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Plants are AUTOTROPHS in that they make their own food and thus sustain themselves without consuming organic molecules derived from any other organisms. Plant cells capture light energy, and convert it to chemical energy. Using this energy, plants make their own organic molecules and are the ultimate source of organic molecules for almost all other organisms. They are often referred to as the PRODUCERS of the biosphere because they produce its food supply. All organisms that produce organic molecules from inorganic molecules using the energy of light are called PHOTOAUTOTROPHS. In this chapter we focus on photosynthesis in plants, which takes place in chloroplasts. The process of photosynthesis most likely originated in a group of bacteria that had infolded regions of the plasma membrane containing such clusters of enzymes and molecules. Chloroplasts appear to have originated from a photosynthetic prokaryote that lived inside a eukaryotic cell. 2
All green parts of a plant have chloroplasts in their cells and can carry out photosynthesis. Their green color is from CHLOROPHYLL,a light absorbing pigment in the chloroplasts that plays a central role in converting solar energy to chemical energy. Chloroplasts are concentrated in the cells of the mesophyll, the green tissue in the interior of the leaf. Carbon dioxide enters the leaf and oxygen exists, by way of tiny pores called STOMATA. Water absorbed by the roots is delivered to the leaves in veins. 3
An envelope of two membranesencloses an inner compartment in the chloroplast, which is filled with a thick fluid called STROMA. Suspended in the stroma is a system of interconnected membranous sacs, called THYLAKOIDS, which enclose another compartment, called the thylakoid space. In some places, thylakoids are concentrated in stacks called GRANA. Built into the thylakoid membranes are the chlorophyll molecules that capture light. 4
In the 1800 s most scientists assumed that plants produce O2by extracting it from CO2. In the 1950 s, scientists tested this hypothesis by using a heavy isotope of oxygen,18o, to follow the fate of oxygen atoms during photosynthesis. EXPERIMENT 1: a plant given carbon dioxide containing 18O gave off no labeled oxygen gas (18O containing). EXPERIMENT 2: a Plant given water containing 18O did produce labeled O2. These experiments showed that the O2 produced during photosynthesis comes from water and not from CO2. It takes two water (H2O) molecules to make each molecule of O2. Additional experiments have revealed that the oxygen atoms in CO2 and the hydrogens in the reactant H2O molecules end up in the sugar molecule and in water that is formed anew. 5
Photosynthesis is a redox(oxidation-reduction) process, just as cellular respiration is. In photosynthesis, water molecules are split apart, yielding O2, they are actually oxidized; that is, they loose electrons, along with hydrogen ions (H+). Meanwhile, CO2 is reduced to sugar as electrons and hydrogen ions are added to it. Overall Cellular respiration harvests energy stored in a glucose molecule by oxidizing the sugar and reducing O2 to H2O. This process involves a number of energy-releasing redox reactions, with electrons losing potential energy as they travel down an energy hill from sugar to O2. In contrast, the food-producing redox reactions of photosynthesis involve an uphill climb. As water is oxidized and CO2 is reduced during photosynthesis, electrons gain energy by being boosted up an energy hill. The light energy captured by chlorophyll molecules in the chloroplast provides the boost for the electrons. Photosynthesis converts light energy to chemical energy and stores it in the chemical bonds of sugar molecules, which can provide energy for later use or raw materials for biosynthesis. 6
Photosynthesis occurs in two stages, each with multiple steps. The LIGHT REACTIONS includethe steps that convert light energy to chemical energy and produce O2. The light reaction occur in the thylakoid membranes. Water is split, providing a source of electrons and giving off O2 gas as a by-product. Light energy absorbed by chlorophyll molecules built into the membranes is sued to drive the transfer of electrons and H+ from water to NADP+, reducing it to NADPH. NADPH is an electron carrier similar to NADH that transports electrons in cellular respiration. In summary the light reactions of photosynthesis are the steps that absorb solar energy and convert it to chemical energy stored in ATP and NADPH. Notice that these reactions produce no sugar; sugar is not made until the CASLVIN CYCL, the second stage of