1. Where It Starts: Photosynthesis
Chapter 6

2. Introduction
Before photosynthesis evolved, Earth’s atmosphere had little free oxygen
Oxygen released during photosynthesis changed the atmosphere
Favored evolution of new metabolic pathways, including aerobic respiration

3. Then and Now

4. 6.1 Sunlight as an Energy Source
Visible light
A small part of a spectrum of electromagnetic energy radiating from the sun
Electromagnetic energy
Travels in waves
Is organized as photons

5. Electromagnetic Spectrum

6. Wavelengths of visible light (in nanometers)
400
500
600
700
Wavelengths of visible light (in nanometers)
Fig. 6.2b, p.94

7. Photosynthetic Pigments
Photosynthesis begins when photons are absorbed by photosynthetic pigment molecules
Pigment molecules absorb only light of particular wavelengths
Photons not captured are reflected as color

8. Pigments Reflect Color

9. Major Photosynthetic Pigments
Chlorophyll a
Main photosynthetic pigment
Absorbs violet and red light (appears green)
Chlorophyll b, carotenoids, phycobilins
Absorb additional wavelengths
Collectively, photosynthetic pigments absorb almost all of wavelengths of visible light

10. Chlorophyll a

11. 6.2 Exploring the Rainbow

12. Engelmann’s Experiment

13. Outcome of T. Engelmann’s experiment.
alga a
Outcome of T. Engelmann’s experiment.
Fig. 6.4a, p.96

14. Absorption Spectra

15. Wavelength (nanometers)80 60
Light absorption (%) 40 20
400
500
600
700
Wavelength (nanometers)
b Absorption spectra for chlorophyll a (solid graph
line) and chlorophyll b (dashed line). Compare these
graphs with the clustering of bacteria shown in (a).
Fig. 6.4b, p.96

16. Wavelength (nanometers)80 60
Light absorption (%) 40 20
400
500
600
700
Wavelength (nanometers)
c Absorption spectra for beta-carotene (solid line)
and one of the phycobilins (dashed line).
Fig. 6.4c, p.96

17. Key Concepts: THE RAINBOW CATCHERS
A great one-way flow of energy through the world of life starts after chlorophylls and other pigments absorb the energy of visible light from the sun’s rays
In plants, some bacteria, and many protists, that energy ultimately drives the synthesis of glucose and other carbohydrates

18. 6.3 Overview of Photosynthesis
Photosynthesis proceeds in two stages
Light-dependent reactions
Light-independent reactions
Summary equation:
6H2O + 6CO O2 + C6H12O6

19. Visual Summary of Photosynthesis

20. end products (e.g., sucrose, starch, cellulose)
Light-
Dependent
Reactions
sunlight
H2O O2
ADP + Pi
ATP
NADP+
NADPH
Light-
Independent
Reactions
Calvin-Benson cycle
CO2
H2O
phosphorylated glucose
end products (e.g., sucrose, starch, cellulose)
Fig. 6.13, p.104

21. Sites of Photosynthesis: Chloroplasts
Light-dependent reactions occur at a much-folded thylakoid membrane
Forms a single, continuous compartment inside the stroma (chloroplast’s semifluid interior)
Light-independent reactions occur in the stroma

22. Sites of Photosynthesis

23. upper epidermis photosynthetic cells leaf vein lower epidermis
a Zooming in on a
photosynthetic cell.
lower epidermis
Fig. 6.6a, p.97

24. Sites of Photosynthesis

25. two outer membranes of chloroplasts stroma part of thylakoid membrane
system bathed in stroma:
thylakoid
compartment,
cutaway view
b Chloroplast structure. No matter how highly folded, its thylakoid membrane system forms a single, continuous compartment in the stroma.
Fig. 6.6b, p.97

26. Sites of Photosynthesis

27. c In chloroplasts, ATP and NADPH form in the light-dependent stage of
sunlight O2
H2O
CO2
CHLOROPLAST
NADPH, ATP
light-
dependent
reactions
light-
independent
reactions
NADP+, ADP
sugars
CYTOPLASM
c In chloroplasts, ATP and NADPH form in the light-dependent stage of
photosynthesis, which occurs at the thylakoid membrane. The second stage,
which produces sugars and other carbohydrates, proceeds in the stroma.
Fig. 6.6c, p.97

