Big+Idea+4


 * Big Idea 4: Photosynthesis**

__Summary__ Photosynthesis is a series of [|metabolic pathways] in an organism, plant, bacteria, algae, etc., that converts carbon dioxide into energy for said organism by utilizing the energy from sunlight. Organisms with the ability of photosynthesis are called [|photoautotrophs], however, organisms that use other organic compounds other than carbon dioxide as their source of carbon are called [|photoheterotrophs]. In most cases, oxygen is released as a waste product.

Photosynthesis is a crucial part of the earth's ecosystem, as it helps recycle carbon dioxide from the air and releases oxygen for other organisms to utilize.

There are two kinds of photosynthesis: light-dependent photosynthesis and light-independent photosynthesis.


 * //words with a * after them: refer to notes at bottom of page//**

__Light-Dependent Photosynthesis__ Light dependent photosynthesis, as the name suggests, requires the presence of sunlight to be able to convert biochemical energyPo into compounds useful to organisms use photosynthesis to maintain homeostasis. Simply put, light dependent photosynthesis uses the energy from sunlight to synthesize food for energy from carbon dioxide. However, not all wavelengths of light are supported by photosynthesis. The carbon dioxide can enter the leaf through an opening called the //stoma*//. The carbon dioxide is then dissolved on a thin layer of water to be diffused through the cell wall.

Light dependent reactions occur in the thylakoid* membrane of chloroplast. Light energy is used synthesize ATP and NADPH*. There are two forms of light dependent reactions: cyclic and non-cyclic. In a non-cyclic reactions, photons are collected by pigment molecules in photosystem II*. When the core chlorophyll molecule has collected enough energy from the accessory pigments*, an electron is released and transferred to a primary electron-acceptor molecule, pheophytin*. Because the electron is very unstable, it is moved from one molecule to another, creating an electron transport chain*, also known as a //Z scheme// (see below), which creates an electrochemical gradient* across the membrane. An ATP synthase* utilizes this potential during photophosphorylation*. The electron is then passed into another photosystem, photosystem I*. The electron is excited* from the light energy collected by the photosystem. A second electron carrier transfers said electron to another electron acceptor. The energy created is used to move hydrogen ions across the membrane into the //thylakoid lumen//. The electron is used to reduce the co-enzyme NADP* into NADPH. The cyclic reaction is same in all respects except the electron stays within photosystem I and only ATP is produced, with no NADPH as a product. Another difference is the displaced electron is passed by electron acceptor molecules back into the photosystem, hence the name cyclic. In the non-cyclic reaction, the chlorophyll molecule regains the lost electron from a water molecule, through a process called photolysis*, which then releases a dioxygen molecule as a waste product.

The overall equation for light dependent reactions within green plants is:
 * 2 H2O + 2 NADP+ + 2 ADP + 2 Pi + light → 2 NADPH + 2 H+ + 2 ATP + O****2**

__Light Independent Reactions__ Processes that make up light independent reactions take place in the stroma, the fluid outside the thykaloid membrane within the chloroplast. There are three overall phases, collectively called the Calvin Cycle. The three phases are carbon fixation, reduction reactions, and ribulose 1,5-diphosphate (RuDP) regeneration. Light independent reactions are also called dark reactions.

In the Calvin Cycle, the first step is carbon fixation. The enzyme RuBisCO* catalyzes* the carboxylation* of of RuDP, by carbon dioxide in a two step reaction. The result is a 3-carbon compound glycerate 3-phosphate. Then, the enzyme phosphoglycerate kinase* catalyzes the phosphorylation* of 3PGA by ATP. The products are 1,3-bisphosphoglycerate * and ADP*. In the next step, the enzyme G3P dehydrogenase* catalyzes the reduction* of of 1,3BPGA by NADPH. The product is glyceraldehyde 3-phosphate, and the NADPH is oxidized, forming NADP.

After the initial carbon fixation. triose phosphate isomerase converts some G3P into dihydroxyacetone phosphate*. Then, adolase* and fructose-1,3 biophosphatase* convert G3P* and DHAP* into fructose-6 phosphate*. One phosphate ion* is lost in the solution. After this step, the fixation of carbon dioxide generates two more G3P. F6P* then has two carbon dioxide molecules removed by transketolase*, giving erythrose-4-phosphate *. The two carbon dioxide molecules are then added to a G3P, giving the ketose* xylulose-5-phosphate* (Xu5P). E4P* and DHAP are converted into sedoheptulose-1,7-bisphosphate* (7cC by the adolase enzyme. 7C is then

__Notes__ Stoma: Pore found on leaves and stem epidermis that allows for gas exchange.

Thylakoid: A membrane bound compartment within the chloroplast organelle. It is the site light dependent reactions for photosynthesis. They are piled up like a stack of coins, grana. Grana operate like one single compartment.