1. Capturing Solar Energy: Photosynthesis7
Capturing Solar Energy: Photosynthesis 1

2. Chapter 7 At a Glance 7.1 What Is Photosynthesis?
7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy?
7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?

3. 7.1 What Is Photosynthesis?
For most organisms, energy is derived from sunlight, either directly or indirectly
Those organisms that can directly trap sunlight do so by photosynthesis
Photosynthesis is the process by which solar energy is trapped and stored as chemical energy in the bonds of a sugar

4. 7.1 What Is Photosynthesis?
Leaves and chloroplasts are adaptations for photosynthesis
Photosynthesis in plants takes place in chlorophyll-containing organelles called chloroplasts, most of which are contained within leaf cells
Chloroplasts contain chemical reactions that are able to convert energy in sunlight into stored energy in sugars
Both the upper and lower surfaces of a leaf consist of a layer of transparent cells, the epidermis

5. Figure 7-1 An overview of photosynthetic structures
cuticle
upper
epidermis
mesophyll
cells
Leaves
stoma
lower
epidermis
stoma
chloroplasts
bundle sheath cells
outer membrane
vascular bundle (vein)
inner membrane
Internal leaf structure
thylakoid
stroma
Chloroplast
channel
interconnecting
thylakoids
Mesophyll cell containing chloroplasts 5

6. 7.1 What Is Photosynthesis?
Leaves and chloroplasts are adaptations for photosynthesis (continued)
The outer surface of both epidermal layers is covered by the cuticle, a transparent, waxy, waterproof covering that reduces the evaporation of water from the leaf
Leaves obtain CO2 for photosynthesis from the air through pores in the epidermis called stomata (singular, stoma)

7. Figure 7-2 Stomata
Stomata open
Stomata closed 7

8. Zebrina Under Microscope
Zebrina (Tradescantia zebrine)

9. 7.1 What Is Photosynthesis?
Leaves and chloroplasts are adaptations for photosynthesis (continued)
Inside the leaf are layers of cells called the mesophyll, where the chloroplasts are located and where photosynthesis occurs
Bundle sheath cells surround the vascular bundles, which lack chloroplasts in most plants
Photosynthesis in plants takes place within chloroplasts, most of which are contained in mesophyll cells

10. Figure 7-1 An overview of photosynthetic structures
cuticle
upper
epidermis
mesophyll
cells
Leaves
stoma
lower
epidermis
stoma
chloroplasts
bundle sheath cells
outer membrane
vascular bundle (vein)
inner membrane
Internal leaf structure
thylakoid
stroma
Chloroplast
channel
interconnecting
thylakoids
Mesophyll cell containing chloroplasts 10

11. 7.1 What Is Photosynthesis?
Leaves and chloroplasts are adaptations for photosynthesis (continued)
Chloroplasts are organelles with a double membrane enclosing a fluid called the stroma
Embedded in the stroma are disk-shaped membranous sacs called thylakoids
The light-dependent reactions of photosynthesis occur in and adjacent to the membranes of the thylakoids

12. 7.1 What Is Photosynthesis?
Leaves and chloroplasts are adaptations for photosynthesis (continued)
Reactions of the Calvin cycle that capture carbon dioxide and produce sugar occur in the stroma
Starting with carbon dioxide (CO2) and water (H2O), photosynthesis converts sunlight energy into chemical energy stored in bonds of glucose and releases oxygen (O2) as a product by the following equation:
6 CO2  6 H2O  light energy  C6H12O6  6O2
carbon water sunlight glucose oxygen
dioxide (sugar)

13. What Is Photosynthesis?
An overview of photosynthesis: light-dependent and light-independent reactions
LIGHT-DEPENDENT REACTIONS (thylakoids)
H2O O2
depleted carriers (ADP, NADP+)
energized carriers (ATP, NADPH)
LIGHT-INDEPENDENT REACTIONS (stroma)
CO2
glucose
Fig. 6-2

14. Figure 7-3 An overview of the relationship between the light reactions and the Calvin cycle6
H2O 6
CO2
energy from
sunlight
ATP
Calvin
cycle
light
reactions
NADPH
ADP
NADP
thylakoid
sugar
chloroplast
(stroma) O2
C6H12O6 14

15. 7.1 What Is Photosynthesis?
Leaves and chloroplasts are adaptations for photosynthesis (continued)
During the light reactions, chlorophyll and other molecules embedded in the chloroplast thylakoid membranes capture sunlight energy and convert some of it into chemical energy stored in the energy-carrier molecules ATP and NADPH

16. 7.1 What Is Photosynthesis?
Leaves and chloroplasts are adaptations for photosynthesis (continued)
In the reactions of the Calvin cycle, enzymes in the stroma use CO2 from the air and chemical energy from the energy-carrier molecules to synthesize a three-carbon sugar that will be used to make glucose

