Photosynthesis Mechanism

Photosynthesis has two types of reaction, first one is light reaction (Hill's reaction) and the other one is dark reaction (Blackman's reaction). In this presentation you learn full mechanism of how plants produce energy for their survival by photosynthesis.

1. PHOTOSYNTHESIS MECHANISM

2. Photophosphorylation mechanism

3. LIGHT DEPENDENT REACTION Photophosphorylation process Hill’s reaction Light reaction Primary photochemical reaction

4. PHOTOPHOSPHORYLATION Takes place in grana of chloroplast. STEPS Absorption of light energy by chloroplast pigment Transfer of light energy from accessory pigments to chlorophyll a Activation of chlorophyll molecules Photolysis of water and evolution of oxygen Electron transport and production of assimilatory powers

5. A. ABSORPTION OF LIGHT BY CHLOROPLAST Different chloroplast pigments absorb light in different regions of the visible spectrum.

6. B. TRANSFER OF LIGHT ENERGY FROM ACCESORY PIGMENT TO CHL - a Accessory & antenna pigments – all pigments except chlorophyll – a. These absorb light and transfer to chlorophyll – a to start the light reaction. Pigment system (I) - Photoreaction centre - P 680 Pigment system (II) – Photoreaction centre – P 700

7. C. ACTIVATION OF CHLOROPHYLL MOLECULES ON RECEIVING PHOTONS BY PS I & II, CHLOROPHYLL MOLECULES GET EXCITED. THEY HAVE MORE ENERGY THAN GROUND STATE ENERGY. AS A RESULT CHLOROPHLLY EXPELS AN ELECTRON AND BECOMES POSITIVELY CHARGED ALONG WITH SOME ENERGY. Chlorophyll – a light excited triplet state of chlorophyll – a Excited triplet state of chlorophyll – a ( chl -a) + + e -

8. D. PHOTOLYSIS OF WATER AND EVOLUTION OF OXYGEN Occurs in Oxygen Evolving Complex (OEC) of PS – II in presence of Mn ++ & Cl - ions. When PS – II is active it receives light. Water molecules splits into H + & OH - ions, this is photolysis of water. H 2 O H + + OH - 2OH - + 2OH - 2H 2 O + O 2 + e - Strong oxidant = Z

9. E. ELECTRON TRANSPORT SYSTEM AND PRODUCTION OF ASSIMILATORY POWERS Released electron travel through number of electron carrier. These again cycled back or used in reduction of NADP+ (Nicotinamide Adenine Dinucleotide phosphate) to NADPH + H+. Extra light energy is used in ATP formation from ADP & inorganic P. Photophosphorylation is of two types :- Non cyclic Cyclic

10. DIFFERENCE BETWEEN PS – I & II

11. NON – CYCLIC PHOTOPHOSPHORYLATION

12. NON - CYCLIC PHOTOPHOSPHORYLATION

13. NON – CYCLIC PHOTOPHOSPHORYLATION

14. CYCLIC PHOTOPHOSPHORYLATION

15. CYCLIC PHOTOPHOSPHORYLATION

16. CYCLIC PHOTOPHOSPHORYLATION

17. PHOTOPHOSPHORYLATION Cyclic photophosphorylation Bundle sheath cells – C4 plants & photosynthetic bacteria. PS – I only. Electron cycle around PS – I. Electron completes and then repeats its cycle. No oxygen evolution Non cyclic photophosphorylation All photosynthetic plants. Both PS – I & II. Electron moves zig-zag from PS-II to PS – I. Electron is drained Into NADPH 2 for dark reaction. Oxygen is evolved.

18. LIGHT REACTION AND DARK REACTION

19. CALVIN CYCLE

20. DARK REACTION The synthesis of glucose in chloroplast by the way of CO2 reduction without the direct influence of light is called Dark reaction. This is the second step in the mechanisms of photosynthesis. It take place in the stroma of the chloroplast. The dark reaction of photosynthesis is purely enzymatic and it is slower than the light reaction. The dark reaction occurs both in the day and night. In Dark reaction, glucose is synthesised from CO2 by using ATP and the assimilatory power(NADPH2) generated in the light reaction. This process is called Carbon fixation or Carbon reduction . In dark reaction, 2 types of cyclic reactions occurs.

