1. Mr. Briner Unit 8.3 Photosynthesis HL
ASM
IB DP Biology
Unit 8.3
Photosynthesis HL

2. Photosynthesis Stages of Photosynthesis
Photosynthesis occurs in two main phases:
Light dependent reaction
Light independent reaction

3. Photosynthesis Photosynthesis Summary Light dependent reaction
Creates electron carriers and ATP
Used to power the Calvin Cycle
Light independent reaction
Creates organic molecules (sugar, etc.)
aka: the Calvin Cycle
Uses energy from the light dependent reactions

4. Photosynthesis
Photosynthesis Summary

5. Photosynthesis

6. Photosynthesis

7. 8.3.U14
The structure of the chloroplast is adapted to its function in photosynthesis.

8. Photosynthesis
Chloroplast structure

9. Photosynthesis Chloroplast structure Thylakoids Stroma
Internal membranes often stacked in grana
Location of the light dependent reaction
Stroma
Surrounding the thylakoids and inside the double membrane
Location of the light independent reaction
Including the Calvin cycle
Often contains starch grains and oil droplets

10. Photosynthesis Chloroplast structure (a) Cell wall (b) Double membrane
(c) Starch grain
(d) Grana
(e) Thylakoid
(f) Stroma

11. Photosynthesis

12. Photosynthesis
Chloroplast structure

13. 8.3.U1
Light-dependent reactions take place in the intermembrane space of the thylakoids.

14. Photosynthesis Stages of Photosynthesis Light Dependent Reaction
Must have light to take place.
Takes place on the thylakoid membranes using the intermembrane space

15. Photosynthesis Stages of Photosynthesis Light Dependent Reaction
Energy of sun is trapped by chlorophyll molecules (reduction)
Photon energy is used to raise the energy of electrons
Which escape the chlorophyll (oxidizing it)
This energy is coupled with reduction of:
ADP + Pi  ATP
NADP+   NADPH + H+ 

16. Photosynthesis

17. 8.3.U2
Light-independent reactions take place in the stroma.

18. Photosynthesis Stages of Photosynthesis Light Independent Reaction
Does not require light
Can occur in both the light and dark periods
Uses the chemical energy from the LDR
Takes place in the stroma
Fixes atmospheric carbon into organic molecules
Such as glucose

19. Photosynthesis

20. 8.3.U4
Absorption of light by photosystems generates excited electrons.

21. Photosynthesis Light dependent reaction Photosystem
System built around chlorophyll to change light energy into chemical energy
Associated with other pigments and proteins
Plants have two Photosystems:
II and I
2 comes first!

22. Photosynthesis Light dependent reaction
Electrons in chlorophyll absorb energy from photons
Raises them from the ground state to higher 'excited' states

23. Photosynthesis Light dependent reaction
Excited electrons have more energy
They are more easily lost from the chlorophyll
This is a form of oxidation

24. 8.3.U3
Reduced NADP and ATP are produced in the light-dependent reactions.

25. Photosynthesis Light dependent reaction
Two main methods of making energy
Non-cyclic photophosphorylation
Passing electrons from PsII to PsI to NADP+
Replacing lost electrons from water photolysis
Cyclic photophosphorylation
Electrons cycle between PsI and a proton pump
Creating ATP

26. Photosynthesis
Light dependent reaction

27. Photosynthesis Light dependent reaction
Non-cyclic photophosphorylation
One-way flow of 2 electrons
From water to PsII
From PsII to ETS
From ETS to PsI
From PsI to NADP+
2 main products
NADPH + H+
ATP

28. Photosynthesis Light dependent reaction: Non-cyclic P.
Photoactivation of Photosystem II:
Photons of light absorbed by pigments
Especially chlorophyll a, at 680 nm
Chlorophyll a is reduced as it gains energy
Chlorophyll a is oxidized when excited e-s move to electron transport system

29. 8.3.U5
Photolysis of water generates electrons for use in the light-dependent reactions.

