11.7. Where are the ATP and NADPH Used?

We learnt that the products of light reaction are ATP, NADPH and O₂. Of these O₂ diffuses out of the chloroplast while ATP and NADPH are used to drive the processes leading to the synthesis of food, more accurately, sugars.

In the light reactions of photosynthesis, energy from light is converted into chemical energy in the form of ATP and NADPH. These two molecules are crucial because they provide the energy and reducing power needed to convert carbon dioxide (CO₂) and water (H₂O) into glucose during the process of synthesizing food. While oxygen (O₂) produced during light reactions is released into the atmosphere, ATP and NADPH are used in the darker biochemical reactions to form organic compounds.

Think of ATP and NADPH as the fuel and electricity needed to run a factory. The factory in this analogy represents the chloroplast, where the raw materials (CO₂ and H₂O) are transformed into products (sugars). Just like a factory needs fuel and electricity to produce its goods, the chloroplast needs ATP and NADPH for photosynthesis.

This is the biosynthetic phase of photosynthesis. This process does not directly depend on the presence of light but is dependent on the products of the light reaction, i.e., ATP and NADPH, besides CO₂ and H₂O.

The biosynthetic phase, also known as the Calvin cycle, occurs in the stroma of the chloroplasts. Although it is commonly referred to as a 'dark reaction' because it does not require light directly, the process actually relies on the ATP and NADPH generated from the light-dependent reactions. This cycle involves fixing carbon dioxide into an organic form, enabling plants to synthesize sugars, which will serve as energy sources for various functions.

Imagine making a homemade smoothie. You don’t need fruit (light) present at all times, but once you have the ingredients prepared (ATP and NADPH), you can blend and have a smoothie whenever you want. Similarly, the Calvin cycle can continue using the products (ATP and NADPH) from the light reactions, even when there is no light available.

You may wonder how this could be verified; it is simple: immediately after light becomes unavailable, the biosynthetic process continues for some time, and then stops. If then, light is made available, the synthesis starts again.

This statement emphasizes that the biosynthetic reactions can continue for a brief period even in the absence of light. This is possible because the ATP and NADPH produced during the last exposure to light can be used momentarily for CO₂ fixation. However, as these reactants are consumed, the process eventually comes to a halt until sunlight is available again to replenish ATP and NADPH.

Consider a battery-powered device. When you use it, it continues to work until the battery runs out. As long as you have power stored (like the ATP and NADPH), it functions even when not plugged in (absence of light). Once the power is depleted, you will have to charge it again with electricity (light) to use it.

Let us now see how the ATP and NADPH are used in the biosynthetic phase. We saw earlier that CO₂ is combined with H₂O to produce (CH₂O) or sugars. It was of interest to scientists to find out how this reaction proceeded, or rather what was the first product formed when CO₂ is taken into a reaction or fixed.

During the Calvin cycle, carbon dioxide combines with a five-carbon sugar known as ribulose bisphosphate (RuBP) to form an unstable six-carbon compound, which quickly splits into two molecules of 3-phosphoglycerate (3-PGA). This signifies the initial step of incorporating CO₂ into organic molecules, a process essential for sugar synthesis.

Imagine a chef preparing a dish. The CO₂ acts as the main ingredient, while RuBP is like a base that helps to bind all the flavors (energy) together. Once combined, they create a new flavor (sugar) essential for the final dish, similar to how the Calvin cycle produces the energy-rich glucose.

The work of Melvin Calvin is exemplary. The use of radioactive 14C by him in algal photosynthesis studies led to the discovery that the first CO₂ fixation product was a 3-carbon organic acid. He also contributed to working out the complete biosynthetic pathway; hence it was called Calvin cycle after him.

Using radioactive isotopes, scientists like Melvin Calvin were able to trace the pathways of carbon assimilation in plants. His experiments demonstrated that the first stable product formed when carbon dioxide is fixed during photosynthesis is a three-carbon compound known as 3-phosphoglycerate (PGA). This significant finding allowed for a deeper understanding of the process of photosynthesis and how plants synthesize sugars.

Think of Melvin Calvin's work like a detective using clues to solve a mystery. By using ‘labeled’ clues (the radioactive carbon), he pieced together the pathway plants take to convert carbon dioxide into sugar, just as a detective arranges bits of evidence to uncover the truth.

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