Chloroplasts Chlorophyll Leaf Structure Biology Photosynthesis Autotrophs Living things that are able to make their own food without using organic molecules derived from any other living thing o Autotrophs that use the energy of light to produce organic molecules are called photoautotrophs Most plants algae other protists and some prokaryotes are photoautotrophs Organelles consisting of photosynthetic pigments enzymes and other molecules grouped together in membranes o The ability to photosynthesize is directly related to the structure of chloroplasts An important light absorbing pigment in chloroplasts Major sites of photosynthesis in green plants Responsible for the green color of plants 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 Stomata are the tiny pores in the leaf that allow carbon dioxide to enter and oxygen to exit Veins in the leaf deliver water absorbed by roots An envelope of two membranes encloses the stroma the dense fluid within the chloroplast A system of interconnected membranous sacs called thylakoids segregates the stroma from another compartment the thylakoid space o Thylakoids are concentrated in stacks called grana Photosynthesis is a Redox Reaction Photosynthesis is a redox oxidation reduction process o Water molecules are split apart by oxidation which means that they lose electrons along with hydrogen ions H o Then CO2 is reduced to sugar as electrons and hydrogen ions are added to it Formula 6CO2 6H20 C6H12O6 6O2 Energy Gain In photosynthesis electrons gain energy by being boosted up an energy hill o Light energy captured by chlorophyll molecules provides the boost for the electrons As a result the light energy is converted to chemical energy which is stored in the chemical bonds of sugar molecules Two Stages of Photosynthesis Photosynthesis occurs in two metabolic stages o One stage involves light reactions In light reactions light energy is converted in the thylakoid membranes to chemical energy and O2 Water is split to provide the O2 as well as electrons H ions reduce NADP to NADPH which is an electron carrier similar to NADPH is temporarily stored and then shuttled into the Calvin cycle where it is used to make sugar Finally the light reactions generate ATP o The second stage is the Calvin cycle which occurs in the stroma of the NADH chloroplast It is a cyclic series of reactions that builds sugar molecules from CO2 and the products of the light reactions During the Calvin cycle CO2 is incorporated into organic compounds a process called carbon fixation NADPH produced by the light reactions provides the electrons for reducing carbon in the Calvin cycle ATP from the light reactions provides chemical energy for the Calvin cycle The Calvin cycle is often called the dark or light independent reactions Sunlight contains energy called electromagnetic energy or radiation o Visible light is only a small part of the electromagnetic spectrum o Electromagnetic energy travels in waves and the wavelength is the distance between the crests of two adjacent waves Light behaves as discrete packets of energy called photons o A photon is a fixed quantity of light energy The shorter the wavelength the greater the energy Radiation Photons Pigments Molecules that absorb light Built into the thylakoid membrane Plant pigments absorb some wavelengths of light and transmit others o We see the color of wavelengths that are transmitted ex Chlorophyll transmits green Pigments in Chloroplasts Chloroplasts contain several different pigments and all absorb light of different wavelengths Chlorophyll a absorbs blue violet and red light and reflects green Chlorophyll b absorbs blue and orange light and reflects yellow green The carotenoids absorb mainly blue green light and reflect yellow and orange Capture Solar Power Pigments in chloroplasts are responsible for absorbing photons capturing solar power causing release of electrons o The electrons jump to a higher energy level the excited state where electrons o The electrons drop back down to their ground state and as they do they are unstable release their excess energy Photosystems The energy released could be lost as heat or light but rather it is conserved as it is passed from one molecule to another molecule o All of the components to accomplish this are organized in thylakoid membranes in clusters called photosystems Photosystems are light harvesting complexes surrounding a reaction center complex The energy is passed from molecule to molecule within the photosystem o Finally is reaches the reaction center where a primary electron acceptor accepts these electrons and consequently becomes reduced o This solar powered transfer of an electron from the electron center pigment to the primary electron acceptor is the first step of the light reactions Type of Photosystems Two types of photosystems have been identified and are called photosystem I and photosystem II o Each type of photosystem has a characteristic reaction center Photosystem II which functions first is called P680 because its pigment absorbs light with a wavelength of 680 nm Photosystem I which functions next is called P700 because it absorbs light with a wavelength of 700nm Electron Transport Chain in Plants and NADPH During the light reactions light energy is transformed into the chemical energy of ATP o To accomplish this electrons removed from water pass from photosystem II to photosystem I and are accepted by NADP o The bridge between photosystems II and I is an electron transport chain that provides energy for the synthesis of ATP NADPH ATP and O2 are the products of the light reactions Chemiosmosis Chemiosmosis is the mechanism that is involved in oxidative phosphorylation in mitochondria and also generates ATP in chloroplasts o ATP is generated because the electron transport chain produces a concentration gradient of hydrogen ions across a membrane ATP synthase couples the flow of H to the phosphorylation of ADP o The chemiosmotic production of ATP in photosynthesis is called photophosphorylation ATP and NADPH Power Sugar Synthesis The Calvin cycle makes sugar within a chloroplast o To produce sugar the necessary ingredients are atmospheric CO2 ATP and NADPH which were generated in the light reactions o Using these three ingredients an energy rich three carbon sugar called glycareldehyde 3 phosphate G3P is produced A plant cell may