30. Photosynthesis Light dependent reaction: Non-cyclic P.
Photolysis of water
Oxidized chlorophyll a lacks electrons
Replaced as H2O is split
H2O  2 H+ + 2 e-s + 1/2 O2

31. Photosynthesis Light dependent reaction: Non-cyclic P.
Photolysis of water
Protons
Remain in thylakoid interior, lowering pH
This contributes to chemiosmotic gradient
Used in phosphorylation of ATP
Two electrons
Replace electrons lost by chlorophyll to ETS
1/2 O2 lost to environment as a waste product

32. 8.3.U6
Transfer of excited electrons occurs between carriers in thylakoid membranes.

33. Photosynthesis Light dependent reaction: Non-cyclic P.
Electron transport system
Proteins embedded in thylakoid membrane

34. Photosynthesis Light dependent reaction: Non-cyclic P.
Electron transport system
Transfer energy along a pathway in series of redox reactions
From PsII to PsI
From PsI to NADP+

35. 8.3.U7
Excited electrons from Photosystem II are used to contribute to generate a proton gradient.

36. Photosynthesis Light dependent reaction: Non-cyclic P.
Electron transport system
Some energy used to pump H+s
From stroma to thylakoid interior
Lowering pH
Contributing to chemiosmotic gradient used in phosphorylation of ATP

37. Photosynthesis Light dependent reaction: Non-cyclic P.
Photoactivation of Photosystem I
Photons of visible light absorbed by pigments
Especially chlorophyll a at 700 nm
Chlorophyll is reduced as it gains energy
Chlorophyll a oxidized when excited e-s move to electron transport system

38. Photosynthesis Light dependent reaction: Non-cyclic P.
Reduction of NADP+
Gain of 2 electrons from ETS
Reduces NADP+  NADPH + H+

39. Photosynthesis
Light dependent reaction: Non-cyclic P.

40. Photosynthesis Light dependent reaction
Non-cyclic photophosphorylation
Light is absorbed by chlorophyll in Photosystem II
Chlorophyll absorbs the light energy
Converts this to chemical energy as electrons
Photosystem II is oxidised
Releasing electrons

41. Photosynthesis
Light dependent reaction Non-cyclic photophosphorylation

42. Photosynthesis Light dependent reaction
Non-cyclic photophosphorylation
Electrons from PSII pass along membrane proteins in thylakoids
Membrane proteins use energy from electrons to pump H+s
From the stroma into the inner thylakoids space

43. Photosynthesis
Light dependent reaction Non-cyclic photophosphorylation

44. Photosynthesis Light dependent reaction
Non-cyclic photophosphorylation
At the same time…
Photosystem I (different chlorophyll combination) absorbs light
Chlorophyll molecule releases electrons
In the oxidation of PS I
PS I is now oxidised

45. Photosynthesis
Light dependent reaction Non-cyclic photophosphorylation

46. Photosynthesis Light dependent reaction
Non-cyclic photophosphorylation
Electrons pass from PSI to other membrane proteins
Called ferrodoxins
These reduce NADP+ to NADPH + H+
NADPH is found in the stroma
Used in the light independent reaction

47. Photosynthesis
Light dependent reaction Non-cyclic photophosphorylation

48. Photosynthesis Light dependent reaction
Non-cyclic photophosphorylation
PS I has been oxidised and lost its electron
To continue absorbing light PsI needs new electrons
Source of electrons for this reduction are those from PsII electron carriers

49. Photosynthesis
Light dependent reaction Non-cyclic photophosphorylation

50. Photosynthesis Light dependent reaction
Non-cyclic photophosphorylation
PsII now also needs electons to maintain light absorption
Source of electrons is photolysis of water
Electrons removed from water for PsII
Also makes of H+ and Oxygen
Oxygen is a waste product of photosynthesis
But very important to aerobic organisms

51. Photosynthesis
Light dependent reaction Non-cyclic photophosphorylation

52. Photosynthesis Light dependent reaction
Non-cyclic photophosphorylation
High concentration of H+ in thylakoid lumen
Diffuse back to the stroma
Through ATP Synthase
Drives the phosphorylation of ADP to ATP

53. Photosynthesis
Light dependent reaction Non-cyclic photophosphorylation

54. Photosynthesis

55. 8.3.U9
Excited electrons from Photosystem I are used to reduce NADP.

