All organisms depend ultimately upon green plants as a source of organic nutrients.. This process occurs whenever radiant energy is received by chlorophyll molecules. By a series of energy- transformations carbon dioxide and water are combined in the production of carbohydrates and oxygen. A net reaction can be written as follows, using glucose as the carbohydrate.
For many years after the raw materials and end products of photosynthesis were known, it was thought that carbon and oxygen separated during the process, in such a reaction, carbon would attach to water, and oxygen would be released.
It was supposed further that the unit was "multiplied" in some fashion to form sugars. As is so often the case in scientific matters, however, the most attractive, plausible, or popular hypothesis does not always turn out to a be a fruitful one.
During the 1930's, it was demonstrated that some bacteria carried on photosynthesis without the liberation of oxygen, and in the early 1940's, studies using readiosotopic tracers indicated that in green plants liberated oxygen did not come from CO.
This was accomplished by incorporating "heavy" oxygen into water molecules and tracing it throughout the process. Contrary to the earlier idea, it was found that the oxygen of H20, not that of COz, became the O, liberated during photosynthesis. About the same time, the biochemist Robert Hill found that exposure to light of green cells in a test tube in the presence of hydrogen acceptors resulted in the liberation of oxygen, but no carbohydrate was synthesized. A little later, it was shown that carbohydrate synthesis would occur in the dark within green ells if they had previously been exposed to light.
Thus, it became obvious that photosynthesis involved two phases: the light, or photo, phase and the dark, or synthetic, phase. We shall discuss these in order. The chlorophyll molecule is so constructed that it can absorb "packets" of light. In the process of doing so, certain of its electrons become energized and actually leave the molecule. The energized state of the electrons represents the transferred radiant energy.
This process of electron separation leaves the chlorophyll molecule in an ionic state. Eventually, electrons will return to the molecule, but only after their energy of excitation has been transferred elsewhere.
Apparently, there are two possible pathways, or cycles, by means of which the electrons may get back to the chlorophyll molecules. Both of these cycles involve oxidation-reduction reactions, and as the electrons are transferred from one acceptor to another, they pass to lower energy levels. In the process, the "excess" energy of the electrons is transferred into high-energy phosphate bonds when ADP is phosphorylated to ATP.
Thus, the electrons return to the chlorophyll molecule in a low-energy state, and the oxidized chlorophyll is in a condition to be reduced again. The ATP thus formed may be used as the phosphorylating agent in the synthetic reactions of photosynthesis.
The cyclic pathways of electron transport outlined above constitute only a part of the light phase of photosynthesis, and it should be noted that the electrons removed initially from chlorophyll are eventually replaced. It should also be noted that in these cyclic pathways the water represented in our initial equation is not involved. Rather, flavins and cytochromes as well as a substance known as vitamin K transport the electrons.
These cyclic pathways leading to the production of ATP are considered to be minor when compared to the entire process of photosynthesis. Furthermore, green plant cells also can on respiration, discussed in a alter topic, in which a relatively high yield of ATP is achieved. Thus, ATP production in the cells is by no means limited to the light phase of photosynthesis.
At this point, let us return to a consideration of the overall photosynthetic equation. It should be apparent now that the products of the reaction, oxygen and carbohydrates, are formed by the splitting of water molecules.
Thus, the hydrogen so released reduces the carbon dioxide to carbohydrate, and molecular oxygen is produced. Here again, we are involved in an oxidation-reduction, but one which is highly unlikely from a thermodynamic viewpoint. The problem is this: a weak oxidant must oxidize a weak reluctant producing a strong oxidant and a strong reluctant. In other words, the C02 and H20 are much more stable than the 02 and the carbohydrate.
Carbon dioxide joins a five-carbon sugar, rib lose diphosphate, which is already present in the cell, to form a very unstable six-carbon compound. The six-carbon compound has a very brief existence; almost immediately it breaks down spontaneously into two molecules of a three- carbon compound, 3-phosphoglyeerie acid. Each molecule of PGA is then reduced to the aldehyde form, phosphogylceraldehyde, by NADP.H2 with the aid of a molecule of ATP.
Thus, it is at this point that the products of the light phase enter into the reduction of carbon dioxide. PGAL is now at the reduction level of a carbohydrate which corresponds to that of an aldehyde, and it may travel any of several different pathways.
It may undergo a series of reactions and eventually be transformed to RDP, it may become modified into glycerol, or it may undergo condensation to form the six-carbon sugar fructose diphosphate, which can undergo dephosphioiylation and certain internal transformations to become glucose. Glucose may then serve as a building block for such saccharine sugars as sunrise or such polysaccharides as starch.
Although PGAL might justly be considered the end product of photosynthesis. PGA is frequently involved in transformation. It may proceed along a pathway leading to the formation of amino acids, which subsequently become involved in protein synthesis, or it may become involved in the formation of fatty acids, which join with glycerol in the formation of fats.
Notice that this scheme shows the entrance of carbon dioxide and the products of the light phase into a cycle involving the compounds we have discussed. Although we have mentioned only a few of the many possible synthetic pathways taken by PGA and PGAL, it should be obvious that the basic organic molecules which serve as nutrient materials for cells of green plants themselves and for the cells of other organisms are produced in photosynthesis.
