1. Chapter 6 Where It Starts–Photosynthesis

2. 6.1 Biofuels Autotrophs make their own food
Harvest energy from the environment and carbon from inorganic molecules
Photosynthesis obtains energy from sunlight
Plants and most autotrophs use photosynthesis to make organic molecules used as food
Heterotrophs get their carbon from organic molecules made by autotrophs

3. 6.1 Biofuels
Fossil fuels (the remains of ancient forests) are a limited resource
Biofuels are made from organic matter that is not fossilized – a renewable resource
Produced mainly from crops such as corn, soybeans, and sugarcane in the U.S.
Researchers are looking for ways to use non-food-plant matter such as switchgrass and agricultural wastes

4. 6.2 Sunlight as an Energy Source
Energy flow through nearly all ecosystems on Earth begins when photosynthesizers intercept energy from the sun
Photosynthetic organisms use pigments to capture the energy of sunlight and convert it to chemical energy – the energy stored in chemical bonds

5. Properties of Light
Visible light is part of an electromagnetic spectrum of energy radiating from the sun
Travels in waves
Organized into photons
Wavelength
The distance between the crests of two successive waves of light (nm)
Shorter wavelength have greater energy

6. Electromagnetic Spectrum of Radiant Energy
range of heat escaping from Earth’s surface
shortest wavelengths (highest energy)
range of most radiation reaching Earth’s surface
longest wavelengths (lowest energy)
visible light
gamma rays
ultraviolet radiation
near-infrared radiation
radiation infrared
x-rays
microwaves
radio waves
Figure 6.2 Properties of light.
A Electromagnetic radiation moves through space in waves that we measure in nanometers (nm). Visible light makes up a very small part of this energy. raindrops or a prism can separate visible light’s different wavelengths, which we see as different colors. About 25 million nanometers are equal to 1 inch.
400 nm
500 nm
600 nm
700 nm

7. Pigments: The Rainbow Catchers
Different wavelengths form colors of the rainbow
Photosynthesis uses wavelengths of nm
Pigment
An organic molecule that selectively absorbs light of specific wavelengths
Chlorophyll a
The most common photosynthetic pigment
Absorbs violet and red light (appears green)

8. Photosynthetic Pigments
Collectively, chlorophyll and accessory pigments absorb most wavelengths of visible light
Certain electrons in pigment molecules absorb photons of light energy, boosting electrons to a higher energy level
Energy is captured and used for photosynthesis

9. Examples of Photosynthetic Pigments
H2C
Produced by
H3C
Pigment
Color
Plants
Protists
Bacteria
Archaea
Chlorophyll a
Other chlorophylls
green green ●
Phycobilins
H3C N
CH3
H3C O N Mg N
CH3
CH3
CH3
CH3 O O N
CH3
H3C O
phycocyanobilin phycoerythrobilin phycoviolobilin
blue red violet ● ●
CH3 O
Carotenoids beta-carotene
orange
● ● ●
lycopene lutein zeaxanthin fucoxanthin
red yellow yellow brown
● ●
Anthocyanins
red, blue ●●
Retinal
violet ●
H3C H3C
H3C CH3
CH3
CH3
CH3
CH3
CH3
CH3
Figure 6.4 Examples of photosynthetic pigments. photosynthetic pigments can collectively absorb almost all visible light wavelengths. Left, the light-catching part of a pigment (shown in color) is the region in which single bonds alternate with double bonds. These and many other pigments (including heme, Section 5.6) are derived from evolutionary remodeling of the same compound. Animals convert dietary beta-carotene into a similar pigment (retinal) that is the basis of vision.
H3C CH3
CH3
CH3 O H
CH3

10. 6.3 Exploring the Rainbow
Photosynthetic pigments work together to harvest light of different wavelengths
Engelmann identified colors of light that drive photosynthesis (violet and red) by using a prism to divide light into colors
Algae using these wavelengths gave off the most oxygen

