Question

Calvin cycle is termed as dark reaction because it (a) Is not dependent on light for \(\mathrm{CO}_{2}\) fixation in stroma (b) Occurs in dark (c) Is by convention only (d) Requires light

Short answer

The Calvin Cycle is termed as a 'dark reaction' because it is not dependent on light for \(\mathrm{CO}_{2}\) fixation in the stroma. Therefore, the correct answer is (a) Is not dependent on light for \(\mathrm{CO}_{2}\) fixation in stroma.

Step-by-step solution

Understanding the Calvin cycle

The Calvin cycle is one of two parts of photosynthesis. While the first part, the light-dependent reaction, requires light and happens in the thylakoid membrane, the second part, the Calvin cycle, does not directly need light and takes place in the stroma of the chloroplast.

Analyzing the options

(a) indicates that the Calvin cycle is not dependent on light for \(\mathrm{CO}_{2}\) fixation in stroma which is true, (b) suggests that the reaction occurs in the dark, which might be misleading as the Calvin cycle can occur in light but it’s not dependent on light, (c) suggests that the term 'dark reaction' is by convention, and (d) states that it requires light, which is incorrect.

Identifying the correct option

Looking at the arguments in step 2, we can say that option (a) is the most accurate one. The Calvin cycle is not directly dependent on light for \(\mathrm{CO}_{2}\) fixation in stroma. Although it does need the ATP and NADPH produced in the light-dependent reactions, it does not require light itself.

Key concepts

Dark Reaction

The term dark reaction can be slightly misleading to students who are learning about photosynthesis for the first time. To clarify, the dark reaction refers to a phase of photosynthesis known as the Calvin cycle. Despite what the name might suggest, the Calvin cycle doesn't necessarily occur in the dark; instead, it operates independently of light, allowing it to function at any time of the day.

Understanding this concept is essential because the Calvin cycle plays a critical role in converting carbon dioxide (CO2) and water into glucose, which plants use for energy and growth. The energy to drive this conversion is supplied by ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), which are produced in the light-dependent reactions of photosynthesis. This separation of light-independent and light-dependent processes is crucial for the efficient functioning of photosynthesis.

CO2 Fixation in Stroma

CO2 fixation is a central part of the photosynthesis process, taking place in the stroma of the chloroplasts, the thick fluid-filled area that surrounds the thylakoids. The stroma offers an enzyme-rich environment for the Calvin cycle to occur. In this process, the enzyme RuBisCO plays a pivotal role in attaching CO2 to a five-carbon sugar named ribulose bisphosphate (RuBP).

This reaction produces a six-carbon compound that promptly splits into two three-carbon compounds. These smaller molecules undergo a series of transformations, eventually leading to the production of glucose. Hence, the fixation of CO2 in the stroma is a biochemical process that leads to the creation of organic molecules that the plant can store or use immediately for energy.

Photosynthesis Process

The full photosynthesis process encompasses the transformation of sunlight into chemical energy, a phenomenal process that sustains the energy needs of nearly all living organisms on Earth. Photosynthesis occurs in two main stages: the light-dependent reactions and the light-independent reactions -- or the Calvin cycle, also known as the dark reaction.

During the light-dependent reactions, which take place in the thylakoid membranes, sunlight is captured by chlorophyll and other pigments. This energy is then used to produce ATP and NADPH while splitting water molecules to release oxygen. Following this, the ATP and NADPH are utilized in the stroma during the Calvin cycle to convert CO2 into glucose. These two stages are intricately linked, with the outputs of the light-dependent reactions directly impacting the efficacy of the Calvin cycle.