Photosynthesis converts sunlight into chemical energy through a series of light-dependent and light-independent reactions. The light reactions use energy from sunlight to produce ATP and NADPH, which provide energy and electrons to drive the Calvin cycle. The Calvin cycle fixes carbon from carbon dioxide into organic three-carbon sugars like glyceraldehyde-3-phosphate, using the ATP and NADPH produced in the light reactions. One sugar molecule is used for growth, while five are recycled to regenerate the starter molecule for the next round of the Calvin cycle.

1. PHOTOSYNTHESIS

2. Photosynthesis -converts sunlight into chemical energy -very complex -general reaction: 6CO 2 + 6H 2 0 -> C 6 H 12 O 6 + 6O 2

3. Light from the sun is composed of wavelengths (colors The shorter the wavelength the higher the frequency, thus the higher the energy The longer the wavelength the lower the energy, thus the lower the energy

4. Sunlight (a.k.a. white light) -sunlight is actually white light made of all wavelength colors -sunlight is visible light -different colors=different wavelengths of light The Visible Spectrum violet-blue-green-yellow-orange-red 380 nm 750 nm

5. The Electromagnetic Spectrum -This is what scientists call radiation waves -Radiation=energy that travels and spreads as it goes Examples: X-rays, gamma rays, visible light, microwaves, etc. -The electromagnetic spectrum is organized according to wavelengths

6. -Wavelengths are measured in nanometers (nm) -Gamma rays have the shortest wavelengths = 10 -5 nm (highest frequency and energy) -Radio waves have the longest wavelengths =10 3 nm (lowest frequency and energy)

7. PHOTONS -Photon=quantum=discreet amounts of light energy -Photons are not objects, but each one has a distinct amount of energy Ex: violet photons contain almost twice as much energy as red photons *violet wavelengths=380 nm=high frequency=high energy *red wavelenghts=750 nm=low frequency=low energy

8. Chlorophyll (the photosynthetic pigment) -Chlorophyll is a green photosynthetic pigment found in chloroplasts of plants -There are two main types of chlorophyll (chlorophyll a and chlorophyll b ) -Green is the least effective color for photosynthesis because it is reflected -What you see is reflected. -Everything else is absorbed **Thus, red and blue are most effective for photosynthesis.

10. Absorption Spectrums Absorption spectrums are graphs that plot a pigment’s light absorption vs. wavelength Absorption spectrum of chlorophyll **Remember: Green wavelengths are between ~ 475 and 600 nm

11. Action spectrums A. Action spectrums tell you how much photosynthesis is occurring at each wavelength B. Made by illuminating chloroplast with different wavelengths of light and then plotting wavelength against some measure of photosynthetic rate C. The photosynthetic rates could be measured by finding oxygen production, carbon dioxide absorption or light absorption Action spectrum for chlorophyll

12. Comparison of absorption and action spectra Absorption spectrum for chlorophyll Action spectrum for chlorophyll *Almost no absorption at green wavelengths *The photosynthetic rate is very low at green wavelengths

13. Light energy and water In photosynthesis, light energy is used to split water molecules This process is called photolysis = when a chemical is broken down by photons Water is split into hydrogen ions, oxygen and electrons by photons ATP is also produced ATP and H ions will be used to fix CO 2 to make organic molecules Photosynthesis relies on water and sunlight for its initial reaction

14. General photosynthesis information There are light dependent and light independent reactions Light dependent reactions require light Light independent reactions do not require light or darkness. -they are independent of light or dark -DO NOT REFER TO LIGHT INDEPENDENT REACTIONS AS DARK REACTIONS (darkness is not required)

15. ASSIGNMENT READ: The light reactions and the Calvin cycle cooperate in converting light energy to the chemical energy of food: an overview (p. 172) Briefly outline the section

16. Light Dependent Reactions Light absorption As chlorophyll absorbs light its electrons are raised to a higher energy level by photons at certain wavelengths The electrons at higher energy levels are said to be excited electrons The excited electrons cause the chlorophyll to become photoactivated Photoactivation is the activation of a particular pigment’s electrons (It is caused by absorbing energy from photons.)

