ribulose bisphosphate (RuBP).
The initial step of the dark reaction involves the fixation of CO2, where CO2 molecules are incorporated into an organic molecule, resulting in the formation of an unstable six-carbon compound. This compound then undergoes a series of enzyme-mediated reactions, leading to the production of glyceraldehyde-3-phosphate (G3P), a three-carbon sugar that can be used to produce glucose and other carbohydrates.
Overall, the dark reaction plays a crucial role in turning atmospheric carbon dioxide into organic compounds that can be utilized by plants for growth and metabolism. It is called the "dark reaction" because it does not require direct light energy and can occur in the absence of light, although it does rely on the products of the light-dependent reactions for its energy source.
The term "dark reaction" is derived from the fact that this phase of the photosynthesis process does not require direct light energy. It was named as such because it was once believed that this phase occurs only in the absence of light, in dark conditions. However, it was later discovered that the dark reaction can still occur in the presence of light, as it is independent of light intensity. Therefore, the term "dark reaction" may be considered a misnomer. Nowadays, this phase of photosynthesis is often referred to as the "light-independent reaction" or the "Calvin cycle", named after the scientist Melvin Calvin who elucidated its mechanism.