1. Photosynthesis: Using Light to Make Food
Chapter 7
Photosynthesis: Using Light to Make Food

2. Introduction Plants, algae, and certain prokaryotes
convert light energy to chemical energy and
store the chemical energy in sugar, made from
carbon dioxide and
water.
© 2012 Pearson Education, Inc. 2

3. AN OVERVIEW OF PHOTOSYNTHESIS
© 2012 Pearson Education, Inc. 5

4. 7.1 Autotrophs are the producers of the biosphere
make their own food through the process of photosynthesis,
sustain themselves, and
do not usually consume organic molecules derived from other organisms.
© 2012 Pearson Education, Inc. 6

5. 7.1 Autotrophs are the producers of the biosphere
Photoautotrophs use the energy of light to produce organic molecules.
Chemoautotrophs are prokaryotes that use inorganic chemicals as their energy source.
Heterotrophs are consumers that feed on
plants or
animals, or
decompose organic material.
© 2012 Pearson Education, Inc. 7

6. 7.1 Autotrophs are the producers of the biosphere
Photosynthesis in plants
takes place in chloroplasts,
converts carbon dioxide and water into organic molecules, and
releases oxygen.
© 2012 Pearson Education, Inc. 8

7. Figure 7.1A-D
Figure 7.1A-D Photoautotroph diversity 9

8. 7.2 Photosynthesis occurs in chloroplasts in plant cells
Chloroplasts are the major sites of photosynthesis in green plants.
Chlorophyll
is an important light-absorbing pigment in chloroplasts,
is responsible for the green color of plants, and
plays a central role in converting solar energy to chemical energy.
© 2012 Pearson Education, Inc. 10

9. 7.2 Photosynthesis occurs in chloroplasts in plant cells
Chloroplasts are concentrated in the cells of the mesophyll, the green tissue in the interior of the leaf.
Stomata are tiny pores in the leaf that allow
carbon dioxide to enter and
oxygen to exit.
Veins in the leaf deliver water absorbed by roots.
© 2012 Pearson Education, Inc. 11

10. Figure 7.2 Zooming in on the location and structure of chloroplasts
Leaf
Leaf Cross Section
Mesophyll
Vein
CO2 O2
Stoma
Mesophyll Cell
Chloroplast
Figure 7.2 Zooming in on the location and structure of chloroplasts
Inner and outer membranes
Granum
Thylakoid
Thylakoid space
Stroma 12

11. Leaf Cross Section Leaf Mesophyll Vein Mesophyll Cell CO2 O2
Figure 7.2_1
Leaf Cross Section
Leaf
Mesophyll
Vein
Mesophyll Cell
Figure 7.2_1 Zooming in on the location and structure of chloroplasts (part 1)
CO2 O2
Stoma
Chloroplast 13

12. 7.2 Photosynthesis occurs in chloroplasts in plant cells
Chloroplasts consist of an envelope of two membranes, which
enclose an inner compartment filled with a thick fluid called stroma and
contain a system of interconnected membranous sacs called thylakoids.
© 2012 Pearson Education, Inc. 14

13. 7.2 Photosynthesis occurs in chloroplasts in plant cells
Thylakoids
are often concentrated in stacks called grana and
have an internal compartment called the thylakoid space, which has functions analogous to the intermembrane space of a mitochondrion.
Thylakoid membranes also house much of the machinery that converts light energy to chemical energy.
Chlorophyll molecules
are built into the thylakoid membrane and
capture light energy.
© 2012 Pearson Education, Inc. 15

14. Inner and outer membranes
Figure 7.2_2
Chloroplast
Inner and outer membranes
Granum
Thylakoid
Thylakoid space
Figure 7.2_2 Zooming in on the location and structure of chloroplasts (part 2)
Stroma 16

15. 7.4 Photosynthesis is a redox process, as is cellular respiration
Photosynthesis, like respiration, is a redox (oxidation-reduction) process.
CO2 becomes reduced to sugar as electrons along with hydrogen ions from water are added to it.
Water molecules are oxidized when they lose electrons along with hydrogen ions.
© 2012 Pearson Education, Inc. 20

16. 7.4 Photosynthesis is a redox process, as is cellular respiration
Cellular respiration uses redox reactions to harvest the chemical energy stored in a glucose molecule.
This is accomplished by oxidizing the sugar and reducing O2 to H2O.
The electrons lose potential as they travel down the electron transport chain to O2.
In contrast, the food-producing redox reactions of photosynthesis require energy.
© 2012 Pearson Education, Inc. 22