56. Photosynthesis Light dependent reaction Cyclic photophosphorylation
When there is a lot of NADPH, PSI does not use electron to make NADPH
It sends its 'excited' electron to a proton pump
Cyclic:
Flow of e-s from PsI to ETS and back
Main product = ATP

57. Light dependent reaction Cyclic photophosphorylation
PS I is oxidised
Releasing an 'excited' electron

58. Light dependent reaction Cyclic photophosphorylation
Electron reduces membrane proton pump
Protons are pumped into the thylakoid space
Generates ATP

59. Light dependent reaction Cyclic photophosphorylation
Electrons are cycled back to PS I
To reduce it

60. Photosynthesis

61. Photosynthesis
ANIMATION

62. 8.3.U8
ATP synthase in thylakoids generates ATP using the proton gradient.

63. Photosynthesis Phosphorylation in terms of chemiosmosis
Accumulation of H+
Due to proton pumping
Causes H+ concentration difference
Between thylakoid space (high concentration) and stroma (low concentration)
Protons diffuse through ATP synthase
ADP reduced to ATP
Same mechanism as in cristae membranes during respiration

64. Photosynthesis
Phosphorylation in terms of chemiosmosis

65. Photosynthesis Phosphorylation in terms of chemiosmosis
Thylakoid interior pH = 4 because:
Photolysis of H2O  2 H+ + 2 e-s + 1/2 O2
ETS H+ pump
H+ diffuse down chemiosmotic gradient
From thylakoid interior (pH = 4) through proton channel into stroma (pH = 8)
Energy released used by ATP synthase to phosphorylate ADP + Pi  ATP

66. Photosynthesis

68. Photosynthesis

69. MAJOR SOURCES
Mr. Briner
ASM
Thank you to my favorite sources of information when making these lectures!
John Burrell (Bangkok, TH)
Dave Ferguson (Kobe, JA)
Brent Cornell (Melbourne, AU)
Andrew Allott – Biology for the IB Diploma
C. J.Clegg – Biology for the IB Diploma
Weem, Talbot, Mayrhofer – Biology for the International Baccalaureate
Howard Hugh’s Medical Institute –
Mr. Hoye’s TOK Website –
And all the contributors at

70. Mr. Briner Unit 8.3 Photosynthesis HL
ASM
IB DP Biology
Unit 8.3
Photosynthesis HL

71. 8.3.U10
In the light-independent reactions a carboxylase catalyses the carboxylation of ribulose bisphosphate (RuBP).

72. Photosynthesis Light independent reaction
Energy trapped from sunlight in the light dependent reaction (ATP and NADPH)
Used to fix carbon from carbon dioxide into organic molecules
Reaction called the Calvin Cycle
Takes place in the stroma
Controlled by enzymes

73. Photosynthesis Light independent reaction
Ribulose Bisphosphate Carboxylase (Rubisco)
Enzyme that allows carbon to be fixed into an initial organic molecule
Rubisco is a link between inorganic (non-living) and organic (living)
Primary productivity

74. Photosynthesis

75. Photosynthesis Light independent reaction: Summary
Ribulose bisphosphate carboxylase (rubisco)
Enzyme that carboxylates ribulose bisphosphate
Forming two glycerate 3-phosphate
RuBP + CO2 rubisco 2 GP

76. Photosynthesis

77. 8.3.U11
Glycerate 3-phosphate is reduced to triose phosphate using reduced NADP and ATP.

78. Photosynthesis Light independent reaction: Summary
Reduction of glycerate 3-phosphate (GP) to triose phosphate (TP)
Energy from ATP and NADPH + H+

79. 8.3.U12
Triose phosphate is used to regenerate RuBP and produce carbohydrates.
8.3.U13
Ribulose bisphosphate is reformed using ATP.

