1. Where It Starts: Photosynthesis
Chapter 6
Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011.

2. 6.1 Green Energy
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. Introduction
Autotroph  organism that makes its own food using carbon from inorganic molecules (CO2) and energy
Heterotroph  organism that obtains energy and carbon form organic compounds
Photosynthesis  metabolic pathway by which most autotrophs capture light energy and use it to make sugars from CO2 and water

4. 6.2 Sunlight as an Energy Source
Electromagnetic energy
Travels in waves
Is organized as photons
Wavelength
Distance between the crests of two successive waves of light
Measured in nanometers (nm): 25 million nm = 1 inch
Visible light  A small part of a spectrum of electromagnetic energy radiating from the sun
Between 380 and 750 nm

5. Electromagnetic Spectrum

6. Photosynthetic Pigments
Photosynthesis begins when photons are absorbed by photosynthetic pigment molecules
Pigment is an organic molecule that absorbs only light of particular wavelengths
Photons not captured are reflected as color

7. Pigments Reflect Color

8. 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
Figure 6.3 in Text

9. Chlorophyll a

10. 6.3 Exploring the Rainbow

11. Engelmann’s Experiment

12. Conclusion: Violet and red light are the best for driving
photosynthesis
Prism
Bacteria
(oxygen
requiring)
alga a
Outcome of T. Engelmann’s experiment.
Fig. 6.4a, p.96

13. Absorption Spectra

14. 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

15. 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

16. 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

17. 6.4 Overview of Photosynthesis
In the Chloroplast!!
Photosynthesis proceeds in two stages
Light-dependent reactions
Light-independent reactions
Summary equation:
6H2O + 6CO O2 + C6H12O6

18. Visual Summary of Photosynthesis

19. 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

20. Sites of Photosynthesis: Chloroplasts
Light-dependent reactions occur at a much-folded thylakoid membrane
Forms a single, continuous compartment inside the stroma
First stage of photosynthesis
light energy + H2O  chemical energy (ATP & NADPH)
Light-independent reactions occur in the stroma (chloroplast’s semifluid interior)
Second stage of photosynthesis
Use ATP & NADPH to assemble sugars from H2O and CO2

21. Sites of Photosynthesis

22. Sites of Photosynthesis

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

24. 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

25. 6.5 Light-Dependent Reactions
In the thylakoid membrane
Light-harvesting complexes
Absorb light energy and pass it to photosystems which then release electrons
Photosystem  a cluster of pigments and proteins that converts light energy to chemical energy in photosynthesis
Electrons enter light-dependent reactions

26. 1. Noncyclic Photophosphorylation
Electrons released from photosystem II flow through an electron transfer chain (ETC)
Electron transfer phosphorylation occurs
Electrons that flow through the ETC set up a hydrogen ion gradient that drives ATP formation
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)
Photolysis  process by which light energy breaks down a molecule

27. Noncyclic Photophosphorylation

28. 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

29. 2. 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

30. 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

31. 6.6 Energy Flow in Photosynthesis

32. 6.6 Energy Flow in Photosynthesis

33. 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

34. 6.7 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
Calvin Benson cycle  light-independent reactions of photosynthesis
Carbon fixation  process by which carbon from an inorganic source gets incorporated into an organic molecule.
Rubisco  carbon fixing enzyme

35. 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

36. Light-Independent Reactions

37. 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

38. 6.8 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  gaps that open on plant surfaces that allow water vapor and gases to diffuse across the epidermis
O2 from photosynthesis cannot escape

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

40. Plant Adaptations to Environment
C3 plants
Photorespiration
Reaction in which rubisco attaches oxygen instead of CO2 to ribulose bisphosphate
Plant loses carbon instead of fixing it.
Extra energy is need to make sugars on dry days

41. Plant Adaptations to Environment
C4 plants
Carbon fixation occurs twice, in two different cells to minimize photorespiration
First reactions release CO2 near rubisco, limit photorespiration when stomata are closed
Examples: Corn, bamboo

42. 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

43. Plant Adaptations to Environment
CAM plants
Type of C4 plant that conserves water by fixing carbon twice, at different times of the day in the same cell
Day time  C4 reactions
Night time  Calvin-Benson cycle
Open stomata and fix carbon at night

44. 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

45. 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
ATP delivers energy, and NADPH delivers electrons and hydrogens to the reaction sites
Details of the reactions vary among organisms

46. Animation: C3-C4 comparison

47. Animation: Calvin-Benson cycle

48. Animation: Energy changes in photosynthesis

49. Animation: Noncyclic pathway of electron flow

50. Animation: Photosynthesis overview

51. Animation: Sites of photosynthesis

52. Animation: Wavelengths of light