1. Chapter 7 Capturing Solar Energy: Photosynthesis
7.1 What is photosynthesis?
7.2 Light-dependent reactions: How is light energy converted to chemical energy?
7.3 Light-independent reactions: How is chemical energy stored in glucose molecules?
7.4 What is the relationship between light-dependent and light-independent reactions?
7.5 Water, CO2, and the C4 pathway

2. 7.1 What Is Photosynthesis?
~ 2 billion years ago, some cells acquired the ability to do photosynthesis (through chance genetic mutation)
Oxygen levels in the atmosphere increased greatly
Free oxygen (O2) is corrosive
But as time went on (more mutation), some organisms used oxygen to break down glucose in cellular respiration

3. (chloroplast) photosynthesis H2O CO2 ATP sugar O2 cellular respiration
Figure :7-1
Title:
Interconnections between photosynthesis and cellular respiration
Caption:
Chloroplasts in green plants use the energy of sunlight to synthesize high-energy carbon compounds such as glucose from low-energy molecules of water and carbon dioxide. Plants themselves, and other organisms that eat plants or one another, extract energy from these organic molecules by cellular respiration, yielding water and carbon dioxide once again. This energy in turn drives all the reactions of life.
cellular
respiration
(mitochondrion)

4. Relationships Photosynthesis: Cellular Respiration:
6 CO2 + 6 H20 + light energy -> C6H12O6 + 6 O2
Cellular Respiration:
C6H12O6 + 6 O2 -> 6 CO2 + 6 H20 + chemical and heat energy

5. 7.1 What Is Photosynthesis?
Leaves and chloroplasts are adaptations for photosynthesis
A leaf is just a few cells thick so the sun can penetrate
Stomata (stoma is singular) adjust to let more or less CO2 in

6. channel interconnecting thylakoids
Leaves
Internal leaf structure
cuticle
upper
epidermis
mesophyll
cells
stoma
lower epidermis
chloroplasts
Figure :7-2
Title:
An overview of photosynthetic structures
Caption:
(a) Photosynthesis occurs primarily in the leaves of land plants. (b) A section of a leaf, showing mesophyll cells where chloroplasts are concentrated and the waterproof cuticle that coats the leaf's upper epidermis. (c) A single chloroplast, showing the stroma and thylakoids where photosynthesis occurs.
Chloroplast in mesophyll cell
bundle sheath
outer membrane
vascular bundle
(vein)
inner membrane
thylakoid
stroma
granum
(stack of
thylakoids)
channel interconnecting thylakoids

7. Internal leaf structure
cuticle
upper
epidermis
mesophyll
cells
Figure :7-2 part b
Title:
An overview of photosynthetic structures part b Internal leaf structure
Caption:
(b) A section of a leaf, showing mesophyll cells where chloroplasts are concentrated and the waterproof cuticle that coats the leaf's upper epidermis.
stoma
lower
epidermis
chloroplasts
bundle sheath
vascular bundle
(vein)

8. FIGURE 7-3 Stoma in leaf of pea plant
Figure 7-3 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

9. Mesophyll cell containing chloroplasts
FIGURE 7-2c An overview of photosynthetic structures
(c) A mesophyll cell packed with green chloroplasts.
Figure 7-2c Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

10. Chloroplast in mesophyll cell
outer membrane
inner membrane
thylakoid
stroma
Figure :7-2 part c
Title:
An overview of photosynthetic structures part c Chloroplast in mesophyll cell
Caption:
(c) A single chloroplast, showing the stroma and thylakoids where photosynthesis occurs.
channel
interconnecting
thylakoids
granum
(stack of
thylakoids

11. 7.1 What Is Photosynthesis?
Light-dependent reactions
Chlorophyll captures sunlight energy and transfer to energy carrier molecules (ATP & NADPH)
Uses H20 and releases O2
Light-independent reactions
Enzymes use ATP & NADPH to drive synthesis of glucose
Uses CO2 and H20 and releases glucose

12. LIGHT-DEPENDENT REACTIONS (thylakoids) H2O O2 depleted carriers
(ADP, NADP+)
energized
carriers
(ATP, NADPH)
Figure: 7-UN1
Title:
Overview of Photosynthesis
Caption:
In light-dependent reactions, chlorophyll and other molecules embedded in the membranes of the thylakoids capture sunlight energy and convert some of it into the chemical energy stored in energy-carrier molecules (ATP and NADPH). Oxygen gas is released as a by-product. In light-independent reactions, enzymes in the stroma use the chemical energy of the carrier molecules to drive the synthesis of glucose or other organic molecules.
LIGHT-INDEPENDENT
REACTIONS
(stroma)
CO2 + H2O
glucose

