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Chapter 4

chapter 4

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Biology 1225
Michael Butler

Overview of Photosynthesis Photosynthesis is the process by which green plants capture sunlight and use the energy to make sugars. Photosynthesis is also performed by many bacteria using chlorophyll (these are the cyanobacteria), the early earth's atmosphere contained very little free oxygen, it was the activity of countless billions of cyanobacteria acting over many millions of years that formed the accumulated oxygen in the atmosphere that allowed aerobic life to evolve. One of the first sugars to be formed is glucose (C H O6).12i6ht energy is converted to the chemical bond energy that holds together the various atoms making up the glucose molecule.The rate at which photosynthesis occurs depends on a number of factors, including the wavelength of the light impinging on the leaf, as well as temperature. From glucose the plant cell is then able to construct all of the other molecules that are required for survival and growth, including starch which is the main way the plant cell stores energy for later use The summary formula for photosynthesis is given on page 85 in the text: 6CO +26H O + l2ght energy ----> C H O6 6 12 + 6O 2 Carbon dioxide is of course a key requirement for formation of glucose in photosynthesis and the rate of photosynthesis depends on the availability of carbon dioxide (CO ), which 2 enters the plant mainly through the stomata, which are openings found mainly on the undersides of the leaves, and diffuses directly into cells within the interior of the leaf. The water enters via the roots and is carried throughout the plant in the veins (We'll have more to say about that later), so water availability is also a factor that influences the rate of photosynthesis. Oxygen (O ) 2s a waste product of photosynthesis, but it is essential for respiration, as will be seen in the next section. You can see from the formula how plants clean and replenish the air. They remove carbon dioxide (a process that counteracts global warming, also a topic for later) and they renew the supply of oxygen. Photosynthesis consists of two separate stages, first the light-dependent reactions and then the light-independent reactions. In the light-dependent reactions, light energy is absorbed by chlorophyll and other pigments in the thylakoid membranes of the chloroplast. Only certain molecules of a specific kind of chlorophyll, called chlorophyll a, can absorb specific wavelengths of light and use this light energy to perform the first stage of photosynthesis. These special chlorophyll a molecules are called reaction centers.(The other pigments, including other kinds of chlorophyll, absorb other wavelengths (colors) of light but they pass this energy over to the reaction centers.) The reaction centers release light-energized electrons to nearby electron acceptors. By an extremely complex process, some of the electrons bring about the synthesis of ATP, and some are incorporated into the bonds of another energy transport molecule called NADPH (see below). As a result of the "consumption" of electrons as ATP and NADPH are formed, the reaction centers become deficient of electrons. Replacement electrons are made available by the splitting of H O mo2ecules into H ions and O . This splitting of water is the source of the oxygen we breathe. 2 The light-independent reactions take place in the stroma of the chloroplast. As the name suggests, light is not necessary for these reactions. However they do require the energy of ATP, and the energized electrons of NADPH, which are formed in the light-dependent stage. In the light-independent reactions, carbon dioxide molecules become incorporated, one by one, into a series of enzymatically controlled chemical reactions called the Calvin Cycle. This is the step that "consumes" carbon dioxide in plants. The cycle uses the ATP and NADPH formed in the light dependent stage and ultimately gives rise to glucose. The + ADP, phosphate and NADP return to the light-dependent stage to be used over again. An overview of Cellular Respiration Harvesting Energy If you ignite a piece of paper in one corner with a match, the flame will pass rapidly over the paper until it is completely consumed. What is really happening here? Paper is composed of cellulose, which in turn is composed of chains of glucose. The burning process involves the breakdown of glucose into carbon dioxide and water, which is accompanied by a rapid release of energy in the form of heat and light. The energy comes from the breaking of the chemical bonds within the glucose. The match provides the activation energy necessary to start the process, but once it begins the energy released by the breakdown in one part of the paper triggers the breakdown in the next and the burning spreads over the paper. Living cells generally "burn" glucose to get energy, producing CO and H O as waste 2 2 products. However the process is controlled and gradual. Rather than releasing the energy all at once in a flash of heat and light, it is extracted in enzymatically controlled steps, and in the process some of it is temporarily stored in ATP. Cellular respiration is an extremely efficient process in comparison with man-made machines. About 40% of the energy in glucose is captured in ATP. The rest is lost as heat. Compare this with the most efficient automobile engine that burns gasoline (a similar process). It converts about 25% of the stored energy in the fuel to movement of the vehicle. The heat produced in cellular respiration is of no apparent use to organisms like plants, but just a
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