Question

Dark reaction of photosynthesis is called so because it (a) Can also occur in dark (b) Does not require light energy (c) Cannot occur during day time (d) Occurs more rapidly at night

Short answer

The most suitable answer is (b) Does not require light energy.

Step-by-step solution

Understand the Photosynthesis process

Photosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. It is divided into two reaction systems: Light-dependent reactions and Light-independent reactions (or Dark reactions). The Light-dependent reactions occur in the presence of light whereas Light-independent reactions don’t necessarily need light to happen.

Analyze the Options

Given the options, (a) says it can also occur in the dark, which is true because Dark reactions are not directly dependent on light. Option (b) states Dark reactions 'do not require' light energy, which is partially correct as while they don’t need sunlight to occur, they still depend on the products of Light reactions (ATP and NADPH). Option (c) implies these reactions cannot occur during the daytime, which is false as they can happen any time, as long as the products from the Light reactions (ATP, NADPH) are available. Lastly, option (d) indicates that Dark reactions occur more rapidly at night, this is not necessarily true as the Dark reactions can take place at any time, irrespective of day or night if the conditions are favourable.

Decide the Most Appropriate Answer

Based on our understanding of Dark reactions and the analysis of the given options, (b) 'Does not require light energy’ is the most suitable response to the question ‘Dark reaction of photosynthesis is called so because’. While it doesn’t require sunlight in a direct way, it is crucial to note that these reactions are still reliant on the products (ATP, NADPH) of the light-dependent reactions, implying light energy is indirectly involved.

Key concepts

Light-independent Reactions

In the fascinating world of photosynthesis, light-independent reactions, also known as dark reactions, play a crucial role. These reactions are not directly driven by sunlight but still depend closely on substances that are generated by the light-dependent reactions. Specifically, the energy carriers ATP and NADPH, produced during the light-dependent phase, act as fuel in these reactions.

The term 'dark reactions' might be a little misleading, as it suggests these processes occur only in darkness. However, that's not the case. Light-independent reactions can occur during the day as well, as long as ATP and NADPH are available to drive the processes forward. This adaptability stems from the reliance on energy and reducing power stored in these molecules rather than on light itself.

It's important to note that while actual sunlight isn't needed during these reactions, the energy from light has already been stored in ATP and NADPH, indicating an indirect dependence on light energy.

Calvin Cycle

The Calvin Cycle is often synonymous with light-independent reactions in photosynthesis. Named after the scientist Melvin Calvin, this cycle takes place in the stroma of chloroplasts. It’s the actual path where carbon dioxide is converted into glucose, thanks to the energy and reducing agents produced in the light-dependent reactions.

The Calvin Cycle consists of several steps:

• Carbon Fixation: Carbon dioxide from the atmosphere is attached to a 5-carbon sugar named ribulose bisphosphate (RuBP), a reaction catalyzed by the enzyme RuBisCO.
• Reduction Phase: ATP and NADPH from the light-dependent reactions provide the necessary energy and electrons to convert the fixed carbon into glyceraldehyde-3-phosphate (G3P), a simple sugar.
• Regeneration of RuBP: Some G3P molecules go on to form glucose, while others are used to regenerate RuBP, allowing the cycle to continue.

This cycle is fundamental in transforming inorganic carbon into organic molecules, which plants then use to grow and thrive.

Photosynthesis Process

Photosynthesis is a critical process for life on Earth, facilitating the conversion of light energy into chemical energy. It primarily takes place in the chloroplasts of plant cells. The entire process of photosynthesis can be broken down into two main phases: light-dependent reactions and light-independent reactions.

During the light-dependent reactions, which occur in the thylakoid membranes of the chloroplasts, energy from sunlight is captured and used to split water molecules into oxygen, protons, and electrons. This not only generates oxygen as a by-product but also forms ATP and NADPH, energy carriers necessary for the subsequent light-independent reactions.

The photosynthesis process is fundamental for plant growth and survival, as it allows plants to create glucose from carbon dioxide and water. This glucose serves as an essential energy source for plants and, indirectly, for other organisms that consume plants. Through photosynthesis, plants also contribute to reducing the amount of carbon dioxide in the atmosphere, playing a crucial role in maintaining ecological balance.