1 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 membranes encloses 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 O2 by 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 include the 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. Electromagnetic spectrum, 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 b absorbs mainly blue and orange light and reflects yellow-green. Chloroplasts also contain 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 pigment molecule 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 chemical energy 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 photoexcited electron 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.