28. Products of Light-Dependent Reactions
Typically, sunlight energy drives the formation of ATP and NADPH
Oxygen is released from the chloroplast (and the cell)

29. Key Concepts: OVERVIEW OF PHOTOSYNTHESIS
Photosynthesis proceeds through two stages in chloroplasts of plants and many types of protists
First, pigments in a membrane inside the chloroplast capture light energy, which is converted to chemical energy
Second, chemical energy drives synthesis of carbohydrates

30. 6.4 Light-Dependent Reactions
Two types of photosystems
In thylakoid membrane
Light-harvesting complexes
Absorb light energy and pass it to photosystems which then release electrons
Electrons enter light-dependent reactions

31. Noncyclic Photophosphorylation
Electrons released from photosystem II flow through an electron transfer chain
At end of chain, they enter photosystem I
Photon energy causes photosystem I to release electrons, which end up in NADPH
Photosystem II replaces lost electrons by pulling them from water (photolysis)

32. Noncyclic Photophosphorylation

33. electron transfer chain
light energy
light energy
electron transfer chain
NADPH
Photosystem II
Photosystem I
THYLAKOID
COMPARTMENT
THYLAKOID
MEMBRANE
oxygen
(diffuses away)
STROMA
Fig. 6.8b, p.99

34. Cyclic Photophosphorylation
Electrons released from photosystem I enter an electron transfer chain, then cycle back to photosystem I
NADPH does not form, oxygen is not released

35. ATP Formation
In both pathways, electron flow through electron transfer chains causes H+ to accumulate in the thylakoid compartment
A hydrogen ion gradient builds up across the thylakoid membrane
H+ flows back across the membrane through ATP synthases
Results in formation of ATP in the stroma

36. 6.5 Energy Flow in Light-Dependent Reactions

37. PHOTOPHOSPHORYLATION
excited
P700
energy
light energy
P700
Photosystem I
CYCLIC
PHOTOPHOSPHORYLATION
a Energy from
light-harvesting
complexes causes
photosystem I to
lose electrons.
b Electrons give up energy
as they pass through an
electron transfer chain. The
energy drives H+ across the
thylakoid membrane, against
its gradient. The electrons
reenter photosystem I.
Fig. 6.9ab, p.100

38. 6.5 Energy Flow in Light-Dependent Reactions

39. PHOTOPHOSPHORYLATION
Photosystem II
excited
P700
excited
P680
NADPH
light energy
energy
light energy
P700
P680
Photosystem I
water
O2 + H+
NONCYCLIC
PHOTOPHOSPHORYLATION
c. Energy from a light- harvesting complex
drives electrons out of
photosystem II. Then,
the photosystem pulls
replacement electrons
d. Electrons from photosystem
II pass through an electron
transfer chain. Energy lost at
each step moves H+ across
the thylakoid membrane. At the
end of the chain, the electrons
enter photosystem
e. NADP+ combines with
hydrogen and with electrons
driven from photosystem II by
energy from a light-harvesting
complex.
Fig. 6.9cde, p.100

40. Key Concepts: MAKING ATP AND NADPH
In the first stage of photosynthesis, sunlight energy is converted to the chemical bond energy of ATP
The coenzyme NADPH forms in a pathway that also releases oxygen

41. 6.6 Light Independent Reactions: The Sugar Factory
Light-independent reactions proceed in the stroma
Carbon fixation: Enzyme rubisco attaches carbon from CO2 to RuBP to start the Calvin–Benson cycle

42. Calvin–Benson Cycle Cyclic pathway makes phosphorylated glucose
Uses energy from ATP, carbon and oxygen from CO2, and hydrogen and electrons from NADPH
Reactions use glucose to form photosynthetic products (sucrose, starch, cellulose)
Six turns of Calvin–Benson cycle fix six carbons required to build a glucose molecule from CO2

