Why do plants go on to manufacture energy-rich sugars when they already have ATP and NADPH for chemical energy from light reactions of photosynthesis?
Plants perform photosynthesis to convert light energy into chemical energy in the form of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), which are crucial for many cellular processes. However, ATP and NADPH are relatively short-lived and cannot be stored for long periods. This is where energy-rich sugars, such as glucose, come into play.
Plants produce energy-rich sugars through a process known as the Calvin cycle or the dark reactions of photosynthesis. This cycle allows them to use ATP and NADPH, along with carbon dioxide (CO2), to build glucose molecules. Here's how it happens:
1. The Calvin cycle starts with the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase oxygenase) catalyzing the fixation of carbon dioxide. Carbon dioxide molecules combine with a 5-carbon compound, RuBP (ribulose-1,5-bisphosphate), resulting in the formation of two 3-carbon molecules called PGA (phosphoglycerate).
2. ATP and NADPH generated during the light reactions of photosynthesis provide the necessary energy and reducing power, respectively, to convert PGA into another 3-carbon molecule called G3P (glyceraldehyde-3-phosphate).
3. Some of the G3P molecules are used to regenerate RuBP, which is essential for the continuation of the Calvin cycle. This step requires ATP.
4. The remaining G3P molecules can then be used to synthesize glucose and other sugars through various metabolic pathways in the plant.
So, even though ATP and NADPH are products of the light reactions and provide energy and reducing power, they are not stable or storable in the long term. Energy-rich sugars, on the other hand, serve as a sustainable energy source that can be stored and used by the plant when needed. These sugars can be converted back into ATP to power cellular processes or used as building blocks for growth, development, or storage in different plant organs.