Chemistry concepts and applications

chemistry and PHOTOSYNTHESIS

Chemically speaking, photosynthesis is the utilization of sunlight by plants for the conversion of carbon dioxide into organic matter. In green plants like trees, shrubs, flowers and conifers, the overall chemical reaction may be represented by the balanced equation CO2 +H2O{CH2O}+O2 in which the newly-formed organic matter is represented by CH2O, a generalized formula for a carbohydrate. The overall process by which green plants convert sunlight to food can be expressed with the equation 6H2O+6CO2C6H12O6+6O2 which translates to "six molecules of water plus six molecules of carbon dioxide produce one molecule of sugar plus six molecules of oxygen" (Farabee, Internet).

The importance of this rather simple process lies in its conversion of energy from radiant (i.e., sunlight) into chemical form (i.e., sugar). Depending slightly on the specific kind of carbohydrate formed, the amount of energy transformed is about "112 kcal per mole of CO2 fixed," and because of this large energy requirement, non-photochemical schemes for fixing CO2 as organic matter is very rare. Thus, the chemical energy which all green plants store by photosynthesis provides the total energy requirement for the plant, including the energy required for the synthesis of a wide variety of other organic substances (Gregory, 67).

All green plants receive their carbon dioxide (CO2) directly from the Earth's atmosphere and under conditions of low light, the rate of photosynthesis is often separate of the CO2 content, but under strong sunlight, the rate is dependent upon "the carbon dioxide partial pressure at low CO2 and reaches a limiting value at high CO2. The average CO2 content of surface air, usually about.03% (considering the current effects of global warming and climate change) is not quite enough for most land plants. Aquatic plants, however, derive their CO2 from dissolved gases in the water.

Overall, the normal primary photosynthetically-produced carbohydrate is known as phosphoglyceraldehyde, being "the phosphate ester of a 3-carbon sugar" molecule (Silverstein, 178). The synthesis of hexoses, a sugar with six carbon atoms in its molecules, and storage polysaccharides, a polymer of ten or more monosaccharides joined by glycosidic links, as well as the formation of fats, proteins and other organic substances during periods of photosynthetic activity, are attributed to a number of secondary processes, far too complicated to explore here. However, our current understanding of the pathway of carbon in photosynthesis comes mainly from "labeling experiments with C14 enriched CO2 (Silverstein, 179).

Generally speaking, the photosynthetic carbon cycle is made up of reactions concerned with the function and re-creation of the actual acceptor molecule reacting with carbon dioxide, assimilation and the photochemical reduction. These CO2 acceptors (5-carbon ribulose diphosphate) is created by condensation and hydrolises, while part of the primary carbohydrates go directly to stored sugars or starch. Through several other complicated chemical processes, the end-product is oxygen which is then released into the atmosphere for use by air-breathing animals.

The chemical process of photosynthesis is made up of light and dark reactions which can be summarized as CO2+4H2O{CH2O}+3H2O+O2 (Bacon, 214). Under very weak sunlight, the rate measured either by CO2 uptake or by O2 production is equal to sunlight intensity. The wavelength of effective sunlight varies widely, beginning with the near ultra-violet to the far red. At stronger illumination, the rate increases with increased intensity and obtains a maximum saturation rate which is independent of other changes in intensity. For example, with most green plants, the maximum is obtained below the natural outside intensity on a bright, sunny day. In a dark period, high energy phosphate compounds are synthesized in the presence of what is called ADP, being adenosine diphosphate, a nucleotide consisting of adenine and ribose with two phosphate groups attached via a high energy bond (Blankenship, 145).

In every single green plant, photosynthetic organisms contain light-sensitive pigments in highly-organized structures known as chloroplasts which can be defined as a photosynthetic plastid containing chlorophyll and other photosynthetic pigments (Blankenship, 152). This ordered arrangement of pigments helps along with the transfer of energy from one molecule to another. As K.E. Bacon points out, chlorophyll is "a magnesium-centered porphyrin containing a hydrophilic 5-membered carbocyclic ring and a lipohilic phytol tail" (176).

The entire unit is known as a photosytem and in green plants, one finds two of these systems, photosystem I and photosystem II, both of which are involved in the light reactions of photosynthesis. Light energy absorbed by these pigments of the antenna complex is "passed to the reaction center chlorophyll molecules from which it passes along an electron-transport chain" (Blankenship, 215). Photosystems II contains a kind of chlorophyll a (P680) which shows maximum light absorption at a wavelength of about 684 nm. When activated by sunlight, a pair of electrons become excited and leave photosystem II and is replaced by electrons from the photolysis of H2O, summarized as 2H2OO2+4H++4e-.

Molecular oxygen is then released and the protons pass into the lumen (the central space that remains in a cell that has lost its living contents) of the thylakoid (an elongated, flattened fluid-filled sac that forms the basic unit of the photosynthetic membrane system). Then, the electrons pass through what is known as an electron transport chain (ETC) in the thylakoid membrane. It is here that the electrons are given more power by sunlight to create even higher energy levels.

They then pass through a second ETC which involves the protein ferredoxin, being a "group of red-brown proteins which contain non-heme iron with sulfur at the active site and function as electron carriers" (Gregory, 223). This extremely complex sequence which today's engineers are attempting to duplicate for use in solar-generated energy systems is also known as noncyclic electron flow (NEF) which is used to drive ATP synthesis (cyclic photophosphorylation). ATP synthesis which is linked to the non-cyclic electron transport chain (NETC) is known as non-cyclic photophorphorylation.

From this point on, the light reactions are utilized to reduce carbon dioxide to carbohydrate. Carbon dioxide is then fixed via a combination of with the 5-carbon sugar ribulose biphosphate (RuBP) which forms two molecules of phosphoglyceric acid (PGA), "a complex lipid similar to acylglycerides, the major foundation of the membranes of cells" (Blankenship, 216).

Thus, this reaction is catalyzed by the enzyme known as ribulose biphosphate carboxylase, and in a series of other reactions, PGA is converted to a succession of carbon sugar phosphates collectively known as the Calvin cycle, named after Melvin Calvin, "an American biochemist best-known for his experiments with the dark reactions of photosynthesis via radioactive carbon" (Bacon, 237). These by-products and chemicals are then used in the synthesis of carbohydrates, fats, protein and other compounds which together generate RuBP.