Photosynthesis and Respiration
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All organisms on earth depend on photosynthesis, in which light energy is used to make sugar and other food molecules from carbon dioxide and water. For example, plants and other photo-synthesizers need only energy from sunlight, carbon dioxide from air, and water from the soil to make the sugar glucose. Photosynthesis is the most important chemical process on earth because it provides food for virtually all organisms, not only for photo-synthesizers but for the organisms that eat them. Plants can capture the energy of the sun by a chemical process called photosynthesis. This chemical reaction can be described by the following simple equation:
6CO2 + 6H2O + light energy = C6H12O6 + 6O2
The product of photosynthesis is a carbohydrate, such as the sugar glucose, and oxygen which is released to the atmosphere. All of the sugar produced in the photosynthetic cells of plants and other organisms is derived from the initial chemical combining of carbon dioxide and water with sunlight. This chemical reaction is catalyzed by chlorophyll acting in concert with other pigment, lipid, sugars, protein, and nucleic acid molecules. Sugars created in photosynthesis can be later converted by the plant to starch for storage, or it can be combined with other sugar molecules to form specialized carbohydrates such as cellulose, or it can be combined with other nutrients such as nitrogen, phosphorus, and sulfur, to build complex molecules such as proteins and nucleic acids.
Light is central to the life of a plant, and its role goes beyond photosynthesis. The sensing of light is as important for plants as vision for animals. Light receptors in plant cells convert light into chemical signals that bring about important stages in the plants life cycle, from the germination of seeds to the production of flowers. However, light is essential to plant reproduction and growth, too much sunlight can be harmful. Sunlight can overheat a plant or overload its photosynthetic machinery. Therefore, one of the major differences between plants and animals on earth is the ability of plants to internally make their own food. In order to produce food for itself a plant requires energy from sunlight, carbon dioxide from the air and water from the soil. If any of these ingredients is lacking, photosynthesis, or food production, will stop. If any factor is removed for a long period of time, the plant will die. Photosynthesis literally means “to put together with light.” Any green plant tissue is capable of photosynthesis. Chloroplasts in these cells contain the green pigment called chlorophyll which traps the light energy. However, leaves are generally the site of most food production due to their special structure. The internal tissue (mesophyll) contains cells with plentiful chloroplasts in an arrangement that allows easy movement of water and air. The protective upper and lower epidermis (skin) layers of the leaf include many stomata that are openings in the leaf formed by two specialized guard cells on either side. Guard cells regulate movement of the gases, (CO2 into and O2 and H2O out of the leaf), involved in photosynthesis. The lower epidermis of the leaf normally contains the largest percentage of stomata. Light Reaction Photosynthesis is the process of turning the energy of sunlight into chemical energy from the raw products of CO2 and H2O. This process is necessary to sustain nearly all forms of life.
Photosynthesis is divided in to two separate reactions known as the light and dark reactions. They take place when light is present but the dark reaction does not require light. The whole process is begun by light reacting with pigments in the leaf causing the splitting of water molecules. This is called proteolysis or the Hill Reaction. Three products are produced in this reaction. Electrons from the hydrogen molecules and remaining H+ ions are used to form two separate energy storage molecules. The air we breathe is from the remaining oxygen portion of H2O. The carbon dioxide molecules are transformed into sugars during the dark reaction using the energy that was formed during the light reaction. Dark Reaction: This part of the photosynthetic process is also called the Calvin Cycle. The Calvin cycle runs during daytime, when the light reactions power the cycles sugar assembly line by supplying it with NADPH and ATP. With one cycle of this reaction 3 carbon atoms are fixed or placed in a sugar molecule. This pathway is called C-3 photosynthesis. This is the way that most broadleaf plants make sugars during the dark reaction. C-3 photosynthesis has a disadvantage though. Oxygen competes with CO 2 for a binding site during the dark reaction. Sometimes sugars are not formed, but energy is still expended to complete the cycle. This is called photorespiration. Another dark reaction pathway is called C-4 photosynthesis because 4 carbons are fixed or placed in a sugar molecule each time the cycle is completed. The dark reaction of C-4 photosynthesis occurs inside of specialized parts of leaf cells in the leaf called the bundle sheath, which exclude the presence of O2. Because there is no oxygen present photorespiration does not occur. The C-4 photosynthetic pathway is what occurs in most monocots or grasses. This is a more efficient pathway and allows grasses to grow faster than broadleaf plants. Crassulacean acid metabolism or CAM photosynthesis is the dark reaction type found in many cactus, succulents, bromeliads, and orchids as well as a few other plants. CAM photosynthesis is similar to C-4 photosynthesis. However, CAM plants open their stomata only during the night to collect CO2, when air temperatures are cooler, thus conserving water because of reduced transpiration. The CO2 is converted into malic acid and then converted back to CO2 during the day when light is present, thus producing sugars, while the stomata are closed and greatly reducing water loss. Plants convert the energy from light into simple sugars, such as glucose. This food may be converted back to water and carbon dioxide, releasing the stored energy through a process called respiration. This energy is required for growth in nearly all organisms. Simple sugars are also converted to other sugars and starches (carbohydrates) which may be transported to the stems and roots for use or storage, or they may be used as building blocks for more complex structures, e.g. oils, pigments, proteins, cell walls, etc. Photosynthesis is dependent on the availability of light. In other words, as sunlight increases in intensity photosynthesis increases. This results in greater food production. Many garden crops, such as tomatoes, respond best to maximum sunlight. Tomato production is cut drastically as light intensities drop. Only two or three varieties of “greenhouse” tomatoes will produce any fruit when sunlight is minimal in fall and spring. Water