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 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

Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes (a) Plants Figure 10.2 Photoautotrophs (c) Unicellular protist 10 µm (e) Purple sulfur bacteria 1.5 µm (b) Multicellular alga (d) Cyanobacteria 40 µm

Heterotrophs 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 Two Stages of Photosynthesis: A Preview Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) The “light” reactions are in the thylakoids The Calvin cycle is in the stroma

Chloroplast Outer membrane Thylakoid Intermembrane space Stroma Granum Inner membrane Figure 10.3 Zooming in on the location of photosynthesis in a plant 1 µm

The “light” reactions overview Capture light energy using chlorophyll Split H2O to electrons, protons and oxygen Release O2 Reduce NADP+ to NADPH Generate ATP from ADP by photophosphorylation

The Calvin cycle overview begins with carbon fixation, incorporating CO2 into organic molecules - forms sugars from these organic molecules, using ATP and NADPH (from light reactions)

Overview of Photosynthesis 1 Light NADP+ ADP + P i Light Reactions Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle Chloroplast Overview of Photosynthesis 1

Overview of Photosynthesis 2 Light NADP+ ADP + P i Light Reactions ATP Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle NADPH Chloroplast Overview of Photosynthesis 2 O2

Overview of Photosynthesis 3 CO2 Light NADP+ ADP + P i Calvin Cycle Light Reactions ATP Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle NADPH Chloroplast Overview of Photosynthesis 3 O2

Overview of Photosynthesis 4 CO2 Light NADP+ ADP + P i Calvin Cycle Light Reactions ATP Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle NADPH Chloroplast Overview of Photosynthesis 4 [CH2O] (sugar) O2

Chlorophyll is the light-capturing substance Surrounded by other pigment molecules, enzymes and electron carriers A “photosystem” Anchored to thylakoid membranes by proteins

Illustration of important properties of chlorophyll Excited state e– Heat Energy of electron Photon (fluorescence) Photon Figure 10.11 Excitation of isolated chlorophyll by light Ground state Chlorophyll molecule (a) Excitation of isolated chlorophyll molecule (b) Fluorescence If illuminated, a solution of chlorophyll will fluoresce, giving off light and heat

(INTERIOR OF THYLAKOID) Photosynthetic Energy Capture Photosystem STROMA Photon Primary electron acceptor Light-harvesting complexes Reaction-center complex e– Thylakoid membrane Figure 10.12 How a photosystem harvests light Pigments-chlorophyll and accessory Transfer of energy Special pair of chlorophyll a molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)

Electron transport chain “Z-Diagram” of photosynthesis Electron transport chain Primary acceptor Primary acceptor 4 7 Electron transport chain Fd Pq e– 2 e– 8 e– H2O e– NADP+ + H+ Cytochrome complex 2 H+ NADP+ reductase + 3 1/2 O2 NADPH Pc e– e– P700 5 P680 Light 1 Light 6 6 ATP Figure 10.13 How linear electron flow during the light reactions generates ATP and NADPH Pigment molecules Photosystem I (PS I) Photosystem II (PS II) This illustrates “non-cyclic” electron flow- electrons enter with water and leave with NADPH

Primary acceptor Primary acceptor Fd Fd NADP+ + H+ Pq NADP+ reductase Cytochrome complex NADPH Pc Figure 10.15 Cyclic electron flow Photosystem I Photosystem II ATP Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH

“Light” reactions of PSI and PSII generate ATP and “reducing power” in the form of NADPH Reducing power allows the conversion of carbon dioxide to carbohydrate CO2 to CH2O requires NADPH reducing power

Carbon fixation conversion of carbon dioxide to carbohydrate Input 3 (Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3 P P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 ATP 6 ADP Calvin Cycle 3 ADP 6 3 P P ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADP+ 6 P i Figure 10.18 The Calvin cycle 5 P G3P 6 P Rubisco is the key enzyme of the cycle Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction Plants that use this cycle exclusively are C3 plants 1 P Glucose and other organic compounds Output G3P (a sugar)

Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis Plants have evolved two alternate types of carbon fixation C4 and Crassulean Acid Metabolism (CAM) fixation

Electron transport chain C3 Photosynthesis: a review CO2 Light NADP+ ADP + P i Light Reactions: Photosystem II Electron transport chain Photosystem I RuBP 3-Phosphoglycerate Calvin Cycle ATP G3P Figure 10.21 A review of photosynthesis Starch (storage) NADPH Chloroplast C3 Photosynthesis: a review O2 Sucrose (export)