Table of Contents

1. Chapter 6 Photosynthesis Table of Contents Section 1 The Light Reactions Section 2 The Calvin Cycle

2. Section 1 The Light Reactions Chapter 6 Objectives • Explainwhy almost all organisms depend on photosynthesis. • Describethe role of chlorophylls and other pigments in photosynthesis. • Summarizethe main events of the light reactions. • Explainhow ATP is made during the light reactions.

3. Definition: • Photosynthesis - process by which organisms capture sunlight energy and store it in organic compounds.

4. Section 1 The Light Reactions Chapter 6 Obtaining Energy • Photosynthesisconverts light energy from the sun into chemical energy in the form of organic compounds through a series of reactions known as biochemical pathways.

5. Section 1 The Light Reactions Chapter 6 Obtaining Energy,continued • Autotrophsuse energy from sunlight or from chemical bonds in inorganic substances to make organic compounds. • Animals and other organisms that must get energy from food instead of directly from sunlight or inorganic substances are calledheterotrophs.

6. Section 1 The Light Reactions Chapter 6 Obtaining Energy • Autotrophs - organisms that manufacture their own food. Most autotrophs perform photosynthesis. • Heterotrophs - organisms that cannot manufacture their own food. • Photosynthesis uses a biochemical pathway meaning a complex series of chemical reactions in which the product of one reaction is consumed in the next reaction.

7. Section 1 The Light Reactions Chapter 6 Comparing Autotrophs and Heterotrophs

8. Section 1 The Light Reactions Chapter 6 Overview of Photosynthesis • The oxygen (O2) and some of the organic compounds produced by photosynthesis are used by cells in a process calledcellular respiration.

9. Section 1 The Light Reactions Chapter 6 Linking Photosynthesis and Respiration

10. Section 1 The Light Reactions Chapter 6 Overview of Photosynthesis, continued • Photosynthesis can be divided into two stages: Light Reactions and Calvin Cycle • In thelight reactions,light energy is converted to chemical energy, which is temporarily stored in ATP and the energy carrier molecule NADPH. • In the Calvin Cycle, organic compounds are formed using CO2 and the chemical energy stored in ATP and NADPH.

11. Section 1 The Light Reactions Chapter 6 Overview of Photosynthesis, continued • Equation of Photosynthesis: 6CO2 + 6H2O light energy C6H12O6 + 6O2

12. Section 1 The Light Reactions Chapter 6 Capturing Light Energy • The light reactions begin with the absorption of light in chloroplasts, organelles found in the cells of plants, some bacteria, and algae.

13. Section 1 The Light Reactions Chapter 6 Capturing Light Energy • Light Reactions • Light reactions are the first set of reactions in photosynthesis. They occur in the thylakoid membrane when chloroplasts absorb light.

14. Section 1 The Light Reactions Chapter 6 Capturing Light Energy • Light Absorption in Chloroplasts • Structure of Chloroplasts a. Chloroplasts are organelles with double membranes. b. Chloroplasts contain flattened sacs called thylakoids. c. Grana are stacks of thylakoids. d. Stroma is a solution that surrounds the thylakoids.

15. Section 1 The Light Reactions Chapter 6 Parts of a Chloroplast

16. Section 1 The Light Reactions Chapter 6 Capturing Light Energy, continued • Light and Pigments • White light from the sun is composed of an array of colors called the visible spectrum. • Pigments absorb certain colors of light and reflect or transmit the other colors.

17. Section 1 The Light Reactions Chapter 6 Capturing Light Energy, continued • Properties of Sunlight and Pigments • The sun’s white light is actually a variety of colors. • Visible Spectrum - the array of colors when the sun’s white light is separated into different wavelengths (ROY G BIV). • Pigment is a compound that absorbs light.

18. Section 1 The Light Reactions Chapter 6 Capturing Light Energy, continued • Chloroplast Pigments • Located in the membrane of thethylakoids of chloroplastsare several pigments, including chlorophylls (such as chlorophyll a and chlorophyll b) and carotenoids.

19. Section 1 The Light Reactions Chapter 6 Spectrum of Light and Plant Pigments

20. Section 1 The Light Reactions Chapter 6 Capturing Light Energy, continued • Chloroplast pigments are in thylakoids a. Chlorophyll “a” - absorbs red light; - directly involved in light reactions. b. Chlorophyll “b” - absorbs blue light; - accessory pigment: assists chlorophyll “a” in capturing sunlight. c. Caratenoids - accessory pigments; Ex: brown, yellow, orange. - enable plant to absorb more of the light spectrum.

