1. Overview: The Process That Feeds the Biosphere
Photosynthesis is the process that converts solar energy into chemical energy
Directly or indirectly, photosynthesis nourishes almost the entire living world

2. Autotrophs sustain themselves without eating anything derived from other organisms
Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules
Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from water and carbon dioxide

4. Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes
These organisms feed not only themselves but also the entire living world

5. LE 10-2 Plants Unicellular protist Purple sulfur bacteria
10 µm
Purple sulfur
bacteria
1.5 µm
Multicellular algae
Cyanobacteria
40 µm

6. Heterotrophs obtain their organic material from other organisms
Heterotrophs are the consumers of the biosphere
Almost all heterotrophs, including humans, depend on photoautotrophs for food and oxygen

7. Concept 10.1: Photosynthesis converts light energy to the chemical energy of food
Chloroplasts are organelles that are responsible for feeding the vast majority of organisms
Chloroplasts are present in a variety of photosynthesizing organisms

8. Chloroplasts: The Sites of Photosynthesis in Plants
Leaves are the major locations of photosynthesis
Their green color is from chlorophyll, the green pigment within chloroplasts
Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast
Through microscopic pores called stomata, CO2 enters the leaf and O2 exits

9. Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf
A typical mesophyll cell has chloroplasts
The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana
Chloroplasts also contain stroma, a dense fluid

10. LE 10-3 Leaf cross section Vein Mesophyll Stomata CO2 O2
Mesophyll cell
Chloroplast
5 µm
Outer
membrane
Thylakoid
Stroma
Granum
Thylakoid
space
Intermembrane
space
Inner membrane
1 µm

11. Tracking Atoms Through Photosynthesis: Scientific Inquiry
Photosynthesis can be summarized as the following equation:
6 CO H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O

12. The Splitting of Water
Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

13. LE 10-4
Reactants:
6 CO2
12 H2O
Products:
C6H12O6
6 H2O
6 O2

14. Photosynthesis as a Redox Process
Photosynthesis is a redox process in which water is oxidized and carbon dioxide is reduced

15. The Two Stages of Photosynthesis: A Preview
Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)
The light reactions (in the thylakoids) split water, release O2, produce ATP, and form NADPH
The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH
The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules

16. LE 10-5_1
H2O
Light
LIGHT
REACTIONS
Chloroplast

17. LE 10-5_2
H2O
Light
LIGHT
REACTIONS
ATP
NADPH
Chloroplast O2

18. LE 10-5_3 H2O CO2 Light NADP+ ADP + CALVIN CYCLE LIGHT REACTIONS ATP
NADPH
Chloroplast
[CH2O]
(sugar) O2

19. Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH
Chloroplasts are solar-powered chemical factories
Their thylakoids transform light energy into the chemical energy of ATP and NADPH

20. The Nature of Sunlight
Light is a form of electromagnetic energy, also called electromagnetic radiation
Like other electromagnetic energy, light travels in rhythmic waves
Wavelength = distance between crests of waves
Wavelength determines the type of electromagnetic energy
Light also behaves as though it consists of discrete particles, called photons

21. The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation
Visible light consists of colors we can see, including wavelengths that drive photosynthesis

22. 1 m (109 nm) 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 103 m Gamma rays
LE 10-6
1 m
(109 nm)
10–5 nm
10–3 nm
1 nm
103 nm
106 nm
103 m
Gamma
rays
Micro-
waves
Radio
waves
X-rays UV
Infrared
Visible light
380
450
500
550
600
650
700
750 nm
Shorter wavelength
Longer wavelength
Higher energy
Lower energy

23. Photosynthetic Pigments: The Light Receptors
Pigments are substances that absorb visible light
Different pigments absorb different wavelengths
Wavelengths that are not absorbed are reflected or transmitted
Leaves appear green because chlorophyll reflects and transmits green light
Animation: Light and Pigments

24. LE 10-7 Light Reflected light Chloroplast Absorbed Granum light
Transmitted
light

25. A spectrophotometer measures a pigment’s ability to absorb various wavelengths
This machine sends light through pigments and measures the fraction of light transmitted at each wavelength

26. Slit moves to pass light of selected wavelength Green light
LE 10-8a
White
light
Refracting
prism
Chlorophyll
solution
Photoelectric
tube
Galvanometer
100
The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light.
Slit moves to pass light
of selected wavelength
Green
light

27. Slit moves to pass light of selected wavelength
LE 10-8b
White
light
Refracting
prism
Chlorophyll
solution
Photoelectric
tube
100
Slit moves to pass light
of selected wavelength
The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light.
Blue
light

28. An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength
The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis
An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process

29. Absorption of light by chloroplast pigments
LE 10-9a
Chlorophyll a
Chlorophyll b
Carotenoids
Absorption of light by
chloroplast pigments
400
500
600
700
Wavelength of light (nm)
Absorption spectra