17. 7.4 Photosynthesis is a redox process, as is cellular respiration
In photosynthesis,
light energy is captured by chlorophyll molecules to boost the energy of electrons,
light energy is converted to chemical energy, and
chemical energy is stored in the chemical bonds of sugars.
© 2012 Pearson Education, Inc. 23

18. 7.5 Overview: The two stages of photosynthesis are linked by ATP and NADPH
Photosynthesis occurs in two metabolic stages.
The light reactions occur in the thylakoid membranes. In these reactions
water is split, providing a source of electrons and giving off oxygen as a by-product,
ATP is generated from ADP and a phosphate group, and
light energy is absorbed by the chlorophyll molecules to drive the transfer of electrons and H+ from water to the electron acceptor NADP+ reducing it to NADPH.
NADPH produced by the light reactions provides the electrons for reducing carbon in the Calvin cycle.
© 2012 Pearson Education, Inc. 25

19. 7.5 Overview: The two stages of photosynthesis are linked by ATP and NADPH
The second stage is the Calvin cycle, which occurs in the stroma of the chloroplast.
The Calvin cycle is a cyclic series of reactions that assembles sugar molecules using CO2 and the energy-rich products of the light reactions.
During the Calvin cycle, CO2 is incorporated into organic compounds in a process called carbon fixation.
After carbon fixation, enzymes of the cycle make sugars by further reducing the carbon compounds.
The Calvin cycle is often called the dark reactions or light-independent reactions, because none of the steps requires light directly.
© 2012 Pearson Education, Inc. 26

20. Chloroplast H2O Light NADP+ ADP P Light Reactions (in thylakoids)
Figure 7.5_s1
H2O
Light
NADP+
ADP P
Light Reactions
(in thylakoids)
Figure 7.5_s1 An overview of the two stages of photosynthesis in a chloroplast (step 1)
Chloroplast 27

21. Chloroplast H2O Light NADP+ ADP P Light Reactions (in thylakoids) ATP
Figure 7.5_s2
H2O
Light
NADP+
ADP P
Light Reactions
(in thylakoids)
ATP
Figure 7.5_s2 An overview of the two stages of photosynthesis in a chloroplast (step 2)
NADPH
Chloroplast O2 28

22. Chloroplast H2O CO2 Light NADP+ ADP P Calvin Cycle Light Reactions
Figure 7.5_s3
H2O
CO2
Light
NADP+
ADP P
Calvin Cycle
Light Reactions
(in stroma)
(in thylakoids)
ATP
Figure 7.5_s3 An overview of the two stages of photosynthesis in a chloroplast (step 3)
NADPH
Chloroplast O2
Sugar 29

23. THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY
© 2012 Pearson Education, Inc. 30

24. 7.6 Visible radiation absorbed by pigments drives the light reactions
Sunlight contains energy called electromagnetic energy or electromagnetic radiation.
Visible light is only a small part of the electromagnetic spectrum, the full range of electromagnetic wavelengths.
Electromagnetic energy travels in waves, and the wavelength is the distance between the crests of two adjacent waves.
© 2012 Pearson Education, Inc. 31

25. 7.6 Visible radiation absorbed by pigments drives the light reactions
Light behaves as discrete packets of energy called photons.
A photon is a fixed quantity of light energy.
The shorter the wavelength, the greater the energy.
© 2012 Pearson Education, Inc. 32

26. Gamma rays Micro- waves Radio waves 650 nm
Figure 7.6A
Increasing energy
105 nm
103 nm
1 nm
103 nm
106 nm
1 m
103 m
Gamma rays
Micro- waves
Radio waves
X-rays UV
Infrared
Visible light
Figure 7.6A The electromagnetic spectrum and the wavelengths of visible light
380
400
500
600
700
750
Wavelength (nm)
650 nm 33

27. 7.6 Visible radiation absorbed by pigments drives the light reactions
absorb light and
are built into the thylakoid membrane.
Plant pigments
absorb some wavelengths of light and
reflect or transmit other wavelengths.
We see the color of the wavelengths that are transmitted. For example, chlorophyll transmits green wavelengths.
© 2012 Pearson Education, Inc. 34

28. Light Reflected light Chloroplast Absorbed light Thylakoid
Figure 7.6B
Light
Reflected light
Figure 7.6B The interaction of light with a chloroplast
Chloroplast
Absorbed light
Thylakoid
Transmitted light 35

