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 CO 2 and other inorganic molecules Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from H 2 O and CO 2 Autotrophs

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

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 O 2 Heterotrophs

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

1 µm Thylakoid space Chloroplast Granum Intermembrane space Inner membrane Outer membrane Stroma Thylakoid

The “light” reactions overview – Capture light energy using chlorophyll – Split H 2 O to electrons, protons and oxygen – Release O 2 – Reduce NADP + to NADPH – Generate ATP from ADP by photophosphorylation

The Calvin cycle overview -begins with carbon fixation, incorporating CO 2 into organic molecules - forms sugars from these organic molecules, using ATP and NADPH (from light reactions)

Light H2OH2O Chloroplast Light Reactions NADP + P ADP i + Overview of Photosynthesis 1

Light H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2 Overview of Photosynthesis 2

Light H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2 Calvin Cycle CO 2 Overview of Photosynthesis 3

Light H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2 Calvin Cycle CO 2 [CH 2 O] (sugar) Overview of Photosynthesis 4

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

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

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

Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 4 Pq Pc Cytochrome complex Electron transport chain 5 ATP Photosystem I (PS I) Light Primary acceptor e–e– P700 6 Fd Electron transport chain NADP + reductase NADP + + H + NADPH 8 7 e–e– e–e– 6 “Z-Diagram” of photosynthesis Photosystem II (PS II) This illustrates “non-cyclic” electron flow- electrons enter with water and leave with NADPH

ATP Photosystem II Photosystem I Primary acceptor Pq Cytochrome complex Fd Pc Primary acceptor Fd NADP + reductase NADPH NADP + + H + 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 CO 2 to CH 2 O requires NADPH reducing power

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

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 C 4 and Crassulean Acid Metabolism (CAM) fixation

Light Reactions: Photosystem II Electron transport chain Photosystem I Electron transport chain CO 2 NADP + ADP P i + RuBP 3-Phosphoglycerate Calvin Cycle G3P ATP NADPH Starch (storage) Sucrose (export) Chloroplast Light H2OH2O O2O2 C 3 Photosynthesis: a review

Note Card What is the “reducing power” of NADPH and how does it link the light reactions with the Calvin Cycle?