21. i ) CALVIN CYCLE OR C3 CYCLE ii) HATCH-SLACK CYCLE OR C4 CYCLE CALVIN CYCLE The synthesis of simple sugars in the dark reaction of photosynthesis discovered by Calvin is called Calvin cycle. Calvin and Benson traced the path of carbon during the synthesis of sugars in a unicellular green alga chlorella using radioactive C14 labelled CO2. For this discovery Calvin was awarded the Nobel prize for the year 1961.

22. The Calvin cycle is a Dark reaction because it does not need sunlight. CALVIN-BENSON CYCLE - refers to the set of light independent redox reaction that takes place in the chloroplast during photosynthesis and carbon fixation that would convert carbon dioxide into the glucose. The carbon and oxygen required for this process are obtained from the ATP and NADPH produced during the photosynthesis process. The conversion of CO2 to carbohydrate is called Calvin cycle or C3 cycle and is named after Melvin Calvin who discovered. The plant undergo Calvin cycle for carbon fixation are known as C3 plant . In C3 cycle, five carbon compound(ribulose 1,5-biphosphate) is reduced by CO2 so that it is also known as reductive pentose phosphate pathway(RPP). OTHER NAMES FOR CALVIN CYCLE:

23. Moreover Calvin cycle is also known as Calvin-Benson- Bassham (CBB) cycle , an attribute to its discoverers: Melvin Calvin, James Bassham and Andrew Benson. Calvin, Bassham and Benson discovered the cycle in 1960s at the university of California in order. They used the radioactive carbon-14 in order to trace the path of the carbon atoms in carbon fixation. They were able to trace the carbon-14 atom soaking up its atmospheric carbon dioxide to is conversion into organic compounds such as carbohydrate.

24. STAGES OF CALVIN CYCLE

25. The Calvin cycle can described under three stages: I) CARBOXYLATION II) REDUCTION III) REGENERATION I) CARBOXYALTION Carboxylation is the fixation of CO2 into a stable organic intermediate. Carboxylation is the most crucial step of the Calvin cycle where CO2 is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two -

26. -molecules of 3-PGA. Since this enzyme also has an oxygenation activity it would be more correct to call it RuBP carboxylase-oxygenase or RuBisCO . II) REDUCTION These are a series of reaction that lead to the formation of glucose. The step involve utilisation of 2 molecules of ATP for phosphorylation and two of NADPH for reduction per CO2 molecule fixed. The fixation of six molecules of CO2 and 6 turns of the cycle are required for the removal of one molecule of glucose from the pathway. III) REGENERATION Regeneration of the CO2 acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted. The regeneration steps require one ATP for phosphorylation to form RuBP.

27. CALVIN CYCLE

28. The synthesis of simple sugars in the dark reaction of photosynthesis discovered by CALVIN is called Calvin cycle. The first stable product of the dark reaction is a 3-carbon compound( 3-phosphoglycerate). Hence, this dark reaction is known as C 3 cycle . In the first step of C 3 cycle, a five carbon compound(ribulose 1,5-biphosphate) is reduced by CO 2 so that it is also known as reductive pentose pathway (RPP). The C 3 cycle occurs in all green plants. Since the path of carbon in the C 3 cycle was elucidated by Melvin Calvin in 1957, the C 3 cycle is called Calvin cycle. Calvin first named this process as carbon assimilation.