17. 5. After photoactivation the electrons quickly return to their ground state 6. When electrons return to their ground state they give off a photon (discreet amount of energy) 7. The photon (energy) is released in the form of heat 8. This process explains the conversion of light energy into heat energy

18. B. Chlorophyll organization and light absorption 1. Chlorophyll is found in the thylakoids which are found in chloroplasts 2. Within the thylakoids chlorophyll is arranged into groups called photosystems 3. There are two photosystems: -Photosystem I – best at 700nm (aka P700) -Photosystem II – best at 680 nm (aka P680)

19. **Both photosystems are identical chlorophyll a molecules, except that they interact with different proteins of the thylakoids 6. Excited electrons that have absorbed photons of light pass from molecule to molecule until they reach the chlorophyll at the center of the photosystem 7. The photosystem (the chlorophyll) will then pass the excited electrons to a chain of electron carriers

20. C. Oxygen production 1. Photosystem II absorbs light 2. Its electrons become excited 3. Photosystem II donates its electrons to an electron transport chain and the flow of electrons will generate an ATP molecule 4. Photosystem II has been oxidized (LEO) 5. To get the electrons back (that were donated) an enzyme in the center of photosystem II breaks a water molecule (photolysis)

21. 6. The water is split into hydrogen ions, oxygen and electrons 7. Electrons are donated to PS II (GER) 8. Oxygen and hydrogen ions are byproducts 9. Oxygen is released to the atmosphere 10. The production of oxygen in photosynthesis is done by photolysis and requires sunlight

22. GET A BOOK FOR YOUR TABLE

23. D. ATP Production 1. an excited electron from the center of PS II is donated and passed along a chain of electron carriers 2. The energy for ATP is generated via a proton gradient that is created as electrons move through an ETC (chemiosmosis)

24. 3. ATP is eventually formed when the H+ ions move through ATP synthase **their energy is harnessed to bring a phosphate group and ADP together 4. The electrons from PS II are eventually donated to PS I (after they go through the ETC) 5. When ATP is produced in this manner it is called non-cyclic photophosphorylation (the book calls it non-cyclic electron flow)

25. E. NADPH Production 1. NADPH = nicotinamide adenine dinucleotide phosphate-oxidase 2. After PS I accepts the electrons that were donated by PS II (the ones that went through the ETC), PS I becomes photoactivated 3. Next PS I donates its excited electrons to NADP+ reductase via another ETC 4. NADP+ reductase is an enzyme that assists in the reduction of NADPH

26. 5. The reduction happens when NADP+ accepts two excited electrons from PS I and a H+ ion from the stroma 6. NADPH is then formed NADP + + H + + 2E - NADP + reductase NADPH ***Non-cyclic electron flow is responsible for generation of NADPH and ATP*** The purpose of NADPH and ATP production is to provide reducing power and chemical energy to drive the Calvin cycle (to make sugar)

27. F. Cyclic photophosphorylation (in the book it is cyclic electron flow) 1. PS II is not involved 2. Produces ATP but not NADPH 3. ATP is made via chemiosmosis (the same way as non-cyclic photophosphorylation) 4. How it works: a. PS I absorbs light b. the excited electrons are given to an electron acceptor c. the electrons pass through an ETC to produce ATP via chemiosmosis d. At the end of the ETC the electrons go back to PS I and the process starts again

28. **Remember in the chloroplast. . .** Chemiosmosis involves the pumping of H+ ions through the membrane. The protons go from the stroma to the thylakoid space. 3. This creates a proton gradient. 4. The protons later flow through ATP synthase (back to the stroma) and their energy is captured in order to join a phosphate with ADP 5. This produces ATP.

29. Go to web animation of light-dependent reactions

30. Assignment #1. Draw and annotate a chloroplast. -Include: grana thylakoids thylakoid membrane stroma ribosomes double membrane circular DNA fat/oil droplets

31. HOMEWORK Complete the coloring worksheet on light reactions TURN IT IN TODAY (2-19) GET YOUR BOOK GO TO P. 180-181

32. Light-independent reactions (light not required) Calvin cycle- 1. takes place in the stroma 2. begins with a 5 carbon sugar called ribulose biphosphate 3. Ribulose biphosphate = RuBP 4. ATP and NADPH from the light dependent reactions drive the Calvin cycle 5. ATP provides the energy 6. NADPH provides reducing power

33. 7. RuBP is a carbon dioxide acceptor 8. The reaction is catalyzed by the enzyme ribulose biphosphate carboxylase 9. RuBP carboxylase=rubisco 10. 3RuBP and 3CO2 form: 6 3-Phosphoglycerate 11. ATP is broken down to convert 6 3-Phosphoglycerate to 6 1,3-Biphosphoglycerate 12. NADPH reduces 6 1,3-Biphosphoglycerate to 6 Glyceraldehyde 3-phosphate

34. 13. Only one of the G3P molecules will be converted to glucose, sucrose, starch, fatty acids or amino acids 14. Five G3P molecules will be converted back to RuBP to keep the Calvin cycle continuing

36. Calvin cycle (more info.) Carbon is: -absorbed as carbon dioxide -released as sugar B. ATP=energy for reactions NADPH=reducing agent C. Net sugar production per turn (3 carbon dioxide and 3 RuBP) is 1 G3P.

