The net process of photosynthesis is described by the following equation: 6CO2 + 6H2O + Light Energy = C6H12O6 + 6O2 This equation simply means that carbon dioxide from the air and water combine in the presence of sunlight to form sugars; oxygen is released as a by-product of this reaction.
During the process of photosynthesis, light penetrates the cell and passes into the chloroplast. The light energy is intercepted by chlorophyll molecules on the granal stacks. Some of the light energy is converted to chemical energy. During this process, a phosphate is added to a molecule to cause the formation of ATP. The third phosphate chemical bond contains the new chemical energy. The ATP then provides energy to some of the other photosynthetic reactions that are causing the conversion of CO2 into sugars. While the above reactions are proceeding CO2 is diffusing into the chloroplast. In the presence of the enzyme Rubisco, one molecule of CO2 is combined with one molecule of RuBP, and the first product of this reaction is two molecules of PGA. The PGA then participates in a cycle of reactions that result in the production of the sugars and in the regeneration of RuBP. The RuBP is then available to accept another molecule of CO2 and to make more PGA.  Which wavelengths of the solar spectrum drive photosynthesis? How do we know the O2 is derived from H2O during photosynthesis?The oxygen product of photosynthesis could originate from either the CO2 or the H2O starting compounds. To determine which of these original compounds contributed to the O2 end product, an isotopic tracer experiment was performed using 18O:How do we know what the first products of photosynthesis are?Another isotopic tracer experiment:14C is a radioactive isotope of carbon. 14CO2 is exposed for a brief period to a green plant that is conducting a photosynthesis in the presence of sunlight. Immediately after exposure to 14CO2, the plant's photosynthetic tissue is killed by immersing it in boiling alcohol, and all of the biochemical reactions cease. The chemical compounds in the dead tissue are all extracted and studied to determine which of them possesses the 14C. Following the briefest exposure to 14CO2, the only chemical compound that possessed 14C was PGA (phosphoglyceric acid, a three carbon molecule). Following longer periods of exposure, much of the 14C was found in a variety of compounds including glucose. By varying the length of the exposure period it was possible to identify the sequence of the reactions leading from PGA to glucose. This research was conducted by Prof. Melvin Calvin and his colleagues at the Univ. of California, Berkeley. Calvin received the Nobel Prize for this work. MetabolismWe have seen how plants convert sunlight into sugars. Now we need to understand how cells can use the products of photosynthesis to obtain energy. There are several possible metabolic pathways by which cells can obtain the energy stored in chemical bonds:Glycolysis:Glycolysis can occur in either the absence or the presence of oxygen. During glycolysis, glucose is broken down to pyruvic acid, yielding 2 ATP of energy. Glycolysis occurs in the cytoplasm of cells, not in organelles, and occurs in all kinds of living organisms. Prokaryote cells use glycolysis and the first living cells most likely used glycolysis.Fermentation:During fermentation, the pyruvic acid produced during glycolysis is converted to either ethanol or lactic acid. This continued use of pyruvic acid during fermentation permits glycolysis to continue with its associated production of ATP.Cellular Respiration:Respiration is the general process by which organisms oxidize organic molecules (e.g., sugars) and derive energy (ATP) from the molecular bonds that are broken.Glucose (a sugar):C 6H12O6Respiration is the opposite of photosynthesis, and is described by the equation: C6H12O6+6O2 ----------> 6CO2+6H2O+36ATP Simply stated, this equation means that oxygen combines with sugars to break molecular bonds, releasing the energy (in the form of ATP) contained in those bonds. In addition to the energy released, the products of the reaction are carbon dioxide and water. In eukaryotic cells, cellular respiration begins with the products of glycolysis being transported into the mitochondria. A series of metabolic pathways (the Krebs cycle and others) in the mitochondria result in the further breaking of chemical bonds and the liberation of ATP. CO2 and H2O are end products of these reactions. The theoretical maximum yield of cellular respiration is 36 ATP per molecule of glucose metabolized. ** Note that photosynthesis is a reduction-oxidation reaction, just like respiration (see the primer on redox reactions from the lecture on Microbes). In respiration energy is released from sugars when electrons associated with hydrogen are transported to oxygen (the electron acceptor), and water is formed as a byproduct. The mitochondria use the energy released in this oxidation in order to synthesize ATP. In photosynthesis, the electron flow is reversed, the water is split (not formed), and the electrons are transferred from the water to CO2 and in the process the energy is used to reduce the CO2 into sugar. In respiration the energy yield is 686 kcal per mole of glucose oxidized to CO2, while photosynthesis requires 686 kcal of energy to boost the electrons from the water to their high-energy perches in the reduced sugar -- light provides this energy. Suggested Readings
In plants, carbon dioxide (CO2) enters the leaves through stomata, where it diffuses over short distances through intercellular spaces until it reaches the mesophyll cells. Once in the mesophyll cells, CO2 diffuses into the stroma of the chloroplast—the site of light-independent reactions of photosynthesis. These reactions actually have several names associated with them. Another term, the Calvin cycle, is named for the man who discovered it, and because these reactions function as a cycle. Others call it the Calvin-Benson cycle to include the name of another scientist involved in its discovery. The most outdated name is “dark reaction,” because light is not directly required ((Figure)). However, the term dark reaction can be misleading because it implies incorrectly that the reaction only occurs at night or is independent of light, which is why most scientists and instructors no longer use it.