11. Photosynthesis and Wavelengths of Light
bacteria
Figure 6.4 Discovery that photosynthesis is driven by particular wavelengths of light.
A Light micrograph of cells in a strand of Chladophora. Theodor Engelmann used this green alga in an early experiment to show that certain colors of light are best for photosynthesis.
B Engelmann directed light through a prism so that bands of colors crossed a water droplet on a microscope slide. The water held a strand of Chladophora and oxygen-requiring bacteria. The bacteria clustered around the algal cells that were releasing the most oxygen—the ones that were most actively engaged in photosynthesis. Those cells were under red and violet light. These results constituted one of the first absorption spectra.
alga
400 nm
500 nm
600 nm
700 nm
Wavelength

12. Absorption Spectra
Most photosynthetic organisms use a combination of pigments to drive photosynthesis
An absorption spectrum shows which wavelengths each pigment absorbs best
Organisms in different environments use different pigments

13. Absorption Spectra chlorophyll b phycoerythrobilin phycocyanobilin
β-carotene
chlorophyll a
Light absorption
Figure 6.4 Discovery that photosynthesis is driven by particular wavelengths of light.
C Absorption spectra of chlorophylls a and b , β-carotene, and two phycobilins reveal the efficiency with which these pigments absorb different wavelengths of visible light. Line color indicates the characteristic color of each pigment.
400 nm
500 nm
600 nm
700 nm
Wavelength

14. 6.4 Overview of Photosynthesis
In photosynthetic eukaryotes, photosynthesis occurs in chloroplasts
An organelle that specializes in photosynthesis in plants and many protists
Photosynthesis occurs in two stages
Light-dependent reactions
Light-independent reactions

15. Summary: Photosynthesis
6CO2 + 6H2O → light energy → C6H12O6 + 6O2

16. First Stage of Photosynthesis – Light-Dependent Reactions
Noncyclic pathway
Light energy is transferred to ATP and NADPH
Water molecules are split, releasing O2
Occurs in the thylakoid membrane
Folded membrane that make up thylakoids
Contains clusters of light-harvesting pigments that absorb photons of different energies and convert light energy into chemical energy

17. Second Stage of Photosynthesis – Light-Independent Reactions
Energy in ATP and NADPH drives synthesis of glucose and other carbohydrates from CO2 and water
Occurs in the stroma
A semifluid matrix surrounded by the two outer membranes of the chloroplast
Sugars are built in the stroma

18. The Chloroplast two outer membranes of chloroplast stroma
part of thylakoid membrane system:
Figure 6.6 Zooming in on a chloroplast, the site of photosynthesis in a plant cell. The micrograph shows chloroplasts in cells of a moss leaf.
thylakoid compartment, cutaway view

19. 6.5 Light-Dependent Reactions
Light-dependent reactions convert light energy to the energy of chemical bonds
Photons boost electrons in pigments to higher energy levels
Light-harvesting complexes absorb the energy
Electrons are released from special pairs of chlorophyll a molecules in photosystems
Electrons may be used in noncyclic or cyclic pathways of ATP formation

20. The Thylakoid Membrane
Figure 6.7 A view of some of the components of the thylakoid membrane as seen from the stroma. Molecules of electron transfer chains and ATP synthases are also present, but not shown for clarity.
photosystem
light-harvesting complex

21. The Noncyclic Pathway – Photosystems and ETCs
Photosystems (type II and type I) contain “special pairs” of chlorophyll a molecules that eject electrons
Electrons lost from photosystem II are replaced by photolysis of water molecules
Process by which light energy breaks down a water molecule into hydrogen and oxygen
Electrons lost from a photosystem enter an electron transfer chain (ETC) in the thylakoid membrane

22. The Noncyclic Pathway – H+ and ATP
In the ETC, electron energy is used to build up a H+ gradient across the membrane
H+ flows through ATP synthase, which attaches a phosphate group to ADP
ATP is formed in the stroma by electron transfer phosphorylation

23. The Noncyclic Pathway – NADPH
Electrons from the first electron transfer chain from photosystem II are accepted by photosystem I
Electrons ejected from photosystem I enter a different electron transfer chain in which the coenzyme NADP+ accepts the electrons and H+, forming NADPH
ATP and NADPH are the energy products of light-dependent reactions in the noncyclic pathway

