Photosynthesis
Photosynthesis is the process by which plants produce Sugar and Oxygen from Carbon dioxide, Water, and the Energy from the sun. Without photosynthesis, life as we know it would not be possible; we, as animals, depend entirely on the Sugars produced by plants to provide us with necessary Energy.
Photosynthesis occurs in two stages:
1) the photochemical reaction and
2) the thermochemical reaction.
[edit] What is photosynthesis
Photosynthesis is the single process which distinguishes between autrophes ("auto" = self, "trophe" = food) and heterotrophes ("hetero" = other, "trophe" = food). Only autotrophies, such as plants, algae, and some prokaryotic organisms, can produce their own food. The remainder of Earth's creatures rely on these autotrophies for their own sustenance. Photosynthesis occurs in the chloroplasts of the plants' cells, especially those located in the leaves. Chloroplasts are specialized organelles that contain chlorophyll, the green pigment which not only gives plants their color, but absorbs the light Energy needed to drive photosynthesis as well. The general equation for photosynthesis is as follows: 6 molecules of Carbon dioxide plus 6 molecules of Water plus light energy yields 6 oxygen molecules plus one molecule of Glucose.
[edit] The photochemical reaction
This reaction is also known as the "light dependent", or simply "light", reaction, because it requires the energy of the sun. The light reaction occurs in the thylakoid membrane of the chloroplasts. The light reaction relies upon two clusters of pigments, known as photosystems I and II, which each possess a different type of chlorophyll and several accessory pigments. These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light. When light hits photosystem II, electrons gain more energy and are carried, via a chain of electron-carrying Proteins, to photosystem I. When the light hits this second photosystem, the electrons are moved again, this time to a molecule of energy-rich NAD. The addition of these electrons reduces NAD to NADH, which will be used in the thermochemical reaction. Meanwhile, the electrons that were moved from photosystem II must be replaced. Water is split, donating its electrons to fill the vacancies in photosystem II and releasing its hydrogen atoms. This creates oxygen, one of the net products of photosynthesis. The electrons needed to replace those removed from photosystem I are provided by photosystem II. The hydrogen ions produced by the splitting of water, supplemented with additional ions from the surrounding area, are pumped back and forth across the thylakoid membrane. This creates a proton gradient, which provides enough energy to create several molecules of energy-packed ATP. Along with the NADH produced by the movement of electrons, the ATP will be used immediately in the thermochemical reaction.
[edit] The Thermochemical Reaction
This reaction is often known as the Calvin Cycle, "light-independent" reaction, or the "dark" reaction, because it does not directly require light energy. It is important to note, however, that the light independent reactions occur for only a brief time after sunset, because they quickly use up their store of ATP and NADH produced in the light reaction. The light independent reaction occurs in the stroma of the chloroplast. The stroma is a thick, syrupy fluid found between the thylakoid membranes. Three "turns" of the Calvin Cycle are required to produce a single molecule of Glucose. 18 ATP and 12 NADH are needed to help transform the carbon dioxide into Sugar. Carbon dioxide enters the cycle and is fixed into Glucose through a series of steps catalyzed by enzymes. ATP provides the Energy for these reactions, while NADH is the reducing agent, attaching high-energy electrons to form the Sugar. After being used, the ATP is converted to ADP and the NADH to NAD, both of which are immediately used in the light reaction.
[edit] Variant Forms of Photosynthesis
Because of the Climate they live in, some plants must slightly alter the process of photosynthesis. Desert plants, for example, must keep passages open to admit the needed Carbon dioxide, but the hot temperatures means water will be evaporated quickly. The passages cannot be closed for too long, because Oxygen will build up in the plant and the gas will be used, instead of carbon dioxide, in photosynthesis. This is a process known as photorespiration uses energy but produces no Sugar. To compensate, a special class of plants known as C4 plants incorporate carbon dioxide into an intermediate enzyme known as PEP carboxylase, which has a very low affinity for oxygen but a very high affinity for Carbon dioxide. This adaptation allows the passages to stay closed while ensuring photorespiration does not occur. Another group of desert plants called CAM plants, open their passages at night and close them during the day, which is opposite to normal plants. Carbon dioxide is stored at night in organic acids and utilized during the day, when sunlight is present to drive the light dependent reactions.