21. B. Electron Transport • Photosystem - a cluster of pigment molecules in the thylakoid membrane. • Two types of photosystems, photosystem I & photosystem II, have similar pigments but different roles in light reactions. 3. Reactions begin when accessory pigments absorb light and pass this energy on to chlorophyll “a”.

22. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy • The pigments are grouped in clusters of a few hundred molecules in the thylakoid membrane. Each cluster and the proteins that the pigment molecules are embedded in are referred to collectively as a photosystem. • By absorbing light, pigment molecules in photosystem I and photosystem II acquire some of the energy carried by the light.

23. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy, continued • In each photosystem, the acquired energy is passed quickly to other pigment molecules until it reaches a specific pair of chlorophyll a molecules. • The acquired energy forces electrons to enter a higher energy level in the two chlorophyll a molecules of photosystem II. These energized electrons are said to be “excited.” The excited electrons have enough energy to leave the chlorophyll a molecules.

24. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy, continued • The acceptor of these electrons from photosystem II is a molecule called the primary electron acceptor, which donates the electrons to the electron transport chain. • As the electrons move from molecule to molecule in this chain, they lose most of the acquired energy. The energy they lose is used to move protons into the thylakoid.

25. Section 1 The Light Reactions Chapter 6

26. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy, continued OCCURING IN PHOTOSYSTEM II: • Step 1. Light energy “excites” electrons in 2 chlorophyll “a” molecules into the next higher e- orbit. • Step 2. These e-‘s move to a primary e- acceptor. • Step 3. The primary electron acceptor donates the e-‘s to the electron transport chain - a series of molecules in the thylakoid membrane through which the e-‘s pass.

27. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy, continued OCCURING IN PHOTOSYSTEM II: * Note: The Energy from the moving electrons is used by the proton pump to move protons from stroma into thylakoid against its concentration gradient.

28. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy, continued • Light is absorbed by photosystem I at the same time it is absorbed by photosystem II. Electrons move from chlorophyll a molecules to another primary electron acceptor. • The electrons lost from photosystem I are replaced by electrons that have passed through the electron transport chain from photosystem II.

29. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy, continued • These electrons are then donated to another electron transport chain, which brings the electrons to the side of the thylakoid membrane that faces the stroma. • In the stroma, the electrons combine with a proton and NADP+. This causes NADP+ to be reduced to NADPH.

30. Section 1 The Light Reactions Chapter 6 • AT THE SAME TIME, THESE STEPS ARE OCCURING IN PHOTOSYSTEM I: • Step 4. Light energy “excites” e- in 2 chlorophyll “a” molecules to move to another primary e- acceptor. (The electrons that are lost in these 2 chlorophyll “a” are replaced by electrons from the photosystem II that have passed through the e- transport chain described in steps 1, 2, & 3.) • Step 5. The primary e- acceptor donates these e-‘s to a different e- transport chain. This chain brings e-‘s to the stroma where the e-‘s combine with a p+ and NADP+ to form NADPH.

31. Section 1 The Light Reactions Chapter 6 • Restoring Photosystem II - these e-‘s must be replaced so the e- transport chain does not stop. a. enzyme inside thylakoid membrane splits H2O into p+, e-, & O2 : 2H2O  4H+ + 4e- + O2 • the e-‘s replace those lost in photosystem II.(from water) • the p+’s remain inside thylakoid. d. the O2 diffuses out of plant.(O2 from water)

32. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy, continued • Replacing Electrons in Light Reactions • Electrons from photosystem II replace electrons that leave photosystem I. Replacement electrons for photosystem II are provided by the splitting of water molecules. • Oxygen produced when water molecules are split diffuses out of the chloroplast and then leaves the plant.

33. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy, continued • Making ATP in Light Reactions • An important part of the light reactions is the synthesis of ATP. During chemiosmosis, the movement of protons through ATP synthase into the stroma releases energy, which is used to produce ATP.

