Chapter 10 Photosynthesis

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 from water and carbon dioxide

Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes These organisms feed not only themselves but also the entire living world

LE 10-2 Plants Unicellular protist Purple sulfur bacteria 10 µm Purple sulfur bacteria 1.5 µm Multicellular algae Cyanobacteria 40 µm

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 oxygen

Concept 10.1: Photosynthesis converts light energy to the chemical energy of food Chloroplasts are organelles that are responsible for feeding the vast majority of organisms Chloroplasts are present in a variety of photosynthesizing organisms

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 Through microscopic pores called stomata, CO2 enters the leaf and O2 exits

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

LE 10-3 Leaf cross section Vein Mesophyll Stomata CO2 O2 Mesophyll cell Chloroplast 5 µm Outer membrane Thylakoid Stroma Granum Thylakoid space Intermembrane space Inner membrane 1 µm

Tracking Atoms Through Photosynthesis: Scientific Inquiry Photosynthesis can be summarized as the following equation: 6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O

The Splitting of Water Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

LE 10-4 Reactants: 6 CO2 12 H2O Products: C6H12O6 6 H2O 6 O2

Photosynthesis as a Redox Process Photosynthesis is a redox process in which water is oxidized and carbon dioxide is reduced

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 water, release O2, produce ATP, and form NADPH 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

LE 10-5_1 H2O Light LIGHT REACTIONS Chloroplast

LE 10-5_2 H2O Light LIGHT REACTIONS ATP NADPH Chloroplast O2

LE 10-5_3 H2O CO2 Light NADP+ ADP + CALVIN CYCLE LIGHT REACTIONS ATP NADPH Chloroplast [CH2O] (sugar) O2

Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH Chloroplasts are solar-powered chemical factories Their thylakoids transform light energy into the chemical energy of ATP and NADPH

The Nature of Sunlight Light is a form of electromagnetic energy, also called electromagnetic radiation Like other electromagnetic energy, light travels in rhythmic waves Wavelength = 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 colors we can see, including wavelengths that drive photosynthesis

1 m (109 nm) 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 103 m Gamma rays LE 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 380 450 500 550 600 650 700 750 nm Shorter wavelength Longer wavelength Higher energy 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 Animation: Light and Pigments

LE 10-7 Light Reflected light Chloroplast Absorbed Granum light Transmitted light

A spectrophotometer measures a pigment’s ability to absorb various wavelengths This machine sends light through pigments and measures the fraction of light transmitted at each wavelength

Slit moves to pass light of selected wavelength Green light LE 10-8a White light Refracting prism Chlorophyll solution Photoelectric tube Galvanometer 100 The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. Slit moves to pass light of selected wavelength Green light

Slit moves to pass light of selected wavelength LE 10-8b White light Refracting prism Chlorophyll solution Photoelectric tube 100 Slit moves to pass light of selected wavelength The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light. Blue light

An absorption spectrum is a graph plotting a pigment’s 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

Absorption of light by chloroplast pigments LE 10-9a Chlorophyll a Chlorophyll b Carotenoids Absorption of light by chloroplast pigments 400 500 600 700 Wavelength of light (nm) Absorption spectra

synthesis (measured Rate of photo- by O2 release) LE 10-9b synthesis (measured Rate of photo- by O2 release) Action spectrum

The action spectrum of photosynthesis was first demonstrated in 1883 by Thomas Engelmann In his experiment, he exposed different segments of a filamentous alga to different wavelengths Areas receiving wavelengths favorable to photosynthesis produced excess O2 He used aerobic bacteria clustered along the alga as a measure of O2 production

Engelmann’s experiment LE 10-9c Aerobic bacteria Filament of algae 400 500 600 700 Engelmann’s experiment

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

LE 10-10 Porphyrin ring: light-absorbing CH3 in chlorophyll a CHO in chlorophyll b Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown

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

Excitation of isolated chlorophyll molecule Fluorescence Excited state e– Heat Energy of electron Photon (fluorescence) Photon Ground state Chlorophyll molecule Excitation of isolated chlorophyll molecule Fluorescence

