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 Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes These organisms feed not only themselves but also most of the living world Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 10.2 Photoautotrophs (a) Plants (c) Unicellular protist Figure 10.2 Photoautotrophs 10 µm (e) Purple sulfur bacteria 1.5 µm (b) Multicellular alga (d) Cyanobacteria 40 µm

Photosynthesis involves two pathways: Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Photosynthesis involves two pathways: Light reactions convert light energy into chemical energy (in ATP and the reduced electron carrier NADPH), splits water, releases O2. Carbon-fixation reactions (aka Calvin Cycle) use the ATP and NADPH, along with CO2, to produce carbohydrates.

Figure 6.15 An Overview of Photosynthesis

The “Photo” in Photosynthesis

Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Light is a form of electromagnetic radiation, which travels as a wave but also behaves as particles (photons). Photons can be absorbed by a molecule, adding energy to the molecule—it moves to an excited state.

Figure 6.16 The Electromagnetic Spectrum

1 m (109 nm) 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 103 m Micro- waves Fig. 10-6 1 m (109 nm) 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 103 m Gamma rays Micro- waves Radio waves X-rays UV Infrared Visible light Figure 10.6 The electromagnetic spectrum 380 450 500 550 600 650 700 750 nm Shorter wavelength Longer wavelength Higher energy Lower energy

Pigments: molecules that absorb wavelengths in the visible spectrum. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Pigments: molecules that absorb wavelengths in the visible spectrum. Chlorophyll absorbs blue and red light; the remaining light is mostly green. Absorption spectrum—plot of light energy absorbed against wavelength. Action spectrum—plot of the biological activity of an organism against the wavelengths to which it is exposed

Figure 6.17 Absorption and Action Spectra

Light Reflected light Chloroplast Absorbed Granum light Transmitted Fig. 10-7 Light Reflected light Chloroplast Figure 10.7 Why leaves are green: interaction of light with chloroplasts Absorbed light Granum Transmitted light

Ex. carotenoids (red, yellow, orange) Ex. anthocyanin (purple) Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy In plants, two chlorophylls absorb light energy chlorophyll a and chlorophyll b. Accessory pigments—absorb wavelengths between red and blue and transfer some of that energy to the chlorophylls. Ex. carotenoids (red, yellow, orange) Ex. anthocyanin (purple) VIDEO 6.1 A green alga, Pediastrum simplex

Figure 6.18 The Molecular Structure of Chlorophyll (Part 1)

Leaves as the Site of Photosynthesis

Chloroplasts: The Sites of Photosynthesis in Plants Leaves are the major locations of photosynthesis Their green color is from chlorophyll, the green pigment within chloroplasts Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast CO2 enters and O2 exits the leaf through microscopic pores called stomata Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

A typical mesophyll cell has 30–40 chloroplasts Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf A typical mesophyll cell has 30–40 chloroplasts The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana Chloroplasts also contain stroma, a dense fluid Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 10-3a Leaf cross section Vein Mesophyll Stomata CO2 O2 Chloroplast Mesophyll cell Figure 10.3 Zooming in on the location of photosynthesis in a plant 5 µm

The Light Reactions

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 light-harvesting complexes The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center A photosystem spans the thylakoid membrane in the chloroplast; it consists of antenna systems and a reaction center. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

When chlorophyll (Chl) absorbs light, it enters an excited state (Chl When chlorophyll (Chl) absorbs light, it enters an excited state (Chl*), then rapidly returns to ground state, releasing an excited electron. A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the Light Reactions Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(INTERIOR OF THYLAKOID) Fig. 10-12 Photosystem STROMA Photon Primary electron acceptor Light-harvesting complexes Reaction-center complex e– Thylakoid membrane Figure 10.12 How a photosystem harvests light Pigment molecules Transfer of energy Special pair of chlorophyll a molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)

The acceptor molecule is reduced. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy When chlorophyll (Chl) absorbs light, it enters an excited state (Chl*), then rapidly returns to ground state, releasing an excited electron. Chl* gives the excited electron to an acceptor and becomes oxidized to Chl+. The acceptor molecule is reduced.

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Photosystem I (PS I) is best at absorbing a wavelength of 700 nm The reaction-center chlorophyll a of PS I is called P700 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Final electron acceptor is NADP+, which gets reduced: Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy The electron acceptor is first in an electron transport system in the thylakoid membrane. Final electron acceptor is NADP+, which gets reduced: ATP is produced chemiosmotically during electron transport (photophosphorylation). ANIMATED TUTORIAL 6.3 Photophosphorylation

Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Two photosystems: Photosystem I absorbs light energy at 700 nm, passes an excited electron to NADP+, reducing it to NADPH. • Photosystem II absorbs light energy at 680 nm, produces ATP, and oxidizes water molecules.

Figure 6.19 Noncyclic Electron Transport Uses Two Photosystems

Linear Electron Flow During the light reactions, there are two possible routes for electron flow: cyclic and linear Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Electron transport chain Fig. 10-13-5 Linear Flow Electron transport chain Primary acceptor Primary acceptor 4 7 Electron transport chain Fd Pq e– 2 e– 8 H2O e– 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)

Cyclic Electron Flow Cyclic electron flow uses only photosystem I and produces ATP; an electron is passed from an excited chlorophyll and recycles back to the same chlorophyll Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle . Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Cyclic Electron Flow Primary acceptor Primary Fd acceptor Fd NADP+ Pq Fig. 10-15 Cyclic Electron Flow 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

Figure 6.20 Cyclic Electron Transport Traps Light Energy as ATP

Some organisms such as purple sulfur bacteria have PS I but not PS II Cyclic electron flow is thought to have evolved before linear electron flow Cyclic electron flow may protect cells from light- induced damage Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

H+ Diffusion Electron transport chain ADP + P Fig. 10-16 Mitochondrion Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE H+ Diffusion Intermembrane space Thylakoid space Electron transport chain Inner membrane Thylakoid membrane Figure 10.16 Comparison of chemiosmosis in mitochondria and chloroplasts ATP synthase Matrix Stroma Key ADP + P i ATP Higher [H+] H+ Lower [H+]

Fig. 10-17 STROMA (low H+ concentration) Cytochrome complex Photosystem II Photosystem I 4 H+ Light NADP+ reductase Light Fd 3 NADP+ + H+ Pq NADPH e– Pc e– 2 H2O 1 1/2 O2 THYLAKOID SPACE (high H+ concentration) +2 H+ 4 H+ To Calvin Cycle Figure 10.17 The light reactions and chemiosmosis: the organization of the thylakoid membrane Thylakoid membrane ATP synthase STROMA (low H+ concentration) ADP + ATP P i H+