Photosynthesis Overview The Process That Feeds the Biosphere

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

Figure 10. 1

• Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes • These organisms feed not only themselves but also most of the living world Bio. Flix: Photosynthesis

Figure 10. 2 (b) Multicellular alga (a) Plants (c) Unicellular protists (d) Cyanobacteria 40 m 10 m (e) Purple sulfur 1 m bacteria

Figure 10. 2 a (a) Plants

Figure 10. 2 b (b) Multicellular alga

Figure 10. 2 c (c) Unicellular protists 10 m

Figure 10. 2 d (d) Cyanobacteria 40 m

Figure 10. 2 e (e) Purple sulfur bacteria 1 m

• 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

• The Earth’s supply of fossil fuels was formed from the remains of organisms that died hundreds of millions of years ago • In a sense, fossil fuels represent stores of solar energy from the distant past

Figure 10. 3

Concept 1: Photosynthesis converts light energy to the chemical energy of food • Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria • The structural organization of these cells allows for the chemical reactions 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 • Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf • Each mesophyll cell contains 30– 40 chloroplasts

• CO 2 enters and O 2 exits the leaf through microscopic pores called stomata • 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 interior fluid

Figure 10. 4 Leaf cross section Chloroplasts Vein Mesophyll Stomata CO O 2 2 Chloroplast Thylakoid Stroma Granum Thylakoid space 1 m Mesophyll cell Outer membrane Intermembrane space Inner membrane 20 m

Figure 10. 4 a Leaf cross section Chloroplasts Vein Mesophyll Stomata CO 2 Chloroplast O 2 Mesophyll cell 20 m

Figure 10. 4 b Chloroplast Thylakoid Stroma Granum Thylakoid space 1 m Outer membrane Intermembrane space Inner membrane

Figure 10. 4 c Mesophyll cell 20 m

Figure 10. 4 d Stroma Granum 1 m

Tracking Atoms Through Photosynthesis: Scientific Inquiry • Photosynthesis is a complex series of reactions that can be summarized as the following equation: 6 CO 2 + 12 H 2 O + Light energy C 6 H 12 O 6 + 6 O 2 + 6 H 2 O

The Splitting of Water • Chloroplasts split H 2 O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product

Figure 10. 5 Reactants: Products: 6 CO 2 C 6 H 12 O 6 12 H 2 O 6 O 2

Photosynthesis as a Redox Process • Photosynthesis reverses the direction of electron flow compared to respiration • Photosynthesis is a redox process in which H 2 O is oxidized and CO 2 is reduced • Photosynthesis is an endergonic process; the energy boost is provided by light

Figure 10. UN 01 becomes reduced Energy 6 CO 2 6 H 2 O C 6 H 12 O 6 6 O 2 becomes oxidized

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 H 2 O Release O 2 Reduce NADP+ to NADPH Generate ATP from ADP by photophosphorylation

• The Calvin cycle (in the stroma) forms sugar from CO 2, using ATP and NADPH • The Calvin cycle begins with carbon fixation, incorporating CO 2 into organic molecules

Figure 10. 6 -1 H 2 O Light NADP +Pi Light Reactions Chloroplast

Figure 10. 6 -2 H 2 O Light NADP +Pi Light Reactions ATP NADPH Chloroplast O 2

Figure 10. 6 -3 CO 2 H 2 O Light NADP +Pi Light Reactions ATP NADPH Chloroplast O 2 Calvin Cycle

Figure 10. 6 -4 CO 2 H 2 O Light NADP +Pi Light Reactions Calvin Cycle ATP NADPH Chloroplast O 2 [CH 2 O] (sugar)

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 is the distance between crests of waves • Wavelength determines the type of electromagnetic energy

• 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 • Light also behaves as though it consists of discrete particles, called photons

10 5 nm 10 3 nm 103 1 nm Gamma X-rays UV nm 106 Infrared nm 1 m (109 nm) Microwaves 103 m Radio waves Visible light 380 450 500 Shorter wavelength Higher energy 550 600 700 650 750 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

