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CAMPBELL BIOLOGY IN FOCUS URRY CAIN WASSERMAN MINORSKY REECE 8 Photosynthesis Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge, Simon Fraser University 2016 Pearson Education, Inc. SECOND EDITION Overview: The Process That Feeds the Biosphere 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 Heterotrophs obtain their organic material from other organisms Consumers of the biosphere Almost all heterotrophs, including humans, depend on photoautotrophs for food and O 2 Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes (b) Multicellular alga (a) Plants These organisms feed not only themselves but also most of the living world (d) Cyanobacteria (c) Unicellular protists 10 m (e) Purple sulfur

1 m bacteria 40 m Concept 8.1: Photosynthesis converts light energy to the chemical energy of food Leaves are the major locations of photosynthesis Their green color is from chlorophyll, the green pigment within chloroplasts Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf Each mesophyll cell contains 3040 chloroplasts CO2 enters and O2 exits the leaf through microscopic pores called stomata Figure 8.3 Leaf cross section Chloroplasts Vein Mesophyll Stomata CO O2 2 Chloroplast Mesophyll cell

Thylakoid Thylakoid Stroma Granum space Outer membrane Intermembrane 20 m space Inner membrane Chloroplast 2016 Pearson Education, Inc. 1 m The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called Chloroplast grana Chloroplasts also contain stroma, a dense interior fluid Stroma

Outer membrane Thylakoid Granum Thylakoid space 1 m Intermembrane space Inner membrane Tracking Atoms Through Photosynthesis: Scientific Inquiry Photosynthesis is a complex series of reactions that can be summarized as the following equation: 6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O The Splitting of Water Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product

Reactants: Products: 6 CO2 C6H12O6 12 H2O 6 H2O 6 O2 Photosynthesis as a Redox Process Photosynthesis reverses the direction of electron flow compared to respiration Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced becomes reduced Energy 6 CO2 6 H2O C6 H12 O6 6 O2 becomes oxidized

Photosynthesis is an endergonic process; the energy boost is provided by light The Two Stages of Photosynthesis: A Preview Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) The light reactions (in the thylakoids) Split H2O Release O2 Reduce NADP+ to NADPH Generate ATP from ADP by photophosphorylation 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 Figure 10.6-4 CO2 H2O Light NADP ADP + Pi

Light Reactions Calvin Cycle ATP NADPH Chloroplast O2 [CH2O] (sugar) Concept 8.2: The light reactions convert solar energy to 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 Wavelength determines the type of electromagnetic energy Light also behaves as though it consists of discrete particles, called photons

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 105 nm 1 nm 103 nm Gamma rays X-rays UV 1m 106 nm 106 nm 103 nm Infrared

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 Wavelengths that are not absorbed are reflected or transmitted Leaves appear green because chlorophyll reflects and transmits green light Light Reflected light Chloroplast Absorbed light Granum

Transmitted light Animation: Light and Pigments 2016 Pearson Education, Inc. A spectrophotometer measures a pigments ability to absorb various wavelengths This machine sends light through pigments and measures the fraction of light transmitted at each TECHNIQUE wavelength Chlorophyll Photoelectric Refracting White light prism Slit moves to pass light of selected wavelength. solution

tube Galvanometer Green light High transmittance (low absorption): Chlorophyll absorbs very little green light. Blue light Low transmittance (high absorption): Chlorophyll absorbs most blue light. An absorption spectrum is a graph plotting a pigments 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 (a) Absorption spectra (b) Action spectrum Absorption of light by chloroplast pigments RESULTS Rate of photosynthesis (measured by O2 release) Figure 10.10 Chlorophyll a Chlorophyll b Carotenoids 400 500 600 Wavelength of light (nm)

400 500 600 700 700 Aerobic bacteria Filament of alga (c) Engelmanns experiment (1883) 400 500 600 700 CH3

CH3 in chlorophyll a CHO in chlorophyll b Porphyrin ring Hydrocarbon tail (H atoms not shown) Chlorophyll a is the main photosynthetic pigment Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll Excitation of Chlorophyll by Light 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 Figure 10.12 Energy of electron e Excited state Heat Photon (fluorescence) Photon Chlorophyll molecule Ground state (a) Excitation of isolated chlorophyll molecule (b) Fluorescence

A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by lightharvesting complexes The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center A primary electron acceptor in the reaction center accepts excited electrons and is reduced as a result Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions Figure 10.13a Lightharvesting complexes Thylakoid membrane Photon Photosystem Reactioncenter complex STROMA

