Chapter 7: Photosynthesis (Outline) Photosynthetic Organisms Flowering Plants as Photosynthesizers Photosynthesis Light Reactions Noncyclic Pathway Calvin Cycle Reactions Carbon Dioxide Fixation Reduction of Carbon Dioxide Regeneration of RuBP C4 CAM

Photosynthetic Organisms Organisms cannot live without energy Almost all the energy that organisms used comes from the solar energy Photosynthesis: A process that captures solar energy and transforms it into chemical energy (carbohydrate) Occurrs in higher plants, algae, mosses, photosynthetic protists and some bacteria (cyanobacteria)

Photosynthetic Organisms Autotrophs – organisms having the ability to synthesize carbohydrates Heterotrophs – are consumers that use carbohydrates produced by photosynthesis

Flowering Plants & Photosynthesis Photosynthesis takes place in the green portions of plants Leaf of flowering plant contains mesophyll tissue with chloroplasts Specialized to carry on photosynthesis CO2 enters leaf through stomata Diffuses into chloroplasts in mesophyll cells In the stroma, CO2 is combined with H2O to form C6H12O6 (sugar) Chlorophyll is capable of absorbing solar energy, which drives photosynthesis

Photosynthesis The process of photosynthesis is an example of a redox reaction (i.e. movement of electrons from one molecule to another) In living things, electrons are accompanied by hydrogen ions (H+ + e) Oxidation is the loss of hydrogen atoms; Reduction is the gain of hydrogen atoms

Photosynthesis Carbon dioxide reduction requires energy and hydrogen atoms Solar energy will be used to generate ATP needed to reduce CO2 to sugar Electrons needed to reduce CO2 will be carried by a coenzyme (NADP+) NADP+ is the active redox coenzyme of photosynthesis When NADP+ is reduced, it accepts 2 e- and H+ When NADP+ is oxidized, it gives up its electrons

Photosynthetic Reactions In the 1930’s, C. B. van Niel studying photosynthesis in bacteria discovered that CO2 was not split in the process of carbohydrate synthesis Researchers later confirmed using the isotope oxygen (18O) that O2 given off from photosynthesis comes from water not CO2 When water splits, O2 is released and the hydrogen atoms (2 e- + H+) are taken up by NADPH

Photosynthetic Reactions Light Reactions: Chlorophyll present in thylakoid membranes absorbs solar energy This energizes electrons Electrons move down electron transport chain Pumps H+ into thylakoids Used to make ATP out of ADP and NADPH out of NADP+ Calvin Cycle Reactions: In the stroma, CO2 is reduced to a carbohydrate Reduction requires the ATP and NADPH produced in the light reactions

Photosynthesis Overview

Photosynthetic Pigments Molecules that absorb some colors in rainbow (visible light) more than others, why? Colors least absorbed reflected/transmitted most Absorption Spectra Graph showing relative absorption of the various colors of the rainbow Chlorophylls a and b (main pigments) are green because they absorb much of the reds and blues of white light Carotenoids are accessory pigments

Phosynthetic Pigments and Photosynthesis

Spectrophotometer An instrument which measures the amount of light of a specified wavelength which passes through a medium Amount of light absorbed at each wavelength is plotted on a graph, resulting in an absorption spectra

Light Reactions The light reactions utilize two photosystems: Photosystem I (PSI) Photosystem II (PSII) A photosystem consists of Pigment complex (molecules of chlorophylls a & b the carotenoids) and electron acceptor molecules that help collect solar energy like an antenna Occur in the thylakoid membranes

The Noncyclic Electron Pathway Uses two photosystems, PS I and PS II Energy enters the system when PS II becomes excited by light Causes an electron to be ejected from the reaction center (chlorophyll a) Electron travels down electron transport chain to PS I PS II takes replacement electron from H2O, which splits, releasing O2 and H+ ions The H+ ions concentrate in thylakoid chambers Which causes ATP production, how?