17. 7.1 What Is Photosynthesis?
Leaves and chloroplasts are adaptations for photosynthesis (continued)
The “photo” part of photosynthesis refers to the capture of sunlight in the thylakoid membranes
The “synthesis” part of photosynthesis refers to the Calvin cycle, which synthesizes sugar from the energy captured in ATP and NADPH in the light reactions

18. 7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy?
The light reactions capture the energy of the sunlight, storing it as chemical energy in ATP and NADPH
These light reactions are made possible by molecules anchored within the membranes of the thylakoid

20. 7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy?
Light is captured by pigments in chloroplasts
The sun emits energy within a broad spectrum of electromagnetic radiation
This electromagnetic spectrum ranges from short-wavelength gamma rays, through ultraviolet, visible, and infrared light, to long-wavelength radio waves

21. 7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy?
Light is captured by pigments in chloroplasts (continued)
Light is composed of individual packets of energy called photons
Visible light has wavelengths with energies strong enough to alter biological pigment molecules such as chlorophyll a
Chlorophyll a is a key light-capturing pigment molecule in chloroplasts, absorbing violet, blue, and red light
Green light, however, is reflected, which is why leaves appear green

22. 7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy?
Light is captured by pigments in chloroplasts (continued)
Chloroplasts also contain accessory pigments, which absorb additional wavelengths of light energy and transfer them to chlorophyll a
Chlorophyll b absorbs blue and red-orange wavelengths of light missed by chlorophyll a
Carotenoids are accessory pigments that absorb blue and green light, and appear yellow or orange to our eyes because they reflect these colors
In autumn, more-abundant, green chlorophyll breaks down before the carotenoids do, revealing their yellow color, which in summer is masked

23. Figure 7-4 Light and chloroplast pigments
100
chlorophyll b 80
carotenoids 60
light absorption (percent)
chlorophyll a 40 20
wavelength (nanometers)
400
450
500
550
600
650
700
750
visible light
micro-
waves
radio
waves
gamma rays
X-rays UV
infrared
higher energy
lower energy 23

24. Figure 7-5 Loss of chlorophyll reveals carotenoid pigments24

25. 7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy?
The light reactions occur in association with the thylakoid membranes
The thylakoid membranes contain many photosystems, each consisting of a cluster of chlorophyll and accessory pigment molecules surrounded by various proteins
Two photosystems, photosystem II (PS II) and photosystem I (PS I), work together during the light reactions

26. 7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy?
The light reactions occur in association with the thylakoid membranes (continued)
Each type of photosystem has a unique electron transport chain located adjacent to it
These electron transport chains (ETC) each consist of a series of electron-carrier molecules embedded in the thylakoid membrane
Within the thylakoid membrane, the overall path of electrons is as follows:
PS II  ETC II  PS I  ETC I  NADP

27. 7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy?
The light reactions occur in association with the thylakoid membranes (continued)
The steps in the light reactions are as follows:
Photons of light are absorbed by pigment molecules clustered in photosystem II
The energized electron is ejected from the chlorophyll molecule
The reaction center within each photosystem consists of a pair of specialized chlorophyll a molecules and a primary electron acceptor molecule embedded in a complex of proteins

28. 7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy?
Photosystem II uses light energy to create a hydrogen ion gradient and split water
The steps in the light reactions are as follows: (continued)
The electron acceptor passes the electron on to the first molecule of ETC II, and it is then passed from one electron-carrier molecule to the next, losing energy as it goes

29. 7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy?
Photosystem II uses light energy to create a hydrogen ion gradient and split water (continued)
The steps in the light reactions are as follows: (continued)
5a. Some of the energy is harnessed to pump hydrogen ions (H) across the thylakoid membrane and into the thylakoid space, where they will be used to generate ATP
5b. The energy-depleted electron leaves ETC II and enters the reaction center of photosystem I, where it replaces an electron ejected when light strikes photosystem I

30. 7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy?
Photosystem II uses light energy to create a hydrogen ion gradient and split water (continued)
The steps in the light reactions are as follows: (continued)
Light energy striking PS I is captured by its pigment molecules and funneled to a chlorophyll a molecule in the reaction center
This ejects an energized electron that is picked up by the primary electron acceptor of PS I

31. 7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy?
Photosystem II uses light energy to create a hydrogen ion gradient and split water (continued)
The steps in the light reactions are as follows: (continued)
From the primary electron acceptor of PS I, the energized electron is passed along ETC I until it reaches NADP
The energy-carrier molecule NADPH is formed when each NADP molecule picks up two energetic electrons, along with one hydrogen ion

32. 7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy?
The hydrogen ion gradient generates ATP by chemiosmosis
The energy of electron movement through the thylakoid membrane creates an H gradient that drives ATP synthesis in a process called chemiosmosis