29. 7.6 Visible radiation absorbed by pigments drives the light reactions
Chloroplasts contain several different pigments, which absorb light of different wavelengths.
Chlorophyll a absorbs blue-violet and red light and reflects green.
Chlorophyll b absorbs blue and orange and reflects yellow-green.
Carotenoids
broaden the spectrum of colors that can drive photosynthesis and
provide photoprotection, absorbing and dissipating excessive light energy that would otherwise damage chlorophyll or interact with oxygen to form reactive oxidative molecules.
© 2012 Pearson Education, Inc. 36

30. 7.7 Photosystems capture solar energy
Pigments in chloroplasts absorb photons (capturing solar power), which
increases the potential energy of the pigment’s electrons and
sends the electrons into an unstable state.
These unstable electrons
drop back down to their “ground state,” and as they do,
release their excess energy as heat.
© 2012 Pearson Education, Inc. 37

31. 7.7 Photosystems capture solar energy
Within a thylakoid membrane, chlorophyll and other pigment molecules
absorb photons and
transfer the energy to other pigment molecules.
In the thylakoid membrane, chlorophyll molecules are organized along with other pigments and proteins into photosystems.
© 2012 Pearson Education, Inc. 39

32. 7.7 Photosystems capture solar energy
A photosystem consists of a number of light- harvesting complexes surrounding a reaction- center complex.
A light-harvesting complex contains various pigment molecules bound to proteins.
Collectively, the light-harvesting complexes function as a light-gathering antenna.
© 2012 Pearson Education, Inc. 40

33. Pair of chlorophyll a molecules
Figure 7.7B
Photosystem
Light
Light-harvesting complexes
Reaction-center complex
Primary electron acceptor
Thylakoid membrane
Figure 7.7B A light-excited pair of chlorophyll molecules in the reaction center of a photosystem passing an excited electron to a primary electron acceptor
Pigment molecules
Pair of chlorophyll a molecules
Transfer of energy 41

34. 7.7 Photosystems capture solar energy
The light energy is passed from molecule to molecule within the photosystem.
Finally it reaches the reaction center where a primary electron acceptor accepts these electrons and consequently becomes reduced.
This solar-powered transfer of an electron from the reaction-center pigment to the primary electron acceptor is the first step in the transformation of light energy to chemical energy in the light reactions.
© 2012 Pearson Education, Inc. 42

35. 7.7 Photosystems capture solar energy
Two types of photosystems (photosystem I and photosystem II) cooperate in the light reactions.
Each type of photosystem has a characteristic reaction center.
Photosystem II, which functions first, is called P680 because its pigment absorbs light with a wavelength of 680 nm.
Photosystem I, which functions second, is called P700 because it absorbs light with a wavelength of 700 nm.
© 2012 Pearson Education, Inc. 43

36. 7.8 Two photosystems connected by an electron transport chain generate ATP and NADPH
In the light reactions, light energy is transformed into the chemical energy of ATP and NADPH.
To accomplish this, electrons are
removed from water,
passed from photosystem II to photosystem I, and
accepted by NADP+, reducing it to NADPH.
Between the two photosystems, the electrons
move down an electron transport chain and
provide energy for the synthesis of ATP.
© 2012 Pearson Education, Inc. 44

37. Figure 7.8A
Electron transport chain Provides energy for synthesis of ATP by chemiosmosis
NADP  H
NADPH
Light
Light
Photosystem I 6
Photosystem II
Stroma 1
Primary acceptor
Primary acceptor 2
Thylakoid membrane 4 5
P680
P700
Figure 7.8A Electron flow in the light reactions: light energy driving electrons from water to NADPH
Thylakoid space 3
H2O 2 1 O2 H  2 45

38. Figure 7.8A_1
Electron transport chain Provides energy for synthesis of ATP by chemiosmosis
Light
Photosystem II
Stroma 1
Primary acceptor 2
Thylakoid membrane 4
P680
Figure 7.8A_1 Electron flow in the light reactions: light energy driving electrons from water to NADPH (part 1)
Thylakoid space 3
H2O 2 1
O2  2 H 46

39. Figure 7.8A_2
Electron transport chain Provides energy for synthesis of ATP by chemiosmosis
NADP  H+
NADPH
Light
Photosystem I 6
Primary acceptor 4 5
Figure 7.8A_2 Electron flow in the light reactions: light energy driving electrons from water to NADPH (part 2)
P700 47