Primary electron acceptor e Transfer of energy Pigment Special pair of molecules chlorophyll a molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) (a) How a photosystem harvests light There are two types of photosystems in the thylakoid membrane: Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm The reaction-center chlorophyll a of PS II is called P680 Photosystem I (PS I) is best at absorbing a

wavelength of 700 nm The reaction-center chlorophyll a of PS I is called P700 Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy Primary acceptor 2H + 1 /2 O2 H2O e 2 E le 4 ctro

n tr ans por Pq tc Primary acceptor hai n E tra lec ns tro p n c h o rt ai n7 Fd e e Cytochrome complex

e 8 NADP reductase 3 Pc e e P700 5 P680 Light 1 Light 6 ATP

Pigment molecules Photosystem II (PS II) Photosystem I (PS I) NADP + H NADPH A mechanical analogy for the light reactions e e e e Mill makes

ATP e n Photo e NADPH Photo n e ATP Photosystem II Photosystem I A Comparison of Chemiosmosis in Chloroplasts and Mitochondria Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the

chemical energy of ATP Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma Key Chloroplast Mitochondrion CHLOROPLAST STRUCTURE

MITOCHONDRION STRUCTURE H Intermembrane space Inner membrane Diffusion Electron transport chain Thylakoid space Thylakoid membrane ATP synthase Matrix Stroma ADP P i

Higher [H ] Lower [H ] H ATP ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH STROMA Cytochrome (low H concentration) complex Photosystem I Photosystem II Light 4 H+ Light NADP reductase 3 Fd

Pq H2O NADPH Pc 2 1 THYLAKOID SPACE (high H concentration) /2 O2 +2 H+ 1 4 H+ To Calvin Cycle Thylakoid membrane STROMA (low H concentration)

NADP + H ATP synthase ADP + Pi ATP H + Concept 8.3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar The Calvin cycle Is similar to the citric acid cycle Occurs in the stroma Generates Glucose Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P) For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2

The Calvin cycle has three phases 1) Carbon fixation 3 molecules of CO2 from atmosphere combine with 3 molecules of RuBP (catalyzed by enzyme Rubisco) 2) Reduction Chemical transformation to 6 molecules of Glyceraldehyde-3-phosphate (1 of which can go on to make other sugars, such as glucose) ATP and NADPH from light rxn. used here 3) Regeneration of the CO2 acceptor (RuBP) 5 Glyceraldehyde-3-phophates continue in cycle to re-make the 3 RuBP molecules Figure 10.19-3 Input

3(Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3P Short-lived intermediate P 6 P 3-Phosphoglycerate P 3P Ribulose bisphosphate (RuBP) 6 ATP 6 ADP 3 ADP

Calvin Cycle 6P P 1,3-Bisphosphoglycerate 3 ATP Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADPH 6 NADP 6 Pi P 5 G3P 6 P Glyceraldehyde 3-phosphate (G3P)

1 P G3P (a sugar) Output Glucose and other organic compounds Phase 2: Reduction The Importance of Photosynthesis: A Review The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells Plants store excess sugar as starch in the chloroplasts and in structures such as roots, tubers, seeds, and fruits In addition to food production, photosynthesis produces the O2 in our atmosphere

2016 Pearson Education, Inc. Figure 8.19 O2 CO2 Mesophyll cell Chloroplast H2O CO2 H2O Light NADP LIGHT REACTIONS: Photosystem II Electron transport chain Photosystem I Electron transport chain ADP

Pi 3-Phosphoglycerate RuBP CALVIN CYCLE ATP NADPH O2 LIGHT REACTIONS Are carried out by molecules in the thylakoid membranes Convert light energy to the chemical energy of ATP and NADPH Split H2O and release O2 2016 Pearson Education, Inc. G3P Starch (storage) Sucrose (export) CALVIN CYCLE REACTIONS Take place in the stroma Use ATP and NADPH to convert

CO2 to the sugar G3P Return ADP, inorganic phosphate, and NADP to the light reactions H2O Sucrose (export) You should now be able to: 1. Describe the structure of a chloroplast 2. Describe the relationship between an action spectrum and an absorption spectrum 3. Trace the movement of electrons in linear electron flow in light reactions 4. Trace the movement of electrons in cyclic electron flow in light reactions 5. Describe the similarities and differences between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts 6. Describe the role of ATP and NADPH in the Calvin cycle

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