1. 1 Photosynthesis

2. 2 All cells can break down organic molecules and use the energy that is released to make ATP. Some cells can manufacture organic molecules from inorganic substances using energy from light (photosynthesis) or from inorganic chemicals (chemosynthesis). Photosynthesis is the ultimate source of almost all organic molecules used by living organisms. It is also the main source of O 2 in the atmosphere.

3. 3 Chloroplasts: Sites of Photosynthesis in Plants Leaves are the major locations of photosynthesis Microscopic pores called stomata allow CO 2 to enter the leaf and O 2 to exit The leaf’s green color is from chlorophyll, the green pigment within chloroplasts Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast Leaf cross section Vein Mesophyll Stomata CO 2 O2O2 Mesophyll cell Chloroplast 5 µm Outer membrane Intermembrane space Inner membrane Thylakoid space Thylakoid GranumStroma 1 µm

4. 4 Chloroplasts: Sites of Photosynthesis in Plants 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 where carbon fixation reactions occur. Leaf cross section Vein Mesophyll Stomata CO 2 O2O2 Mesophyll cell Chloroplast 5 µm Outer membrane Intermembrane space Inner membrane Thylakoid space Thylakoid GranumStroma 1 µm

5. 5 Overall Reaction Photosynthesis: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2 During photosynthesis, H is removed from H2O (leaving O2 as a waste product), energized by light, and then used to reduce CO2 to form glucose.

6. 6 The Splitting of Water Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules 6 CO 2 12 H 2 O Reactants: Products: C 6 H 12 O 6 6H2O6H2O 6O26O2 Figure 10.4

7. 7 Photosynthesis Photosynthesis occurs in 2 stages: Light dependent stage or energy capturing reactions Light independent “Dark” stage or carbon fixation reactions (also called the Calvin cycle) H2OH2O LIGHT REACTIONS Chloroplast Light ATP NADPH O2O2 NADP + CO 2 ADP P + i CALVIN CYCLE [CH 2 O] (sugar)

8. 8 The Nature of Sunlight Light is a form of electromagnetic energy, also called electromagnetic radiation Like other electromagnetic energy, light travels in waves Wavelength ( ) = distance between crests of waves Wavelength determines the type of electromagnetic energy

9. 9 The entire range of electromagnetic energy, or radiation Visible light consists of colors we can see, including wavelengths that drive photosynthesis Visible light Gamma rays X-rays UV Infrared Micro- waves Radio waves 10 –5 nm 10 –3 nm 1 nm 10 3 nm10 6 nm 1 m (10 9 nm) 10 3 m 380 450 500550600 650 700 750 nm Longer wavelength Lower energy Shorter wavelength Higher energy The Electromagnetic Spectrum

10. 10 Visible Spectrum The portion of the electromagnetic spectrum that we can see White light contains all of the visible spectrum Colors are the reflection of specific within the visible spectrum not reflected are absorbed Composition of pigments affects their absorption spectrum

11. 11 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 Chloroplast Light Reflected light Absorbed light Transmitted light Granum

12. 12 The Absorption Spectra Of 3 Pigments In Chloroplasts (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. Absorption of light by chloroplast pigments Chlorophyll a Wavelength of light (nm) Chlorophyll b Carotenoids

13. 13 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 Chlorophyll a C CH CH 2 C C C C C C N N C H3CH3C C C C C C C C C N C C C C N MgH H3CH3C H C CH 2 CH 3 H C H H CH 2 H CH 3 C O O O O O CHO in chlorophyll a 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

14. 14 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 Excited state Energy of election e–e– Heat Photon (fluorescence) Chlorophyll molecule Ground state Photon

15. 15 Photosystems (PS) A PS is a collection of pigments and proteins found within the thylakoid membrane that harness the energy of an excited electron to do work Captured energy is transferred between PS molecules until it reaches the chlorophyll a molecule at the reaction center At the reaction center are 2 molecules Chlorophyll a Primary electron acceptor The chlorophyll a is oxidized as the electron is passed to primary electron acceptor which is reduced Thylakoid Photon Light-harvesting complexes Photosystem Reaction center STROMA Primary electron acceptor e–e– Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) Thylakoid membrane

16. 16 Photosystems 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 P700 + Photosystem II (PS II) O2O2 [CH 2 O] (sugar) ee Primary acceptor 2 H + 1⁄21⁄2 H2OH2O ee ee Pq Cytochrome complex Pc ATP Primary acceptor ee Photosystem I (PS I) Light 6 6 5 Fd NADP + reductase NADPH NADP + + 2 H + + H + 4 P680 O2O2 ee ee

17. 17 Photosystems – Electron Flow ATP Photosystem II e–e– e–e– e–e– e–e– Mill makes ATP e–e– e–e– e–e– Photon Photosystem I Photon NADPH

18. 18 Electron Flow in Photosystems There two routes for the path of electrons stored in the primary electron acceptors depending on the photosynthetic organism Noncyclic electron flow - Plants, algae, cyanobacteria Cyclic electron flow - Bacteria other than cyanobacteria Both pathways begin with the capturing of photon energy and utilize an electron transport chain with cytochromes for chemiosmosis

19. 19 Noncyclic Electron Flow Uses both Photosystem II and I Electrons from Photosystem II are removed and replaced by electrons donated from water Synthesizes ATP and NADPH Electron donation converts water into O2 and 2H+ 1.Light excites electrons 2.The electrons energize the reaction center as they are passed to the primary acceptor 3.H2O split via enzyme catalysed reaction forming 2H+, 2e-, and O2. Electrons move to fill orbital vacated by removed electron Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2 e–e– e–e– + 2 H + H2OH2O O2O2 1/21/2

