CHAPTER 8 PHOTOSYNTHESIS Brenda Leady, University of Toledo Prepared by Brenda Leady, University of Toledo Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Trophic organization Heterotroph Autotroph Must eat food, organic molecules from their environment, to sustain life Autotroph Make organic molecules from inorganic sources Photoautotroph Use light as a source of energy Green plants, algae, cyanobacteria

CO2 + H2O + light energy → C6H12O6 + O2 + H2O Photosynthesis Energy within light is captured and used to synthesize carbohydrates CO2 + H2O + light energy → C6H12O6 + O2 + H2O CO2 is reduced H2O is oxidized Energy from light drives this endergonic reaction

Biosphere Regions on the surface of the Earth and in the atmosphere where living organisms exist Largely driven by the photosynthetic power of green plants Cycle where cells use organic molecules for energy and plants replenish those molecules using photosynthesis Plants also produce oxygen

Chloroplast Organelles in plants and algae that carry out photosynthesis Chlorophyll- green pigment Majority of photosynthesis occurs in leaves in central mesophyll Stomata- carbon dioxide enters and oxygen exits leaf

Chloroplast anatomy Outer and inner membrane Intermembrane space Thylakoid membrane contains pigment molecules Thylakoid membrane forms thylakoids Enclose thylakoid lumen Granum- stack of thylakoids Stroma- fluid filled region between thylakoid membrane and inner membrane

2 stages of photosynthesis Light reactions Take place in thylakoid membranes Produce ATP, NADPH and O2 Calvin cycle Occurs in stroma Uses ATP and NADPH to incorporate CO2 into organic molecules

Light energy Type of electromagnetic radiation Travels as waves Short to long wavelengths Also behaves as particles- photons Shorter wavelengths have more energy

Photosynthetic pigments absorb some light energy and reflect others Leaves are green because they reflect green wavelengths Absorption boosts electrons to higher energy levels Wavelength of light that a pigment absorbs depends on the amount of energy needed to boost an electron to a higher orbital

After an electron absorbs energy, it is an excited state and usually unstable Releases energy as Heat Light Excited electrons in pigments can be transferred to another molecule or “captured” Captured light energy can be transferred to other molecules to ultimately produce energy intermediates for cellular work

Pigments Chlorophyll a Chlorophyll b Carotenoids

Absorption vs. action spectrum Absorption spectrum Wavelengths that are absorbed by different pigments in the plant Action spectrum Rate of photosynthesis by whole plant at specific wavelengths

Photosystems Thylakoid membrane Photosystem I (PSI) Photosystem II (PSII)

Photosytem II (PSII) 2 main components Light-harvesting complex or antenna complex Directly absorbs photons Energy transferred via resonance energy transfer Reaction center P680 →P680* Relatively unstable Transferred to primary electron acceptor Removes electrons from water to replace oxidized P680 Oxidation of water yields oxygen gas

Photosystem II (PSII) Redox machine Recent research in biochemical composition of protein complex and role of components 3 dimensional structure determined in 2004 using x-ray crystallography

Electron releases some of its energy along the way Electrons accepted by primary electron acceptor is PSII to a pigment molecule in the reaction center of PSI Electron releases some of its energy along the way Establishes H+ electrochemical gradient ATP synthesis uses chemiosmotic mechanism similar to mitochondria

Photosystem I (PSI) Key role to make NADPH Light striking light-harvesting complex of PSI transfers energy to a reaction center High energy electron removed from P700 and transferred to a primary electron acceptor NADP+ reductase NADP+ + 2 electrons + H + → NADPH P700+ replaces its electrons from plastocyanin No splitting water, no oxygen gas formed

Summary O2 produced in thylakoid lumen by oxidation of H2O by PSII 2 electrons transferred to P680+ ATP produced in stroma by H+ electrochemical gradient Splitting of water places H+ in the lumen High-energy electrons move from PSII to PSI, pumping H+ into the lumen Formation of NADPH consumes H+ in the stroma NADPH produced in the stroma from high-energy electrons that start in PSII and boosted in PSI NADP+ + 2 electrons + H + → NADPH

Cyclic and noncyclic electron flow Electrons begin at PSII and eventually transfer to NADPH Linear process produces ATP and NADPH in equal amounts Cyclic photophosphorylation Electron cycling releases energy to transport H+ into lumen driving synthesis of ATP

The cytochrome complexes of mitochondria and chloroplasts have evolutionarily related proteins in common Homologous genes are similar because they are derived from a common ancestor Comparing the electron transport chains of mitochondria and chloroplasts reveals homologous genes Family of cytochrome b-type proteins plays similar but specialized roles

Calvin cycle ATP and NADPH used to make carbohydrates Somewhat similar to citric acid cycle CO2 incorporated into carbohydrates Precursors to all organic molecules Energy storage

CO2 incorporation Also called Calvin-Benson cycle Requires massive input of energy For every 6 CO2 incorporated, 18 ATP and 12 NADPH used Glucose is not directly made

3 phases Carbon fixation Reduction and carbohydrate production CO2 incorporated in RuBP using rubisco 6 carbon intermediate splits into 2 3PG Reduction and carbohydrate production ATP is used to convert 3PG into 1,3-bisphosphoglycerate NADPH electrons reduce it to G3P 6 CO2 → 12 G3P 2 for carbohydrates 10 for regeneration Regeneration of RuBP 10 G3P converted into 6 RuBP using 6 ATP

The Calvin cycle was determined by isotope labeling methods 14C-labeled CO2 injected into cultures of green algae Allowed to incubate different lengths of time Separated newly made radiolabeled molecules using two-dimensional paper chromatography Autoradiography- radiation from 14C-labeled molecules makes dark spots on the film Identified 14C-labeled spots and the order they appeared Calvin awarded Nobel Prize in 1961

Variations in photosynthesis Certain environmental conditions can influence both the efficiency and way the Calvin cycle works Light intensity Temperature Water availability

Photorespiration RuBP + CO2 → 2 3PG Rubisco can also be an oxygenase Rubisco functions as a carboxylase C3 plants make 3PG Rubisco can also be an oxygenase Adds O2 to RuBP eventually releasing CO2 Photorespiration Using O2 and liberating CO2 is wasteful More likely in hot and dry environment Favored when CO2 low and O2 high

C4 plants C4 plants make a 4 carbon compound in the first step of carbon fixation Hatch-Slack pathway Leaves have 2-cell layer organization Mesophyll cells CO2 enters via stomata and 4 carbon compound formed (PEP carboxylase does not promote photorespiration) Bundle-sheath cells 4 carbon compound transferred that releases steady supply CO2

In cooler climates, C3 plants use less energy to fix CO2 In warm dry climates C4 plants have the advantage in conserving water and preventing photorespiration In cooler climates, C3 plants use less energy to fix CO2 90% of plants are C3

CAM plants Some C4 plants separate processes using time Crassulacean Acid Metabolism CAM plants open their stomata at night CO2 enters and is converted to malate Stomata close during the day to conserve water Malate broken down into CO2 to drive Calvin cycle