34. Section 1 The Light Reactions Chapter 6 • Chemiosmosis • 1. Chemiosmosis is a process where ATP is synthesised in a light reaction. • protons are inside thylakoid from the splitting of H2O. • protons are also pumped from stroma into thylakoids. • this results in more p+s inside thylakoid than outside in stroma, thus, a concentration gradient. • this concentration gradient of p+’s is potential energy. • p+’s diffuse down concentration gradient from inside thylakoid to outside in stroma. • ATP synthase is the carrier protein in membrane for p+’s to leave. • this energy from p+’s is harnessed by ATP synthase for: • ADP + P  ATP

35. Section 1 The Light Reactions Chapter 6

36. Section 2 The Calvin Cycle Chapter 6 Objectives • Summarize the main events of the Calvin cycle. • Describe what happens to the compounds that are made in the Calvin cycle. • Distinguish between C3, C4, and CAM plants. • Summarize how the light reactions and the Calvin cycle work together to create the continuous cycle of photosynthesis. • Explain how environmental factors influence photosynthesis.

37. Section 2 The Calvin Cycle Chapter 6 Carbon Fixation • The ATP and NADPH produced in the light reactions drive the second stage of photosynthesis, theCalvin cycle. • In the Calvin cycle, CO2 is incorporated into organic compounds, a process calledcarbon fixation.

38. Section 2 The Calvin Cycle Chapter 6 Carbon Fixation, continued • The Calvin cycle, which occurs in the stroma of the chloroplast, is a series of enzyme-assisted chemical reactions that produces a three-carbon sugar. • Most of the three-carbon sugars (G3P) generated in the Calvin cycle are converted to a five-carbon sugar (RuBP) to keep the Calvin cycle operating. But some of the three-carbon sugars leave the Calvin cycle and are used to make organic compounds, in which energy is stored for later use.

39. Section 2 The Calvin Cycle Chapter 6

40. Section 2 The Calvin Cycle Chapter 6 II. Calvin Cycle • The second set of reactions in photosynthesis. • Produces organic compounds • Uses the energy in ATP and NADPH made during the light reactions. • Takes place in the stroma.

41. Section 2 The Calvin Cycle Chapter 6 • Carbon Fixation • Carbon fixation is taking C from CO2 to make organic compounds. • Step 1. CO2 diffuses into stroma and combines with RuBP (5 Carbon molecule) to form 2 molecules of PGA. • Step 2. Each PGA is converted into PGAL ( 3 Carbon molecule). • Step 3. Most PGAL is converted back to RuBP to restore the Calvin Cycle. Some PGAL is used to make organic compounds. 2. These 3 steps must occur 3 times to form 1 PGAL molecule.

42. Section 2 The Calvin Cycle Chapter 6 B. Organic Products From Calvin Cycle • Amino Acids • Lipids • Carbohydrates (CH2O is general formula for carbohydrates) a. glucose c. sucrose e. starch b. fructose d. glycogen f. cellulose 4. General Formula for Photosynthesis: CO 2 + H2 O + light energy  (CH2O) + O2

43. Section 2 The Calvin Cycle Chapter 6 II. Calvin Cycle • NADP – Nicotinamide adenine dinucleotide phosphate • RuBP – Ribulose Biphosphate • PGA – Phosphoglycerate • PGAL – Glyceraldehyde Phosphate

44. Section 2 The Calvin Cycle Chapter 6 The Calvin Cycle

45. Section 2 The Calvin Cycle Chapter 6 The Calvin Cycle

46. Section 2 The Calvin Cycle Chapter 6 The Calvin Cycle

47. Section 2 The Calvin Cycle Chapter 6 Alternative Pathways • The C4 Pathway • Some plants that evolved in hot, dry climates fix carbon through the C4 pathway. These plants have their stomata partially closed during the hottest part of the day. • Certain cells in these plants have an enzyme that can fix CO2 into four-carbon compounds even when the CO2 level is low and the O2 level is high. These compounds are then transported to other cells, where the Calvin cycle ensues.

48. Section 2 The Calvin Cycle Chapter 6 Alternative Pathways, continued • The CAM Pathway • Some other plants that evolved in hot, dry climates fix carbon through the CAM pathway. These plants carry out carbon fixation at night and the Calvin cycle during the day to minimize water loss.

49. Section 2 The Calvin Cycle Chapter 6

50. Section 2 The Calvin Cycle Chapter 6 Factors That Affect Photosynthesis • Light Intensity • The rate of photosynthesis increases as light intensity increases, because more electrons are excited in both photosystems. • However, at some point all of the available electrons are excited, and the maximum rate of photosynthesis is reached. The rate then stays level regardless of further increases in light intensity.