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Фотосинтез - Coggle Diagram

- - - - Photoautotrophs not only feed us; they also clothe us (think cotton), house us (think wood), and provide energy for warmth, light, transport, and manufacturing.
        
        kelp, a large alga that forms extensive underwater “forests” off the coast of California.
        
        the remarkable ability of these organelles to harness light energy and use it to drive the synthesis of organic compounds emerges from the structural organization and interactions of their component parts
        
        Photosynthetic bacteria have infolded regions of the plasma membrane containing such molecules.
- - - - Membranes within the chloroplast form the framework for many of the reactions of photosynthesis, just as mitochondrial membranes provide the structure for much of the energy-harvesting machinery in cellular respiration
        
        In the chloroplast, an envelope of two membranes encloses an inner compartment, which is filled with a thick fluid called stroma. Suspended in the stroma is a system of inter-connected membranous sacs, called thylakoids, which enclose another internal compartment, called the thylakoid space. In many places, thylakoids are concentrated in stacks called grana (singular, granum).
        
        The precise arrangements of these membranes and compart-ments are essential to the process of photosynthesis
- - - - It was almost 20 years before van Niel’s hypothe-sis was confirmed. Using a heavy isotope of oxygen, O-18, scientists were able to follow the fate of oxy-gen atoms during photosynthesis.
        
        O-18 has two more neutrons in the nucleus of its atom than the more common isotope O-16, and this slight difference in mass can be experimentally measured. To determine the source of the O2 released by photosynthesis, researchers produced CO2 and H2O containing O-18, thus “labeling” the two reac-tant molecules.
        
        the O2 released during photosynthesis comes from water and not from CO2.
        
        the mid-1940s, American bio-chemist Melvin Calvin and his colleagues began using radioactive C-14 to trace the sequence of the intermedi-ate molecules formed in the cyclic pathway that produces sugar from CO2. They worked for 10 years to elucidate this cycle, which is now called the Calvin cycle. Calvin received the Nobel Prize in 1961 for this work.
- - - - In summary, the light reactions absorb solar energy and convert it to chemical energy stored in both ATP and NADPH.
        Notice that these reactions produce no sugar; sugar is not made until the Calvin cycle, which is the second stage of photosynthesis.
        
        The Calvin cycle occurs in the stroma of the chloroplast.
        It is a cyclic series of reactions that assembles sugar molecules using CO2 and the energy-rich products of the light reactions.
        The incorporation of carbon from CO2 into organic com-pounds, shown in the figure as CO2 entering the Calvin cycle, is called carbon fixation. After carbon fixation, the carbon compounds are reduced to sugars.
        As the figure suggests, it is NADPH produced by the light reactions that provides the electrons for reducing carbon com-pounds in the Calvin cycle. And ATP from the light reactions provides chemical energy that powers several of the steps of the Calvin cycle. The Calvin cycle is sometimes referred to as the dark reactions, or light-independent reactions, because none of the steps requires light directly. However, in most plants, the Calvin cycle occurs during daylight, when the light reactions power the cycle’s sugar assembly line by supplying it with NADPH and ATP .
        The word photosynthesis encapsulates the two stages.
        Photo, from the Greek word for “light,” refers to the light reac-tions; synthesis, meaning “putting together,” refers to sugar construc tion by the Calvin cycle
- - - - A photon has a fixed quantity of energy, and the shorter the wavelength of light, the greater the energy of its photons. In fact, the photons of wavelengths that are short-er than those of visible light have enough energy to damage molecules such as proteins and nucleic acids. This is why ultra-violet (UV) radiation can cause sunburns and skin cancer.
        
        Light-absorbing mol-ecules called pigments, built into the thylakoid membranes, absorb some wavelengths of light and reflect or transmit other wavelengths. We do not see the absorbed wavelengths; their energy has been absorbed by pigment molecules. What we see when we look at a leaf are the green wavelengths that are not absorbed but are transmitted and reflected by the pigments.
        
        Chlorophyll a, which participates directly in the light reac-tions, absorbs mainly blue-violet and red light. A very similar molecule, chlorophyll b, absorbs mainly blue and orange light. Chlorophyll b broadens the range of light that a plant can use by conveying absorbed energy to chlorophyll a, which then puts the energy to work in the light reactions.
        
        more important function seems to be photoprotection: Some carotenoids absorb and dissipate excessive light energy that would otherwise damage chlorophyll or interact with oxygen to form reactive oxida-tive molecules that can damage cell molecules.
        
        Each type of pigment absorbs certain wavelengths of light because it is able to absorb the specific amounts of energy in those photons
- - - - A light-harvesting complex consists of various pigment molecules bound to proteins. The number and vari-ety of pigment molecules can harvest light over a larger sur-face area and a larger portion of the spectrum than could any single pigment molecule alone.
        
        When a pigment molecule absorbs a photon, the energy is transferred from molecule to molecule, somewhat like a human “wave” at a sporting event, until it is passed into the reaction-center complex
        
        The reaction-center complex contains a pair of spe-cial chlorophyll a molecules and a molecule called the prima-ry electron acceptor, which, as its name indicates, is capable of accepting electrons and becoming reduced.
        