40. 7.8 Two photosystems connected by an electron transport chain generate ATP and NADPH
The products of the light reactions are
NADPH,
ATP, and
oxygen.
© 2012 Pearson Education, Inc. 49

41. 7.9 Chemiosmosis powers ATP synthesis in the light reactions
Interestingly, chemiosmosis is the mechanism that
is involved in oxidative phosphorylation in mitochondria and
generates ATP in chloroplasts.
ATP is generated because the electron transport chain produces a concentration gradient of hydrogen ions across a membrane.
© 2012 Pearson Education, Inc. 50

42. 7.9 Chemiosmosis powers ATP synthesis in the light reactions
In photophosphorylation, using the initial energy input from light,
the electron transport chain pumps H+ into the thylakoid space, and
the resulting concentration gradient drives H+ back through ATP synthase, producing ATP.
© 2012 Pearson Education, Inc. 51

43. Electron transport chain
Figure 7.9
Chloroplast
To Calvin Cycle H+
ATP
Light
Light
ADP P
Stroma (low H+ concentration) H+
NADP+ H+
NADPH H+ H+
Thylakoid membrane
Figure 7.9 The production of ATP by chemiosmosis H+ H+ H+ H+
H2O H+ 1 2 H+
Thylakoid space (high H+ concentration) O H+ H+ H+ H+ H+ H+
Electron transport chain H+ H+
Photosystem II
Photosystem I
ATP synthase 52

44. Electron transport chain
Figure 7.9_1
To Calvin Cycle H+
ATP
Light
Light
ADP P H+
NADPH
NADP+ H+ H+ H+ H+ H+ H+ H+
H2O H+ H+
Figure 7.9_1 The production of ATP by chemiosmosis (partial) 1 2
O2 2 H+ H+ H+ H+ H+ H+
Electron transport chain H+ H+
Photosystem II
Photosystem I
ATP synthase 53

46. Step Release of one molecule of G3P 6 NADP 6 P 5 P G3P G3P
Figure 7.10B_s4
Step Carbon fixation 1
Input: 3
CO2
Rubisco 1 3 P P 6 P
RuBP
3-PGA
Step Reduction 2 6
ATP 3
ADP 6
ADP P
Calvin Cycle 4 2 3
ATP 6
NADPH
Step Release of one molecule of G3P 3 6
NADP
Figure 7.10B_s4 Details of the Calvin cycle, which takes place in the stroma of a chloroplast (step 4) 6 P 5 P
G3P
G3P 3
Glucose and other compounds
Step Regeneration of RuBP 4
Output: 1 P
G3P 55

47. Hand in 1st Lab work sheet by the end of the period
Sophomore biology
Hand in 1st Lab work sheet by the end of the period
Testing a new variable – Design your own experiment
Lab Write - up
1. What question are we trying to answer?
2. Make a hypothesis
3. Form a materials list
4. Outline an experimental procedure
5. Run your experiment
6. Present your data – graph – qualitative
7. Analyze your data
8. Make a conclusion

48. 7.9 Chemiosmosis powers ATP synthesis in the light reactions
How does photophosphorylation compare with oxidative phosphorylation?
Mitochondria use oxidative phosphorylation to transfer chemical energy from food into the chemical energy of ATP.
Chloroplasts use photophosphorylation to transfer light energy into the chemical energy of ATP.
© 2012 Pearson Education, Inc. 57

49. THE CALVIN CYCLE: REDUCING CO2TO SUGAR
© 2012 Pearson Education, Inc. 58

50. CO2 ATP NADPH Input Calvin Cycle Output: G3P Figure 7.10A
Figure 7.10A An overview of the Calvin cycle
Output:
G3P 60

51. 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle
The steps of the Calvin cycle include
carbon fixation,
reduction,
release of G3P, and
regeneration of the starting molecule ribulose bisphosphate (RuBP).
© 2012 Pearson Education, Inc. 61

52. Step Carbon fixation Input: 3 CO2 Rubisco 3 P P 6 P RuBP 3-PGA
Figure 7.10B_s1
Step Carbon fixation 1
Input: 3
CO2
Rubisco 1 3 P P 6 P
RuBP
3-PGA
Calvin Cycle
Figure 7.10B_s1 Details of the Calvin cycle, which takes place in the stroma of a chloroplast (step 1) 62

