Photosynthesis

1. Photosynthesis

2. Photosynthesis • 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 O2 in the atmosphere.

3. Leaf cross section Vein Mesophyll Stomata O2 CO2 Mesophyll cell Chloroplast 5 µm Outer membrane Thylakoid Intermembrane space Thylakoid space Stroma Granum Innermembrane 1 µm Chloroplasts: Sites of Photosynthesis in Plants • Leaves are the major locations of photosynthesis • Microscopic pores called stomata allow CO2 to enter the leaf and O2 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

4. Leaf cross section Vein Mesophyll Stomata O2 CO2 Mesophyll cell Chloroplast 5 µm Outer membrane Thylakoid Intermembrane space Thylakoid space Stroma Granum Innermembrane 1 µm 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.

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. Reactants: 12 H2O 6 CO2 6 H2O 6 O2 C6H12O6 Products: Figure 10.4 The Splitting of Water • Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

7. H2O CO2 Light NADP+ ADP + P i CALVIN CYCLE LIGHT REACTIONS ATP NADPH Chloroplast [CH2O] (sugar) O2 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)

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. 1 m (109 nm) 10–3 nm 103 nm 106 nm 10–5 nm 103 m 1 nm Gamma rays Micro- waves Radio waves X-rays Infrared UV Visible light 650 750 nm 500 550 600 700 450 380 Shorter wavelength Longer wavelength Higher energy Lower energy The Electromagnetic Spectrum • The entire range of electromagnetic energy, or radiation • Visible light consists of colors we can see, including wavelengths that drive photosynthesis

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. Light Reflected light Chloroplast Absorbed light Granum Transmitted light 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

12. Chlorophyll a Chlorophyll b Carotenoids Absorption of light by chloroplast pigments Wavelength of light (nm) 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.

13. CH3 in chlorophyll a in chlorophyll b CHO CH2 CH3 CH H C C C Porphyrin ring: Light-absorbing “head” of molecule; note magnesium atom at center C C CH3 C H3C C CH2 C C N N H C Mg C H N C C N H3C C C CH3 C C C C C H H CH2 H C C O CH2 O O C O O CH3 CH2 Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown Chlorophyll a • 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

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

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

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 [CH2O] (sugar) O2 Primary acceptor Primary acceptor 4 Fd e Pq e e e H2O NADP+ + 2 H+ Cytochrome complex 2 H+ NADP+ reductase + O2 NADPH 1⁄2 Pc e + H+ P700 5 e Light P680 Light 6 6 ATP Photosystem I (PS I) Photosystem II (PS II)

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

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. H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH O2 [CH2O] (sugar) Primary acceptor e– H2O 2 H+ + O2 1/2 e– e– Energy of electrons Light P680 Photosystem II (PS II) 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+ • Light excites electrons • The electrons energize the reaction center as they are passed to the primary acceptor • H2O split via enzyme catalysed reaction forming 2H+, 2e-, and O2. Electrons move to fill orbital vacated by removed electron

20. O2 [CH2O] (sugar) Electron Transport chain Primary acceptor Primary acceptor Electron transport chain Fd e– Pq e– e– e– NADP+ H2O Cytochrome complex 2 H+ + 2 H+ NADP+ reductase + O2 NADPH 1/2 Pc e– + H+ P700 Energy of electrons e– Light P680 Light ATP Photosystem I (PS I) Photosystem II (PS II) Noncyclic Electron Flow • Each excited electron is passed along to an electron transport chain • ETC produces ATP through chemiosmotic phosphorylation • The electron is now lower in energy and enters photosystem I where it is re-energized utilizing sunlight • 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

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

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

23. Mitochondrion Chloroplast CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE Diffusion H+ Thylakoid space Intermembrane space Electron transport chain Membrane ATP synthase Key Stroma Matrix Higher [H+] Lower [H+] ADP + P i ATP 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. 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 H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH O2 [CH2O] (sugar) STROMA (Low H+ concentration) Cytochrome complex Photosystem I Photosystem II NADP+ reductase Light Light 2 H+ NADP+ + 2H+ Fd NADPH + H+ Pq Pc H2O O2 1/2 THYLAKOID SPACE (High H+ concentration) 2 H+ +2 H+ To Calvin cycle Thylakoid membrane ATP synthase STROMA (Low H+ concentration) ADP + ATP P i H+

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 CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)

26. H2O CO2 Input Light (Entering one at a time) 3 NADP+ CO2 ADP CALVIN CYCLE LIGHT REACTIONS ATP Phase 1: Carbon fixation NADPH Rubisco O2 [CH2O] (sugar) 3 P P Short-lived intermediate P 6 3 P P 3-Phosphoglycerate Ribulose bisphosphate (RuBP) 6 ATP 6 ADP 3 ADP CALVIN CYCLE 6 P P 3 ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADP+ 6 P i P 5 G3P P 6 Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction 1 P G3P (a sugar) Glucose and other organic compounds Output The Calvin Cycle • The Calvin cycle has three phases: • Carbon Fixation – attached to 5C sugar • Reduction of CO2, NADPH oxidation • Regeneration of the CO2 acceptor (RuBP)

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. 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. 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 CO2

30. Summary 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)

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. H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH Electron Transport chain [CH2O] (sugar) O2 Primary acceptor 7 Primary acceptor 4 Electron transport chain Fd e Pq e 8 e 2 e H2O NADP+ + 2 H+ Cytochrome complex 2 H+ NADP+ reductase + O2 NADPH 3 1⁄2 Pc e + H+ 5 P700 e Energy of electrons Light P680 Light 1 1 6 6 ATP Photosystem I (PS I) Photosystem II (PS II)