Photosynthesis is a vital process carried out by green plants, algae, and some bacteria, which allows them to convert light energy from the sun into chemical energy stored in glucose (a type of sugar). This process occurs primarily in the chloroplasts of plant cells and can be broken down into two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle).
Light-Dependent Reactions
The first stage occurs in the thylakoid membranes of the chloroplasts. Here, sunlight is captured by chlorophyll, the green pigment in plants. When chlorophyll absorbs light energy, it excites electrons, which are then transferred through a series of proteins known as the electron transport chain. This transfer of energy helps to convert light energy into two important energy-carrying molecules: ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).
Additionally, water molecules (H2O) are split in a process called photolysis, releasing oxygen (O2) as a byproduct. The energy released during this splitting also contributes to the formation of ATP and NADPH.
Light-Independent Reactions (Calvin Cycle)
The second stage of photosynthesis occurs in the stroma of the chloroplasts. ATP and NADPH produced in the light-dependent reactions are used in the Calvin cycle to convert carbon dioxide (CO2) from the atmosphere into glucose. Through a series of chemical reactions, carbon dioxide is combined with a 5-carbon sugar (ribulose bisphosphate or RuBP) and with the help of the enzyme rubisco, is transformed into a 3-carbon sugar (glyceraldehyde-3-phosphate or G3P). Eventually, these G3P molecules are rearranged to form glucose.
Energy Transformation
In summary, photosynthesis is an excellent example of energy transformation. Light energy from the sun is converted into chemical energy stored in glucose. The process starts with the absorption of photons by chlorophyll, exciting electrons and leading to the production of ATP and NADPH. These energy carriers are then used to power the conversion of carbon dioxide into glucose. Thus, photosynthesis not only sustains the plant itself but also serves as the foundation of energy for nearly all life on Earth, as animals consume plants for energy. Through this remarkable conversion process, energy flows from the sun to plants and, subsequently, to the entire food web.