Photosynthesis is essential to all life on Earth. Not only do photosynthetic autotrophies produce the sugar needed for other organisms, they also produce the vital element Oxygen that all animals must breathe to survive.
[edit] Overall reaction of photosynthesis
In chemical terms, photosynthesis is a light-energized oxidation–reduction process. (Oxidation refers to the removal of electrons from a molecule; reduction refers to the gain of electrons by a molecule.) In plant photosynthesis, the energy of light is used to drive the oxidation of Water (H2O), producing Oxygen gas (O2), hydrogen ions (H+), and electrons. Most of the removed electrons and hydrogen ions ultimately are transferred to carbon dioxide (CO2), which is reduced to organic products. Other electrons and hydrogen ions are used to reduce nitrate and sulfate to amino and sulfhydryl groups in Amino acids, which are the building blocks of Proteins. In most green cells, Carbohydrates—especially starch and the Sugar sucrose—are the major direct organic products of photosynthesis. The overall reaction in which Carbohydrates—represented by the general formula (CH2O)—are formed during plant photosynthesis can be indicated by the following equation:
light
CO2 + 2H2O ---------------> (CH2O) + O2 + H2O
green plants
This equation is merely a summary statement, for the process of photosynthesis actually involves numerous complex reactions. These reactions occur in two stages: the “light” stage, consisting of photochemical (i.e., light-dependent) reactions; and the “dark” stage, comprising chemical reactions controlled by enzymes (organic catalysts). During the first stage, the energy of light is absorbed and used to drive a series of electron transfers, resulting in the synthesis of the energy-rich compound adenosine triphosphate (ATP) and the electron donor reduced nicotine adenine dinucleotide phosphates (NADPH). During the dark stage, the ATP and NADPH formed in the light reactions are used to reduce Carbon dioxide to organic carbon compounds. This assimilation of inorganic carbon into organic compounds is called carbon fixation.
During the 20th century, comparisons between photosynthetic processes in green plants and in certain photosynthetic sulfur Bacteria provided important information about the photosynthetic mechanism. Sulfur bacteria use hydrogen sulfide (H2S) as a source of hydrogen atoms and produce sulfur instead of Oxygen during photosynthesis. The overall reaction is
light
CO2 + 2H2S ---------------> (CH2O) + S2 + H2O
sulphur bacteria
In the 1930s Dutch biologist Cornelis van Niel recognized that the utilization of carbon dioxide to form organic compounds was similar in the two types of photosynthetic organisms. Suggesting that differences existed in the light-dependent stage and in the nature of the compounds used as a source of hydrogen atoms, he proposed that hydrogen was transferred from hydrogen sulfide (in Bacteria) or water (in green plants) to an unknown acceptor (called A), which was reduced to H2A. During the dark reactions, which are similar in both Bacteria and green plants, the reduced acceptor (H2A) reacted with carbon dioxide (CO2) to form carbohydrate (CH2O) and to oxidize the unknown acceptor to A. This putative reaction can be represented as:
CO2 + 2H2A ---------------> (CH2O) + A2 + H2O
Van Niel’s proposal was important because the popular (but incorrect) theory had been that oxygen was removed from carbon dioxide (rather than hydrogen from water) and that carbon then combined with water to form Carbohydrates (rather than the hydrogen from water combining with CO2 to form CH2O).
By 1940 chemists were using heavy isotopes to follow the reactions of photosynthesis. Water marked with an isotope of oxygen (18O) was used in early experiments. Plants that photosynthesized in the presence of water containing H218O produced oxygen gas containing 18O; those that photosynthesized in the presence of normal water produced normal oxygen gas. These results provided strong support for van Niel’s theory that the Oxygen gas produced during photosynthesis is derived from water.