Figure 10. 8 Light Reflected light Chloroplast Absorbed light Granum 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

Figure 10. 9 TECHNIQUE Refracting Chlorophyll Photoelectric solution tube White prism Galvanometer light Slit moves to pass light of selected wavelength. Green light Blue light High transmittance (low absorption): Chlorophyll absorbs very little green light. Low transmittance (high absorption): Chlorophyll absorbs most 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

(a) Absorption spectra (b) Action spectrum (c) Engelmann’s experiment Absorption of light by chloroplast pigments RESULTS Rate of photosynthesis (measured by O 2 release) Figure 10. 10 Chlorophyll a Chlorophyll b Carotenoids 400 500 600 Wavelength of light (nm) 400 500 600 700 Aerobic bacteria Filament of alga 400 500 600 700

• The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. Engelmann • In his experiment, he exposed different segments of a filamentous alga to different wavelengths • Areas receiving wavelengths favorable to photosynthesis produced excess O 2 • He used the growth of aerobic bacteria clustered along the alga as a measure of O 2 production

• 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

Figure 10. 11 CH 3 in chlorophyll a CHO in chlorophyll b Porphyrin ring Hydrocarbon tail (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

Energy of electron Figure 10. 12 e Excited state Heat Photon Chlorophyll molecule Photon (fluorescence) Ground state (a) Excitation of isolated chlorophyll molecule (b) Fluorescence

Figure 10. 12 a (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 light-harvesting complexes • The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center

Figure 10. 13 Photosystem Light. Reactionharvesting center complexes complex e STROMA Primary electron acceptor Thylakoid membrane Photon Chlorophyll Pigment Special pair of molecules chlorophyll a Protein molecules THYLAKOID SPACE subunits (INTERIOR OF THYLAKOID) (b) Structure of photosystem II (a) How a photosystem harvests light Transfer of energy STROMA THYLAKOID SPACE

Figure 10. 13 a Thylakoid membrane Photon Photosystem Light. Reactionharvesting center complexes complex STROMA Primary electron acceptor e Transfer of energy Special pair of chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) (a) How a photosystem harvests light

Thylakoid membran Figure 10. 13 b Chlorophyll Protein subunits (b) Structure of photosystem II STROMA THYLAKOID SPACE

• 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

• 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 P 680

• Photosystem I (PS I) is best at absorbing a wavelength of 700 nm • The reaction-center chlorophyll a of PS I is called P 700

Daily Questions 12. 15. 14 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. What is Photosynthesis? What is a photosystem? Explain what happens to chlorophyll when light excites it. What is the pigment responsible for Photosynthesis? What are the 2 parts of Photosynthesis? How is Photosynthesis a Redox Reaction? What is the visible spectrum? What are 2 accessory pigments? What is the difference between PS I and PS II? Why do plants appear green?

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

• A photon hits a pigment and its energy is passed among pigment molecules until it excites P 680 • An excited electron from P 680 is transferred to the primary electron acceptor (we now call it P 680+)

Figure 10. 14 -1 Primary acceptor e 2 P 680 1 Light Pigment molecules Photosystem II (PS II)

• P 680+ is a very strong oxidizing agent • H 2 O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P 680+, thus reducing it to P 680 • O 2 is released as a by-product of this reaction

Figure 10. 14 -2 Primary acceptor 2 H + 1/ O 2 2 H 2 O e 2 3 e e P 680 1 Light Pigment molecules Photosystem II (PS II)

• Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I • Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane • Diffusion of H+ (protons) across the membrane drives ATP synthesis

Figure 10. 14 -3 Primary acceptor 2 1/ 2 H + O 2 H 2 O e 2 3 Ele ct ron Pq 4 tran spo rt c Cytochrome complex hai n Pc e e 5 P 680 1 Light ATP Pigment molecules Photosystem II (PS II)

• In PS I (like PS II), transferred light energy excites P 700, which loses an electron to an electron acceptor • P 700+ (P 700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain

Figure 10. 14 -4 Primary acceptor 2 H + 1/ O 2 2 H 2 O e 2 3 Ele ct ron Pq Primary acceptor 4 tran spo rt c Cytochrome complex hai n e Pc e e 5 P 680 P 700 Light 1 Light 6 ATP Pigment molecules Photosystem II (PS II) Photosystem I (PS I)

• Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd) • The electrons are then transferred to NADP+ and reduce it to NADPH • The electrons of NADPH are available for the reactions of the Calvin cycle • This process also removes an H+ from the stroma

Figure 10. 14 -5 Primary acceptor 2 1/ 2 H + O 2 H 2 O e 2 3 Ele ct ron Pq Primary acceptor 4 tran spo rt c Cytochrome complex hai n e E tra lect ch ns ron ai po n rt 7 Fd e e NADP reductase Pc e e 5 P 680 P 700 Light 1 Light 6 ATP Pigment molecules Photosystem II (PS II) 8 Photosystem I (PS I) NADP + H NADPH

Figure 10. 15 e e e Mill makes ATP NADPH e Photo n n Photo e e e ATP Photosystem II Photosystem I

Cyclic Electron Flow • Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH • No oxygen is released • Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle

Figure 10. 16 Primary acceptor Fd Pq NADP reductase Cytochrome complex Pc Photosystem II ATP Fd Photosystem I NADP + H NADPH

• 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

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

Figure 10. 17 MITOCHONDRION STRUCTURE Intermembrane space Inner membrane Matrix Key Chloroplast Mitochondrion [H ] Higher Lower [H ] H Electron transport chain ATP synthase ADP P i CHLOROPLAST STRUCTURE Diffusion Thylakoid space Thylakoid membrane Stroma 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 H 2 O to NADPH

Figure 10. 18 STROMA (low H concentration) Photosystem II 4 H+ Light Cytochrome complex Photosystem I Light NADP reductase 3 Fd Pq H 2 O 1 THYLAKOID SPACE (high H concentration) O 2 +2 H+ Pc 4 H+ To Calvin Cycle Thylakoid membrane STROMA (low H concentration) NADPH 2 1/ 2 NADP + H ATP synthase ADP + Pi ATP H+

Concept 3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO 2 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 CO 2 and leaves as a sugar named glyceraldehyde 3 -phospate (G 3 P) • For net synthesis of 1 G 3 P, the cycle must take place three times, fixing 3 molecules of CO 2 • The Calvin cycle has three phases – Carbon fixation (catalyzed by rubisco) – Reduction – Regeneration of the CO 2 acceptor (Ru. BP)

Figure 10. 19 -1 Input (Entering one 3 CO 2 at a time) Phase 1: Carbon fixation Rubisco 3 P P Ribulose bisphosphate (Ru. BP) 3 P Short-lived intermediate P 6 P 3 -Phosphoglycerate

Figure 10. 19 -2 Input (Entering one 3 CO 2 at a time) Phase 1: Carbon fixation Rubisco 3 P Short-lived intermediate 3 P P Ribulose bisphosphate (Ru. BP) Calvin Cycle P 6 P 3 -Phosphoglycerate 6 ATP 6 ADP 6 P P 1, 3 -Bisphoglycerate 6 NADPH 6 NADP 6 Pi 6 P Glyceraldehyde 3 -phosphate (G 3 P) 1 G 3 P (a sugar) Output P Glucose and other organic compounds Phase 2: Reduction

Figure 10. 19 -3 Input (Entering one 3 CO 2 at a time) Phase 1: Carbon fixation Rubisco 3 P Short-lived intermediate 3 P P Ribulose bisphosphate (Ru. BP) 3 ADP 3 Calvin Cycle ATP Phase 3: Regeneration of the CO 2 acceptor (Ru. BP) 5 G 3 P P 6 P 3 -Phosphoglycerate 6 ATP 6 ADP 6 P P 1, 3 -Bisphoglycerate 6 NADPH 6 NADP 6 Pi P 6 P Glyceraldehyde 3 -phosphate (G 3 P) 1 G 3 P (a sugar) Output P Glucose and other organic compounds Phase 2: Reduction