A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes A photosystem consists of a reaction center surrounded by light-harvesting complexes The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center

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

(INTERIOR OF THYLAKOID) LE 10-12 Thylakoid Photosystem STROMA Photon Light-harvesting complexes Reaction center Primary electron acceptor Thylakoid membrane e– Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)

There are two types of photosystems in the thylakoid membrane Photosystem II functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm Photosystem I is best at absorbing a wavelength of 700 nm The two photosystems work together to use light energy to generate ATP and NADPH

Noncyclic Electron Flow During the light reactions, there are two possible routes for electron flow: cyclic and noncyclic Noncyclic electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH

LE 10-13_1 Primary acceptor e– Energy of electrons Light P680 H2O CO2 Light NADP+ ADP LIGHT REACTIONS CALVIN CYCLE ATP NADPH O2 [CH2O] (sugar) Primary acceptor e– Energy of electrons Light P680 Photosystem II (PS II)

LE 10-13_2 Primary acceptor H2O e– 2 H+ + O2 1/2 Energy of electrons CO2 Light NADP+ ADP LIGHT REACTIONS CALVIN CYCLE ATP NADPH O2 [CH2O] (sugar) Primary acceptor H2O e– 2 H+ + 1/2 O2 e– e– Energy of electrons Light P680 Photosystem II (PS II)

LE 10-13_3 Primary Electron transport chain acceptor Pq H2O e– CO2 Light NADP+ ADP LIGHT REACTIONS CALVIN CYCLE ATP NADPH O2 [CH2O] (sugar) Primary acceptor Electron transport chain Pq H2O e– Cytochrome complex 2 H+ + 1/2 O2 e– Pc e– Energy of electrons Light P680 ATP Photosystem II (PS II)

LE 10-13_4 Primary acceptor Primary Electron transport chain acceptor H2O CO2 Light NADP+ ADP LIGHT REACTIONS CALVIN CYCLE ATP NADPH O2 [CH2O] (sugar) Primary acceptor Primary acceptor Electron transport chain Pq e– H2O e– Cytochrome complex 2 H+ + 1/2 O2 e– Pc e– P700 Energy of electrons Light P680 Light ATP Photosystem I (PS I) Photosystem II (PS II)

LE 10-13_5 ADP Electron Transport chain Primary acceptor Primary H2O CO2 Light NADP+ ADP LIGHT REACTIONS CALVIN CYCLE ATP NADPH O2 Electron Transport chain [CH2O] (sugar) Primary acceptor Primary acceptor Electron transport chain Fd Pq e– e– H2O e– e– NADP+ Cytochrome complex 2 H+ NADP+ reductase + 2 H+ + 1/2 O2 NADPH Pc e– + H+ Energy of electrons e– P700 Light P680 Light ATP Photosystem I (PS I) Photosystem II (PS II)

NADPH Mill makes ATP Photosystem II Photosystem I LE 10-14 e– ATP e– Photon e– Photon Photosystem II Photosystem I

Cyclic Electron Flow Cyclic electron flow uses only photosystem I and produces only ATP Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle

Primary acceptor Primary Fd acceptor Fd NADP+ Pq NADP+ reductase LE 10-15 Primary acceptor Primary acceptor Fd Fd NADP+ Pq NADP+ reductase Cytochrome complex NADPH Pc Photosystem I ATP Photosystem II

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 The spatial organization of chemiosmosis differs in chloroplasts and mitochondria

LE 10-16 Mitochondrion Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST Diffusion Intermembrane space Thylakoid space Electron transport chain Membrane Key ATP synthase Matrix Stroma Higher [H+] Lower [H+] ADP + P i ATP H+

Animation: Calvin Cycle The current model for the thylakoid membrane is based on studies in several laboratories Water is split by photosystem II on the side of the membrane facing the thylakoid space The diffusion of H+ from the thylakoid space back to the stroma powers ATP synthase ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place Animation: Calvin Cycle