[edit] Basic products of photosynthesis
As has been stated, Carbohydrates are the most important direct organic product of photosynthesis in the majority of green plants. The formation of a simple Carbohydrates, Glucose, is indicated by a chemical equation,
light
6CO2 + 12H2O ---------------> C6H12O6 + 6O2 + 6H2O
green plants
Little free Glucose is produced in plants; instead, Glucose units are linked together to form starch or are joined with fructose, another sugar, to form sucrose (see carbohydrate).
Not only carbohydrates, as was once thought, but also amino acids, proteins, lipids (or fats), pigments, and other organic components of green tissues are synthesized during photosynthesis. Minerals supply the elements (e.g., Nitrogen, N; Phosphorus, P; Sulfur, S) required to form these compounds. Chemical bonds are broken between oxygen (O) and carbon (C), hydrogen (H), nitrogen, and sulfur, and new bonds are formed in products that include gaseous oxygen (O2) and organic compounds. More energy is required to break the bonds between Oxygen and other elements (e.g., in Water, nitrate, and sulfate) than is released when new bonds form in the products. This difference in bond energy accounts for a large part of the light energy stored as chemical energy in the organic products formed during photosynthesis. Additional Energy is stored in making complex molecules from simple ones.
[edit] Evolution of the process
Although life and the quality of the Atmosphere today depend on photosynthesis, it is likely that green plants evolved long after the first living cells. When the Earth was young, electrical storms and Solar radiation probably provided the Energy for the synthesis of complex molecules from abundant simpler ones, such as Water, ammonia, and Methane. The first living cells probably evolved from these complex molecules (see life: The primitive atmosphere). For example, the accidental joining together (condensation) of the Amino acids glycine and the fatty acid acetate may have formed complex organic molecules known as porphyrins; these molecules, in turn, may have evolved further into coloured molecules called pigments; e.g., chlorophylls of green plants, bacteriochlorophyll of photosynthetic Bacteria, hemin (the red pigment of blood), and cytochromes, a group of pigment molecules essential in both photosynthesis and cellular respiration.
Primitive coloured cells then had to evolve mechanisms for using the light energy absorbed by their pigments. At first, the energy may have been used immediately to initiate reactions useful to the cell. As the process for utilization of light energy continued to evolve, however, a larger part of the absorbed light energy probably was stored as chemical energy, to be used to maintain life. Green plants, with their ability to use light energy to convert Carbon dioxide and Water to Carbohydrates and Oxygen, are the culmination of this evolutionary process.
The first oxygenic (oxygen-producing) cells probably were the cyanophytes, or “blue-green algae,” which appeared about 2,000,000,000 to 3,000,000,000 years ago. These microscopic organisms are believed to have greatly increased the Oxygen content of the atmosphere, making possible the development of aerobic (oxygen-using) organisms. Cyanophytes are prokaryotic cells; that is, they contain no distinct, membrane-enclosed subcellular particles (organelles), such as nuclei and chloroplasts. Green plants, by contrast, are composed of eukaryotic cells, in which the photosynthetic apparatus is contained within membrane-bound chloroplasts. There is a theory that the first photosynthetic eukaryotes were red Algae that may have developed when nonphotosynthetic eukaryotic cells engulfed cyanophytes. Within the host cells, these cyanophytes are thought to have evolved into chloroplasts. Alternatively, the ancestors of chloroplasts in green plants may have been another oxygenic prokaryote like Prochloron, an organism that has been found only growing symbiotically inside ascidians.
There are a number of photosynthetic Bacteria that are not oxygenic (e.g., the sulfur bacteria previously discussed). The evolutionary pathway that led to these Bacteria diverged from the one that resulted in oxygenic organisms. In addition to the absence of oxygen production, nonoxygenic photosynthesis differs from oxygenic photosynthesis in two other ways: light of longer wavelengths is absorbed and used by pigments called bacteriochlorophylls, and reduced compounds other than water (such as hydrogen sulfide or organic molecules) provide the electrons needed for the reduction of Carbon dioxide.
[edit] How Does Photosynthesis Work in Plants?