The Noncyclic Electron Pathway PS I captures light energy and ejects the energized electron (e-) from the reaction center Transferred permanently by electron acceptors to a molecule of NADP+ Causes NADPH production NADP+ + 2 e- + H+ NADPH and ATP produced are used by the Calvin cycle reaction (reduction of CO2) in the stroma

Organization of the Thylakoid Membrane PS II: Pigment complex and electron-acceptors Adjacent to an enzyme that oxidizes water Oxygen is released as a gas (by-product) Electron transport chain: Consists of Pq (plastoquinone) and cytochrome complexes Carries electrons from PS II to PS I Pumps H+ from the stroma into thylakoid space PS I: Pigment complex and electron acceptors Adjacent to enzyme that reduces NADP+ to NADPH ATP synthase complex: Has a channel for H+ flow Which drives ATP synthase to join ADP and Pi

Organization of a Thylakoid

ATP Production Thylakoid space acts as a reservoir for H+ ions Each time water is oxidized, two H+ remain in the thylakoid space Electrons yield energy Used to pump H+ across thylakoid membrane Move from stroma into the thylakoid space Flow of H+ back across thylakoid membrane Energizes ATP synthase Enzymatically produces ATP from ADP + Pi This method of producing ATP is called chemiosmosis

Calvin Cycle Reactions: Overview of C3 Photosynthesis A cyclical series of reactions Utilizes atmospheric carbon dioxide to produce carbohydrates Known as C3 photosynthesis Involves three phases: Carbon dioxide fixation Carbon dioxide reduction RuBP Regeneration

Calvin Cycle Reactions: Phase 1: Carbon Dioxide Fixation CO2 enters the stroma of the chloroplast via the stomata of the leaves CO2 is attached to 5-carbon RuBP molecule Result in an intermediate 6-carbon molecule This splits into two 3-carbon molecules (3PG) Reaction is accelerated by RuBP Carboxylase (Rubisco) CO2 now “fixed” because it is part of a carbohydrate

Calvin Cycle Reactions: Phase 2: Carbon Dioxide Reduction ATP phosphorylates each 3PG molecule and creates 1,3-bisphosphoglycerate (BPG) BPG is then reduced by NADPH to glyceraldehyde-3-phosphate (G3P) NADPH and some ATP used for the reduction reactions are produced in light reactions

Calvin Cycle Reactions: Phase 3: Regeneration of RuBP RuBP used in CO2 fixation must be replaced Every three turns of Calvin Cycle, Five G3P molecules are used To remake three RuBP molecules 5 X 3 (carbons in G3P) = 3 X 5 (carbons in RuBP)

The Calvin Cycle

C3 Plants C3 plants include more than 95% percent of the plant species on earth (e.g. trees) Called C3 because the CO2 is first incorporated into a 3-carbon compound In hot dry conditions, the photosynthetic efficiency of C3 plants suffers because of photorespiration Stomata must close to avoid wilting CO2 decreases and O2 increases O2 starts combining with RuBP instead of CO2 CO2 + RUBP Rubisco    2 3PG

C4 Photosynthesis: C4 Plants Use a supplementary method of CO2 uptake forming a 4-carbon molecule instead of the two 3-carbon molecules In C4 plants CO2 is inserted into a 3-carbon molecule called phosphoenolpyruvate (PEP) forming Oxaloacetate, a C4 molecule in the mesophyll cells Oxaloacetate is converted into malate, which is transported into the bundle sheath cells Here the 4-carbon molecule is broken down into CO2, which enters the Calvin cycle to form sugars & starch

Chloroplast distribution in C4 vs. C3 Plants

CO2 Fixation in C4 vs. C3 Plants In hot and dry climates C4 plants avoid photorespiration as PEP in mesophyll cells does not bind to O2 Net productivity of C4 is about 2-3 times of C3 plants In cool, moist conditions, CO2 fixation in C3 is three times more efficient than in C4

CAM Photosynthesis Called CAM after the plant family in which it was first found (Crassulaceae) CAM plants partition carbon fixation by time Crassulacean-Acid Metabolism During the night through their open stomata CAM plants use PEPCase to fix CO2 Forms C4 molecules (malate) Stored in large vacuoles in mesophyll cells During daylight NADPH and ATP are available Stomata closed for water conservation C4 molecules release CO2 to Calvin (C3) cycle

CO2 Fixation in a CAM Plant Adaptive Value: Better water use efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.)