29. The Calvin cycle involves the following: The CO 2 is accepted by ribulose biphosphate( a 5-carbon compound present in the stroma) to form an unstable intermediate 6- carbon compound. Ribulose biphosphate + CO 2 Rubisco 6-carbon compound 2. This 6-carbon compound reacts with one H 2 O molecule and splits into two molecules of 3-phosphoglyceric acid( 3-phosphoglycerate). Both of these reaction are catalysed by the enzyme Ribulose biphosphate carboxylase( Rubisco). 6 carbon compound + H 2 O Rubisco 3-phosphoglyceric acid + 2H + 3- phosphoglyceric acid is the first stable product of dark reaction of photosynthesis. Thus 12 molecules of 3-phosphoglyceric acid and 12H +

30. are formed from 6CO 2 ,6H 2 O and 6 RuBP molecules. 3. Each 3-phosphoglyceric acid molecules is phosphorylated by an ATP to produce a 1,3-diphosphoglyceric acid and ADP. This reaction is catalysed by the enzyme phosphoglycerate kinase. 3-phosphoglyceric acid + ATP phosphoglycerate 1,3-diphosphoglyceric acid + ADP kinase 4. The 1,3diphosphoglyceric acid is reduced by an NADPH 2 to form a 3-phosphoglyceraldhyde molecule and NADP. One H 3 PO 4 molecule is released free. This reaction is catalysed by the enzyme triose phosphate dehydrogenase. 1,3-diphosphoglyceric acid Triose phosphate 3-phosphoglyceraldehyde dehydrogenase NADP + H 3 PO 4

31. Thus 12 molecules of 3-phosphoglyceraldehyde are formed from the 12 3-phosphoglyceraldehyde molecules by consuming 12 ATP and 12 NADPH 2. 5. In this step, the simple sugar called fructose 6 phosphate is synthesised from the 3-phosphoglyceraldehyde. It involves the following steps Five molecules of 3-phosphoglyceraldehyde isomerise into dihydroxyacetone phosphates. 3-phosphoglyceraldehyde Triose phosphate Dihydroxyacetone isomerase phosphate

32. Three 3-phosphoglyceraldehyde molecules and three dihydroxyacetone phosphate then unite in the presence of the enzyme aldolase to form fructose 1,6-diphosphate. 3-phosphoglyceraldehyde + Dihydroxyacetone phosphate aldolase Fructose 1,6 diphosphate The fructose 1,6-diohosphate is converted into fructose 6-phosphate by the activity of the enzyme Fructose 1,6-phosphatase. Thus three fructose 6-phosphate molecules are formed. Fructose 1,6-diphosphate Fructose 1,6 phosphatase fructose 6-phosphate Of these one molecule is used to produce glucose 6-phosphate which in turn converted into sucrose while the other two molecules are use

33. in the regeneration of RuBP. Fructose 6 phosphate phosphohexose Glucose 6-phosphate isomerase Glucose 6 phosphate + H2O phosphatase Glucose + Phosphate 6. Two molecules of 3-phosphoglyceraldyde react with two fructose-6- phosphate in the presence of enzyme transketolase to form erythrose 4-phosphate and xylulose-5-phosphate 3-phosphoglyceraldehyde + fructose-6-phosphate Transketolase Erythrose 4-phosphate + xylulose-5-phosphate 7. Two Erythrose 4-phosphate molecules combine with two dihydroxyacetone phosphate in the presence of the enzyme aldose to form 2 Sedoheptulose-7-phosphate.

34. Erythrose-4-phosphate + Dihydroxyacetone phosphate Aldolase sedoheptulos1,7- diphosphate 8. Sedoheptulose1,7-diphosphate loses one phosphate group by the action of the enzyme Sedoheptulose1,7-phosphatase to form sedoheptulose-7-phosphate. Sedoheptulose1,7-diphosphate+2H2O sedoheptulose1,7-phosphatase + iP 9. Two sedoheptulose-7-phosphate molecules react with two molecules of 3-phosphoglyceraldehyde in the presence of transketolase to form two xylulose—phosphate and two ribose-5-phosphate molecules. Sedoheptulose-7-phosphate + 3-phosphoglyceraldehyde Transketolase Xylulose-5-phosphate + Ribose5-phosphate

35. 10. Two ribose 5-phosphate molecules are converted into ribulose5-phosphate by the enzyme ribulose 5-phosphate isomerase. Similarly, the four xylulose 5-phosphate molecules are converted into ribulose 5-phosphate by the enzyme ribulose 50phosphate epimerase. Ribose 5-phosphate Ribulose 5-phosphate Ribulose 5-phosphate isomerase Xylulose 5-phosphate Ribulose 5-phosphate Ribulose 5-phosphate epimerase 11. The six Ribulose 5-phosphate molecules undergo phosphorylation with 6ATP to form six molecules of ribulose 1,5-biphosphate and 6ADP. The reaction is catalysed by the enzyme phosphor ribulose kinase.

