Photosynthesis Photosynthesis is the process that converts solar energy into chemical energy Directly or indirectly photosynthesis nourishes almost the entire living world Autotrophs sustain themselves without eating anything derived from other organisms Autotrophs are the producers of the biosphere producing organic molecules from CO2 and other inorganic molecules Almost all plants are photoautotrophs using the energy of sunlight to make organic molecules from H2O and CO2 Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Photosynthesis occurs in plants algae certain other protists and some prokaryotes These organisms feed not only themselves but also most of the living world Heterotrophs obtain their organic material from other organisms Heterotrophs are the consumers of the biosphere Almost all heterotrophs including humans depend on photoautotrophs for food and O2 The structural organization of these cells allows for the chemical reactions of photosynthesis Leaves are the major locations of photosynthesis Their green color is from chlorophyll the green pigment within chloroplasts Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast CO2 enters and O2 exits the leaf through microscopic pores called stomata Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Chloroplasts are found mainly in cells of the mesophyll the interior tissue of the leaf A typical mesophyll cell has 30 40 chloroplasts The chlorophyll is in the membranes of thylakoids connected sacs in the chloroplast thylakoids may be stacked in columns called grana Chloroplasts also contain stroma a dense fluid Photosynthesis can be summarized as the following equation 6 CO2 12 H2O Light energy C6H12O6 6 O2 6 H2O Fig 10 3 Leaf cross section Vein Mesophyll Stomata Chloroplast CO2 O2 Mesophyll cell Outer membrane Thylakoid Stroma Granum Thylakoid space Intermembrane space Inner membrane 1 m 5 m Chloroplasts split H2O into hydrogen and oxygen incorporating the electrons of hydrogen into sugar molecules Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced 6 CO2 12 H2O C6H12O6 6 O2 6 H2O Photosynthesis consists of the light reactions the photo part and Calvin cycle the synthesis part Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings 1 The light reactions in the thylakoids Split H2O Release O2 Reduce NADP to NADPH Generate ATP from ADP by photophosphorylation 2 The Calvin cycle in the stroma forms sugar from CO2 using ATP and NADPH The Calvin cycle begins with carbon fixation incorporating CO2 into organic molecules Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 10 5 4 CO2 H2O Light NADP ADP P i Light Reactions Calvin Cycle ATP NADPH Chloroplast O2 CH2O sugar The light reactions convert solar energy to the chemical energy of ATP and NADPH Chloroplasts are solar powered chemical factories Their thylakoids transform light energy into the chemical energy of ATP and NADPH Light is a form of electromagnetic energy also called electromagnetic radiation Like other electromagnetic energy light travels in rhythmic waves Wavelength is the distance between crests of waves Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Wavelength determines the type of electromagnetic energy The electromagnetic spectrum is the entire range of electromagnetic energy or radiation Visible light consists of wavelengths including those that drive photosynthesis that produce colors we can see Light also behaves as though it consists of discrete particles called photons Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 10 6 10 nm 10 nm 3 5 103 nm 1 nm Gamma X rays rays UV 106 nm Infrared 1m 109 nm Microwaves 103 m Radio waves Visible light 380 450 500 Shorter wavelength Higher energy 550 600 650 700 750 nm Longer wavelength Lower energy Photosynthetic Pigments The Light Receptors Pigments are substances that absorb visible light Different pigments absorb different wavelengths There are 3 types of pigments 1 Chlorophyll a 2 Chlorophyll b 3 Carotenoids Wavelengths that are not absorbed are reflected or transmitted Leaves appear green because chlorophyll reflects and transmits green light Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 10 7 Light Reflected light Chloroplast Absorbed light Granum Transmitted light RESULTS Absorption of light by chloroplast pigments Fig 10 9 Chlorophyll a Carotenoids 400 a Absorption spectra Chlorophyll b 500 600 700 b Action spectrum Rate of photosynthesis measured by O2 release Wavelength of light nm Aerobic bacteria Filament of alga c Engelmann s experiment 400 500 600 700 An absorption spectrum is a graph plotting a pigment s light absorption versus wavelength The absorption spectrum of chlorophyll a suggests that violet blue and red light work best for photosynthesis An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process Chlorophyll a is the main photosynthetic pigment Accessory pigments such as chlorophyll b broaden the spectrum used for photosynthesis Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll When a pigment absorbs light it goes from a ground state to an excited state which is unstable When excited electrons fall back to the ground state photons are given off an afterglow called fluorescence If illuminated an isolated solution of chlorophyll will fluoresce giving off light and heat Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 10 11 Energy of electron e Excited state Heat Photon fluorescence Photon Chlorophyll molecule Ground state a Excitation of isolated chlorophyll molecule b Fluorescence 1 Light reactions takes place in photosystems in the thylakoid membrane Photosystem consists of a reaction center complex surrounded by light harvesting complexes A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a The light harvesting complexes funnel the energy of photons to the reaction center There are two types of photosystems in the thylakoid membrane Photosystem II Photosystem I Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 10 12 Photosystem STROMA Light harvesting