Modern Concept Of Photosynthesis - 1 | Agriculture Optional Notes for UPSC PDF Download

Photosynthesis

Photosynthesis, the remarkable biological phenomenon by which plants harness sunlight to produce oxygen and convert carbon dioxide into starch and sugars, is a multifaceted process that has captivated scientists for decades. In this article, we will explore the intricate journey of photosynthesis, shedding light on the two main phases, the evidence supporting them, and the factors influencing their efficiency.

The Two Phases of Photosynthesis

Scientists have established that photosynthesis comprises two fundamental phases:

• Light Phase or Photochemical Reactions (Light-Dependent Reactions): In this initial stage, chlorophyll molecules in plant cells capture energy from sunlight and convert it into chemical energy, which is then stored in molecules such as ATP and NADPH⁺H⁺.
• Dark Phase or Chemical Dark Reactions (Light-Independent Reactions): During the second stage, carbohydrates are synthesized from carbon dioxide, utilizing the energy stored in ATP and NADPH formed during the light-dependent reactions.

Supporting Evidence for Light and Dark Reactions

To substantiate the existence of these phases, researchers have employed several experiments and studies:

• Physical Separation of Chloroplast: Chloroplasts can be divided into grana and stroma fractions. When light is provided to the grana fraction without CO₂, ATP and NADPH₂ are produced. These assimilatory powers are then utilized in the stroma fraction to generate carbohydrates.
• Experiments with Intermittent Light: The rate of photosynthesis is faster in intermittent light, where light and dark periods alternate. This is due to the discrepancy in the speed of light and dark reactions, preventing the accumulation of ATP and NADPH₂.
• Temperature Coefficient Studies: The temperature coefficient (Q₁₀) reveals that photosynthesis involves both dark and light reactions. When photosynthesis is rapid, Q₁₀ is greater than 2, but it drops to around 1 when photosynthesis slows down.
  

Light Reaction (Photochemical Reactions)

• The light reaction takes place within the grana, a part of the chloroplast, and encompasses processes that rely on the presence of light.
• The primary outcome of the light reaction is the generation of assimilatory compounds, mainly ATP and NADPH₂.
• In 1939, Robin Hill conducted pioneering research demonstrating that chloroplasts extracted from the leaves of Stellaria media and Lamium album, when suspended in a test tube containing suitable electron acceptors like potassium ferroxalate (in some cases, this chemical alone is sufficient) and potassium ferricyanide, release oxygen as a result of the photochemical splitting of water.
• It's important to note that in these conditions, no carbon dioxide is consumed, and no carbohydrates are produced. However, the light-driven reduction of the electron acceptors is accompanied by the release of oxygen.
  

The process of water splitting in photosynthesis is referred to as photolysis, and it's commonly known as the Hill reaction in honor of its discoverer.

The Hill reaction provides evidence for two key points:

• Oxygen is released from water as a byproduct during photosynthesis.
• Electrons required for the reduction of carbon dioxide (CO₂) are sourced from water. This results in the production of a reduced substance (hydrogen donor), which later participates in reducing CO₂.

Dichlorophenol indophenol, a dye, was employed by Hill in his renowned Hill reaction.
As explained by Arnon in 1961, this process involves the conversion of light energy into chemical energy. This energy is stored in the form of ATP, a process referred to as photophosphorylation within chloroplasts. Additionally, the electron acceptor NADP⁺ is transformed into NADPH, a hydrogen donor. This conversion of NADP⁺ to NADPH is known as photoreduction or the generation of reducing power NADPH.