Green plants use photosynthesis to create Energy from Carbon dioxide and Sunlight. This energy, in the form of Glucose, is used by the plant to grow and fuel the necessary reproductive activities of the plant. Excess Glucose is stored in the leaves, stem and roots of the plant. The stored Glucose provides food for higher organisms that eat the plants. A byproduct of the process of photosynthesis is Oxygen, which is released into the atmosphere in exchange for the Carbon dioxide used during the chemical reaction of photosynthesis.
Photosynthesis in plants requires a combination of Carbon dioxide, Water and light energy. The light energy used in photosynthesis is typically derived from the sun but is also effective when provided by artificial lighting. The leaves of a plant have the primary burden of creating food for the plant through the process of photosynthesis. The leaves of a plant are spread flat to catch as many of the sun's rays as possible, in order to facilitate the absorption of light energy.
Within the leaves are mesophyll cells which contain chloroplasts. Photosynthesis occurs within these structures, which contain the substance chlorophyll. Chlorophyll, along with other pigments present in the chloroplast, absorbs the light energy of all colors but green for use in the photosynthesis process. The remaining green light is reflected back off of the plant, resulting in green color characteristic of a plant using photosynthesis for energy. Once the light has been absorbed, it must be stored as ATP, or adenosine triphosphate, in order to be used in the next phase of photosynthesis.
During the final stage of photosynthesis, which is considered to be light-independent, carbon dioxide is converted into glucose. This chemical change requires the ATP that was stored in the first part of the photosynthesis cycle. The ATP is combined with carbon dioxide in what is known as the Calvin cycle. This combination creates a compound called glyceraldehyde 3-phosphates, which combines with another glyceraldehyde 3-phosphate compound as it is produced, to produce one Glucose molecule.
[edit] Photosynthesis and Respiration
The importance of Photosynthesis can be understood with respect to our breathing process. The breathing process keeps us alive and Photosynthesis provides us Oxygen to breathe in.
The process of Photosynthesis and Respiration are inter-related and serve one another. While Photosynthesis requires carbon-dioxide and releases oxygen to produce glucose, Respiration needs oxygen while inhaling and releases carbon-dioxide while exhaling.
Photosynthesis happens during the day time when the sun shines because the plants require Sunlight to produce Energy. On the other hand Respiration happens all the time as long as a living creature is alive.
However, unlike other living creatures, the plants breathe once in a day. During night, when there is no sunlight, the stomata (pores through which sunlight and carbon-dioxide enter the leaves) are closed and the leaves breathe releasing carbon-dioxide in the Air.
[edit] Photosynthesis and Environment
The level of carbon-dioxide in the environment largely depends on the process of Photosynthesis. The process of Photosynthesis again depends on the number of plants and trees we have. Excessive increase or decrease in the level of carbon-dioxide can bring forth disastrous results on the planet earth.
Industrial revolutions and technical progress have led to too many factories, production houses, buildings, roads etc thereby increasing the use of fuel and release of industrial waste and carbon-dioxide which can be very harmful for the environment.
Just the way, an increase in the carbon-dioxide level may harm the environment; similarly decrease in the level may cause the planet to freeze as CO2 helps in keeping our planet warm and live-able. Photosynthesis helps in maintaining the balance of the carbon-dioxide level in nature by taking in CO2 in the day time (and simultaneously supplying oxygen for other living beings) and breathing it out in the night.
[edit] The role of photosynthesis in control of the environment
How does photosynthesis in temperate and tropical forests and in the sea affect the quantity of greenhouse gases in the atmosphere? This is an important and controversial issue today. As mentioned above, photosynthesis by plants removes carbon dioxide from the atmosphere and replaces it with oxygen. Thus, it would tend to ameliorate the effects of carbon dioxide released by the burning of fossil fuels. However, the question is complicated by the fact that plants themselves react to the amount of carbon dioxide in the atmosphere. Some plants, appear to grow more rapidly in an atmosphere rich in carbon dioxide, but this may not be true of all species. Understanding the effect of greenhouse gases requires a much better knowledge of the interaction of the plant kingdom with carbon dioxide than we have today. Burning plants and plant products such as petroleum releases carbon dioxide and other byproducts such as hydrocarbons and nitrogen oxides. However, the pollution caused by such materials is not a necessary product of solar energy utilization. The artificial photosynthetic reaction centers discussed above produce energy without releasing any byproducts other than heat. They hold the promise of producing clean energy in the form of electricity or hydrogen fuel without pollution. Implementation of such solar energy harvesting devices would prevent pollution at the source, which is certainly the most efficient approach to control.