24. Noncyclic Pathway of Photosynthesis
light energy
light energy
ATP
synthase
photosystem II
electron transfer chain
photosystem I
electron transfer chain
ADP, Pi e e– e– e– e–
NADPH e e– e–
thylakoid compartment H
stroma
Light energy ejects elec- trons from a photosystem II.
The photosystem pulls replacement electrons from water molecules, which then break apart into oxygen and hydrogen ions. The oxygen leaves the cell as O2.
The electrons enter an electron transfer chain in the thylakoid membrane.
4 Energy lost by the electrons as
they move through the chain is used to actively transport hydrogen ions from the stroma into the thylakoid compart- ment. A hydrogen ion gradient forms across the thylakoid membrane.
Light energy ejects elec- trons from a photosystem I. Replacement electrons come from an electron transfer chain.
The ejected electrons move through a second electron transfer chain, then combine with NADP+ and H+, so NADPH forms.
Hydrogen ions in the thylakoid compartment follow their gradient across the thylakoid membrane by flowing through the interior of ATP synthases.
Hydrogen ion flow causes ATP synthases to phosphorylate ADP, so ATP forms in the stroma.
Figure 6.9 Light-dependent reactions, noncyclic pathway. ATP and oxygen gas are produced in this
pathway. Electrons that travel through two different electron transfer chains end up in NADPH.

25. The Cyclic Pathway
When NADPH accumulates in the stroma, the noncyclic pathway stalls
A cyclic pathway runs in type I photosystems to make ATP
Electrons are cycled back to photosystem I and NADPH does not form
Allows light-dependent reactions to continue when the noncyclic path stops

26. Pathways of Light-Dependent Reactions
Figure 6.8 Summary of the inputs and outputs of the two pathways of light-dependent reactions.

27. 6.6 Light-Independent Reactions
Energy flow in the light-dependent reactions is an example of how organisms harvest energy from their environment
Produces NADPH to pass to the light-independent reactions
The cyclic, light-independent reactions of the Calvin-Benson cycle are the “synthesis” part of photosynthesis
Enzyme-mediated reactions that build sugars in the stroma of chloroplasts

28. Carbon Fixation
Extraction of carbon atoms from inorganic sources (atmosphere) and incorporating them into an organic molecule
Builds glucose from CO2
Uses bond energy of molecules formed in light-dependent reactions (ATP, NADPH)

29. The Calvin-Benson Cycle
The enzyme rubisco attaches CO2 to RuBP
Forms two 3-carbon PGA molecules
PGAL is formed
PGAs receive a phosphate group from ATP, and hydrogen and electrons from NADPH
Two PGAL combine to form a 6-carbon sugar
Rubisco is regenerated

30. Calvin– Benson Cycle other molecules glucose 1 2 4
Figure 6.11 The Calvin–Benson cycle. This sketch shows a cross-section of a chloroplast with the reactions cycling in the stroma. The steps shown are a summary of six cycles of the Calvin–Benson reactions. Black balls signify carbon atoms. Appendix V details the reaction steps.
1 Six CO2 diffuse into a photosynthetic cell, and then into a chloroplast. Rubisco attaches each to a RuBP molecule. The resulting intermediates split, so twelve
molecules of PGA form.
2 Each PGA molecule gets a phosphate group from ATP, plus hydrogen and electrons from NADPH. Twelve PGAL form.
3 Two PGAL may combine to form one six-carbon sugar (such as glucose).
4 The remaining ten PGAL receive phosphate groups from ATP. The transfer primes them for endergonic reactions that regenerate the 6 RuBP.
glucose 3
other molecules
Stepped Art