photosynthesis. The CALVIN CYCLE occurs in the stroma of the chloroplast. It is a cyclic series of reactions that assemble sugar molecules using CO2 and the energy-containing products of the light reactions. In the 1940 s, Calvin and his colleagues traced the path of carbon in the cycle, using the radioactive isotope 14C to label the carbon in CO2. The incorporation of carbon from CO2 into organic compounds is called CARBON FIXATION. After carbon fixation, enzymes of the cycle make sugars by further reducing the carbon compounds. It is NADPH produced by the light reactions that provides the electrons for reducing carbon in the Calvin cycle. And ATP from the light reaction 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. 7
Sunlight is a type of energy called ELECTROMAGNETIC ENERGY or RADIATION. Electromagneticspectrum, is the full range of electromagnetic wavelengths from the very short gamma rays to the very long-wavelength radio waves. Visible light-the radiation your eyes see as different colors-is only a small fraction of the spectrum. It consists of wavelengths from about 380 nm to about 750 nm. The distance between the crests of two adjacent waves is called a WAVELENGTH. Shorter wavelength have more energy than longer ones. The theory of light as waves explains most of light s properties. However, light also behaves as discrete packets of energy called photons. PHOTONS is a fixed quantity of light energy, and as you have just learned, the shorter the wavelength, the greater the energy. 8
Light-absorbing molecules 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 the pigment transmits and reflects. Different pigments absorb light of different wavelengths, and chloroplasts contain several kinds of pigments. Chlorophyll a, which participates directly in the light reactions, absorbs mainly blue-violet and red light. A very similar molecule chlorophyll babsorbs mainly blue and orange light and reflects yellow-green. Chloroplasts alsocontain a family of pigments called CAROTENOIDS, which seem to be used in photoprotection: They absorb and dissipate excessive light energy that would otherwise damage chlorophyll or interact with oxygen to form reactive oxidative molecules that can damage cell molecules. 9
When a pigmentmolecule absorbs a photon, one of the pigment s electron s jumps to an energy level farther from the nucleus. In this location, the electron has more potential energy, and we say that the electron has been raised from a ground state to an excited state. The excited state is very unstable. Chlorophyll in its native habitat of the thylakoid membrane, passes off its excited electron to a neighboring molecule before it has a chance to drop back to the ground state. In the thylakoid membrane, chlorophyll molecules are organized along with other pigments and proteins into clusters called photosystems. A photosystem consists of a number of light-harvesting complexes surrounding a reaction center complex. The light-harvesting complexes consist of pigment molecules bound to proteins. The REACTION CENTER COMPLEX contains a pair of chlorophyll a molecules and a molecule called the primary electron acceptor, which is capable of accepting electrons and becoming reduced. Two types of photosystems have been identified, and they cooperate in the light reactions. They are referred to as Photosystem 1 and photosystem 2, with photosystem 2 acting first. In photosystem 2 the chlorophyll a of the reaction center is called P680 because the light it absorbs best is red light with a wavelength of 680nm. The reaction center chlorophyll of photosystem 1 is called P700 nm. 10
In the light reactions, light energy is transformed into the chemicalenergy of ATP and NADPH. In this process, electrons removed from water molecules pass from photosystem 2 to photosystem 1 to NADP+. Between the two photosystems, the electrons move down an electron transport chain and provide energy for the synthesis of ATP. 1.) A pigment molecule in a light-harvesting complex absorbs a photon of light. The energy is passed to other pigment molecules and finally to the reaction center of photosystem 2, where it excites an electron of chlorophyll P680 to a higher energy state. 2.) This electron is captured by the primary electron acceptor. 3.) Water is split, and its electrons are supplied one by one to P680, each replacing an electron lost to the primary electron acceptor. The oxygen atom combines with an oxygen oxygen from another split water molecule to form a molecule of O2. 4.) Each photoexcitedelectron passes from photosystem 2 to photosystem 1 via an electron transport chain. The exergonic fall of electrons provides energy for the synthesis of ATP by pumping H+ across the membrane. 5.) Light energy excites an electron of chlorophyll P700 in the reaction center of photosystem 1. The primary electron center captures the electron, and an electron from the bottom of the electron transport chain replaces the lost electron in P700. 6.) The excited electron of photosystem 1 is passed through a short electron transport chain to NADP+, reducing it to NADPH. NADPH, ATP, and O2 are the products of the light reactions. 11