13. How Is Light Energy Converted to Chemical Energy?
Sun emits energy in a broad spectrum of electromagnetic radiation
A packet of energy is called a photon, and the level of energy corresponds to a wavelength
Light can be absorbed, reflected or transmitted

14. Sun emits energy in a broad spectrum of electromagnetic radiation
Visible light ("rainbow colors")
Micro-
waves
Radio
waves
Gamma rays
X-rays UV
Infrared
Visible light
Figure :7-3 part a
Title:
Light, chloroplast pigments, and photosynthesis part a Visible light ("rainbow colors")
Caption:
(a) Visible light, a small part of the electromagnetic spectrum (top line), consists of wavelengths that correspond to the colors of the rainbow.
400
450
500
550
600
650
700
750
Wavelength (nanometers)

15. How Is Light Energy Converted to Chemical Energy?
During photosynthesis, light is first captured by pigments in chloroplasts
Chloroplasts contain different pigments that can absorb certain wavelengths of photons

16. light absorption (percent)
Absorbance of photosynthetic pigments
100
chlorophyll b 80 60
carotenoids
light absorption (percent) 40
chlorophyll a
Figure :7-3 part b
Title:
Light, chloroplast pigments, and photosynthesis part b Absorbance of photosynthetic pigments
Caption:
(b) Chlorophyll (blue and green curves) strongly absorbs violet, blue, and red light. Carotenoids (orange curve) absorb blue and green wavelengths. Question Based on the information in this graph, what color are carotenoids? What color is phycocyanin? 20
400
500
600
700
wavelength (nanometers)

17. Chloroplast Pigments
Chlorophyll a and b strongly absorb violet, blue and red light
Carotenoids absorb blue and red light
So what colors do you see?
Carotenoids are also vitamin A and forms the visual pigment in our eyes (captures light energy so we can see!)

18. FIGURE 7-6 Loss of chlorophyll reveals yellow carotenoids
Figure 7-6 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

19. The Light-Dependent Reactions Occur Within the Thylakoid Membranes
Thylakoid membranes contain photosystems I and II
These are highly organized assemblies of proteins, chlorophyll and accessory pigment molecules
The pigment molecules absorb light energy (photon)

20. A mechanical analogy for the light reactions
Mill
makes
ATP e–
Photon
Photosystem II
Photosystem I
NADPH
Figure 10.14 
The light reactions use the solar power of photons absorbed by PS II and PS I to provide chemical energy in the form of ATP and reducing power in the form of the electrons carried by NADPH to the carbohydrate-synthesizing reactions of the Calvin cycle.
Copyright © 2005 Pearson Prentice Hall, Inc.

21. energy level of electrons 2e– NADPH
sunlight
energy level of electrons
2e–
NADPH
within thylakoid membrane
+ H+
NADP+
electron transport chain
2e–
synthesis
energy to drive
ATP
photosystem I
2e–
Figure :7-4
Title:
The light-dependent reactions of photosynthesis
Caption:
1 Light is absorbed by photosystem II, and the energy is passed to electrons in the reaction-center chlorophyll molecules. 2 Energized electrons leave the reaction center. 3 The electrons move into the adjacent electron transport chain. 4 The chain passes the electrons along, and some of their energy is used to drive ATP synthesis by chemiosmosis. Energy-depleted electrons replace those lost by photosystem I. 5 Light strikes photosystem I, and the energy is passed to electrons in the reaction-center chlorophyll molecules. 6 Energized electrons leave the reaction center. 7 The electrons move into the electron transport chain. 8 The energetic electrons from photosystem I are captured in molecules of NADPH. 9 The electrons lost from the reaction center of photosystem II are replaced by electrons obtained from splitting water, a reaction that also releases oxygen, and H+ used to form NADPH. Question If these reactions produce ATP and NADPH, then why do plant cells need mitochondria?
photosystem II
reaction
center 1
/2 O2 + 2 H+
H2O
2e–

