Question

Photosynthesis is a (a) Single-step process (b) Two-step process (c) Three-step process (d) Multi-step process

Short answer

The process of photosynthesis is a \(b)\) Two-step process

Step-by-step solution

Understanding Photosynthesis

Photosynthesis is an essential process where green plants and some other organisms use sunlight to synthesize foods with the presence of carbon dioxide and water. It generally includes two stages: the light-dependent reactions and the light-independent reactions (or the Calvin Cycle).

Categorising the Steps of Photosynthesis

Given the main stages of photosynthesis mentioned in step 1, namely the light-dependent reactions and the Calvin Cycle, photosynthesis can logically be classified as a two-step process.

Key concepts

Light-dependent reactions

During photosynthesis, light-dependent reactions occur as the first stage. These reactions take place in the thylakoid membranes of the chloroplasts. Here, sunlight is absorbed by chlorophyll, a green pigment found in plants. The absorbed light energy is crucial as it splits water molecules into oxygen, protons, and electrons. This process is known as photolysis.

• Photolysis of water releases oxygen as a by-product.
• Electrons and protons are also generated, which are essential for the next stages of photosynthesis.

Alongside these reactions, energy carrier molecules ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) are produced. These molecules store energy and reducing power which are utilized in the subsequent Calvin Cycle. Thus, through the light-dependent reactions, solar energy is captured and converted into chemical energy, setting the stage for the next phase of photosynthesis.

Calvin Cycle

The Calvin Cycle, often referred to as the light-independent reactions, marks the second stage of photosynthesis. Although these reactions do not directly require light, they are dependent on the products of the light-dependent reactions, namely ATP and NADPH. The Calvin Cycle occurs in the stroma of chloroplasts.
During this cycle, atmospheric carbon dioxide is fixed into organic molecules in a series of intricate steps:

• Carbon Dioxide Fixation: Carbon dioxide is incorporated into a 5-carbon sugar molecule, ribulose bisphosphate (RuBP).
• Reduction Phase: ATP and NADPH delivered from light-dependent reactions are used to reduce 3-phosphoglycerate into glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar.
• Regeneration of RuBP: Some G3P molecules go towards synthesizing glucose, while others regenerate RuBP, allowing the cycle to continue.

This process effectively transforms the energy-rich products of the light-dependent reactions into glucose, a stable energy storage form, thus highlighting the interconnectedness and efficiency of the photosynthetic process.

Two-step process

Photosynthesis is accurately described as a two-step process. This classification is due to its dual phases: the light-dependent reactions and the Calvin Cycle. Starting with the capture of light energy in the initial step, the process progresses to carbon fixation and sugar synthesis in the second step.
In the first step, light-dependent reactions harness and convert sunlight into chemical energy in the form of ATP and NADPH.
The second step, the Calvin Cycle, uses this chemical energy to fix carbon dioxide into organic compounds like glucose. Understanding photosynthesis as a two-step process illustrates the streamlined flow of energy from light absorption to sugar creation.

• Each step relies on the other: Light-dependent reactions supply the necessary energy carriers for the Calvin Cycle.
• The Calvin Cycle depends on products from the light-dependent reactions to synthesize complex sugars.

The interrelation between these steps emphasizes the complex yet efficient nature of photosynthetic processes in sustaining plant and, consequently, life on Earth.