Photosynthesis

From Chempedia

Sara Fiskum
Molly Green
Calley Gruenhagen
Lisa Habermann
Matt Hayes

Photosynthesis is a chemical reaction in which light energy is converted to chemical energy in glucose. Plants use this process to synthesize sugars and oxygen gas, which are essential components of life. The basic chemistry behind photosynthesis is an oxidation-reduction reaction in which oxygen is oxidized and hydrogen, ATP, and NADP are reduced. Photosynthesis can be represented by the following chemical equation: 6CO₂ + 12H₂O + hv -> C₆H₁₂O₆ + 6O₂ + 6H₂O (Carbon Dioxide + Water + Light Energy -> Glucose + Oxygen + Water) There are two general processes that occur during photosynthesis: Photolysis and the Calvin Cycle. These processes can be distinguished by where they take place and their dependency on light. Photolysis requires light, therefore is referred to as a light-dependant reaction and takes place in the stroma. The Calvin Cycle does not require light and is known as a light-independent reaction, which takes place outside the stroma. Both reactions occur simultaneously.

Two important points to mention before explaining the photosystems and reactions are the antenna complex and the reaction system. The antenna complex is the location in which the light energy is absorbed, which causes an electron to be excited. The energy received in the complex is transmitted to a nearby chlorophyll molecule, which results in another electron becoming excited. When the energy is transmitted, the original excited electron falls back to its original ground state. The antenna complex then transmits the energy to a reaction center. In the reaction center, excited electrons are transferred to a molecule that acts as the electron accepter. Once this molecule is reduced by the electron, the electromagnetic energy that was absorbed from light is transformed into chemical energy. When light excites the electrons in chlorophyll to a higher energy state, the reaction is exothermic.

In the light-dependant reaction, plants use chlorophyll to absorb light. Chlorophyll is made up of green pigment molecules. Therefore, chlorophyll reflects green and yellow light and absorbs blue, violet, and red light. The light-dependant reaction occurs in two places; photosystem I and photosystem II. The process begins in photosystem II, where the absorbed light energy excites the electrons in the chlorophyll, which raises the electrons potential energy. Excited electrons are unstable and quickly return to their ground states, but in doing so the electrons release energy to the next chlorophyll molecule. "When an electron in the reaction center chlorophyll is excited energetically, the electron binds to pheophytin and the reaction center chlorophyll is oxidized" (Freeman, 217). Pheophytin is an electron acceptor. The chlorophyll b is now lacking electrons and a water molecule is split to replace the missing electrons. The remaining oxygen atom then binds with another oxygen atom (due to diatomic properties), which creates oxygen gas as a byproduct. This process can be represented by the following equation: 2 H₂O -> O₂ + 4 H⁺ + 4 e⁻ The remaining H+ atoms are stored in the stroma for later use. In the electron transport chain, the electrons go from an excited state to ground state using a series of oxidation-reduction reactions. This releases energy and lowers the potential energy as the electron transport chain proceeds. The thylakoid membrane captures the energy and helps drive the reaction to produce ATP. At the end of the electron transport chain, chlorophyll "a" receives the electron. The end result of photosystem II is photophosphorylation, a process that produces adenosine triphosphate (ATP), which is the main energy source for cells. The high H+ concentration inside the stroma also provides fundamental energy required to produce ATP. In accordance with the second law of thermodynamics, the H+ ions have a tendency to leave the highly concentrated area. To do so they must flow out of an ATP synthase that acts as a "micro mill," spinning and creating energy to bond additional phosphate groups to ADP to form ATP. When electrons reach the end of the electron transport chain in photosystem II, they are passed onto the protein called plastocyanin. The protein then picks up the electrons and diffuses them into the thylakoid membrane and donates the electrons to photosystem I. The protein then transports the electron to an enzyme inside the thylakoid membrane called NADP. NADP then combines with two electrons and the hydrogen atom that was left over from the splitting of water in photosystem II. The production of NADPH is the end result of photosystem I. This process can be represented by the following equation: NADP⁺ + H⁺ + 2 e⁻ -> NADPH

Light-Dependent reaction The light-independent reaction does not depend on light, but it could not proceed without the light-dependent reactions. The light-independent reactions produce sugars by combining carbon dioxide with the ATP, NADPH, and H⁺ that were produced in the light reactions.

The light-independent reaction uses the process called the Calvin Cycle. First, CO₂ from the air enters through the thylakoid membrane. Three CO₂ molecules attach to a 5-carbon sugar molecule, which makes 18 total carbon atoms. Each of the now 6-carbon molecules, known as 3-phosphoglycerate, breaks apart into two molecules due to instability. Each molecule of 3-phosphoglycerate gains a phosphate from ATP, which makes a 3-diphosphoglycerate. The 3-diphosphoglycerate is reduced by NADPH to form phosphoglyceraldehyde (PGAL), a 3-carbon sugar. PGAL stores more energy than any of the other molecules. The product from the reduction of one 3-diphosphoglycerate is an NADP molecule and a phosphate ion, which is reused in later reactions. Now there are six molecules of PGAL. One molecule of PGAL is removed from the system. The other five PGAL molecules are then converted back into three 5-carbon molecules (3-phosphoglycerate) with the addition of three phosphates from ATP, which reduces ATP to ADP. One molecule of PGAL is produced and the other molecules are reused in another synthesis. Two PGAL molecules combine to produce glucose. Therefore, two Calvin Cycles must be completed in order to produce one molecule of glucose. Plants not only use glucose for energy, but it can also be combined with other glucoses to form cellulose as well as starch. The overall chemical reaction that occurs during the light-independent reaction is as follows: H⁺ + ATP + NADPH₂ + CO₂ -> PGAL (x2)  C₆H₁₂O₆

The Calvin Cycle

Photosynthesis is a very important process. Many factors can affect the photosynthetic process; including water, temperature, nitrogen and carbon dioxide concentrations, and leaf morphology. These factors determine the speed of the photosynthetic process as well amount of products formed. The major products produced in photosynthesis are oxygen, glucose, ATP, and NADPH. Oxygen is necessary in respiration for many animals, glucose is used as energy, and ATP and NADPH are used to drive many biological processes.

References Biology, Campbell 6^th edition Freeman, Scott. 2005. Biological Science. 2^nd Edition, Pearson Prentice Hall. NJ. pp 217.

"Photosynthesis." Lubey\’s BioHELP! 20 Sept. 2005 <http://www.borg.com/\~lubehawk/ photosyn.htm>.

"Photosynthesis." Lubey\’s BioHELP! 20 Sept 2005 <http://[[[-http://www.borg.com/~lubehawk/ photochm.htm]www.borg.com/~lubehawk/ photochm.htm-http://www.borg.com/~lubehawk/ photochm.htm]www.borg.com/~lubehawk/ photochm.htm-http://www.borg.com/~lubehawk/ photochm.htm]www.borg.com/~\~lubehawk/ photochm.htm>.

"Photosynthesis." Wikipedia "The Free Encyclopedia". 20 Sept. 2005 <www.wikipedia.org>.

Roast, Thomas L., et al. Plant Biology. P. 147-162. Canada: Thomson Brooks, 2006.

Weinig, Dr. Cynthia. "Photosynthesis." Classroom Office Bldg B35, 29 September.

Diagrams:

Light Reaction And Calvin Cycle Diagrams: http://www.biologycorner.com/bio3/notes-lightdependent.html.

Thylakoid membrane Diagram: www.cs.csbsju.edu/.../ figures/thylakoid.htm.