Chemiosmosisis also the mechanism that generates ATP in a chloroplast. Recall that the process of chemiosmosis drives ATP synthesis using the potential energy of a concentration gradient of hydrogen ions across a membrane. The gradient is created when an electron transport chain uses the energy released as it passes electrons down the chain to pump hydrogen ions across a membrane. During the electron transport chain hydrogen ions are pumped across the membrane from the stroma into the thylakoid space. This generates the concentration gradient across the membrane. In photosynthesis, this chemiosmotic production of ATP is called PHOTOPHOSPHORYLATION because the initial energy input is light energy. Notice that in the light-driven flow of electrons through the two photosystems, the final electron acceptor is NADP+, not O2 as in cellular respiration. 12
The Calvin cycle functions like a sugar factory within a chloroplast. Inputs to this allimportant food-making process are CO2 (from the air) and ATP and NADPH (both generated by the light reactions). Using carbon from CO2, energy from ATP, and highenergy electrons from NADPH, the calvincycle constructs and energy-rich, three carbon sugar, glyceraldehyde-3-phosphate (G3P). A pplantcell can use G3P to make glucose and other organic molecules as needed. The starting material is a five-carbon sugar named ribulosebisphosphate(rubp). 1.) CARBON FIXATION-In the carbon fixation step, the enzyme rubiscoattaches CO2 to RuBP. 2.) REDUCTION-In the next step, a reduction, NADPH reduces the organic acid 3-PGA to G3P with the assistance of ATP. To make a molecule of G3P, the cycle must incorporate the carbon atoms from three molecules of CO2. The cycle actually incorporates one carbon at a time, but we show it starting with three CO2 molecules so that we end up with a complete G3P molecule. 3.) RELEASE OF ONE MOLECULE OFG3P-For every three CO2 molecules fixed, one G3P molecules leaves the cycle as product, and the remaining five G3P molecules are rearranged. 4.) REGENERATION OF RuBPUsing energy from ATP to regenerate three molecules of RuBP. Note that for the net synthesis of one G3P molecule, the Calvin cycle consumes 9 ATP and 6 NADPH molecules, which were provided by the light reactions. 13
Thephotosystems transfer photo excited electrons through electron transport chain, where energy is harvested to make ATP, and to NADP+, where electrons are stored in the highenergy compound NADPH. The chloroplast s sugar factory is the Calvin cycle, the second stage of photosynthesis. In the stroma, the enzyme rubiscocombines CO2 with RuBP, and ATP and NADPH are used to reduce 3-PGA to G3P. 14
Closingstomata on a hot, dry day is an adaption that reduces water loss, but it also prevents CO2 from entering the leaf and O2 from leaving. As a result, CO2 levels get very low in the leaf, while O2 from reactions builds up. These conditions reduce called photorespiration. Such plants are called C3 PLANTS because the first organic compound produced is the three-carbon compound 3-PGA. When stomata closes to reduce water loss and O2 builds up in a leaf, rubiscoadds O2 instead of CO2 to RuBP. A two-carbon product of this reaction is then broken down by the cell to CO2 and H2O, a process known as PHOTORESPIRATION. Unlike photosynthesis, photorespiration yields no sugar, and unlike cellular respiration, it produces no ATP. C4 PLANTS: Precede the Calviincycle by first fixing CO2 into a four-carbon compound. When the weather is hot and dry, a C4 plant keeps its stomata mostly closed, thus conserving water. At the same time, it continues making sugars by photosynthesis CAM PLANTS: these species are adapted to very dry climates. A CAM plant conserves water by opening its stomata and admitting CO2 only at night. When CO2 enters the leaves, it is fixed into a four-carbon compound, as in C4 plants. The four-carbon compound in a CAM plant banks CO2 at night and releases it to the Calvin cycle during the day. In C4 plants, carbon fixation and the Calvin cycle occur in different types of cells. In CAM plants, these process occur in the same cells, but at different times. 15