80. Photosynthesis Light independent reaction: Summary
Regeneration of RuBP
83% of 3-carbon TP is used to regenerate 5- carbon RuBP
Using energy from ATP

81. Photosynthesis Light independent reaction: Summary
Synthesis of carbohydrates and more
Enzymes convert TP to various products:
Monosaccharides
Glucose, Fructose
Disaccharides
Sucrose
Polysaccharides
Starch
Other products
Lipids, Amino acids, Nucleic acids

82. Photosynthesis Light independent reaction Carbon Fixation
Single carbon in carbon dioxide is added to Ribulose bisphosphate (5C)
Forming 2 molecules of Glycerate-3-phosphate (GP)

83. Photosynthesis Light independent reaction Reduction
GP is reduced to a Triose-phosphate(TP)
In the process NADPH and ATP are oxidised
Providing the energy

84. Photosynthesis Light independent reaction Reduction
TP is used to manufacture a variety of organic molecules
Including Glucose

85. Photosynthesis Light independent reaction Regeneration
Some of the TP is used to regenerate RuBP
Allows more carbon dioxide to be fixed from the atmosphere

86. Photosynthesis

88. 8.3.a1 Calvin’s experiment to elucidate the carboxylation of RuBP.
Mr. Briner
ASM
8.3.a1
Calvin’s experiment to elucidate the carboxylation of RuBP.

89. Photosynthesis Calvin’s experiment
Melvin Calvin, an American biochemist, figured out the steps of the LIR
‘Calvin Cycle’ is named after him
Famous “lollipop” experiment because his equipment looked like an upside-down lollipop

90. Photosynthesis Calvin’s experiment
Calvin used two types of Carbon in his experiments:
Normal C (in the plants)
Radioactive 14C (in CO2)

92. Photosynthesis Calvin’s experiment
Plants put in water to do photosynthesis
CO2 with radioactive 14C is added
Plants will use the radioactive 14C when doing the LIR
Shine light on plants to start photosynthesis

93. Photosynthesis Calvin’s experiment
Kill the plants after different periods of time to stop photosynthesis
Analyze dead plants to see which compounds have 14C
Using chromatography
First GP, then TP, then RuBP and Glucose
Calvin used this information to develop the eponymous cycle

94. TOK QUESTION: Abstract ART?
The lollipop apparatus used to work out the biochemical details of the Calvin cycle shows considerable creativity.
To what extent is the creation of an elegant procedure similar to the creation of a work of art?

95. 8.3.s1
Annotation of a diagram to indicate the adaptations of a chloroplast to its function.

96. Photosynthesis Chloroplast structure and function
Chloroplast interior separated into thylakoids and stroma
Thylakoids = site of light dependent reactions
Large surface area to maximize light absorption
Small space inside thylakoids allows for proton accumulation
Thylakoid interior acidic (pH = 4) with high proton concentration allowing for chemiosmotic gradient

97. Photosynthesis Chloroplast structure and function
Chloroplast interior separated into thylakoids and stroma
Stroma = site of light independent reactions
Stroma is basic (pH = 8)
Where Calvin cycle enzymes function optimally

98. Photosynthesis Chloroplast structure and function
Thylakoid membranes hold photosystem pigments
Pigments anchor in thylakoid membranes by hydrophobic/ hydrocarbon tails
Photolysis enzymes on inner surface of thylakoid membrane
Splits water into oxygen and hydrogen

99. Photosynthesis Chloroplast structure and function
Intrinsic proteins bound in thylakoid membranes allow electron transport
ETC proteins between photosystems II and I pump protons into thylakoid interior
Adding to chemiosmotic gradient
NADP reductase bound to outer thylakoid membrane
Reduces NADP to NADPH for Calvin cycle

100. Photosynthesis Chloroplast structure and function
ATP synthase in the thylakoid membrane
Allows proton flow down chemiosmotic gradient to power photophosphorylation
ATP production in stroma

101. Photosynthesis Chloroplast structure and function
Chloroplast DNA and ribosomes allow for protein synthesis
Chloroplast starch granules for storage of photosynthetic products

102. Photosynthesis
Chloroplast structure and function

103. MAJOR SOURCES
Mr. Briner
ASM
Thank you to my favorite sources of information when making these lectures!
John Burrell (Bangkok, TH)
Dave Ferguson (Kobe, JA)
Brent Cornell (Melbourne, AU)
Andrew Allott – Biology for the IB Diploma
C. J.Clegg – Biology for the IB Diploma
Weem, Talbot, Mayrhofer – Biology for the International Baccalaureate
Howard Hugh’s Medical Institute –
Mr. Hoye’s TOK Website –
And all the contributors at