33. 7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy?
The hydrogen ion gradient generates ATP by chemiosmosis (continued)
Chemiosmosis occurs in three steps
As the energized electron travels along ETC II, some of the energy it liberates is used to pump hydrogen ions into the thylakoid space
This pumping creates a high concentration of H inside the space relative to the surrounding stroma
H flows down its concentration gradient through a thylakoid channel protein called ATP synthase, generating ATP from ADP

34. Figure 7-7 Events of the light reactions occur in and near the thylakoid membrane
chloroplast
(stroma)
CO2
light
energy
H is pumped into
the thylakoid space H
electron
transport
chain I
Calvin
cycle
electron transport chain II
NADP e H
NADPH
sugar e e e
ATP
synthase
C6H12O6 e
ADP e
photosystem II
photosystem I Pi H H H
ATP 2 H H
H2O H O2 H H H
The flow of H down
its concentration gradient
powers ATP synthesis
A high H concentration is
created in the thylakoid space
thylakoid
membrane
(thylakoid space) 34

35. 7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?
The Calvin cycle captures carbon dioxide
ATP and NADPH synthesized from light reactions are used to power the synthesis of a simple sugar (gyceraldehyde-3-phosphate, or G3P)
This is accomplished through a series of reactions occurring in the stroma called the Calvin cycle

36. 7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?
The Calvin cycle captures carbon dioxide (continued)
The Calvin cycle occurs in three steps
Carbon fixation
The synthesis of G3P
The regeneration of ribulose bisphosphate (RuBP)

37. Figure 7-9 The Calvin cycle fixes carbon from CO2 and produces the simple sugar G3P
H2O
CO2
ATP
Calvin
cycle
light
reactions
NADPH
ADP
NADP
sugar O2
C6H12O6 3
Carbon fixation
combines three CO2
with three RuBP using
the enzyme rubisco
CO2 3 6
RuBP
PGA 6
ATP
Calvin
cycle 6
ADP 3
ADP 6
NADPH 3
ATP 6
NADP 5 6
G3P
G3P
Energy from ATP
and NADPH is used
to convert the six
molecules of PGA to
six molecules of G3P
Using the energy
from ATP, the five
remaining molecules
of G3P are converted
to three molecules
of RuBP 1
G3P
One molecule of
G3P leaves the cycle 1 1 1
G3P
G3P
glucose
Two molecules of G3P combine
to form glucose and other molecules 37

38. 7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?
The Calvin cycle captures carbon dioxide (continued)
Carbon fixation
During carbon fixation, carbon from CO2 is incorporated, or “fixed,” into a larger organic molecule
The enzyme rubisco combines three CO2 molecules with three RuBP molecules, forming three unstable 6-carbon molecules that each quickly split in half
Six molecules of a 3-carbon product, phosphoglyceric acid (PGA), result
The generation of this 3-carbon PGA molecule is called the C3 pathway

39. 7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?
The Calvin cycle captures carbon dioxide (continued)
The synthesis of G3P occurs in a series of reactions using energy received by ATP and NADPH (generated by light reactions)
Energy donated by ATP and NADPH is used to convert six PGA molecules into six of the 3-carbon sugar molecule G3P

40. 7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?
The Calvin cycle captures carbon dioxide (continued)
The regeneration of RuBP
ATP from the light reactions is used with five of the six G3P molecules formed to regenerate the 5-carbon RuBP necessary to repeat the cycle
The remaining G3P molecule, which is the end product of photosynthesis, exits the cycle

41. 7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?
The Calvin cycle captures carbon dioxide (continued)
Why do some plants use alternate pathways for carbon fixation?
When plant stomata are closed in hot environments to prevent water loss, oxygen builds up in the plant cells and RuBP combines with it, rather than CO2, in a wasteful process called photorespiration
This process prevents the Calvin cycle from synthesizing sugar, and plants may die under these circumstances

42. 7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?
The Calvin cycle captures carbon dioxide (continued)
Flowering plants have evolved two different mechanisms to circumvent wasteful photorespiration
The C4 pathway
Crassulacean acid metabolism (CAM)

43. Figure E7-1 The C4 pathway and the CAM pathway
CO2
night
day
mesophyll
cell
CO2
PEP
(3C)
PEP
carboxylase
mesophyll
cell
pyruvate
(3C)
oxaloacetate
(4C)
PEP
(3C)
pyruvate
(3C)
PEP
carboxylase
malate
(4C)
oxalo-
acetate
(4C)
CO2
rubisco
pyruvate
(3C)
malate
(4C)
Calvin
cycle
malate
(4C)
malate
(4C)
CO2
sugar
rubisco
Calvin
cycle
malic acid
(4C)
bundle
sheath
cell
central vacuole
sugar
C4 plants
CAM plants 43