30. synthesis (measured Rate of photo- by O2 release)
LE 10-9b
synthesis (measured
Rate of photo-
by O2 release)
Action spectrum

31. The action spectrum of photosynthesis was first demonstrated in 1883 by Thomas Engelmann
In his experiment, he exposed different segments of a filamentous alga to different wavelengths
Areas receiving wavelengths favorable to photosynthesis produced excess O2
He used aerobic bacteria clustered along the alga as a measure of O2 production

32. Engelmann’s experiment
LE 10-9c
Aerobic bacteria
Filament
of algae
400
500
600
700
Engelmann’s experiment

33. Chlorophyll a is the main photosynthetic pigment
Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis
Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll

34. oo Porphyrin ring: light-absorbing
CH3
in chlorophyll a
CHO
in chlorophyll b
Porphyrin ring:
light-absorbing
“head” of molecule; note magnesium atom at center
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of chloroplasts; H atoms not shown

35. Excitation of Chlorophyll by Light
When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable
When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence
If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat

36. Excitation of isolated chlorophyll molecule Fluorescence
Excited
state e–
Heat
Energy of electron
Photon
(fluorescence)
Photon
Ground
state
Chlorophyll
molecule
Excitation of isolated chlorophyll molecule
Fluorescence

37. A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes
A photosystem consists of a reaction center surrounded by light-harvesting complexes
The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center

38. A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a
That’s the first step in the light reactions

39. (INTERIOR OF THYLAKOID)
LE 10-12
Thylakoid
Photosystem
STROMA
Photon
Light-harvesting
complexes
Reaction
center
Primary electron
acceptor
Thylakoid membrane e–
Transfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)

40. There are two types of photosystems in the thylakoid membrane
Photosystem II functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm
Photosystem I is best at absorbing a wavelength of 700 nm

41. Noncyclic Electron Flow
During the light reactions, there are two possible routes for electron flow: cyclic and noncyclic
Noncyclic electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH

42. LE 10-13_1 Primary acceptor e– Energy of electrons Light P680
H2O
CO2
Light
NADP+
ADP
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH O2
[CH2O] (sugar)
Primary
acceptor e–
Energy of electrons
Light
P680
Photosystem II
(PS II)

43. LE 10-13_2 Primary acceptor H2O e– 2 H+ + O2 1/2 Energy of electrons
CO2
Light
NADP+
ADP
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH O2
[CH2O] (sugar)
Primary
acceptor
H2O e–
2 H+ +
1/2 O2 e– e–
Energy of electrons
Light
P680
Photosystem II
(PS II)

44. LE 10-13_3 Primary Electron transport chain acceptor Pq H2O e–
CO2
Light
NADP+
ADP
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH O2
[CH2O] (sugar)
Primary
acceptor
Electron transport chain Pq
H2O e–
Cytochrome
complex
2 H+ +
1/2 O2 e– Pc e–
Energy of electrons
Light
P680
ATP
Photosystem II
(PS II)

45. LE 10-13_4 Primary acceptor Primary Electron transport chain acceptor
H2O
CO2
Light
NADP+
ADP
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
Electron transport chain Pq e–
H2O e–
Cytochrome
complex
2 H+ +
1/2 O2 e– Pc e–
P700
Energy of electrons
Light
P680
Light
ATP
Photosystem I
(PS I)
Photosystem II
(PS II)

46. LE 10-13_5 ADP Electron Transport chain Primary acceptor Primary
H2O
CO2
Light
NADP+
ADP
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH O2
Electron
Transport
chain
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
Electron transport chain Fd Pq e– e–
H2O e– e–
NADP+
Cytochrome
complex
2 H+
NADP+
reductase
+ 2 H+ +
1/2 O2
NADPH Pc e–
+ H+
Energy of electrons e–
P700
Light
P680
Light
ATP
Photosystem I
(PS I)
Photosystem II
(PS II)

47. NADPH Mill makes ATP Photosystem II Photosystem I LE 10-14 e– ATP e–
Photon e–
Photon
Photosystem II
Photosystem I

48. Cyclic Electron Flow
Cyclic electron flow uses only photosystem I and produces only ATP
Cyclic electron flow generates surplus ATP, because more ATP than NADPH is needed by the Calvin cycle

49. Primary acceptor Primary Fd acceptor Fd NADP+ Pq NADP+ reductase
LE 10-15
Primary
acceptor
Primary
acceptor Fd Fd
NADP+ Pq
NADP+
reductase
Cytochrome
complex
NADPH Pc
Photosystem I
ATP
Photosystem II

50. A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy
Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP
The spatial organization of chemiosmosis differs in chloroplasts and mitochondria

51. LE 10-16 Mitochondrion Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST
Diffusion
Intermembrane
space
Thylakoid
space
Electron
transport
chain
Membrane
Key
ATP
synthase
Matrix
Stroma
Higher [H+]
Lower [H+]
ADP + P i
ATP H+

