How do plants produce high energy sugars through the process of photosynthesis?    

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trophyhunter1 | College Teacher | (Level 1) Educator Emeritus

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Photosynthesis is a chemical reaction that green plants and algae carry out in organelles known as chloroplasts in their cells. In this reaction, light energy is absorbed by the producer and some of it is converted to chemical energy stored in organic molecules such as the sugar glucose. A plant needs the raw materials carbon dioxide (taken in by stomates, tiny pores in the leaves) and water (transported up to the leaves by xylem tissue inside the veins) and sunlight.

Photosynthesis occurs in two stages--the light reaction and the light independent reaction. In the light reaction, visible light is absorbed by chlorophyll, a green pigment inside chloroplasts--organelles in the cells of any green part of the plant, and this energy is used to cause water to split into hydrogen and oxygen atoms. In this step, light energy is converted to chemical energy in the form of NADPH a source of electrons which are used in the electron transport chain later on and ATP, a high energy molecule used by cells to perform life functions like photosynthesis. Once the water molecule is split, the oxygen exits the leaves through tiny pores known as stomates. It is important to note that during the light reaction, no sugar is produced. This happens during the light-- independent reaction.

In the light--independent reaction, the H+ ions that formed when water was split will be transferred to the carbon dioxide, reducing it to a sugar, a high energy compound. The carbon dioxide undergoes carbon fixation and is converted to an organic compound. The NADPH from the light reactions reduces the fixed carbon. ATP from the light reaction provides energy for this step. The carbon dioxide that was taken in by the stomates reacts with the hydrogen and forms a carbohydrate known as glucose. It is a monosaccharide (simple sugar) and contains stored chemical energy in its bonds. Its formula is C6H12O6 and glucose can be used to synthesize other larger organic molecules like sucrose (a disaccharide) or starch (a polysaccharide). These molecules can provide energy for the plant for respiration, growth and other life processes. 

The complete formula for photosynthesis is 6CO2 +12 H20+light--> C6H12O6+ 6O2+6H20. In words, six molecules of carbon dioxide plus twelve molecules of water plus light yield one molecule of glucose plus six molecules of oxygen plus six molecules of water.