31. The Calvin-Benson Cycle – Illustrated
6 CO2
12 ATP
12 PGA
6 RuBP
6 ADP
12 ADP + 12 Pi
Calvin– Benson Cycle 6
ATP
12 NADPH
Figure 6.11 The Calvin–Benson cycle. This sketch shows a cross-section of a chloroplast with the reactions cycling in the stroma. The steps shown are a summary of six cycles of the Calvin–Benson reactions. Black balls signify carbon atoms. Appendix V details the reaction steps.
1 Six CO2 diffuse into a photosynthetic cell, and then into a chloroplast. Rubisco attaches each to a RuBP molecule. The resulting intermediates split, so twelve
molecules of PGA form.
2 Each PGA molecule gets a phosphate group from ATP, plus hydrogen and electrons from NADPH. Twelve PGAL form.
3 Two PGAL may combine to form one six-carbon sugar (such as glucose).
4 The remaining ten PGAL receive phosphate groups from ATP. The transfer primes them for endergonic reactions that regenerate the 6 RuBP.
4 Pi
12 NADP+
12 PGAL
10 PGAL
sugar

32. 6.7 Adaptations: Alternative Carbon-Fixing Pathways
Environments differ, and so do the details of photosynthesis
C3 plants
C4 plants
CAM plants

33. Stomata
Small openings through the waxy cuticle covering epidermal surfaces of leaves and green stems
Allow CO2 in and O2 out
Close on dry days to minimize water loss
Closing, however, prevents gas exchange as well

34. Photorespiration
When stomata close, O2 levels rise and CO2 levels decline
Reduces efficiency of sugar production in Calvin-Benson cycle
Photorespiration
At high O2 levels, rubisco attaches to oxygen instead of carbon
Inefficient way to produce sugars as it requires more ATP and NADPH
CO2 is produced rather than fixed

35. C3 Plants Plants that use only the Calvin–Benson cycle to fix carbon
Forms 3-carbon PGA in mesophyll cells
Used by most plants, but inefficient in dry weather when stomata are closed
Example: barley

36. Adaptations to Photorespiration
mesophyll cell
CO2 O2
2PG
RuBP
Calvin– Benson Cycle
PGA
Figure 6.13 Adaptations of C4 and CAM plants minimize photorespiration.
A Photorespiration. When stomata close during photosynthesis, O2 accumulates in tissues and CO2 declines. Rubisco initiates more photorespiration, shunting resources away from the sugar producing reactions of the Calvin–Benson cycle.
ATP
NADPH
sugars

37. C4 Plants
Plants that have an additional set of reactions for sugar production on dry days when stomata are closed; compensates for inefficiency of rubisco
Forms 4-carbon oxaloacetate in mesophyll cells, then bundle-sheath cells make sugar
Examples: corn, switchgrass, bamboo

38. Adaptations to Photorespiration
mesophyll cell
CO2 from inside plant
oxaloacetate C4
Cycle
bundle-sheath cell
CO2
Figure 6.13 Adaptations of C4 and CAM plants minimize photorespiration.
B In C4 plants, oxygen also builds up in leaves when stomata close during photosynthesis. An additional pathway in these plants keeps the CO2 concentration high enough in bundle sheath cells to prevent photorespiration.
RuBP
Calvin– Benson Cycle
PGGA
sugars

39. CAM Plants Crassulacean acid metabolism
Plants with an alternative carbon-fixing pathway that allows them to conserve water in climates where days are hot
Forms 4-carbon oxaloacetate at night, which is later broken down to CO2 for sugar production
Example: succulents, cactuses

40. Adaptations to Photorespiration
mesophyll cell
CO2 froom outside plant
oxaloacetaate
C4 Cycle
night
day
CO2
Figure 6.13 Adaptations of C4 and CAM plants minimize photorespiration.
C CAM plants open stomata and fix carbon using a C4 pathway at night. When the plant’s stomata are closed during the day, the organic compound made during the night is converted to
CO2 that enters the Calvin–Benson cycle.
RuBP
Calvin– Benson Cycle
PGGA
sugars

41. Points to Ponder
Are “greening” efforts such as using biofuels and low-carbon imprint practices too little, too late? Why or why not?
Why is radiation with greater or lesser wavelengths than visible light not generally used in biological processes?
What conflicting “needs” confront a plant living in a hot, dry environment? What is the frequent result in C3 plants? How is the problem avoided in C4 plants?