52. LE 10-17 STROMA (Low H+ concentration) Cytochrome complex
H2O
CO2
Light
NADP+
ADP
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH
STROMA
(Low H+ concentration) O2
[CH2O] (sugar)
Cytochrome
complex
Photosystem II
Photosystem I
Light
NADP+
reductase
Light
2 H+ Fd
NADP+ + 2H+
NADPH
+ H+ Pq Pc
H2O
THYLAKOID SPACE
(High H+ concentration)
1/2 O2
+2 H+
2 H+ To
Calvin
cycle
Thylakoid
membrane
ATP
synthase
STROMA
(Low H+ concentration)
ADP +
ATP P i H+

53. Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO2 to sugar
The Calvin cycle starts and ends with the same molecule (RuBP).
The cycle makes sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH
Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)
For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2

54. Phase 1: Carbon fixation Ribulose bisphosphate
LE 10-18_1
H2O
CO2
Input
Light 3
(Entering one
at a time)
NADP+
ADP
CO2
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
Phase 1: Carbon fixation
NADPH
Rubisco O2
[CH2O] (sugar) 3 P P
Short-lived
intermediate 3 P P 6 P
Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate 6
ATP
6 ADP
CALVIN
CYCLE

55. LE 10-18_2 Input 3 (Entering one at a time) CO2
H2O
CO2
Input
Light 3
(Entering one
at a time)
NADP+
ADP
CO2
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
Phase 1: Carbon fixation
NADPH
Rubisco O2
[CH2O] (sugar) 3 P P
Short-lived
intermediate 3 P P 6 P
Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate 6
ATP
6 ADP
CALVIN
CYCLE 6 P P
1,3-Bisphosphoglycerate 6
NADPH
6 NADP+ 6 P i 6 P
Glyceraldehyde-3-phosphate
(G3P)
Phase 2:
Reduction 1 P
G3P
(a sugar)
Glucose and
other organic
compounds
Output

56. LE 10-18_3 Input 3 (Entering one at a time) CO2
H2O
CO2
Input
Light 3
(Entering one
at a time)
NADP+
ADP
CO2
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
Phase 1: Carbon fixation
NADPH
Rubisco O2
[CH2O] (sugar) 3 P P
Short-lived
intermediate 3 P P 6 P
Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate 6
ATP
6 ADP
3 ADP
CALVIN
CYCLE 3 6 P P
ATP
1,3-Bisphosphoglycerate 6
NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+ 6 P i 5 P
G3P 6 P
Glyceraldehyde-3-phosphate
(G3P)
Phase 2:
Reduction 1 P
G3P
(a sugar)
Glucose and
other organic
compounds
Output

57. Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates
Dehydration is a problem for plants, sometimes requiring tradeoffs with other metabolic processes, especially photosynthesis
On hot, dry days, plants close stomata, which conserves water but also limits photosynthesis
The closing of stomata reduces access to CO2 and causes O2 to build up
These conditions favor a seemingly wasteful process called photorespiration

58. Photorespiration: An Evolutionary Relic?
In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound
In photorespiration, rubisco adds O2 to the Calvin cycle instead of CO2
Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar

59. Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2
In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle

60. C4 Plants
C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells
These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

61. LE 10-19 Mesophyll cell Mesophyll cell CO2 Photosynthetic
cells of C4 plant
leaf
PEP carboxylase
Bundle-
sheath
cell
The C4 pathway
Oxaloacetate (4 C)
PEP (3 C)
Vein
(vascular tissue)
ADP
Malate (4 C)
ATP
C4 leaf anatomy
Pyruvate (3 C)
Bundle- sheath
cell
Stoma
CO2
CALVIN
CYCLE
Sugar
Vascular
tissue

62. CAM Plants
CAM plants open their stomata at night, incorporating CO2 into organic acids
Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle

63. LE 10-20 Sugarcane Pineapple C4 CAM CO2 CO2 Mesophyll cell
CO2 incorporated
into four-carbon
organic acids
(carbon fixation)
Night
Organic acid
Organic acid
Bundle-
sheath
cell
CO2
CO2
Day
Organic acids
release CO2 to
Calvin cycle
CALVIN
CYCLE
CALVIN
CYCLE
Sugar
Sugar
Spatial separation of steps
Temporal separation of steps

64. The Importance of Photosynthesis: A Review
The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds
Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells
In addition to food production, photosynthesis produces the oxygen in our atmosphere

65. Photosystem II Electron transport chain Photosystem I
Light reactions
Calvin cycle
H2O
CO2
Light
NADP+
ADP + P i
RuBP
3-Phosphoglycerate
Photosystem II
Electron transport
chain
Photosystem I
ATP
G3P
Starch
(storage)
NADPH
Amino acids
Fatty acids
Chloroplast O2
Sucrose (export)