[edit] Photosynthesis and Life
Photosynthesis is directly related to the life and survival of all the other living creatures on earth. It not just supplies oxygen without which breathing and being alive would be difficult, but it also supplies food and energy to all.
Among all the living organisms on planet earth only plants are capable of producing their own food and deriving energy from it. No other living creature can produce their food and thus, depend on plants or other creatures which feed on plants to survive. Therefore, by producing energy the plants supply all the necessary nutrients and energy directly and/or indirectly to the other living creatures. The production of this energy is possible through Photosynthesis.
[edit] Factors that influence the rate of photosynthesis
The rate of photosynthesis is defined in terms of the rate of Oxygen production either per unit mass (or area) of green plant tissues or per unit weight of total chlorophyll. The amount of light, the Carbon dioxide supply, the temperature, the water supply, and the availability of minerals are the most important environmental factors that directly affect the rate of photosynthesis in land plants. The rate of photosynthesis also is determined by the plant species and its physiological state—e.g., its health, its maturity, and whether or not it is in flower..
[edit] Light intensity and temperature
As has been mentioned, the complex mechanism of photosynthesis includes a photochemical, or light-dependent, stage and an enzymatic, or dark, stage that involves chemical reactions. These stages can be distinguished by studying the rates of photosynthesis at various degrees of light saturation (i.e., intensity) and at different temperatures. Over a range of moderate temperatures and at low to medium light intensities (relative to the normal range of the plant species), the rate of photosynthesis increases as the intensity increases and is independent of temperature. As the light intensity increases to higher levels, however, the rate becomes increasingly dependent on temperature and less dependent on intensity; light “saturation” is achieved at a specific light intensity, and the rate then is dependent only on temperature if all other factors are constant. In the light-dependent range before saturation, therefore, the rate of photosynthesis is determined by the rates of photochemical steps. At high light intensities, some of the chemical reactions of the dark stage become rate-limiting. At light saturation, rate increases with temperature until a point is reached beyond which no further rate increase can occur. In many land plants, moreover, a process called photorespiration occurs at high light intensities and temperatures. Photorespiration competes with photosynthesis and limits further increases in the rate of photosynthesis, especially if the supply of water is limited (see below Photorespiration).
[edit] Carbon dioxide
Included among the rate-limiting steps of the dark stage of photosynthesis are the chemical reactions by which organic compounds are formed using carbon dioxide as a carbon source. The rates of these reactions can be increased somewhat by increasing the carbon dioxide concentration. During the past century, the level of carbon dioxide in the Atmosphere has been rising due to the extensive combustion of fossil fuels. The atmospheric level of Carbon dioxide climbed from about 0.028 percent in 1860 to 0.0315 percent by 1958 (when improved measurements began), and to 0.034 percent by 1981. This increase in carbon dioxide directly increases plant photosynthesis, but the size of the increase depends on the species and physiological condition of the plant. Furthermore, if increasing levels of atmospheric Carbon dioxide result in climatic changes, including increased global temperatures as some meteorologists predict, these changes will affect photosynthesis rates.
[edit] Water
For land plants, water availability can function as a limiting factor in photosynthesis and plant growth. Besides the requirement for Water in the photosynthetic reaction itself, water is transpired from the leaves; that is, water evaporates from the leaves to the atmosphere via the stomates. These stomates are small openings through the leaf epidermis, or outer skin; they permit the entry of carbon dioxide but also allow the exit of water vapour. The stomates open and close according to the physiological needs of the leaf. In hot and arid Climates the stomates may close to conserve water, but this closure limits the entry of Carbon dioxide and hence the rate of photosynthesis, while the wasteful process of photorespiration may increase. If the level of carbon dioxide in the atmosphere increases, more carbon dioxide could enter through a smaller opening of the stomates, so that more photosynthesis could occur with a given supply of Water.