20. 20 Noncyclic Electron Flow 4.Each excited electron is passed along to an electron transport chain 5.ETC produces ATP through chemiosmotic phosphorylation 6.The electron is now lower in energy and enters photosystem I where it is re- energized utilizing sunlight 7.This e- is then passed to a different electron transport system that includes ferridoxin. The enzyme NADP+ reductase assists in the oxidation of ferridoxin and subsequent reduction of NADP+ to NADPH Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) Energy of electrons O2O2 e–e– e–e– + 2 H + H2OH2O O2O2 1/21/2 Pq Cytochrome complex Electron transport chain Pc ATP P700 e–e– Primary acceptor Photosystem I (PS I) e–e– e–e– Electron Transport chain NADP + reductase Fd NADP + NADPH + H + + 2 H + Light

21. 21 Cyclic Electron Flow Uses Photosystem I only Electrons from Photosystem I are recycled Synthesizes ATP only 1.Electron in Photosystem I is excited and transferred to ferredoxin that shuttles the electron to the cytochrome complex. 2.The electron then travels down the electron chain and re-enters photosystem I Photosystem I Photosystem II ATP Pc Fd Cytochrome complex Pq Primary acceptor Fd NADP + reductase NADP + NADPH Primary acceptor

22. 22 Photosystems – Electron Flow ATP Photosystem II e–e– e–e– e–e– e–e– Mill makes ATP e–e– e–e– e–e– Photon Photosystem I Photon NADPH

23. 23 MITOCHONDRION STRUCTURE Intermembrane space Membrane Electron transport chain Mitochondrion Chloroplast CHLOROPLAST STRUCTURE Thylakoid space Stroma ATP Matrix ATP synthase Key H+H+ Diffusion ADP +P H+H+ i Higher [H + ] Lower [H + ] Comparison of Chemiosmosis in Chloroplasts & Mitochondria Both the Mitochondria and Chloroplast generate ATP via a proton motive force resulting from an electrochemical inbalance across a membrane Both utilize an electron transport chain primarily composed of cytochromes to pump H + across a membrane. Both use a similar ATP synthase complex Source of “fuel” for the process differs Location of the H + “reservoir” differs

24. 24 STROMA (Low H + concentration) Light Photosystem II Cytochrome complex 2 H + Light Photosystem I NADP + reductase Fd Pc Pq H2OH2O O2O2 +2 H + 1/21/2 2 H + NADP + + 2H + + H + NADPH To Calvin cycle THYLAKOID SPACE (High H + concentration) STROMA (Low H + concentration) Thylakoid membrane ATP synthase ATP ADP + P H+H+ i [CH 2 O] (sugar) O2O2 NADPH ATP ADP NADP + CO 2 H2OH2O LIGHT REACTIONS CALVIN CYCLE Light Overview 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

25. 25 The Calvin Cycle The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle During the carbon fixation reactions (Calvin cycle) energy from ATP and hydrogen from NADPH are used to reduce CO2 and form glucose. Carbon enters the cycle as CO 2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)

26. 26 The Calvin cycle has three phases: Carbon Fixation – attached to 5C sugar Reduction of CO2, NADPH oxidation Regeneration of the CO 2 acceptor (RuBP ) The Calvin Cycle [CH 2 O] (sugar) O2O2 NADPH ATP ADP NADP + CO 2 H2OH2O LIGHT REACTIONS CALVIN CYCLE Light Input CO 2 (Entering one at a time) Rubisco 3PP Short-lived intermediate Phase 1: Carbon fixation 6 P 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 PP Ribulose bisphosphate (RuBP) 3 6 NADP + 6 6 NADPH P i 6P 1,3-Bisphosphoglycerate P 6 P Glyceraldehyde-3-phosphate (G3P) P1 G3P (a sugar) Output Phase 2: Reduction Glucose and other organic compounds 3 3 ADP ATP Phase 3: Regeneration of the CO 2 acceptor (RuBP) P 5 G3P

27. 27 Carbon Fixation Starts with CO2 A molecule of CO2 is converted from its inorganic form to an organic molecule (fixation) through the attachment to a 5C sugar (ribulose bisphosphate or RuBP). Catalysed by the enzyme RuBP carboxylase (Rubisco).

28. 28 Reduction The formed 6C sugar immediately cleaves into 3- phosphoglycerate Each 3-phosphoglycerate molecule receives an additional phosphate group forming 1,3- Bisphosphoglycerate (ATP phosphorylation) NADPH is oxidized and the electrons transferred to 1,3- Bisphosphoglycerate cleaving the molecule as it is reduced forming Glyceraldehyde 3-phosphate

29. 29 Regeneration The final phase of the cycle is to regenerate RuBP Glyceraldehyde 3- phosphate is converted to RuBP through a series of reactions that involve the phosphorylation of the molecule by ATP For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO 2

30. 30 Summary Light CO 2 H2OH2O Light reactionsCalvin cycle NADP + RuBP G3P ATP Photosystem II Electron transport chain Photosystem I O2O2 Chloroplast NADPH ADP +P i 3-Phosphoglycerate Starch (storage) Amino acids Fatty acids Sucrose (export)

31. 31 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

32. 32 P700 + CO 2 Photosystem II (PS II) H2OH2O Light LIGHT REACTIONS CALVIN CYCLE O2O2 NADPH [CH 2 O] (sugar) ee Primary acceptor 2 H + 1⁄21⁄2 H2OH2O ee ee 1 Energy of electrons Pq Cytochrome complex Pc ATP Electron transport chain NADP + Primary acceptor ee Photosystem I (PS I) Light 6 6 2 ADP ATP 5 Fd Electron Transport chain 7 NADP + reductase NADPH NADP + + 2 H + 8 + H + 1 3 4 P680 O2O2 ee ee