Photosynthesis happens in two major parts: the Light Reaction, and the Dark reaction. The light reaction requires sunlight and involves the absorption of light energy, and the conversion of light energy into chemical energy. The dark reaction, also known as the Calvin Cylce, does not require sunlight and is the process that takes the chemical energy and stores it in the form of usable glucose.
Plant cells have chloroplasts in them (refer to picture), which consist of groups of stacked thylakoids surrounded by a membrane. The space surrounding the thylakoids is called the stoma. Embedded in the membrane of thylakoids is chlorophyll, a green pigment that absorbs light. Chlorophylls form two types of clusters, called photosystems (PS) I and II. (shown in picture)
When sunlight comes in contact with a plant, photons from the sunlight hit the chloroplast to start the light reaction. Solar photons take only eight minutes to reach plants from the sun. When electrons attached to a chlorophyll are stuck by a photon, they become excited. These electrons are then so excited that they are transferred from one PS II to one PS I. An electron transport system is formed between the photosystems, and more electrons are transferred. Water is oxidized into O2 and H+ in an enzyme by PS II. Electrons are also formed in the oxidation that replace the electrons lost in the transferring of electrons. The protons (H+) made in the oxidation process accumulate in the thylakoid space, making a difference in the concentration gradient. When electrons reach PS I, NADP+ is reduced to NADPH, which is used later in the Calvin Cycle. The difference in concentration gradients of protons causes potential energy to build up, and then protons are forced through the ATP synthase, which reduces ADP to ATP.
The Calvin Cycle (shown in picture) uses the NADPH and the ATP created in the light reaction, along with some carbon dioxide to form sugars that the plant or fungi can use. To start the calvin cycle, three carbon dioxides are combined with three 5-carbon sugars (RuBp) with the help of the catalyst rubisco, which immediately split into six molecules of a 3-carbon acid. This first phase is known as Carbon Fixation. Next, the six 3-carbon acids are rearranged with the energy from one ATP and one NADPH and creates G3P. One G3P is half of a glucose. Then, the remaining 5-carbon sugars are rearranged with the help of ATP to create more RuBp, which joins with other carbon dioxides to continue the Calvin Cycle.
The rate of photosynthesis depends on light levels, water levels, nutrient levels, temperature, and carbon dioxide availability.
Extra Information:
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