        When an electron from a reaction-center chlorophyll a is boosted to a higher energy level, it is immediately captured by the primary electron acceptor. This is the first step in the transformation of light energy to chemical energy in the light reactions.
        
        Two types of photosystems have been identified, and they cooperate in the light reactions. They are referred to as photosystem I and photosystem II, in order of their discovery, although photosystem II actually functions first in the sequence of steps that make up the light reactions.
        
        Each of the two types of photosystems has a characteristic reaction-center complex. Now let’s see how the two photosystems work together in the light reactions to generate ATP and NADPH.
- - - - What is the source of the elec-trons that are moving through the photosystems to NADPH?
        Don’t the light reactions produce O2—where does that happen? And how does the flow of electrons down that ramp produce ATP?
        The electrons that end up reducing NADP+ to NADPH orig-inally come from water. An enzyme in the thylakoid space splits H2O into 2 electrons, 2 hydrogen ions (H+), and 1 oxygen atom (1 2 O2). The oxygen atom immediately joins with another oxygen to form O2.
        
        water is the source of the O2 produced in photosynthesis, and these oxygen molecules diffuse out of the thylakoids, the chloroplast, and the plant cell, finally exiting the leaf through its stomata. The all-important electrons from water are passed, one by one, to the reaction center chlorophyll a molecules in photosystem II, replacing the photoexcited electron that was just captured by the primary electron acceptor. From photosystem II, the elec-trons pass through an electron transport chain to the reaction center chlorophyll a molecules in photosystem I, again replac-ing photoexcited electrons that had been captured by its pri-mary electron acceptor. Although the illustration shows these electrons being dropped in a bucket, they actually are passed through a short electron transport chain to NADP+, reducing it to NADPH.
        Now that we have accounted for NADPH and O2, all that is left is ATP . Making ATP in the light reactions involves an electron transport chain and chemiosmosis—the same players and process you met in the synthesis of ATP in cel-lular respiration. Recall that in chemiosmosis, the potential energy of a concentration gradient of H+ across a membrane powers ATP synthesis. This gradient is created when an electron transport chain uses the energy released as it passes electrons down the chain to pump H+ across a membrane. The energy of the concentration gradient drives H+ back across the membrane through ATP syn-thase, spinning this rotary motor and phosphorylating ADP to produce ATP
        
        The next module, which presents a slightly more realistic model of the light reactions than this mechanical analogy, should help you visualize how photosystem II, the electron transport chain, photo-system I, and ATP synthase function together within the thylakoid membranes of a chloroplast to produce NADPH and ATP .
- - - - As you can see in step ➊, carbon fixation, the enzyme rubisco attaches CO2 to RuBP . (Recall that carbon fixation refers to the initial incorporation of CO2 into organic com-pounds.) This unstable six-carbon molecule splits into two three-carbon molecules. In step ➋, reduction, ATP and NADPH are used to reduce the three-carbon molecule to G3P .
        For this to be a cycle, RuBP must be regenerated. In step ➌, release of one molecule of G3P , you can see that for every three CO2 molecules fixed, one G3P molecule leaves the cycle as product. In step ➍, regeneration of RuBP , the remaining five G3P molecules are rearranged, using energy from ATP , to regenerate three molecules of RuBP .
        For the synthesis of one G3P molecule, the Calvin cycle consumes nine ATP and six NADPH molecules, which were pro-vided by the light reactions. Neither the light reactions nor the Calvin cycle alone can make sugar from CO2. Photosynthesis is an emergent property of the structural organization of a chloro-plast, which integrates the two stages of photosynthesis.
- - - - the Free-Air CO2 Enrichment (FACE) experiment set up in Duke University’s experimental forest, scientists monitored the effects of elevated CO2 levels on an intact forest ecosystem over a period of 15 years. Six study sites were established, each 30 m in diameter and ringed by 16 towers (Figure 7.13a). In three of the plots, the towers released air containing CO2 con-centrations about 1½ times present-day levels. Monitoring instruments on a tall tower in the center of each plot adjusted the distribution of CO2 to maintain a stable concentration.
        
        poison ivy in the elevated CO2 plots showed an average annual growth increase of 149% compared with control plots.
        This increase is much greater than the increase for woody plants that similar studies have documented. Indeed, over a 12-year monitoring period, the trees in the FACE experimental plots showed only about a 15% yearly increase in biomass compared with those in the control plots.
        There was one other significant finding of the poison ivy study. A chemical analysis showed that the high-CO2 plants produced a more potent form of poison ivy’s allergenic com-pound, urushiol. Thus, poison ivy is predicted to become both more abundant and more toxic (“itchy”) as atmospheric CO2 levels rise.
        
        Scientists calculate that the CO2 released by human activities has increased the average temperature of the planet by about 1ºC (1.8ºF) since 1900.
        
        After more than 20 years of meetings and negotiations, the 2015 United Nations Climate Change Conference in Paris produced an agreement among 195 countries to limit global warming by the year 2100 to less than 2°C (3.6°F).