PHOTOSYNTHESIS

1. PHOTOSYNTHESIS Outline 3.1-3.4

2. Overview • Photosynthesis is a series of reactions that convert light energy into chemical energy • This is the only system in which chemical energy is being created.

3. The overall reaction for photosynthesis is: 6CO2+12H2OC6H12O6+6H2O+6O2

4. Overview • A photon (unit of light energy) boosts an electron of a special molecule to a higher energy level. • There are reactions that are light dependent and reactions that are light independent

5. Overview • There are three main stages to photosynthesis: • Capturing sunlight energy • Synthesizing ATP and NADPH • The Calvin Cycle

6. The molecules that absorb the light are called pigments. • Photosynthesis is carried out by a number of different organisms all containing the green-colored pigment, chlorophyll.

7. Photosynthetic Pigments • Chlorophylls • Magnesium located at the centre of the complex ring structure called a porphyrin ring (alternating single and double carbon bonds).

8. Reflect photons in the green range so they appear green to the human eye • Chlorophylls a and b absorb primarily violet-blue and red light • Chlorophyll a is the only pigment that can transfer the energy of light to the carbon fixation reactions of photosynthesis; chlorophyll b is an accessory pigment.

9. Photosynthetic Pigments • Carotenoids • carbon rings linked by alternating single and double bonds • Reflect photons in the yellow-to-red range so they appear yellow-orange to the human eye • beta-carotene is the orange pigment in carrots

10. Play an energy absorbing role rather than a photosynthetic one – protect the chlorophyll from too much light by absorbing light that would otherwise damage the chlorophyll pigment • similar compounds are thought to protect the human eye from excessive light

11. Absorption Spectrum

12. Chloroplasts • Photosynthetic factories of plants and algae • Light dependant reactions occur on photosynthetic membranes, called thylakoids • Thylakoids that are stacked upon one another are called grana • Stroma is a semi-liquid material that bathes the interior of the chloroplast (around the thylakoids and grana)

13. Photosynthesis Research • Ancient times they thought that plants obtained all their food from the soil • In early 1600’s, a physician (J. B. Van Helmont) set out to determine if this was true

14. Early Research • He found that after 5 years the mass of the tree had increased substantially, but the mass of the soil around the tree had only decreased a little

15. He then incorrectly concluded that water was responsible for the tree’s increase in mass

16. A Century Later… • In 1771, Joseph Priestly, an English scientist, discovered (by accident) that gases play a role in photosynthesis • He burned a candle in a closed container until it went out; he then placed a living mint plant under the container next to the candle • After 10 days, the candle could begin to burn again • He showed that plants release a gas into the air that supports combustion

17. 1796 Gas Identified • A Dutch doctor (Jan Ingenhousz) identified the gas released by plants as oxygen • He was also the first to realize that sunlight is essential to the process of photosynthesis, and that CO2 is the source of carbon for plants • He mistakenly assumed that the oxygen given off by plants came from the CO2 that the plants took in.

18. A Century Later… • In the 1930’s, a graduate student (C.B. Van Neil) showed that the oxygen produced in photosynthesis came from water and not CO2 • CO2(g) +2H2S(g)+ light [CH2O](aq)+H2O(l) + 2S(s)

19. Blackman’s Experiments • Measured the effect of changes in • light intensity • CO2 concentration • temperature

20. At low light intensities, rate of photosynthesis increases by increasing light intensity regardless of temperature (fig. 8 p. 150) • At high light intensities, rate of photosynthesis can be increased by increasing temperature, but not by further increasing light intensity

21. At a controlled temperature, the rate of photosynthesis decreases as CO2 levels decrease.

22. Concluded that there are 2 stages of photosynthesis: 1.) light-dependent(Thylakoid membrane) (a) capturing light energy (b) using light energy to make ATP and NADPH 2.) light-independent(Stroma) (c) using free energy of ATP and reducing power of NADPH to make glucose from CO2

24. Light Dependant Reactions • Require chlorophyll • Occur on the thylakoid membrane in chloroplasts • Primary goal is to produce ATP and NADPH (energy used to make glucose in Calvin cycle) • 2 photosystems: • Photosystem I (cyclic) • Photosystem II (non-cyclic)

25. Photosystems • A cluster of independent pigment molecules • chlorophyll a and accessory pigment molecules • Consist of an antenna complexand a reaction center • Antenna pigment will absorb a photon and transfer the energy from pigment to pigment until it reaches the reaction center • Chlorophyll a is at the reaction center

27. Photosystem I • Cyclic photophosphorylation • P700 (reaction centre chlorophyll) channels energy to the electron acceptor: ferrodoxin • The chlorophyll a molecule at the reaction centre of PSI is called P700 because its absorption spectrum peaks at a wavelength of 700 nm (red light)