22. electron transport chain
sunlight
2e–
NADPH
NADP+
+ H+
2e–
electron transport chain
energy level of electrons
within thylakoid membrane
2e–
Figure :7-4
Title:
The light-dependent reactions of photosynthesis
Caption:
1 Light is absorbed by photosystem II, and the energy is passed to electrons in the reaction-center chlorophyll molecules. 2 Energized electrons leave the reaction center. 3 The electrons move into the adjacent electron transport chain. 4 The chain passes the electrons along, and some of their energy is used to drive ATP synthesis by chemiosmosis. Energy-depleted electrons replace those lost by photosystem I. 5 Light strikes photosystem I, and the energy is passed to electrons in the reaction-center chlorophyll molecules. 6 Energized electrons leave the reaction center. 7 The electrons move into the electron transport chain. 8 The energetic electrons from photosystem I are captured in molecules of NADPH. 9 The electrons lost from the reaction center of photosystem II are replaced by electrons obtained from splitting water, a reaction that also releases oxygen, and H+ used to form NADPH. Question If these reactions produce ATP and NADPH, then why do plant cells need mitochondria?
photosystem I
energy to drive
reaction
center
ATP
synthesis
2e–
photosystem II
H2O
1/2 O2 + 2 H+

23. How Is Light Energy Converted to Chemical Energy?
Photosystem II generates ATP
Photosystem I generates NADPH
Splitting water maintains the flow of electrons through the photosystems
Chemiosmosis: creating the hydrogen ion gradient
Chemiosmosis: ATP synthesis
Oxygen is a by-product of photosynthesis

24. thylakoid chloroplast
FIGURE 7-8 (part 1) Events of the light-dependent reactions occur in and near the thylakoid membranes
chloroplast
Figure 7-8 (part 1) Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

25. electrons powers active transport of H+ by ETC. Energy-carrier
Energy from energized
electrons powers active
transport of H+ by ETC.
Energy-carrier
molecules power
the C3 cycle.
ETC
PSII
PSI
stroma
ETC C3
cycle
FIGURE 7-8 (part 2) Events of the light-dependent reactions occur in and near the thylakoid membranes
Energy from
energized
electrons powers
NADPH synthesis.
thylakoid space
High H+ concentration
generated by active
transport.
H+ channel coupled
to ATP-synthesizing
enzyme.
Flow of H+ down
concentration gradient
powers ATP synthesis.
Figure 7-8 (part 2) Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

26. 1 Energy is released as water flows downhill. 2 Energy is harnessed to
rotate turbine.
FIGURE E7-1 Energy stored in a water "gradient" can be used to generate electricity
3 Energy of rotating
turbine is used to
generate electricity.
Figure E7-1 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

27. photosystem II thylakoid membrane H+ H+ 2e– H+ H+ (thylakoid interior)
Figure: E7-1
Title:
Chemiosmosis: creating the hydrogen ion gradient
Caption:
The energy released from the exergonic reaction of these electron transfers in Photosystem II is used to power active transport of hydrogen ions across the thylakoid membrane from the stroma into the thylakoid interior.
(stroma)

28. (high H+ concentration in thylakoid) H+ H+
(low H+ concentration
in stroma) H+ H+ H+ H+
(high H+ concentration
in thylakoid) H+ H+
Figure: E7-2
Title:
Chemiosmosis: ATP Synthesis
Caption:
The thylakoid membrane does not allow hydrogen ions to leak out, except at specific protein channels that are coupled to ATP-synthesizing enzymes. When hydrogen ions flow through these channels, down their gradients of charge and concentration, the energy released drives the synthesis of ATP. H+
ADP
ATP P

29. Figure :7-5
Title:
Oxygen is a by-product of photosynthesis
Caption:
The bubbles released by the leaves of this aquatic plant (Elodea) are composed of oxygen, a by-product of photosynthesis.

30. Light-Independent Reactions: How Is Chemical Energy Stored in Glucose Molecules?
The C3 cycle captures carbon dioxide
The C3 (3 carbons) cycle of carbon fixation
It is also called the Calvin-Benson cycle
Carbon is fixed during the C3 cycle is used to synthesize glucose
You do not need to know the details of this cycle, nor Figure 7-10 (ed. 7 Figure 7-6)

31. 6 CO2 6 H2O 6 12 RuBP PGA C3 cycle 12 6 12 G3P glucose
Figure :7-6
Title:
The C3 cycle of carbon fixation
Caption:
1 Six molecules of RuBP react with 6 molecules of CO2 and 6 molecules of H2O to form 12 molecules of PGA. This reaction is carbon fixation, the capture of carbon from CO2 into organic molecules. 2 The energy of 12 ATPs and the electrons and hydrogens of 12 NADPHs are used to convert the 12 PGA molecules to 12 G3Ps. 3 Two G3P molecules are available to synthesize glucose or other organic molecules. This occurs outside the chloroplast and is not part of the C3 cycle. 4 Energy from 6 ATPs is used to rearrange 10 G3Ps into 6 RuBPs, completing one turn of the C3 cycle.
G3P 12
glucose
(or other organic
compounds)

