Photosynthesis

Photosynthesis in Eukaryotes: Photosynthesis by green plants is the first step that moves essential elements of nitrogen, carbon, sulfur, and sugars for cellular function and protein synthesis into the food web. Consuming green plants and organisms that eat them is how essential elements such as nitrogen and minerals, are moved from one trophic layer to another to return through decomposers.



Photosynthesis depends on several different kinds of chlorophyll in two systems of a unit in which one type of chlorophyll 700 in system I produces one energy unit called NADPH, and another type of chlorophyll 680 in system II produces an energy unit called ATP. Particles of light are called photons. The light spectrum is made of different wave lengths, including some that our eyes can not detect, like infrared and near infrared, but which plants can use depending on their pigment. Each type of chlorophyll and associated carotinoids react to different wavelengths of light and are packed together to pass the associated photon energy from one to the other.



There are two kinds of photosynthesis, cyclic and non cyclic. In noncyclic photosynthesis there are two different strategies for handling the carbon dioxide C3 and C4. C4 works where light intensity is high and concentrates carbon dioxide around the veins in the leaf. A different type of molecule is made at the start of the Calvin cycle. What follows is a general explanation for noncyclic photosynthesis for C3 plants followed by an explanation of cyclic photosynthesis.





Noncyclic photosynthesis



In noncyclic photosynthesis in one phase, light is used to create NADP and ATP with electrons coming from splitting water. In a separate step, carbon dioxide is reduced to build sugars by combining carbon and oxygen in carbon dioxide with the hydrogen from the water. Oxygen and water are byproducts.



Think of the first part of photosynthesis as basketball team passing the ball down the court to the player who is in position to make the basket.



In noncyclic photosynthesis, chlorophyll 700 in System I is energized by photons of light. It transfers two electrons to a proton acceptor then to ferredoxin. Ferredoxin transfers electrons to NADP+ which adds a proton making it NADPH. This charges part of the system. At the same time, chlorophyll 680 in System II is activated by a photon and gives up two electrons to an electron acceptor which passes it to other receptors. The electron loses energy as it passes from receptor to receptor. This energy is used to make ATP. The spent electrons are passed to System I to replace the ones used to make NADPH. System II pulls the missing electrons it needs from water.



The second part of photosynthesis uses the NADPH and ATP, in a process called the Calvin Cycle to reduce carbon dioxide to build carbohydrates. Six molecules of carbon dioxide added to hydrogen, NADPH, and ATP from the first steps and ribulose biophosphate (a five carbon sugar) yields a six carbon sugar and enough ribulose biophosphate to start the Calvin Cycle again.



The Calvin Cycle is a series of steps which, when completed, produces 1/6 of a glucose (sugar) molecule. The cycle begins with a five carbon sugar phosphate called ribulose biphosphate. An enzyme, ribulose biphosphate carboxylase, combines carbon dioxide and the sugar, ribulose biphosphate to form an unstable six carbon molecule.



The six carbon molecule splits to form two 3-carbon molecules called PGA. The PGA gains energy from ATP and is reduced by the hydrogen from NADPH to make PGAL (an energy rich compound). After 6 cycles, two PGAL molecules leave the cycle to be used to make complex sugars. Ten PGAL molecules remain to be rearranged to form ribulose biphosphate, the 5-carbon sugar that began the process. Byproducts are oxygen and water.



Cyclic photosynthesis



Cyclic photosynthesis uses only System I chlorophyll. In cyclic photosynthesis water is not oxidized, so no oxygen is released. Cyclic photosynthesis produces ATP. In cyclic photosynthesis, light hits the chlorophyll and an electron goes to a reduction enzyme ferredoxin where it is passed from one molecule to another losing energy along the way. Unlike noncyclic photosynthesis the spent electron returns to the same system. Hydrogen sulfide is often the electron donor in making carbohydrates.





Photosynthesis in Prokaryotes: Prokaryotes have no organelle in their cells. So there are no chloroplasts in photosynthetic prokaryotes but they do have a thylakoid like structure made from the plasma membrane around the edge of the cell. In these bacteria cyclic photosynthesis is used to produce ATP but hydrogen sulfide, hydrogen or carbon molecules may replace water as the electron donor. Photosynthetic bacteria absorb best in the near infrared spectrum so are able to carry on photosynthesis in what appears to be very low light. Some bacteria are photoheterotrophs, using light but carbon comes from organic molecules.

Last updated: April 10, 2015

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