29. Cyclic – the same electron that was excited and accepted by ferredoxin is returned to P700 • ATP is made – protons are moved into the thylakoid space by the electron flow; these protons are pumped across the membrane to make ATP • No NADPH – electrons are not released to generate NADPH

30. Photosystem II • Non-cyclic photophosphorylation • Plants evolved and needed a way to “fix” carbon from CO2 in order to make glucose • Energy is transmitted in same manner as photosystem I (proton gradient makes ATP) but the absorption peak is 680nm so it is referred to as P680. • NADPH is produced and used as the reducing power in the Calvin Cycle

31. Photosynthesis in Plants • Photon of light is captured by PSII • Photon excites an electron from PSII which then passes through an electron transport chain • Movement of electron moves protons across the membrane to generate ATP • Electron reaches PSI and replaces the electron lost from PSI • Electron is accepted from PSI by ferredoxin • Ferredoxin gives two electrons to reduce NADP+ to NADPH

33. If PSII is non-cylic, how does PSII replace the electron that is lost to the electron transport chain-PSI-ferredoxin-NADPH?

34. There is a protein Z that is associated with PSII • Protein Z is able to split a water molecule (H2O) into H+ and O2 • O2 is released by the plant as a waste product • H+’s are added to the proton concentration gradient that was established during the passage of electrons in PSII. • P680 becomes an oxidant (electron seeker) and will get the electron from protein Z that is released when a water molecule is split

36. Light Independent Reactions • Calvin Cycle • uses CO2 (as a carbon source) and ATP and NADPH (produced in the light dependent reactions) to synthesize G3P • G3P can be converted to sugar (glucose) and used by the cell immediately, or it can be converted to starch to be stored by the cell

37. Calvin Cycle • Occurs in the stroma of the chloroplast • Cyclic (like Krebs Cycle) – the end product is also the starting material 1.) Carbon fixation 2.) Reduction reactions 3.) RuBP regeneration It takes six turns of the Calvin cycle to produce one 6 carbon sugar

39. Carbon Fixation • CO2 (1-carbon) adds to RuBP (5-carbon) • Catalyzed by the enzyme RuBPcarboxylase/oxygenase Rubisco • Slow process (3 substrates processed every second vs. 1000 for a normal enzyme) • Instantly splits into two 3-carbon molecules (PGA) • Photosynthesis using the Calvin Cycle is known as C3 photosynthesis

40. Reduction Reactions • P is added to PGA forming 1, 3 BPG. • NADPH reduces 1,3 BPG to G3P (PGAL) • NADPH becomes NADP+ • glyceraldehyde-3-phosphate (PGAL)

41. Regeneration of RuBP • One G3P exits the Calvin Cycle, the remaining 5 G3P molecules are rearranged to create 3 RuBP molecules • It is important to regenerate the RuBP so more molecules of CO2 can be fixed to make G3P (converted to glucose or starch

42. Photorespiration • Plants also undergo photorespiration • RuBPcarboxylase oxidizes RuBP • CO2 is released without production of ATP or NADPH  it essentially undoes photosynthesis. • RuBPcarboxylase is an enzyme that works using light • Photorespiration occurs when O2 is more abundant than CO2

44. C3 Plants • Plants that undergo photosynthesis using light dependent and Calvin cycle reactions • Lose between 25-50% of fixed carbon due to photorespiration • Depends on temperature: oxidative activity increases as the temperature increases • temperatures above 28oC are severe

45. Alternate Ways to Fix Carbon 1.) C4 Photosynthesis - sugar cane and corn plants 2.) Crassulacean Acid Metabolism (CAM) - cacti, pineapples

46. C4 Plants • CO2 (1-carbon), is added to phosphoenolpyruvate (PEP) 3-carbon • Oxaloacetate (4-carbon) is formed and converted into another 4-carbon molecule (malate) which travels from the mesophyll cells to the bundle sheath cells

48. Bundle sheath cells are impermeable to CO2 and therein hold the CO2 within them. • Carbon and oxygen atoms are removed from the malate to create CO2, which is then added to the Calvin cycle. • the increased CO2 inhibits photorespiration.

49. It takes 30 molecules of ATP for a C4 plant to make one glucose (as compared to 18 ATP in C3 plants) • However, it is required, otherwise most CO2 would not react and oxidation of RuBP carboxylase would occur.

50. CAM Plants • Used by plants in hot regions • Crassulacean acid metabolism • Stomata are open during the night and closed during the day to avoid water loss but allow CO2 uptake • C4 and C3 takes place in the same cells • C4 photosynthesis occurs at night • C3 photosynthesis occurs during the day.