1. Photosynthesis

2. Introduction/ Overview
Powered by light, the green parts of plants produce organic compounds and O2 from CO2 and H2O.
Using glucose as our target product, the equation describing the net process of photosynthesis is:
6CO2 + 12H2O + light energy -> C6H12O6 + 6O2 + 6 H2O
The chemical reactants for the reaction are carbon dioxide (g) and water, the products of the reaction are glucose, oxygen (g) and water.
Photosynthesis consists of two independent pathways called the light-dependent reaction (light reaction) and the light-independent reaction (dark reaction).

4. Introduction/Overview of Photosynthesis

5. Introduction/ Overview
Light Reactions: the energy in sunlight is trapped, O2 is released, and both ATP and NADPH + H+ (hydrogen-carrier molecule) are formed
Dark Reactions: the ATP and NADPH + H+ react with CO2 from the atmosphere and form glucose
The entire process results in the transformation of light energy from the sun into energy stored in the bonds of the glucose molecule.

6. Structure of a Chloroplast**
Each chloroplast has two membranes around a central aqueous space, the stroma.
In the stroma are membranous sacs, the thylakoids.
These have an internal aqueous space, the thylakoid space.
Thylakoids may be stacked into columns called grana.

7. Structure of a Chloroplast
Any green part of a plant has chloroplasts.
However, the leaves are the major site of photosynthesis for most plants.
There are about half a million chloroplasts per square millimeter of leaf surface.
The color of a leaf comes from chlorophyll, the green pigment in the chloroplasts.
Chlorophyll plays an important role in the absorption of light energy during photosynthesis.
O2 exits and CO2 enters the leaf through microscopic pores, stomata, in the leaf.

8. Structure of a Chloroplast
The light reactions take place in the thylakoid membrane
The dark reactions take place in the stroma

9. Structure of a Chloroplast

11. Photosynthesis and Light
The thylakoids convert light energy into the chemical energy of ATP and NADPH.
The entire range of electromagnetic radiation is the electromagnetic spectrum.
The most important segment for life is a narrow band between 380 to 750 nm, visible light.
While the sun radiates a full electromagnetic spectrum, the atmosphere selectively screens out most wavelengths, permitting only visible light to pass in significant quantities.
While light travels as a wave, many of its properties are those of a discrete particle, the photon.
Photons are not tangible objects, but they do have fixed quantities of energy.
The amount of energy packaged in a photon is inversely related to its wavelength.
Photons with shorter wavelengths pack more energy.

12. Electromagnetic Spectrum

13. Why are plants green?

14. Photosynthetic Pigments
When light meets matter, it may be reflected, transmitted, or absorbed. A pigment is a substance which absorbs specific wavelengths of visible light and reflects others.
Different pigments absorb photons of different wavelengths.
A leaf looks green because chlorophyll, the dominant pigment, absorbs red and blue light, while transmitting and reflecting green light.

15. Photosynthetic Pigments
Chlorophyll a, the primary photosynthetic pigment, absorbs best in the red and blue wavelengths, and least in the green. (green in color)
Chlorophyll b, with a slightly different structure than chlorophyll a, has a slightly different absorption spectrum (but also red and blue) and funnels the energy from these wavelengths to chlorophyll a. (green in color)
Carotenoids can funnel the energy from other wavelengths to chlorophyll a and also participate in photoprotection against wavelengths of light (ultraviolet) that could possibly damage the plant cells. (Yellow, orange, red in color)
Xanthrophylls are another type of pigment which are involved in light absorption. (Red in color)
**Note- Chlorophyll b, the Cartotenoids and Xanthrophylls are known as accessory pigments, which absorb other wavelengths of light that Chlorophyll a cannot.

16. Structure of Chlorophyll

18. Absorption vs. Action Spectra Why is there a difference?

20. Light Reactions

21. Light Reactions
The purpose of this reaction is to generate energy storage molecules (ATP) and reducing molecules (NADPH) which will be used to create sugars by reducing carbon dioxide in the second stage of photosynthesis the light independent reaction.
A side product of this reaction is the production of oxygen gas that is produced when electrons and H+ are removed from water during the process called photolysis.
ATP can be generated in two manners. The cyclic pathway involving only one photosystem and the non-cyclic pathway, which involves photolysis and both photosystems.
NADPH is produced only by the non-cyclic pathway, which involves photolysis and both photosystems.

22. Light Reactions Also called Light Dependent Reactions
Pigments that are in the chloroplasts intercept light and begin the light reactions of photosynthesis.
The light reactions occur in two photosystems (located in the thylakoid membrane):
Photosystem: a unit of several hundred chlorophyll a molecules and associated acceptor molecules
-photosystem I (PSI)- 700 nm
-photosystem II (PSII)- 680 nm

23. Photosystems Photosystems I and II are in the thylakoid membrane.
A photosystem acts like a light-gathering “antenna complex” consisting of a few hundred chlorophyll a, chlorophyll b, and carotenoid molecules.
Both work together to use light to generate ATP and NADPH.

24. Two types of photosystems**
Photosystem I-site of cyclic production of ATP also involved in non-cyclic pathway, absorbs light at 700 nm.
Photosystem II- Beginning of non-cyclic pathway. Noncyclic electron flow produces ATP and NADPH in roughly equal quantities. Absorbs light at 680 nm.
II actually occurs first. It’s named that because it was discovered 2nd.

25. Photophosphorylation and Chemiosmosis
Photophosphorylation is the production of ATP, from the energy of light.
Takes place in the thylakoids.
In chemiosmosis, ATP is produced from ADP and Pi when hydrogen ions pass out of the thylakoid through ATP Synthase.