43. Light-Independent Reactions

44. stroma
Fig. 6.10b, p.101

45. a CO2 in air spaces inside a leaf diffuses into a photosynthetic
cell. Six times, rubisco attaches a
carbon atom from CO2 to the RuBP
that is the starting compound for
the Calvin–Benson cycle.
f It takes six turns of the
Calvin–Benson cycle (six
carbon atoms) to produce
one glucose molecule and
regenerate six RuBP.
6CO2
b Each PGA molecule gets
a phosphate
group from ATP, plus hydrogen
and electrons
from NADPH.
The resulting
intermediate is
called PGAL.
e Ten of the PGAL
get phosphate groups from ATP. In terms of energy, this primes them for an uphill
run—for the
endergonic synthesis reactions that
regenerate RuBP.
6 RuBP
12 PGA 12
ATP
6 ADP
Calvin-Benson
cycle
12 ADP + 12 Pi 6
ATP 12
NADPH
4 Pi
12 NADP+
d The phosphorylated
glucose enters reactions that form carbohydrate
products—mainly sucrose, starch, and cellulose.
c Two of the twelve PGAL
molecules combine to form
a molecule of glucose with
an attached phosphate group.
10 PGAL
12 PGAL
1 Pi
phosphorylated glucose
Fig. 6.10, p.101

46. 6.7 Adaptations: Different Carbon-Fixing Pathways
Environments differ
Plants have different details of sugar production in light-independent reactions
On dry days, plants conserve water by closing their stomata
O2 from photosynthesis cannot escape

47. Plant Adaptations to Environment
C3 plants
High O2 level; Rubisco attaches to O2 instead of CO2 to RuBP; Photorespiration reduces efficiency of sugar production

48. a C3 plants. On dry days, stomata close
CO2
glycolate
RuBP
Calvin-Benson cycle
PGA
sugar
ATP
NADPH
a C3 plants. On dry days, stomata close
and oxygen accumulates in air spaces
inside leaves. The high concentration of
oxygen makes rubisco attach oxygen
instead of carbon to RuBP. Cells lose
carbon and energy as they make sugars.
Fig. 6.11a2, p.102

49. Plant Adaptations to Environment
C4 plants
Carbon fixation occurs twice
First reactions release CO2 near rubisco, limit photorespiration when stomata are closed

50. C4 cycle Calvin-Benson cycle
CO2 from inside plant C4
cycle
oxaloacetate
CO2
RuBP
Calvin-Benson cycle
PGA
sugar
b C4 plants. Oxygen also builds
up in the air spaces inside the leaves
when stomata close. An additional
pathway in these plants keeps the
CO2 concentration high enough to
prevent rubisco from using oxygen.
Fig. 6.11b2, p.102

51. Plant Adaptations to Environment
CAM plants
Open stomata and fix carbon at night

52. C4 cycle Calvin-Benson cycle
CO2 from outside plant C4
cycle
oxaloacetate
night
day
CO2
RuBP
Calvin-Benson cycle
PGA
sugar
c CAM plants open stomata and
fix carbon with a C4 pathway at night.
When stomata are closed during the
day, organic compounds made during
the night are converted to CO2 that
enters the Calvin–Benson cycle.
Fig. 6.11c2, p.102

53. Key Concepts: MAKING SUGARS
Second stage is the “synthesis” part of photosynthesis
Enzymes speed assembly of sugars from carbon and oxygen atoms, both from carbon dioxide
Reactions use ATP and NADPH that form in the first stage of photosynthesis

54. Key Concepts: MAKING SUGARS (cont.)
ATP delivers energy, and NADPH delivers electrons and hydrogens to the reaction sites
Details of the reactions vary among organisms

55. 6.8 A Burning Concern
Photoautotrophs remove CO2 from atmosphere; metabolic activity of organisms puts it back
Human activities disrupt the carbon cycle
Add more CO2 to the atmosphere than photoautotrophs can remove
Imbalance contributes to global warming

56. Fossil Fuel Emissions

57. Animation: C3-C4 comparison
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58. Animation: Calvin-Benson cycle
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59. Animation: Energy changes in photosynthesis
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60. Animation: Noncyclic pathway of electron flow
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61. Animation: Photosynthesis overview
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62. Animation: Sites of photosynthesis
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63. Animation: Wavelengths of light
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