BSC2010 Test 3 Study Guide Photosynthesis Study Guide Chapter 10 organisms that use light as a source of energy to synthesize organic are self feeders They sustain themselves without eating anything derived Photosynthesis is the conversion of light energy from the sun that is then converted into chemical energy and then converted into sugar and other organic molecules The actual conversion is photosynthesis Autotrophs from other living beings Photoautotrophs substances Plants multicellular algea kelp some unicellular protists euglena prokaryotes called cyanobacteria and other photosynthetic prokaryotes such as purple sulfur bacteria Heterotrophs obtain their organic material by the second major mode of nutrition They live on compounds produced by other organisms They are the consumers Leaves are the major sites of photosynthesis in plants Chlorophyll branes the green pigment located within chloroplasts resides in thylakoid mem It is the light energy absorbed by chlorophyll that drives the synthesis of organic molecules in the chloroplast the dense fluid within the chloroplast enclosed by a two membrane envelope the tissue in the interior of the leaf Where chloroplasts are found mainly how carbon dioxide enters the leaf and oxygen exits they are microscopic Mesophyll Stomata pores Stroma Thylakoids an elaborate system of interconnected membraneous sacs segregating the stroma from another compartment the interior of the thylakoids or thylakoid mem branes Granum columns of stacked thylakoid sacs summary of complex series of chemical reactions Tracking Atoms through Photosynthesis 6CO2 12H20 Light Energy C6H12O6 6O2 6H2O we use glucose to simplify the relationship between photosynthesis and respira tion but the direct product of photosynthesis is is a 3 Carbon sugar that we used ot make glucose 12 Molecules are consumed and 6 are newly formed during photosynthesis We can simplify by indicating only the net consumption of water 6CO2 6H2O Light Energy C6H12O6 6O2 Now lets put it in simplest form CO2 H2O CH2O O2 Photosynthesis is the reverse direction of electron flow of cell respiration Water is split and electrons are transferred along with hydrogen ions from water to carbon dioxide reducing it to sugar Bc the E increase in potential energy as they move from water to sugar it requires energy In this case light The Two Stages of Photosynthesis Light reactions the photo part of photosynthesis they are the steps that convert solar energy into chemical energy Occurring in the thylakoid membranes within the chloro plast Water is split providing a source of electrons and protons H and giving off O2 as a by product Light absorbed by chlorophyll drives a transfer of electrons and hydrogen ions from water to an acceptor called NADP where they are temporarily stored The light reactions use solar power to reduce NADP to NADPH by adding a pair of electrons along with an H The light reactions also generate ATP using chemiosmosis to power the addi tion of a phosphate group to ADP photophosphorylation Thus light energy chemical energy in the form of NADPH a source of elec trons as reducing power that can be passed along to an electron acceptor re ducing it AND ATP In the thylakoids molecules of NADP and ADP pick up electrons and phos phate and then are released to the stroma as NADPH and ATP No sugar is produced in light reactions the synthesis part occurring in the stroma of the chloroplast Starts with Calvin Cycle incorporating CO2 from the air into organic molecules already present in the chloro plast This initial incorporation of carbon into organic compounds is Carbon Fixation The Calvin cycle then reduces the fixed carbon to carbohydrate by the addition of electrons The reducing power is provided by NADPH which acquired its electrons during light reactions To convert CO2 to carbohydrate the calvin cy cle also requires chemical energy in the form of ATP which is also generated by light reactions Calvin cycle makes sugar but can do so only with help of NADPH and ATP from light reaction None of the steps in Calvin require light directly though The Nature of Sunlight the entire range of radiation The distance between crests of electromagnetic waves 380 nm to 750 nm in wavelength it can be detected as various colors by Light is a form of energy known as electromagnetic energy also called electromag netic radiation Wavelength Electromagnetic Spectrum Visible light the human eye Photons are not tanigble objects but they act like objects in that each of them has a fixed quantity of energy The amount of energy is inversely related to the wavelength of the light the shorter the wavelength the greater the energy of each photon of that light Visible light drives photosynthesis Pigment Substances that absorb visible light Photosynthetic Pigments The Light Receptors Different pigments absorb light of different wavelengths and the wavelenghts that are absorbed disappear If the pigment is illuminated with white light the color we see is the color most reflected or transmitted by the pigment If a pigment absorbs all wavelengths it appears black We see green when we look at a leaf because chlorophyll absorbs violet blue and red light while transmitting and reflecting green light Spectrophotometer of the pigment and measures the fraction of light transmitted at each wavelength Absorption spectrum a graph plotting a pigment s light absorption vs wavelength directs beams of light of different wavelengths through a solution The absorption spectra of chloroplast pigments provide clues to the relative ef fectiveness of different wavelengths for driving photosynthesis since light can perform work in chloroplast only if it is absorbed Chlorophyll A participates directly in the light reactions Is blue green the spectrum suggests that violet blue and red light work best for photo synthesis The absorption spectrum for Chlorophyll A alone underestimates the ef fectiveness of certain wavelengths in driving photosynthesis This is partly because accessory pigments with different absorption spectra are also photosynthetically important in chloroplasts and broaden the spectrum of colors that can be used for photosynthesis Chlorophyll B Carotenoids shades of yellow and orange because they absorb violet and blue green light the accessory pigment is olive green a group of accessory pigments hydrocarbons that are various Carotenoids may broaden the spectrum of colors that can drive photo synthesis Photoproctection and dissipate
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