Photosynthesis

1. Photosynthesis Photosynthesis is a process by which green plants and other organisms such as algae and some bacteria synthesize their own food in the presence of light

2. Historical perspective

3. T.W. Engelmann Experiment C.B. Van Niel • H2 from a suitable oxidisable compound reduces CO2 to carbohydrates. H2O is the H2 donor and is oxidized to O2. In purple ad green sulphur bacteria H2S is the H2 donor and the oxidation product is sulphur or sulphate. • Inferred that O2 evolved by green plants comes from water not from CO2. This was proved by radio isotopic technique • 6CO2 + 12H2O  C6H12O6 + 6H2O + 6O2

4. Site for photosynthesis • Green plants – mostly in leaves to a lesser extends in green stems and floral parts • Specilized cells in leaves called mesophyll cells – these cells contain chloroplasts which are located at the outer margin with their broad surface parallel to the cell wall of mesophyll cells. This helps in easy diffusion of CO2 Chloroplasts in Onion Root Cells

5. Chloroplast Which part of the chloroplast contain the machinery for photochemical reactions of photosynthesis? Thylakoids contain pigment require for capturing solar energy to initiate photosynthesis Pigment is a substance that absorbs light of different wavelength

6. Photosynthesis is a two stage process 1. Light Reactions Light Dependent Process, requires the direct energy of light to make energy carrier molecules that are used in the second process. The grana, the stroma lamellae trap light energy synthesis ATP and NADPH 2. Dark Reactions The Light Independent Process, reactions are not directly light driven but are dependent on products of light reaction (ATP, NADPH) to form C-C covalent bonds of carbohydrates. This does not mean it occurs in darkness.

7. Light reactions Dark reactions

8. Light absorption properties of chlorophyll Chlorophyll absorbs light in the violet and blue wavelength and also in red region of the visible spectrum. This portion of the spectrum between 400 nm and 700 nm is referred to as PAR (photsynthetically active radiation) Spectrum of sun light Chlorophyll reflect the green light, hence, impart green colour to leaves

9. Structure of chlorophyll It is a large molecule composed of four 5 membered rings called pyrrole rings and a central core of magnesium. A side chain called phytol chain extends from one of the pyrrole ring. The long side chain is made of insoluble carbon and hydrogen atom which help to anchor the chlorophyll molecules with thylakoids Phyrrol Phytol Mg Molecular model of chlorophyll

10. Types of chlorophyll In plants mostly there are two kinds – chlorophyll a and b. They are similar in their molecular structure except that the CH3 group in chlorophyll a is replaced by CHO group in chlorophyll b

11. Different pigments in leaf Paper chromatography is used to separate leaf pigments Four important pigments are: 1. Chlorophyll a (blue-green) 2. Chlorophyll b (yellow-green) 3. Xanthophyll (yellow) 4. Carotenoids (yellow-orange)

12. Absorption spectrum of pigments A curve obtained by plotting the amount of absorption of different wavelengths of light by a particular pigment Absorption spectrum Action spectrum A curve showing the rate of photosynthesis at different wavelength of light

13. What is light reaction? • Photochemical phase – light absorption • Water splitting • Oxygen release • Formation of ATP and NADPH

14. Photosystem • Photosystem are arrangements of chlorophyll and other pigments packed into thylakoids. • Many Prokaryotes have only one photosystem, Photosystem II (so numbered because, while it was most likely the first to evolve, it was the second one discovered). • Eukaryotes have Photosystem II plus Photosystem I. • Photosystem I uses chlorophyll a, in the form referred to as P700. • Photosystem II uses a form of chlorophyll a known as P680. Both "active" forms of chlorophyll a function in photosynthesis due to their association with proteins in the thylakoid membrane. • Photosystem is LHC – light harvesting complex

15. (antennae) Reaction centres PS1 – The reaction centre, chlorophyll a, has absorption peak at 700 nm called P700 PS2 – The reaction centre, chlorophyll a, has absorption peak at 680 nm called P680

16. Lamellae of the grana have both PSI and PSII • The Stroma lamellae lack PSII and NADP reductase enzyme • Cyclic photophosphorylation occurs only when light of wave length above 680 nm is available for excitation

17. Non-Cyclic Photo phosphorylation

18. Cyclic Photophosphorylation

19. Chemiosmotic hypothesis • Spliting of water molecules takes place on inner side of membrane, the H+ produced during this process accumulate within lumen of thylakoid

20. Chemiosmotic hypothesis 2. Electrons move through photosystems, protons are transported across the membrane (into lumen) by cytochrome complex which is a H+ carrier. Electrons are transported to the electron carrier present on the inner side of the membrane. The protons are released into the lumen. H+ carrier

21. Chemiosmotic hypothesis 3. The NADP reductase enzyme is located on the stroma side of membrane. H+ are required for reduction of NADP+. The protons are removed from the stroma. Electrons are also required. They come from PSI.