36. Thus,the Calvin cycle is completed. Ribulose 5-phosphate + ATP phosphor ribulose kinase ribulose 1,5-diphosphate + ADP SIGNIFICANCE OF CALVIN CYCLE Calvin cycle enables the plant to accumulate food material in them. Plant growth and yield are determined by the rate of dark reaction. The vast reserve of energy in the form of coal, oil, peat and dung are rely the outcomes of Calvin cycle in plants. The entire living world depends on food synthesised in plants via Calvin cycle

37. Hatch and slack pathway ( C4 pathway )

38. HATCH AND SLACK PATHWAY

39. DISCOVERY The first experiments indicating that some plants do not use C 3 carbon fixation but instead produce malate and aspartate in the first step of carbon fixation were done in the 1950s and early 1960s by Hugo Peter Kortschak and Yuri Karpilov . The C 4 pathway was elucidated by Marshall Davidson Hatch and Charles Roger Slack , in Australia, in 1966; it is sometimes called the Hatch–Slack pathway.

40. HATCH AND SLACK

41. PEP carboxylase is located in the mesophyll cells, on the leaf exterior near the stomata. There is no rubisco in the mesophyll cells. CO2 entering the stomata is rapidly fixed by PEP carboxylase into a 4-carbon compound, called malate, by attaching the CO2 to PEP. The malate is then transported deeper into the leaf tissue to the bundle sheath cells, which are both far away from the stomata (and thus far away from oxygen) and contain rubisco.  Once inside the bundle sheath cells, malate is decarboxylated to release pyruvate and CO2; the CO2 is then fixed by rubisco as part of the Calvin cycle, just like in C3 plants.  Pyruvate then returns to the mesophyll cells, where a phosphate from ATP is used to regenerate PEP.  Thus in C4 plants, C4 carbon fixation has a net added cost of 1 ATP for every CO2 delivered to rubisco; however, C4 plants are less likely to die of dehydration compared to C3 plants in dry conditions. C4 PATHWAY

42. C 4 PATHWAY Inseveral plants like Sugarcane , Maize ,Euphorbia , Amaranthus , Sorghum , first stable product in dark reaction is oxaloacetic acid (4C). First Co2 fixation occur in mesophyll chloroplast , while second in bundle sheath . Two aspect of c4 model are: (a) Asparate formers (b) Malate formers

43. There are two important adaptations that allow C4 plants to do this: => First, C4 plants use an alternate enzyme for the first step of carbon fixation. This enzyme is called phosphoenolpyruvate (PEP) carboxylase , and it has no oxygenase activity and has a much higher affinity for CO2 than rubisco. As the name “PEP carboxylase” suggests, the enzyme attaches CO2 to a compound called phosphoenolpyruvate (PEP). =>Second, C4 plants have specialized leaf anatomy with two different types of photosynthetic cells: mesophyll cells (on the exterior of the leaf, near stomata) and bundle sheath cells (in the interior of the leaf, far away from stomata).  Rubisco is located in bundle sheath cells, but not in mesophyll cells.(KRANZ ANATOMY)

44. KRANZ ANATOMY Cross section of maize leaf

45. KRANZ ANATOMY The word Kranz means “wreath” or “ring”. Kranz anatomy is a specialized structure in C 4 plants where the mesophyll cells are clustered around the bundle-sheath cells in a ring-like fashion. The number of chloroplasts in the bundle-sheath cells is more than that in the mesophyll cells. This is found in C 4 grasses such as maize and a few dicots. The Kranz anatomy is developed in three different steps: Initiation of procambium Bundle sheath and mesophyll cells specification Chloroplast development and integration of the C 4 cycle

46. DIFFERENCE BETWEEN C 3 AND C 4 PATHWAY

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