Why are the reactions in the 1st and 2nd stages of photosynthesis called "light reactions."
Because sun light is needed to make the chemical reactions take place.
http://en.wikipedia.org/wiki/Light_reaction
The reactions in the first and second stages of photosynthesis are called "light reactions" because they require light energy to occur. These reactions take place in the thylakoid membranes within the chloroplasts of plant cells.
To understand why they are called "light reactions," let's look at the process step by step:
1. Light absorption: In the first stage, light energy is absorbed by pigments in the chlorophyll molecules, such as chlorophyll a and b. These pigments are located in the thylakoid membranes of the chloroplasts.
2. Electron transport: The absorbed light energy excites electrons in the chlorophyll molecules. These energized electrons are then transferred through a series of electron transport chains within the thylakoid membrane.
3. ATP and NADPH production: As the excited electrons move through the electron transport chains, they release energy. This energy is utilized to generate a molecule called ATP (adenosine triphosphate), which is an essential energy carrier in cells. Additionally, NADP+ (nicotinamide adenine dinucleotide phosphate) is reduced to form NADPH, another molecule that carries energy.
4. Oxygen release: During the light reactions, water molecules are also split, releasing oxygen gas as a byproduct. This process is called photolysis, and it provides the oxygen necessary for aerobic respiration in organisms.
Overall, the term "light reactions" indicates that these stages of photosynthesis require light energy to initiate the complex series of biochemical reactions. Without light, these reactions would not occur, and photosynthesis could not take place.