In summary, photosynthesis is an extremely complex process involving many separate reactions. Like virtually all reactions which within occur living systems, they are catalyzed by a complex of specific enzymes. Although the light and dark phases of photosynthesis can be separated experimentally, they are closely interrelated in the overall metabolism of any given photiosynthetic cell.
In addition to photosynthesis, the plant cell carriers on respiration, during which large amounts of ATP are formed, and this ATP supplies energy for many of the synthetic reactions we have mentioned. In other words, the ATP formed during the light phase of photosynthesis is not nearly sufficient to drive the many endergonic reactions carried on in the plant.
Nevertheless, our original equation is accurate as a summary equation, because every energetic reaction is driven by energy which is ultimately supplied by sunlight.
Synthesis common to all cells there are numerous compounds not obtained by cells as prefabricated nutrients; rather, they are synthesized within the cells themselves. These are primarily the organic macromolecules which constitute the bulk of cell contents exclusive of water.
In all cells except the photo synthetic cells discussed above, and the chemosynthetic bacterial cells mentioned previously, the raw materials employed in synthetic reactions comes from the digestion of prefabricated materials taken into the organism. The energy necessary to drive these endergonic reactions also comes from these prefabricated materials, in their respiration.
Photosynthesis is the process by which plants and other things make food. It is a chemical process that uses sunlight to turn carbon dioxide into sugars the cell can use as energy. As well as plants, many kinds of algae, protists and bacteria use it to get food. Photosynthesis is very important for life on Earth. Most plants either directly or indirectly depend on it. The exception are certain organisms that directly get their energy from chemical reactions; these organisms are called chemoautotrophs.
Photosynthesis can happen in different ways, but there are some parts that are common.
- 6 CO2(g) + 6 H2O + photons → C6H12O6(aq) + 6 O2(g)
- carbon dioxide + water + light energy → glucose + oxygen
Reactions[change | change source]
Photosynthesis has two main sets of reactions. Light-dependent reactions need light to work; and light-independent reactions, which do not need light to work.
Light-dependent reactions[change | change source]
Main article: Light-dependent reaction
Light energy from the sun is used to split water molecules (photolysis). The sunlight hits chloroplasts in the plant, causing an enzyme to break apart the water. Water, when broken, makes oxygen, hydrogen, and electrons in the pathway of Dolai's S-state diagrams. Ref. Dolai, U (2017) "Chemical Scheme of Water -Splitting Process during Photosynthesis by the way of Experimental Analysis ". IOSR Journal of Pharmacy and Biological Sciences. 12(6): 65-67. doi:10.9790/3008-1206026567. ISSN 2319-7676.
Hydrogen, along with electrons energized by light, converts NADP into NADPH which is then used in the light-independent reactions. Oxygen diffuses out of the plant as a waste product of photosynthesis, and ATP is synthesized from ADP and inorganicphosphate. This all happens in the grana of chloroplasts.
Light-independent reactions[change | change source]
Main article: Light-independent reaction
During this reaction, sugars are built up using carbon dioxide and the products of the light-dependent reactions (ATP and NADPH) and various other chemicals found in the plant in the Calvin Cycle. Therefore, the light-independent reaction cannot happen without the light-dependent reaction. Carbon dioxide diffuses into the plant and along with chemicals in the chloroplast, ATP, and NADPH, glucose is made and finally, transported around the plant by translocation.
Factors affecting photosynthesis[change | change source]
There are three main factors affecting photosynthesis:
Light intensity[change | change source]
If there is little light shining on a plant, the light-dependent reactions will not work efficiently. This means that photolysis will not happen quickly, and therefore little NADPH and ATP will be made. This shortage of NADPH and ATP will lead to the light-independent reactions not working as NADPH and ATP are needed for the light-independent reactions to work.
Carbon dioxide levels[change | change source]
Carbon dioxide is used in the light-independent reactions. It combines with NADPH and ATP and various other chemicals (such as Ribulose Bisphosphate) to form glucose. Therefore, if there is not enough carbon dioxide, then there will be a buildup of NADPH and ATP and not enough glucose will be formed.
Temperature[change | change source]
There are many enzymes working in photosynthetic reactions – such as the enzyme in photolysis. These enzymes will not work as well, or stop working at all at high or low temperatures and therefore, so will the light-dependent and light-independent reactions. Tropical plants have a higher temperature optimum than the plants adapted to temperate climates.
Early evolution[change | change source]
The first photosynthetic organisms probably evolved early in the history of life. They may have used reducing agents such as hydrogen or hydrogen sulfide as sources of electrons, rather than water.Cyanobacteria appeared later, and the excess oxygen they produced contributed to the oxygen catastrophe. This made the evolution of complex life possible.
Effectiveness[change | change source]
Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts, which is about six times larger than the current power used by human civilization. Photosynthetic organisms also convert around 100–115 thousand million metric tonnes of carbon into biomass per year.
Related pages[change | change source]
References[change | change source]