Chapter 8: Photosynthesis: Capturing Energy

Photosynthesis: – absorb and convert light energy into stored chemical energy of organic molecules

Electromagnetic Spectrum Wavelength – all radiation travels in waves Visible spectrum – – 760 nm (red) - 380nm (purple)

UV Fig Visible light Infrared Micro- waves Radio waves X-rays Gamma rays 10 3 m 1 m (10 9 nm) 10 6 nm 10 3 nm 1 nm 10 –3 nm 10 –5 nm nm Longer wavelength Lower energyHigher energy Shorter wavelength

Light Behaves as waves and particles Photons = particles/packets of energy E= hc/λ

2 ways excited electrons can behave (when absorb photon of light) 1 st shifts to higher-energy orbital, THEN 1) atom can return to ground state (e- are in normal, lowest energy levels) – Energy lost as heat or light (fluorescence) 2) e- can leave atom and be accepted by e- acceptor molecule – photosynthesis

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

PhotosynthesisPhotosynthesis in Chloroplasts Chlorophyll - green pigment, in chloroplasts, mesophyll Chloroplast – – Outer membrane – Inner membrane – encloses stroma Stroma (fluid-filled, enzymes to make carbs.)

Thylakoids – – in stroma, 3 rd sys. Of membranes – forms interconnected flat, disclike sacs Thylakoid lumen – – fluid-filled space inside of thylakoid Grana = thylakoid stacks

Fig Leaf cross section Vein Mesophyll Stomata CO 2 O2O2 Chloroplast Mesophyll cell Outer membrane Intermembrane space 5 µm Inner membrane Thylakoid space Thylakoid Granum Stroma 1 µm

Fig. 10-3a 5 µm Mesophyll cell Stomata CO 2 O2O2 Chloroplast Mesophyll Vein Leaf cross section

Fig. 10-3b 1 µm Thylakoid space Chloroplast Granum Intermembrane space Inner membrane Outer membrane Stroma Thylakoid

Chlorophyll Thylakoid membrane Main pigment of photosynthesis Absorbs mostly blue/red wavelengths Green – green light is scattered/reflected

Fig Reflected light Absorbed light Light Chloroplast Transmitted light Granum

2 main parts of Chlorophyll 1) complex ring = porphyrin ring – Joined smaller rings of C and N – Absorbs light energy – Magnesium in center 2) long side chain – Hydrocarbons – Extremely nonpolar

Fig Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center in chlorophyll a CH 3 Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown CHO in chlorophyll b

Types of Chlorophylls Chlorophyll a – Most important – Bright green – Initiate light-dependent reactions Chlorophyll b – Accessory pigment – Yellow-green – Different functional group on porphyrin ring – shifts λ of light that is absorbed/reflected Carotenoids – Accessory – yellow, orange

Spectrums Absorption spectrum – plot of a PIGMENT’S absorption of light of different λ Action spectrum – gives relative effectiveness of different λs of light in photosynthesis (PROCESS) – Rate of photosynthesis is measured at each λ for leaf cells/tissues exposed to monochromatic light

Photosynthesis simplified: 6 CO H 2 O + Light energy  C 6 H 12 O O H 2 O Redox – e- transferred from e- donor (reducing agent) to an e- acceptor (oxidizing agent) Many complex steps 2 parts: – Light-dependent (photo) – thylakoids – Carbon fixation (synthesis) - stroma

Reactants: Fig CO 2 Products: 12 H 2 O 6 O 2 6 H 2 O C 6 H 12 O 6

Redox

Light Fig H2OH2O Chloroplast Light Reactions NADP + P ADP i +

Light Fig H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2

Light Fig H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2 Calvin Cycle CO 2

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

Overview of light-dependent reactions Chlorophyll captures light energy 1 e- moves to higher state e- transferred to acceptor molecule, replaced by e- from water Water is split Oxygen released Need some energy for – ADP  ATP – NADP+  NADPH

Overview of Carbon fixation Fix C atoms from CO2 to existing C skeletons No direct light needed – “dark” reactions Depends on products of light-reactions

Light-Dependent Reactions - Details Light energy  chemical energy Summary equation:

Photosystems I and II Reaction center + many antenna complexes Antenna complex (light-harvesting) = – units of chlorophylls a + b and accessory pigments organized with pigment-binding proteins in thylakoid membranes – Absorbs light energy and transfers it to reaction center Reaction center = – complex of chlorophyll molecules + proteins – Light energy  chemical energy by series of e- transfers

Photosystem I – chlorophyll a – 700 nm (P700) Photosystem II – chlorophyll a 680 nm (P680) Pigment absorbs light energy Energy passed from 1 pigment molecule to another until it reaches P700 or P680 at reaction center e- raised to higher energy level e- donated to e- acceptor

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

Noncyclic electron transport Makes ATP and NADPH Continuous linear process – 1 way flow of e- from water to NADP+ – Water  photolysis  e- to P680  ETC (e- lose energy)  P700  ETC  NADP+ See diagram

A photon hits a pigment and its energy is passed among pigment molecules until it excites P680 An excited electron from P680 is transferred to the primary electron acceptor Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Pigment molecules Light P680 e–e– 2 1 Fig Photosystem II (PS II) Primary acceptor

P680 + (P680 that is missing an electron) is a very strong oxidizing agent H 2 O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680 +, thus reducing it to P680 O 2 is released as a by-product of this reaction Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Photolysis “light-splitting” Catalyzed by manganese-containing enzyme; breaks water into 2e-, 2p + and O Each e- donated to P680 p + released into thylakoid lumen 2 water must split to yield 1 O 2  atmosphere 2H 2 O  O 2 + 4H +

Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 Fig 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Fig Photosystem II (PS II)

In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor P700 + (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Fig Photosystem II (PS II)

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

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 Fig Photosystem II (PS II)

Fig Mill makes ATP e–e– NADPH Photon e–e– e–e– e–e– e–e– e–e– ATP Photosystem IIPhotosystem I e–e–

Cyclic Electron Transport (simplest light- dependent reaction) Makes ATP, no NADPH Only Photosystem I Cyclic – energized e- that originate from P700 eventually return to P700 Light – continuous flow of e- through ETC in thylakoid membrane

e- passed from 1 acceptor to another, e- lose energy (some energy used to pump protons across thylakoid membranes) ATP synthase uses proton gradient to make ATP NADPH not made, water not split, O2 not made

Fig ATP Photosystem II Photosystem I Primary acceptor Pq Cytochrome complex Fd Pc Primary acceptor Fd NADP + reductase NADPH NADP + + H +

ATP synthesis By chemiosmosis Photosystem II – as e- passed down ETC, some energy releases (exergonic) Some energy not released  drives synthesis of ATP (endergonic) Synthesis of ATP (P +ADP) is coupled with e- energized by light (photo), process = photophosphorylation

Fig Light Fd Cytochrome complex ADP + i H+H+ ATP P synthase To Calvin Cycle STROMA (low H + concentration) Thylakoid membrane THYLAKOID SPACE (high H + concentration) STROMA (low H + concentration) Photosystem II Photosystem I 4 H + Pq Pc Light NADP + reductase NADP + + H + NADPH +2 H + H2OH2O O2O2 e–e– e–e– 1/21/

Light Reactions

Carbon Fixation Requires ATP + NADPH – Energy used to form organic molecules from CO2 Summary equation:

Calvin Cycle Most plants use – C 3 In stroma – 13 reactions 3 phases: – CO 2 uptake – Carbon reduction – RuBP regeneration

CO 2 Uptake One reaction CO 2 + ribulose biphosphate (RuBP) [5-C] Enzyme = ribulose biphosphate carboxylase/oxygenase (Rubisco) Product = unstable 6-C intermediate Immediately  2 phosphoglycerate (PGA) (3-C each)  C 3 pathway

Carbon Reduction phase 2 steps Energy from ATP and NADPH converts PGA molecules to glyceraldehyde-3-phosphate (G3P) For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO 2 6C enter as CO 2, 6C leave as 2 – G3P (can form glucose or fructose) 2 – G3P removed from cycle, 10 G3P remain = 30 C atoms total

RuBP regeneration phase 10 reactions 30 C rearranged into 6 ribulose phosphate (+P)  RuBP (5-C where cycle started)

Fig 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

Fig 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

Fig 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

Summary of Carbon Fixation Inputs: – 6 CO 2 – P from ATP – e- (as hydrogen) from NADPH End – 6C hexose molecule remaining G3P  make RuBP which combines with more CO 2

Calvin Cycle

Photosynthesis Summary Video

C 4 and CAM plants Initial carbon fixation step differs – precedes Calvin Cycle; does not replace it

C 4 Pathway Fixes CO 2 at low concentration 1 st - fix CO 2 into 4C oxaloacetate – in mesophyll cells (Calvin in bundle sheath cells) PEP carboxylase – catalyzes reaction – CO 2 + phosphoenolpyruvate (PEP) (3C)  oxaloacetate

Oxaloacetate +NADPH  usually malate (into bundle sheath)  decarboxylation  pyruvate (3C) + CO 2 Malate + NADP+  Pyruvate + CO 2 + NADPH CO 2 combines with RuBP  Calvin Cycle

C3-C4 pathway – extra energy for pyruvate  PEP ( 30 ATPs per hexose) – Increases CO2 conc. – stomate don’t need to be open as much  promotes rapid growth C3 alone (18 ATPs per hexose)

Fig C 4 leaf anatomy Mesophyll cell Photosynthetic cells of C 4 plant leaf Bundle- sheath cell Vein (vascular tissue) Stoma The C 4 pathway Mesophyll cell CO 2 PEP carboxylase Oxaloacetate (4C) Malate (4C) PEP (3C) ADP ATP Pyruvate (3C) CO 2 Bundle- sheath cell Calvin Cycle Sugar Vascular tissue

Fig a Stoma C 4 leaf anatomy Photosynthetic cells of C 4 plant leaf Vein (vascular tissue) Bundle- sheath cell Mesophyll cell

Fig b Sugar CO 2 Bundle- sheath cell ATP ADP Oxaloacetate (4C) PEP (3C) PEP carboxylase Malate (4C) Mesophyll cell CO 2 Calvin Cycle Pyruvate (3C) Vascular tissue The C 4 pathway

CAM plants Fix CO2 at night Xeric plants Crassulacean acid metabolism (CAM) NIGHT = Use PEP carboxylase to fix CO2  oxaloacetate  malate  stored in vacuoles DAY = CO2 removed from malate and ready for Calvin cycle

C3 and C4 – different location C3 and CAM – different times, same cell

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 2 in our atmosphere Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

You should now be able to: 1.Describe the structure of a chloroplast 2.Describe the relationship between an action spectrum and an absorption spectrum 3.Trace the movement of electrons in linear electron flow 4.Trace the movement of electrons in cyclic electron flow 5.Describe the role of ATP and NADPH in the Calvin cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings