Chapter 10 Photosynthesis Overview The Process That Feeds the Biosphere The chloroplasts of plants use a process called photosynthesis to capture light energy from the sun and convert it to chemical energy stored in sugars and other organic molecules Plants and other autotrophs are the producers of the biosphere Photosynthesis nourishes almost all the living world directly or indirectly Autotrophs produce their organic molecules from CO2 and other inorganic raw materials obtained from the environment Autotrophs are the producers of the biosphere Autotrophs can be separated by the source of energy that drives their metabolism Photoautotrophs use light as a source of energy to synthesize organic compounds Chemoautotrophs harvest energy from oxidizing inorganic substances such as sulfur and ammonia Heterotrophs live on organic compounds produced by other organisms Almost all heterotrophs are completely dependent on photoautotrophs for food and for oxygen a These organisms are the consumers of the biosphere The most obvious type of heterotrophs feeds on other organisms Animals by product of photosynthesis Concept 10 1 Photosynthesis converts light energy to the chemical energy of food Chloroplasts The Sites of Photosynthesis in Plants Leaves are the major site of photosynthesis for most plants Chlorophyll plays an important role in the absorption of light energy during photosynthesis The color of a leaf comes from chlorophyll All green parts of a plant have chloroplasts Chlorophyll the green pigment in the chloroplasts Chloroplasts are found mainly in mesophyll the tissues in the interior of the leaf Stomata pore that allows gas exchange between the environment and interior of the plant Veins deliver water from the roots and carry off sugar from mesophyll cells to nonphotosynthetic areas of the plant Chlorophyll is located in the thylakoids Each chloroplast has two membranes Stroma dense fluid within chloroplasts In the stroma is an elaborate system of interconnected membranous sacs the thylakoids The interior of the thylakoids forms another compartment the thylakoid space Thylakoids may be stacked into columns called grana The Two Stages of Photosynthesis A Preview Photosynthesis is two processes each with multiple stages The light reactions photo convert solar energy to chemical energy Light Dependent In the light reactions light energy absorbed by chlorophyll in the thylakoids drives the transfer of electrons and hydrogen from water to NADP nicotinamide adenine dinucleotide phosphate forming NADPH NADPH an electron acceptor provides reducing power via energized electrons to the Calvin cycle O2 is released as a by product The light reaction generates ATP in a process called photophosphorylation Thus light energy is initially converted to chemical energy in the form of two compounds NADPH and ATP The Calvin cycle synthesis uses energy from the light reactions to incorporate CO2 from the atmosphere into sugar Light Independent 1 o The cycle begins with the incorporation of CO2 into organic molecules a process known as carbon fixation o The fixed carbon is reduced with electrons provided by NADPH and ATP from the light reactions also powers parts of the Calvin cycle o Calvin cycle that makes sugar but only with the help of ATP and NADPH from the light reactions o Calvin cycle occurs in the stroma Concept 10 2 The light reactions convert solar energy to the chemical energy of ATP and NADPH The thylakoids convert light energy into the chemical energy of ATP and NADPH Light is a form of energy known as electromagnetic radiation The distance between crests of electromagnetic waves is called the wavelength The entire range of electromagnetic radiation is the electromagnetic spectrum Visible light is the radiation that drives photosynthesis When light meets matter it may be reflected transmitted or absorbed Different pigments absorb photons of different wavelengths and the wavelengths that are between 380 to 750 nm is the band of visible light A leaf looks green because chlorophyll the dominant pigment absorbs red and blue light while absorbed disappear transmitting and reflecting green light The light reaction can perform work with those wavelengths of light that are absorbed There are several pigments in the thylakoid that differ in their absorption spectra An absorption spectrum plots a pigment s light absorption versus wavelength Chlorophyll a the dominant pigment most absorption Chlorophyll b accessory pigment 2nd most absorption Carotenoids accessory pigment hydrocarbons Least Absorption Only chlorophyll a participates directly in the light reaction but accessory photosynthetic pigments absorb light and transfer energy to chlorophyll a Chlorophyll b with a slightly different structure than chlorophyll a has a slightly different absorption spectrum and funnels the energy from these wavelengths to chlorophyll a Carotenoids can funnel the energy from other wavelengths to chlorophyll a and also participate in photoprotection against excessive light These compounds absorb and dissipate excessive light energy that would otherwise damage chlorophyll When a molecule absorbs a photon one of that molecule s electrons is elevated to an orbital with more potential energy The electron moves from its ground state to an excited state The only photons that a molecule can absorb are those whose energy matches exactly the energy difference between the ground state and excited state of this electron Because this energy difference varies among atoms and molecules a particular compound absorbs only photons corresponding to specific wavelengths Thus each pigment has a unique absorption spectrum A Photosystem A Reaction Center Associated with Light Harvesting Complexes Chlorophyll excited by absorption of light energy produces very different results in an intact chloroplast than it does in isolation molecules into photosystems Photosystems are composed of a reaction center surrounded by a light harvesting complex In the thylakoid membrane chlorophyll is organized along with proteins and smaller organic 2 a Light harvesting complex consists of pigment molecules which may include chlorophyll a chlorophyll b and carotenoid molecules bound to particular proteins Together these light harvesting complexes act like light gathering antenna complexes for the reaction center The Reaction Center is a protein complex that includes two special chlorophyll a molecules and a molecule called the primary electron acceptor