53. Step Carbon fixation Input: 3 CO2 Rubisco 3 P P 6 P RuBP 3-PGA
Figure 7.10B_s2
Step Carbon fixation 1
Input: 3
CO2
Rubisco 1 3 P P 6 P
RuBP
3-PGA
Step Reduction 2 6
ATP 6
ADP P
Calvin Cycle 2 6
NADPH 6
NADP
Figure 7.10B_s2 Details of the Calvin cycle, which takes place in the stroma of a chloroplast (step 2) 6 P
G3P 63

54. Step Release of one molecule of G3P 6 NADP 6 P 5 P G3P G3P
Figure 7.10B_s3
Step Carbon fixation 1
Input: 3
CO2
Rubisco 1 3 P P 6 P
RuBP
3-PGA
Step Reduction 2 6
ATP 6
ADP P
Calvin Cycle 2 6
NADPH
Step Release of one molecule of G3P 3 6
NADP
Figure 7.10B_s3 Details of the Calvin cycle, which takes place in the stroma of a chloroplast (step 3) 6 P 5 P
G3P
G3P 3
Glucose and other compounds
Output: 1 P
G3P 64

55. Step Release of one molecule of G3P 6 NADP 6 P 5 P G3P G3P
Figure 7.10B_s4
Step Carbon fixation 1
Input: 3
CO2
Rubisco 1 3 P P 6 P
RuBP
3-PGA
Step Reduction 2 6
ATP 3
ADP 6
ADP P
Calvin Cycle 4 2 3
ATP 6
NADPH
Step Release of one molecule of G3P 3 6
NADP
Figure 7.10B_s4 Details of the Calvin cycle, which takes place in the stroma of a chloroplast (step 4) 6 P 5 P
G3P
G3P 3
Glucose and other compounds
Step Regeneration of RuBP 4
Output: 1 P
G3P 65

56. 7.11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climates
Most plants use CO2 directly from the air, and carbon fixation occurs when the enzyme rubisco adds CO2 to RuBP.
Such plants are called C3 plants because the first product of carbon fixation is a three-carbon compound, 3-PGA.
© 2012 Pearson Education, Inc. 66

57. 7.11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climates
In hot and dry weather, C3 plants
close their stomata to reduce water loss but
prevent CO2 from entering the leaf and O2 from leaving.
As O2 builds up in a leaf, rubisco adds O2 instead of CO2 to RuBP, and a two-carbon product of this reaction is then broken down in the cell.
This process is called photorespiration because it occurs in the light, consumes O2, and releases CO2.
But unlike cellular respiration, it uses ATP instead of producing it.
© 2012 Pearson Education, Inc. 67

58. 7.11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climates
C4 plants have evolved a means of
carbon fixation that saves water during photosynthesis while
optimizing the Calvin cycle.
C4 plants are so named because they first fix CO2 into a four-carbon compound.
When the weather is hot and dry, C4 plants keep their stomata mostly closed, thus conserving water.
© 2012 Pearson Education, Inc. 68

59. 7.11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climates
Another adaptation to hot and dry environments has evolved in the CAM plants, such as pineapples and cacti.
CAM plants conserve water by opening their stomata and admitting CO2 only at night.
CO2 is fixed into a four-carbon compound,
which banks CO2 at night and
releases it to the Calvin cycle during the day.
© 2012 Pearson Education, Inc. 69

61. PHOTOSYNTHESIS REVIEWED AND EXTENDED
© 2012 Pearson Education, Inc. 71

62. 7.12 Review: Photosynthesis uses light energy, carbon dioxide, and water to make organic molecules
Most of the living world depends on the food-making machinery of photosynthesis.
The chloroplast
integrates the two stages of photosynthesis and
makes sugar from CO2.
© 2012 Pearson Education, Inc. 72

63. 7.12 Review: Photosynthesis uses light energy, carbon dioxide, and water to make organic molecules
About half of the carbohydrates made by photosynthesis are consumed as fuel for cellular respiration in the mitochondria of plant cells.
Sugars also serve as the starting material for making other organic molecules, such as proteins, lipids, and cellulose.
Excess food made by plants is stockpiled as starch in roots, tubers, seeds, and fruits.
© 2012 Pearson Education, Inc. 73

64. Calvin Cycle (in stroma) Electron transport chain
Figure 7.12
H2O
CO2
Chloroplast
Light
NADP
ADP P
Light Reactions
RuBP
Photosystem II
Calvin Cycle (in stroma)
3-PGA
Electron transport chain
Thylakoids
Photosystem I
ATP
Stroma
Figure 7.12 A summary of photosynthesis
NADPH
G3P
Cellular respiration
Cellulose
Starch O2
Sugars
Other organic compounds 74