Chapter 10 Photosynthesis is the conversion of light energy to chemical energy that is stored in sugar and other organic molecules 6 CO 2 12 H 2 O light energy C 6 H 12O 6 6 O 2 6 H 2 O Autotrophs are organisms that sustain themselves without eating anything derived from other human beings They are known as producers Heterotrophs obtain organic material by consuming the compounds produced by other organisms Decomposers consume the remains of dead organisms There are two stages of photosynthesis the conversion of solar energy to chemical energy and the Calvin cycle which is where chemical energy is transformed to organic molecules of food Light reactions capture solar energy to form ATP and allow for the electron transfer from H2O to NADP resulting in NADPH The Calvin Cycle uses ATP and NADPH to form sugar from CO2 Chloroplasts are the sites of photosynthesis in cells Mesophyll which is the tissue in the interior of the leaf is where the chloroplasts are found The stomata are microscopic pores that allow CO2 to enter and O2 to exit The stroma which is where the Calvin Cycle occurs is a dense fluid surrounded by an envelope of two membranes Thylakoids which is where light reactions occur are sacs that are suspended in the stroma Inside the thylakoids is the thylakoid space and grana are stacks of thylakoid Chlorophyll is the green pigment in chloroplasts that absorbs light for photosynthesis The chloroplast splits H2O into H2 and O The electron and hydrogen ions from it are transferred to CO2 which is reduced to a sugar The electrons increase in potential energy and require more energy which is endergonic and supplied by light There are two processes to photosynthesis light reactions and the Calvin Cycle Light reactions convert the solar energy into chemical energy by splitting water into electrons hydrogen ions and oxygen The light absorbed by the chlorophyll drives the electron and hydrogen ion to NADP Solar power then reduces NADP to NADPH by transferring the ion and electron Photophosphorylation occurs which is the generation of ATP from ADP and inorganic phosphorylation through chemiosmosis Light energy is transferred to chemical energy in the form of NADPH and ATP Light reactions do not produce sugar The Calvin Cycle involves carbon fixation which takes in CO2 from the air and develops into organic compounds present in the chloroplast NADPH reduces carbon to a carbohydrate by adding electrons ATP is the energy used to convert CO2 to CH2O The Calvin Cycle is the process that makes sugar with the help of ATP and NADPH from light reactions Pigments absorb photons which are discrete particles of light and each absorb different wavelengths In chloroplasts there are three types of pigments chlorophyll a which directly participates in light reactions chlorophyll b an accessory pigment and carotenoids which is a group of accessory pigments The action spectrum profiles the effectiveness of different wavelengths and confirms the best ones for photosynthesis Photoprotection is the process of absorbing and dissipating excessive light energy that would otherwise damage chlorophyll or interact with O2 forming oxidative molecules that are dangerous When a molecule absorbs a photon of light a molecule s electron is elevated to an orbital with more potential energy The ground state of an electron is its normal orbital The excited state is an absorbed photon which boosts the electron to an orbital of higher energy Photons absorb only energy that is equal to the difference between the ground and excited state The excited state is very unstable and causes the electron to quickly drop back releasing heat in an isolated molecule A photosystem is composed of a reaction center complex surrounded by several lightharvesting complexes The RCC is the organized association of proteins holding a special pair of chlorophyll a molecules The LHC consist of various pigment molecules chlorophyll a chlorophyll b and or carotenoids bound to proteins Having a large number and variety of pigment molecules allows the photosystem to harvest light over a larger surface area The LHC is essentially an antenna for the RCC Pigment molecules absorb photons and the energy is transferred from pigment molecule to pigment molecule within the LHC until passed to the RCC The primary electron acceptor is in the RCC and is a molecule capable of accepting electrons and being reduced A pair of chlorophyll a molecules in the RCC enables the use of light energy to boost the electron to a higher energy level and transfer it to a different molecule the PEA The transfer of the electron from the RCC chlorophyll a pair to the PEA is the first step of light reactions There are two types of photosystems Photosystem I P700 chlorophyll a pair and Photosystem II P680 which have different RCC Light energizes two photosystems to allow for ATP and NADPH synthesis because of the flow of electrons Linear Electron Flow occurs during light reactions and is the flow of electrons through photosystems and thylakoid components First a photon strikes a pigment molecule in the LHC of PSII The electron is raised to a higher energy level but then drops back down stimulating an electron in a nearby pigment This continues until it reaches the P680 pair The electron in the pair is transferred to the PEA and P680 becomes P680 An enzyme catalyzes the splitting of H2O into two electrons two hydrogen ions and oxygen The electron is supplied one by one to the P680 replacing the electron that was transferred to the PEA The hydrogen ions are released into the thylakoid lumen and oxygen combines with another oxygen to form O2 The photoexcited electron from the PSII PEA travels through the ETC to the PSI PEA The exergonic electron drops to a lower level supplying the energy for ATP synthesis The electron passes through the cytochrome complex the hydrogen ion is released into the lumen and a proton gradient is formed for chemiosmosis Light energy enters the PSI RCC causing the P700 excited electron to transfer to the PEA The photoexcited electron goes through redox reactions from the PEA of PSI down the second ETC NADP reductase catalyzes the transfer of electrons from Fd to NADP Two electrons are required for the reduction to NADPH The energy level in it is greater than in water making the electron more available for the Calvin Cycle Hydrogen ions are also removed from the stroma The Cyclic Electron Flow is an alternate path for photoexcited electrons and uses PSI