Light reactions harness energy from the sun to produce chemical bonds, ATP, and NADPH. These energy-carrying molecules are made in the stroma where carbon fixation takes place.
The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration.
In the stroma, in addition to CO2,two other components are present to initiate the light-independent reactions: an enzyme called ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), and three molecules of ribulose bisphosphate (RuBP), as shown in (Figure). RuBP has five atoms of carbon, flanked by two phosphates.
The Calvin cycle has three stages. In stage 1, the enzyme RuBisCO incorporates carbon dioxide into an organic molecule, 3-PGA. In stage 2, the organic molecule is reduced using electrons supplied by NADPH. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue. Only one carbon dioxide molecule is incorporated at a time, so the cycle must be completed three times to produce a single three-carbon GA3P molecule, and six times to produce a six-carbon glucose molecule.
Which of the following statements is true?
RuBisCO catalyzes a reaction between CO2 and RuBP. For each CO2 molecule that reacts with one RuBP, two molecules of another compound 3-phospho glyceric acid (3-PGA) form. PGA has three carbons and one phosphate. Each turn of the cycle involves only one RuBP and one carbon dioxide and forms two molecules of 3-PGA. The number of carbon atoms remains the same, as the atoms move to form new bonds during the reactions (3 C atoms from 3CO2 + 15 C atoms from 3RuBP = 18 C atoms in 6 molecules of 3-PGA). This process is called carbon fixation, because CO2 is “fixed” from an inorganic form into organic molecules.
ATP and NADPH are used to convert the six molecules of 3-PGA into six molecules of a chemical called glyceraldehyde 3-phosphate (G3P). That is a reduction reaction because it involves the gain of electrons by 3-PGA. (Recall that a reduction is the gain of an electron by an atom or molecule.) Six molecules of both ATP and NADPH are used. For ATP, energy is released with the loss of the terminal phosphate atom, converting it into ADP; for NADPH, both energy and a hydrogen atom are lost, converting it into NADP+. Both of these molecules return to the nearby light-dependent reactions to be reused and re-energized.
Interestingly, at this point, only one of the G3P molecules leaves the Calvin cycle and is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant. Because the G3P exported from the chloroplast has three carbon atoms, it takes three “turns” of the Calvin cycle to fix enough net carbon to export one G3P. But each turn makes two G3Ps, thus three turns make six G3Ps. One is exported while the remaining five G3P molecules remain in the cycle and are used to regenerate RuBP, which enables the system to prepare for more CO2 to be fixed. Three more molecules of ATP are used in these regeneration reactions.
This link leads to an animation of the Calvin cycle. Click stage 1, stage 2, and then stage 3 to see G3P and ATP regenerate to form RuBP.
PhotosynthesisDuring the evolution of photosynthesis, a major shift occurred from the bacterial type of photosynthesis that involves only one photosystem and is typically anoxygenic (does not generate oxygen) into modern oxygenic (does generate oxygen) photosynthesis, employing two photosystems. This modern oxygenic photosynthesis is used by many organisms—from giant tropical leaves in the rainforest to tiny cyanobacterial cells—and the process and components of this photosynthesis remain largely the same. Photosystems absorb light and use electron transport chains to convert energy into the chemical energy of ATP and NADH. The subsequent light-independent reactions then assemble carbohydrate molecules with this energy. In the harsh dry heat of the desert, plants must conserve every drop of water must be used to survive. Because stomata must open to allow for the uptake of CO2, water escapes from the leaf during active photosynthesis. Desert plants have evolved processes to conserve water and deal with harsh conditions. Mechanisms to capture and store CO2 allows plants to adapt to living with less water. Some plants such as cacti ((Figure)) can prepare materials for photosynthesis during the night by a temporary carbon fixation/storage process, because opening the stomata at this time conserves water due to cooler temperatures. During the day cacti use the captured CO2 for photosynthesis, and keep their stomata closed.
The harsh conditions of the desert have led plants like these cacti to evolve variations of the light-independent reactions of photosynthesis. These variations increase the efficiency of water usage, helping to conserve water and energy. (credit: Piotr Wojtkowski)
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