[edit] Minerals
Several minerals are required for healthy plant growth and for maximum rates of photosynthesis. Nitrate or ammonia, sulfate, phosphates, Iron, Magnesium, and Potassium are required in substantial amounts for the synthesis of Amino acids, Proteins, coenzymes, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), chlorophyll and other pigments, and other essential plant constituents. Smaller amounts of such elements as Manganese, Copper, and chlorine are required in photosynthesis. Some other trace elements are needed for various nonphotosynthetic functions in plants.
[edit] Internal factors
Each plant species adapts to a range of environmental factors. Within this normal range of conditions, complex regulatory mechanisms in the plant’s cells adjust the activities of enzymes (i.e., organic catalysts). These adjustments maintain a balance in the overall photosynthetic process and control it in accordance with the needs of the whole plant. With a given plant species, for example, doubling the Carbon dioxide level might cause a temporary increase of nearly two fold in the rate of photosynthesis; a few hours later, however, the rate might fall to the original level because photosynthesis had made more sucrose than the rest of the plant could use. By contrast, another plant species provided with such Carbon dioxide enrichment might be able to use more sucrose and would continue to photosynthesize and to grow faster throughout most of its life cycle.
[edit] Applications
Several photosynthesis-based (and, some distant) opportunities for the human race are highly promising.
[1] As mentioned earlier, the primary reaction of photosynthesis is highly efficient. Thus, attempts are being made to produce artificial systems to just do that and to produce chemical energy in artificial systems (e.g., in membrane vesicles, the liposomes). A research group at Arizona State University has succeeded in producing ATP (the energy currency of life) in such systems.
[2] Since water is available in huge quantities on our Earth, and since hydrogen is a clean fuel, another effort is being made at Golden (Colorado) and Berkeley (California) to use the green alga Chlamydomonas reinhardtii to trick it in converting water into oxygen and hydrogen. The problem is that hydrogen production machinery is sensitive to Oxygen. Thus, researchers are attempting to separate in time the two processes. We await results of such research.
[3] Genetic Engineering is another powerful approach that is being used to produce plants that are, for example, resistant to frost; insects; drought and pathogens (Disease causing organisms), etc. A specific example is the development of a cotton variety that would be resistant to caterpillars that are eating the leaves and destroying the crops. The best hope for the developing countries is, of course, the increased yield of plants under marginal lands (such as in dry and saline soils).
[4] Another exciting approach, also by genetic engineering, is to construct plants that have added nutritional values (such as plants that make lots of Vitamin E; crop plants that are rich in specific proteins; Rice containing Iron in the form of ferritin; and canola plants that produce palm oil). Such engineering approaches can increase quality and quantity of food to meet the needs of the increasing World Population.
[5] A highly exciting new application is constructing plants that produce medicines (plants have been doing it since they came to be on our Earth, but now we can direct them to make what we need and in quantities we need). In particular, efforts at Boyce Thompson Institute aim to produce vaccines in Bananas that will revolutionize their delivery to children in developing countries . It will be affordable and would increase the life expectancy and health of millions. What a delightful thought! .
Sun shines each day and does not charge us any money for the light it gives us. The light falling on Earth is very clean and will be there for a very long time as long as the Sun lasts. In addition to the current applications, mentioned above, Photosynthesis-based technology could also include
(1) using the concept of efficient "energy capture" to our artificial systems just as plants have been doing: thousands of chlorophyll a molecules (antenna) serving one center where the process occurs efficiently;
(2) the use of compounds, produced by plants, in triggering reactions that kill cancer cells; and
(3) the use of photosynthetic organisms in cleaning of aqueous surface environments (lakes, etc). An excellent example is in the use of cyanoBacteria that literally eat up the nitrates from ground water and clear it for us. The opportunities that photosynthesis-based technology provides us are enormous. The success, however, requires a concerted effort on the part of biophysicists, biochemists, molecular biologists, plant physiologists, microbiologists, geneticists, agronomists, physicists, chemists, bio-technologists and engineers to come together and ask what they can do for the World, not what the World can do for them.