22. Chemiosmotic hypothesis 4. These processes result in increased H+ concentration in lumen and decreases concentration in stroma. This creates a proton gradient. Proton gradient is important because the breakdown of this gradient leads to release of energy

23. Chemiosmotic hypothesis 5. The H+ move through the trans membrane Channel of ATPase to stroma. ATPase enzyme has two parts – F0 and F1. As the H+ pass through F0 and F1 complex, it releases enough energy to produce ATP. There is a conformation change in F1 which activates the enzyme. F0 F1

24. Dark Reaction - Biosynthetic phase Melvin Calvin, Ernest Orlando Lawrence Berkeley National Laboratory Using carbon-14, and the new techniques of ion exchange, paper chromatography, and radio autography, Calvin and his many associates mapped the complete path of carbon in photosynthesis. The accomplishment brought him the Nobel prize in chemistry in 1961.

25. Phase 1: Carbon Fixation CO2 comes into the stroma of the chloroplast. Rubisco catalyzes the bonding of CO2 to RuBP to create an unstable 6-carbon molecule that instantly splits into two 3-carbon molecules of 3-PG.

26. Phase 2: Reduction ATP phosphorylates each 3-PG molecule and creates 1,3-bisphosphoglycerate.  This in turn results in the loss of the terminal phosphate group from ATP thus making ADP.

27. NADPH reduces 1,3-bisphosphoglycerate which causes the phosphate group to break off once again.  The molecule then picks up a proton (H+) from the medium to become glyceraldehyde-3-phosphate.  The broken off phosphate group also gains a proton to become H3PO4.  NADPH is oxidized by this process and becomes NADP+.

28. Phase 3: Regeneration For every six molecules of G3P created five molecules continue on to phase 3 while one leaves to be used for organic compounds.  ATP is once again needed.  However, this time it phosphorylates G3P to regenerate RuBP after some rearrangement.

29. Photo respiration When carbon dioxide levels decline below the threshold for RuBP carboxylase, RuBP is catalyzed with oxygen instead of carbon dioxide. The product of that reaction forms glycolic acid, a chemical that can be broken down by photorespiration, producing neither NADH nor ATP, in effect dismantling the Calvin Cycle. O2 2-Phosphoglycolate RuBP + CO2 Rubisco 3-Phosphoglycerate Calvin cycle

30. C4-Pathway The C4 pathway is designed to efficiently fix CO2 at low concentrations and plants that use this pathway are known as C4 plants. These plants fix CO2 into a four carbon compound (C4) called oxaloacetate. This occurs in cells called mesophyll cells. Examples: Maize, Sugar cane, Pearl millet, Amaranth

31. C4-Plants • C4 plants require presence of two types of photosynthetic cells - Mesophyll cells and bundle sheath cells. • It contains dimorphic chloroplast. • Chloroplast in mesophyll cells are granal and in bundle sheath cells are agranal. • Rubisco is present only in bundle sheath cells C3 plant C4 plant (Kranz anatomy)

32. C4-Pathway The C4 pathway is designed to efficiently fix CO2 at low concentrations and plants that use this pathway are known as C4 plants. These plants fix CO2 into a four carbon compound (C4) called oxaloacetate. This occurs in cells called mesophyll cells.

33. C4-Pathway 1. CO2 is fixed to a three-carbon compound called phosphoenolpyruvate to produce the four-carbon compound oxaloacetate. The enzyme catalyzing this reaction, PEP carboxylase, fixes CO2 very efficiently so the C4 plants don't need to to have their stomata open as much. The oxaloacetate is then converted to another four-carbon compound called malate in a step requiring the reducing power of NADPH.

34. C4-Pathway 2. The malate then exits the mesophyll cells and enters the chloroplasts of specialized cells called bundle sheath cells. Here the four-carbon malate is decarboxylated to produce CO2, a three-carbon compound called pyruvate, and NADPH. The CO2 combines with ribulose bisphosphate and goes through the Calvin cycle.

35. C4-Pathway 4. The pyruvate re-enters the mesophyll cells, reacts with ATP, and is converted back to phosphoenolpyruvate, the starting compound of the C4 cycle.