Concept 4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates • Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis • On hot, dry days, plants close stomata, which conserves H 2 O but also limits photosynthesis • The closing of stomata reduces access to CO 2 and causes O 2 to build up

Photorespiration: An Evolutionary Relic? • In most plants (C 3 plants), initial fixation of CO 2, via rubisco, forms a three-carbon compound (3 phosphoglycerate) • In photorespiration, rubisco adds O 2 instead of CO 2 in the Calvin cycle, producing a two-carbon compound • Photorespiration consumes O 2 and organic fuel and releases CO 2 without producing ATP or sugar

• Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O 2 and more CO 2 • Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle • 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

C 4 Plants • C 4 plants minimize the cost of photorespiration by incorporating CO 2 into four-carbon compounds in mesophyll cells • This step requires the enzyme PEP carboxylase • PEP carboxylase has a higher affinity for CO 2 than rubisco does; it can fix CO 2 even when CO 2 concentrations are low • These four-carbon compounds are exported to bundle-sheath cells, where they release CO that is

Figure 10. 20 The C 4 pathway C 4 leaf anatomy Mesophyll cell PEP carboxylase Mesophyll cell Photosynthetic Bundlecells of C 4 sheath plant leaf cell Vein (vascular tissue) Oxaloacetate (4 C) PEP (3 C) ADP ATP Malate (4 C) Stoma Bundlesheath cell Pyruvate (3 C) CO 2 Calvin Cycle Sugar Vascular tissue CO 2

Figure 10. 20 a C 4 leaf anatomy Photosynthetic cells of C 4 plant leaf Mesophyll cell Bundlesheath cell Vein (vascular tissue) Stoma

Figure 10. 20 b The C 4 pathway Mesophyll PEP carboxylase cell Oxaloacetate (4 C) Malate (4 C) Bundlesheath cell PEP (3 C) ADP ATP Pyruvate (3 C) CO 2 Calvin Cycle Sugar Vascular tissue CO 2

• In the last 150 years since the Industrial Revolution, CO 2 levels have risen greatly • Increasing levels of CO 2 may affect C 3 and C 4 plants differently, perhaps changing the relative abundance of these species • The effects of such changes are unpredictable and a cause for concern

CAM Plants • Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon • CAM plants open their stomata at night, incorporating CO 2 into organic acids • Stomata close during the day, and CO 2 is released from organic acids and used in the Calvin cycle

Figure 10. 21 Sugarcane Pineapple C 4 CO 2 CAM CO 2 Mesophyll Organic acid cell 1 CO 2 incorporated Organic acid (carbon fixation) CO 2 Bundlesheath cell Calvin Cycle 2 CO 2 released to the Calvin cycle Sugar (a) Spatial separation of steps Calvin Cycle Night Day Sugar (b) Temporal separation of steps

Figure 10. 21 a Sugarcane

Figure 10. 21 b Pineapple

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 structures such as roots, tubers, seeds, and fruits • In addition to food production, photosynthesis produces the O in our atmosphere

Figure 10. 22 H 2 O Light CO 2 NADP + Pi Light Reactions: Photosystem II Electron transport chain Photosystem I Electron transport chain Ru. BP ATP NADPH 3 -Phosphoglycerate Calvin Cycle G 3 P Starch (storage) Chloroplast O 2 Sucrose (export)

Figure 10. UN 02 El El ec Primary acceptor H 2 O O 2 Photosystem II tr on ch tr ai an n sp Pq o Primary acceptor NADP rt Cytochrome complex Pc ATP ec tr on ch tr ai an Fd n spo r reductase Photosystem I t NADP + H NADPH

Figure 10. UN 03 3 CO 2 Carbon fixation 3 5 C Regeneration of CO 2 acceptor 6 3 C Calvin Cycle 5 3 C Reduction 1 G 3 P (3 C)

Figure 10. UN 04 p. H 7 p. H 4 p. H 8 ATP

Figure 10. UN 05

Figure 10. UN 06

Figure 10. UN 07

Figure 10. UN 08