LE 10-17 STROMA (Low H+ concentration) Cytochrome complex H2O CO2 Light NADP+ ADP LIGHT REACTIONS CALVIN CYCLE ATP NADPH STROMA (Low H+ concentration) O2 [CH2O] (sugar) Cytochrome complex Photosystem II Photosystem I Light NADP+ reductase Light 2 H+ Fd NADP+ + 2H+ NADPH + H+ Pq Pc H2O THYLAKOID SPACE (High H+ concentration) 1/2 O2 +2 H+ 2 H+ To Calvin cycle Thylakoid membrane ATP synthase STROMA (Low H+ concentration) ADP + ATP P i H+

Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO2 to sugar The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P) For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2

The Calvin cycle has three phases: Carbon fixation (catalyzed by rubisco) Reduction Regeneration of the CO2 acceptor (RuBP) Play

Phase 1: Carbon fixation Ribulose bisphosphate LE 10-18_1 H2O CO2 Input Light 3 (Entering one at a time) NADP+ ADP CO2 LIGHT REACTIONS CALVIN CYCLE ATP Phase 1: Carbon fixation NADPH Rubisco O2 [CH2O] (sugar) 3 P P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE

LE 10-18_2 Input 3 (Entering one at a time) CO2 H2O CO2 Input Light 3 (Entering one at a time) NADP+ ADP CO2 LIGHT REACTIONS CALVIN CYCLE ATP Phase 1: Carbon fixation NADPH Rubisco O2 [CH2O] (sugar) 3 P P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 6 P P 1,3-Bisphosphoglycerate 6 NADPH 6 NADP+ 6 P i 6 P Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction 1 P G3P (a sugar) Glucose and other organic compounds Output

LE 10-18_3 Input 3 (Entering one at a time) CO2 H2O CO2 Input Light 3 (Entering one at a time) NADP+ ADP CO2 LIGHT REACTIONS CALVIN CYCLE ATP Phase 1: Carbon fixation NADPH Rubisco O2 [CH2O] (sugar) 3 P P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 ATP 6 ADP 3 ADP CALVIN CYCLE 3 6 P P ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADP+ 6 P i 5 P G3P 6 P Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction 1 P G3P (a sugar) Glucose and other organic compounds Output

Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates Dehydration is a problem for plants, sometimes requiring tradeoffs with other metabolic processes, especially photosynthesis On hot, dry days, plants close stomata, which conserves water but also limits photosynthesis The closing of stomata reduces access to CO2 and causes O2 to build up These conditions favor a seemingly wasteful process called photorespiration

Photorespiration: An Evolutionary Relic? In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound In photorespiration, rubisco adds O2 to the Calvin cycle instead of CO2 Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar

Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2 In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle

C4 Plants C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

LE 10-19 Mesophyll cell Mesophyll cell CO2 Photosynthetic cells of C4 plant leaf PEP carboxylase Bundle- sheath cell The C4 pathway Oxaloacetate (4 C) PEP (3 C) Vein (vascular tissue) ADP Malate (4 C) ATP C4 leaf anatomy Pyruvate (3 C) Bundle- sheath cell Stoma CO2 CALVIN CYCLE Sugar Vascular tissue

CAM Plants CAM plants open their stomata at night, incorporating CO2 into organic acids Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle

LE 10-20 Sugarcane Pineapple C4 CAM CO2 CO2 Mesophyll cell CO2 incorporated into four-carbon organic acids (carbon fixation) Night Organic acid Organic acid Bundle- sheath cell CO2 CO2 Day Organic acids release CO2 to Calvin cycle CALVIN CYCLE CALVIN CYCLE Sugar Sugar Spatial separation of steps Temporal separation of steps

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 In addition to food production, photosynthesis produces the oxygen in our atmosphere

Photosystem II Electron transport chain Photosystem I Light reactions Calvin cycle H2O CO2 Light NADP+ ADP + P i RuBP 3-Phosphoglycerate Photosystem II Electron transport chain Photosystem I ATP G3P Starch (storage) NADPH Amino acids Fatty acids Chloroplast O2 Sucrose (export)