26. Non-cyclic (linear) Pathway
Noncyclic electron flow (linear), the predominant route, produces both ATP and NADPH.
Inputs are light and water.
Light strikes Photosystem II, 2 photons absorbed.
Electrons from chlorophyll pass along to the primary acceptor- plastoquinone.
Water is split (photolysis) into H+ and O-. H+ ions replace those in the chlorophyll.
As the electrons are moved by electron transport to the cytochrome complex and the carrier plastocyanin, and on to Photosystem I, H+ ions are pumped across the thylakoid membrane from the stroma. ATP formed by chemiosmosis.
Electrons pass to Photosystem I, where they are energized, carried by ferredoxin to NADP+ Reductase, and produce NADPH, outside the thylakoid in the stroma.
Oxygen’s link up and are released.
NADPH and ATP are then used later in the Calvin Cycle.

27. Linear Diagram of Light Reactions

28. Cyclic Pathway Only involves Photosystem I. ATP is the product.
These exist because the Calvin Cycle uses more ATP than NADPH.
Ferredoxin cycles some H+’s back into the cytochrome complex, which pumps them out into the thylakoid space.
The Hydrogens then are transformed into ATP by ATP Synthase. This process is called chemiosmosis.

29. Cyclic Diagram

30. Results of Light Dependent Reaction
Noncyclic electron flow pushes electrons from water, where they are at low potential energy, to NADPH, where they have high potential energy.
This process also produces ATP.
Oxygen is a byproduct and is released as a waste product.
Noncyclic electron flow uses proton pumps to move H+ protons across the thylakoid membrane to establish a proton gradient which is used to generate ATP by chemiosmotic phosphorylation.
Cyclic electron flow converts light energy to chemical energy in the form of ATP.
This ATP and NADPH will be used to drive the light independent reaction and the formation of glucose.

31. Diagram of Light Reactions
Site of cyclic ATP
production

32. Mechanical Analogy of the Light Reactions

33. NADPH
NADP+ is an electron acceptor
NADP+ + 2 e- + 2 H NADPH + H+

34. Primary Electron Acceptor ETC Proton Pump Thylakoid Lumen ATP Synthase
PSI
PSII
Thylakoid
Lumen
Water (H2O)
Oxygen (O2)
Hydrogen Ion (H+)
Electron
ATP Synthase

35. Step 1

36. Step 2

37. Step 3

38. Step 3

39. Step 3

40. Step 3

41. Step 3

42. Step 3

43. ~ e
low
high
Step 4

44. Step 4

45. Step 4

46. Step 4

47. Step 4

48. Step 4

49. Step 5

50. Step 5

51. Step 5

52. Step 5

53. Step 5

54. NADP+
NADPH
+ H+
Step 5

55. high
low
Step 6

56. Step 6

57. Step 6

58. Step 6

59. Step 6

60. ADP + P ~ e
Chemiosmosis
ATP
Step 6

61. NADP+
ADP + P
Reactants

62. O2 is
released
into the
atmosphere
NADPH
+ H+ DR
ATP
Products

63. Dark Reaction

64. Dark Reaction
The dark reaction is also known as the Calvin Cycle, after an American scientist (Melvin Calvin) who figured out the pathway.
The dark reaction is also known as the C3 Cycle because the first stable products of this pathway are molecules that contain three carbon atoms.
The Calvin Cycle occurs in the stroma.
Do not require light, but happen in day and night.
Powered by ATP and NADPH.

66. The Calvin Cycle

67. Stage 1- Carbon Fixation
In the carbon fixation phase, each CO2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (RuBP).
This is catalyzed by RuBP carboxylase or rubisco.
The six-carbon intermediate splits in half to form two molecules of
3-phosphoglycerate per CO2.

68. Carbon Fixation

69. Stage 2- Reduction
Reduction is the second phase. During reduction, each 3-phosphoglycerate receives another phosphate group from ATP to form 1,3 bisphosphoglycerate.
A pair of electrons from NADPH reduces each 1,3 bisphosphoglycerate to G3P or phosphoglyceraldehyde (PGAL)- a 3-C sugar
The electrons reduce a carboxyl group to a carbonyl group.
One of these sugars leaves to eventually become glucose or other compounds, the others move on.

70. Stage 2- Reduction

71. Stage 3- Regeneration
In the last phase, regeneration of the CO2 acceptor (RuBP), these five G3P molecules are rearranged to form 3 RuBP molecules.
To do this, the cycle must spend three more molecules of ATP (one per RuBP) to complete the cycle and prepare for the next.

72. Stage 3- Regeneration
PGA
PGAL

73. ~ ~ 6 CO2 unstable compound 6 RuBP 6 ATP 12 PGA e 12 ATP 6 ADP + P e
10 PGAL
12 ADP + P
Glucose C
12 PGAL C
12 NADPH + H+ e-
2 PGAL C
12 NADP+

74. Results of Photosynthesis
For each six CO2 molecules that enter the cycle one glucose molecule is produced.
To produce 1 glucose, 18 ATP’s and 12 NADPH’s are used.
About 30% of the energy available in ATP and NADPH is finally present in the glucose produced.

77. Limiting Factors in Photosynthesis
Temperature plays a role in affecting the rate of photosynthesis. Enzymes involved in the photosynthetic process are directly affected by the temperature of the organism and its environment. Photosynthesis increases to a certain point, then shows a decrease.

78. Limiting Factors
Light Intensity is also a limiting factor, if there is no sunlight, then the photolysis of water cannot occur without the light energy required.

79. Limiting Factors
Carbon Dioxide concentration also plays a factor, due to the supplies of carbon dioxide required in the Calvin cycle stage.
Water availability also plays a role, since with no water, the plant cannot survive.

80. Graphs of the factors

81. The C4 Cycle

82. C4
vs.
CAM

83. The End!