37. Phases of the Calvin Cycle Carbon fixation: 1. Every RuBP is attached to a CO 2 3RuBP Rubisco 3CO 2 (a 5 carbon sugar) converts to a very unstable 6 carbon molecule that is immediately converted to 6 3-carbon molecules 6 glycerate-3-phosphate **For every RuBP and CO 2 pair, two three carbon molecules are formed 3RuBP + 3CO2 -> 6 glycerate-3-phosphate

38. B. Reduction 1. molecules of glycerate-3-phosphate are phosphorylated to glycerate-1,3-biphosphate *when ATP is hydrolyzed to ADP 2. glycerate-1,3-biphosphate is reduced when NADPH donates its electrons *NADPH ->NADP+ 3. 6 molecules of triose-phosphate are produced *one is removed from the Calvin cycle and used by the plant to produce sugar *the other 5 are recycled back into the Calvin cycle and converted back to 3RuBP

39. C. Regeneration (of RuBP) 1. 5 triose-phosphate (G3P) molecules go through a complex series of reactions to form 3 RuBP 2. The Calvin cycle starts over 3. CO2 will be received by RuBP again

40. More on the Calvin Cycle Start with 15 total carbons in 3 RuBP *Remember RuBP is a 5 C molecule 3 CO 2 is added for a total of 18 carbons 1 triose-phosphate (G3P) is released (a 3 C molecule is released) The other 5 triose-phosphate molecules are recycled back into 3RuBP(15 C are recycled)

41. E. Net gain of carbons=3 (the single triose phosphate that was released) Energy consumed during the Calvin cycle =9 ATP and 6 NADPH ATP and NADPH will regenerate in the light-dependent reaction There must always be light dependent reactions for light independent reactions to occur The products of the light reactions are used as fuel for the Calvin cycle

42. 1. Outline photosynthesis (light-dependent and light-independent) 2. Explain how the light-independent reaction depends on the light –dependent reaction

43. Measuring Photosynthesis *can be done three ways Production of oxygen Aquatic plants release oxygen in bubbles during photosynthesis If the bubbles are collected, their volume can be measured

44. B. Carbon dioxide absorption EX: 1. Leaves take in CO 2 from the air 2. You could pot a plant in an enclosed environment and measure the CO 2 before and after EX: 1. Aquatic plants absorb CO 2 from the water 2. If plants take up CO 2 , the pH of the water will rise 3. You could use pH indicators to measure pH before and after

45. C. Increase in Biomass (measure the increase in sugar molecules) 1. Measure how much mass a plant gains over time 2. to do this the plant must be completely dehydrated (dead) 3. It is best to measure batches of plants -select a few from each bunch to be dehydrated at different times) 4. Biomass is an indirect measurement of the photosynthetic rate

46. Limiting Factors in Photosynthesis For photosynthesis to occur the following criteria must be met: -suitable temperature -presence of: chlorophyll light carbon dioxide water

47. B. Changes to one limiting factor will change the rate of photosynthesis C. Limiting factors are those that are near their minimum or maximum level D. Limiting factors determine the rate-limiting step For example : If light intensity is the limiting factor, the light dependent reaction will limit the rate of photosynthesis. The limiting-step will be the reduction reaction in the Calvin cycle (when the products of the light dependent stage are needed)

48. E. Light as a limiting factor 1. At low light NADPH and ATP are not produced (b/c they are light-dependent products) 2. If NADPH and ATP are not produced the Calvin cycle will stop at the reduction and phosphorylation reactions The effect of light intensity on photosynthesis Rate of photosynthesis Light intensity

49. *At high intensity photosynthesis plateaus *Light intensity is directly proportional to the rate of photosynthesis *Light is not usually the limiting factor

50. F. Carbon dioxide as a limiting factor 1. If there is little or no carbon dioxide the Calvin cycle is limited at carbon fixation 2. RuBP and NADPH will acculmulate **Carbon dioxide is often a limiting factor because it is never at a high concentration in the atmosphere

51. Rate of photosynthesis Carbon dioxide concentration The effect of carbon dioxide on photosynthesis *There is no photosynthesis when carbon dioxide is low *Carbon dioxide and photosynthesis are directly proportional *At high carbon dioxide concentrations photosynthesis plateaus

52. G. Temperature as a limiting factor 1. At low temperature the enzymes that catalyze the reactions work slowly 2. At high temperature rubisco is ineffective (it is denatured) 3. Carbon fixation becomes the rate-limiting step

53. Rate of photosynthesis Temperature The effect of temperature on photosynthesis *As temperature increases so does the rate of photosynthesis *After the optimum temperature is surpassed the rate quickly falls

54. Review Questions Compare action spectra and absorption spectra. What are the functions of ATP and NADPH produced in non-cyclic photophosphorylation What is the purpose of cyclic photophosphorylation? What is the advantage of non-cyclic photophosphorylation over cyclic photophosphorylation? What is the purpose of the Calvin cycle? Explain the relationship between the structure of the chloroplast and its function.