CHAPTER 9 Photosynthesis Capturing Light Energy 9 1 Light And Photosynthesis in photosynthesis spectrum because most life depends on light it s important to know the nature of light and it s role Visible light is a small part of the range of radiation called the electromagnetic all radiation in the spectrum travels as waves wavelength distance from one wave peak to the next gamma rays x rays UV rays Visible Light Infared Micro waves TV and Radio Waves short wavelengths long wavelengths 380 760 is the wavelength range for visible light light is composed of small particles or packets of energy called photons energy of a photon is inversely proportional to it s wavelength shorter has more energy longer has less when a molecule absorbs a photon of light energy one of its electrons becomes ener gized which means that the electron shifts from a lower energy atomic orbital to a high energy orbital that is more distant from the atomic nucleus one of 2 things happens to the energized electron the atom may return to it s ground state the condition in hich all its elec trons are in their normal lowest energy levels the energized electron may leave the atom and be accepted by an electron accep tor molecule which becomes reduced in the process this is what happens in photosynthesis 9 2 Chloroplasts lie mainly inside the leaf in the cells of the mesophyll a layer with many air spaces and a very high concentration of water vapor each mesophyll cell has 20 to 100 chloroplasts stomata microscopic pores that allows the interior of the leaf to exchange gases with the outside stroma a fluid filled region enclosed in the inner membrane which contains most of the enzymes required to produce carbohydrate molecules a third system of membranes suspended in the stroma that forms an interconnected set of flat disc like sacs called thylakoids the thylakoid membrane encloses the thylakoid lumen a fluid filled interior space in some regions thylakoid sacs are arranged in stacks called grana pigments substances that absorb visible light ex chlorophyll chlorophyll may be excited by light directly by energy passed to it from the source or in directly by energy passed to it from accessory pigments that have become excited by light Chlorophyll the main pigment of photosynthesis absorbs light primarily in the blue red region because it s green is found in the thylakoid membrane has 2 main parts porphyrin ring a complex ring made up of joined smaller rings composed of carbon and nitrogen atoms absorbs light energy contains an atom of magnesium in the center of the ring also contains a long hydrocarbon side chain that makes the molecules extremely non polar and anchors the chlorophyll in the membrane all chlorophyll molecules in the thylakoid membrane are associated with specific chlorophyll binding proteins facilitate the transfer of energy from one molecule to another chlorophyll a the pigment that initiates the light dependent reactions of photosynthesis chloyophyll b an accessory pigment that also participates in photosynthesis Carotenoids other accessory photosynthetic pigment yellow and orange absorb different wavelengths of light than chlorophyll thereby expanding the spectrum of light that provides energy for photosynthesis when a carotenoid molecule is excited its energy can be transferred to chlorophyll a are antioxidants that inactivate highly reactive forms of oxygen generated in the chloro plasts lengths wavelengths of light absorption spectrum a plot of a pigments absorption of light of different wave action spectrum of photosynthesis is a grpah of the relative effectiveness of different to obtain measure the rate of photosynthesis at each wavelength for leaf cells or tissues exposed to monochromatic light light of one wavelength 9 3 Overview of Photosynthesis Overall reaction of photosynthesis can be summarized as carbon dioxide water light energy and chlorophyll glucose oxygen ATP and NADPH are the products of the light dependent reactions light energy is converted to chemical energy in the light dependent reactions which are associated with the thylakoids ton of light begin as chlorophyll captures light energy which causes one of its electrons to move to a higher energy state some energy is used to phosphorylate ADP forming ATP in addition the coenzyme NADP becomes reduced forming NADPH 9 4 The Light Dependent Reactions light dependent reactions of photosynthesis begin when chlorophyll a and or accessory pigments absorb light antenna complexes organized with pigment binding proteins in the thylakoid membrane into units called each antenna complex absorbs light and transfers it to the reaction center which consists of chlorophyll molecules and proteins including electron transfer components that participate directly in photosynthesis light energy is converted to chemical energy in the reaction centers by a series of electron transfer reactions when a pigment molecule absorbs light energy that energy is passed through a process called resonance directly from one pigment molecule to another within the antenna com plex until it reaches the reaction center noncyclic electron transport a pigment molecule in an antenna complex associated with photosystem I absorbs a pho the absorbed energy is transferred from one pigment molecule to another until it reaches the reaction center where it excites an electron in a molecule of P700 this energized electron is transferred to a primary electron acceptor a spe cial molecule of chlorophyll a which is the first of several electron acceptors in a series the energized electron is passed along an electron transport chain from one electron acceptor to another until it s passed to the ferredoxin and iron containing protein ferredoxin transfers the electron to NADP in the presence of the enzyme ferredoxin NADP reductase how is the electron that has been donated to the electron transport chain replaced this occurs through photolysis light splitting of water a process that not only yields electrons but also is the source of almost all the oxygen in earth s atmosphere name is somewhat misleading makes it seem like water is broken by light when really light splits water indirectly by causing P680 to become oxidized Noncyclic electron transport is a continuous linear process in the presence of light there is a continuous one way flow of electrons from the ulti mate electron source H2 to the terminal electron acceptor NADP as electrons are transferred along the electron