32. 6 6 6 CO2 C 6 H2O 6 C C C C C 12 C C C RuBP PGA C3 cycle 12 ATP 12 ADP
Figure :7-6
Title:
The C3 cycle of carbon fixation
Caption:
1 Six molecules of RuBP react with 6 molecules of CO2 and 6 molecules of H2O to form 12 molecules of PGA. This reaction is carbon fixation, the capture of carbon from CO2 into organic molecules. 2 The energy of 12 ATPs and the electrons and hydrogens of 12 NADPHs are used to convert the 12 PGA molecules to 12 G3Ps. 3 Two G3P molecules are available to synthesize glucose or other organic molecules. This occurs outside the chloroplast and is not part of the C3 cycle. 4 Energy from 6 ATPs is used to rearrange 10 G3Ps into 6 RuBPs, completing one turn of the C3 cycle. 6
ADP 12
NADPH 12 C C C 6
ATP 12
NADP+
G3P C C C C C C
glucose
(or other organic
compounds)

33. energy from sunlight O2 CO2 H2O ATP NADPH Light-dependent
reactions occur
in thylakoids.
Light-
independent
reactions
(C3 cycle) occur
in stroma.
Figure :7-7
Title:
A summary diagram of photosynthesis
Caption:
The light-dependent reactions in the thylakoids convert the energy of sunlight into the chemical energy of ATP and NADPH. Part of the sunlight energy is also used to split H2O, forming O2. In the stroma, the light-independent reactions use the energy of ATP and NADPH to convert CO2 and H2O to glucose. The depleted carriers, ADP and NADP+, return to the thylakoids to be recharged by the light-dependent reactions. Question Could a plant survive in an oxygen-free atmosphere?
ADP
NADP+
H20
chloroplast
glucose

34. Water, CO2, C3 and C4 Pathways
cuticle
upper
epidermis
mesophyll
cells
Figure :7-2 part b
Title:
An overview of photosynthetic structures part b Internal leaf structure
Caption:
(b) A section of a leaf, showing mesophyll cells where chloroplasts are concentrated and the waterproof cuticle that coats the leaf's upper epidermis.
stoma
lower
epidermis
chloroplasts
bundle sheath
vascular bundle
(vein)

35. 7.5 Water, CO2, and the C4 Pathway
When stomata are closed to conserve water, CO2 cannot enter, and O2 cannot leave
A wasteful process called photorespiration occurs when it is too hot for C3 plants
Glucose is not made, and seedlings may die.
Some plants have a C4 pathway
Compare pathways of C3 and C4 plants

36. C3 plants use the C3 pathway within chloroplast in mesophyll cell
CO2 O2
PGA
CO2 C3
CYCLE
RuBP
G3P
glucose
bundle-
sheath
cells
C4 plants use the C4 pathway
within chloroplast in mesophyll cell
CO2
PEP
4-carbon
molecule
AMP C4
Pathway
ATP
Figure :7-8
Title:
Comparison of C3 and C4 plants
Caption:
(a) In C3 plants, only the mesophyll cells carry out photosynthesis. All carbon fixation occurs by the C3 pathway. With low CO2 and high O2 levels, photorespiration dominates in C3 plants, because the enzyme that should catalyze the RuBP plus CO2 reaction catalyzes the RuBP plus O2 reaction instead. (b) In C4 plants, both the mesophyll cells and bundle sheath cells contain chloroplasts and participate in photosynthesis. CO2 is combined with PEP by a more selective enzyme, and the carbon is shuttled into bundle sheath cells by a four-carbon molecule, which releases CO2 into the bundle sheath cells. Higher CO2 levels allow efficient carbon fixation (with little photorespiration) in the C3 pathway of the bundle sheath cells. Notice that the regeneration of PEP requires energy from ATP. Question Why do C3 plants have an advantage over C4 plants under conditions that are not hot and dry?
pyruvate
CO2 O2
bundle-
sheath
cells
PGA
CO2 C3
CYCLE
RuBP
G3P
glucose
within chloroplast in bundle-sheath cell

37. 7.5 Water, CO2, and the C4 Pathway
C4 plants reduce photorespiration by means of a two-stage carbon-fixation process
C3 and C4 plants are each adapted to different environmental conditions