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

2. The Light Reactions: Converting Solar Energy to Chemical Energy
Figure 7.0_1
Chapter 7: Big Ideas
The Light Reactions: Converting Solar Energy to Chemical Energy
An Overview of Photosynthesis
Figure 7.0_1 Chapter 7: Big Ideas
The Calvin Cycle: Reducing CO2 to Sugar
Photosynthesis Reviewed and Extended 2

3. Can these organisms hold the key to our future energy
needs?
Figure 7.0_2 Single-celled algae 3

4. AN OVERVIEW OF PHOTOSYNTHESIS
© 2012 Pearson Education, Inc. 4

5. All Living Organisms Need Energy and Nutrients
Autotrophs = self sufficient; able to convert inorganic nutrients to organic molecules
Autotrophs are primary producers of all ecosystems!!
Photoautotrophs use the energy of light to produce organic molecules from CO2 and water.
Plants, algae (type of protist), some prokaryotes
Convert light energy to chemical energy
Chemoautotrophs are prokaryotes that use inorganic chemicals as their energy source.
Heterotrophs are consumers that feed on organic compounds
Animals, fungi, some prokaryotes and protists
© 2012 Pearson Education, Inc. 5

6. Figure 7.1A-D
Figure 7.1A-D Photoautotroph diversity 6

7. Photosynthesis occurs in chloroplasts in plant cells
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 7

8. Stomata are tiny pores in the leaf that allow
carbon dioxide to enter and
oxygen to exit.
Chloroplasts are concentrated in the cells of the mesophyll, the green tissue in the interior of the leaf.
Leaf Cross Section
Leaf
Veins in the leaf deliver water absorbed by roots.
Mesophyll
Vein
Mesophyll Cell
Figure 7.2_1 Zooming in on the location and structure of chloroplasts (part 1)
CO2 O2
Stoma
Chloroplast 8

9. Inner and outer membranes
Chloroplast = double
membrane
Inner and outer membranes
Granum = stack of thylakoids
Thylakoid
Thylakoid space
Stroma
Figure 7.2_2 Zooming in on the location and structure of chloroplasts (part 2)
Stroma = thick fluid surrounding thylakoids = site of Calvin cycle!!!
Thylakoids = interconnected system of membranous sacs
Thylakoid membranes = site of chlorophyll and light reactions!!! 9

10. Summary Equation for Photosynthesis
Light energy 6
CO2 6
H2O
C6H12O6 6 O2
Carbon dioxide
Water
Oxygen gas
Photosynthesis
Glucose
Water is the source of O2 product
6 CO2  12 H2O → C6H12O6  6 H2O  6 O2
Scientists traced the process of photosynthesis
using isotopes
Student Misconceptions and Concerns
Students may not connect the growth in plant mass to the fixation of carbon during the Calvin cycle. It can be difficult for many students to appreciate that molecules in air can contribute significantly to the mass of plants.
Teaching Tips
Many students do not realize that glucose is not the direct product of photosynthesis. Although glucose is often shown as a final product of photosynthesis, a three-carbon sugar is directly produced (G3P, as the authors note later in Module 7.10). A plant can use G3P to make many types of organic molecules, including glucose.
© 2012 Pearson Education, Inc. 10

11. 7.4 Photosynthesis is an endergonic, redox process
Reduced = gain of electrons; oxidized = loss of electrons
CO2 becomes reduced to sugar.
Water molecules are oxidized to O2
Photosynthesis is endergonic:
light energy is captured by chlorophyll molecules to excite electrons
light energy is converted to chemical energy, and
chemical energy is stored in the chemical bonds of sugars.
Becomes reduced
Teaching Tips
In our world, energy is frequently converted to a usable form in one place and used in another. For example, electricity is generated by power plants, transferred to our homes, and used to run computers, create light, and help us prepare foods. Consider relating this common energy transfer to the two-stage process of photosynthesis.
Becomes oxidized 11

12. Photosynthesis occurs in two stages
Light
NADP+
ADP P
Light Reactions
The light reactions occur in the thylakoid membranes.
Light energy splits H2O to O2, releasing high energy electrons
Movement of electrons used to ATP is generate from ADP and P
Electrons end up on NADP+ reducing it to NADPH.
(in thylakoids)
Figure 7.5_s1 An overview of the two stages of photosynthesis in a chloroplast (step 1)
Chloroplast 12

13. Photosynthesis occurs in two stages
Light
NADP+
ADP P
Light Reactions
The light reactions occur in the thylakoid membranes.
Light energy splits H2O to O2, releasing high energy electrons
Movement of electrons used to ATP is generate from ADP and P
Electrons end up on NADP+ reducing it to NADPH.
(in thylakoids)
ATP
Figure 7.5_s2 An overview of the two stages of photosynthesis in a chloroplast (step 2)
NADPH
Chloroplast O2 13

14. Calvin cycle occurs in the stroma of the chloroplast.
H2O
Calvin cycle occurs in the stroma of the chloroplast.
CO2
Light
Cyclic series of reactions that assembles sugar molecules using CO2 and ATP, NADPH of the light reactions.
NADPH provides the electrons for reducing carbon in the Calvin cycle.
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)
Carbon fixation = incorporation of C into organic compounds
NADPH
Chloroplast O2
Sugar 14

15. EATING THE SUN -
THE LIGHT REACTIONS
© 2012 Pearson Education, Inc. 15

16. 7.6 Visible radiation drives the light reactions
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
Student Misconceptions and Concerns
1. The authors note that electromagnetic energy travels through space in waves that are like ripples made by a pebble dropped in a pond. This wave imagery is helpful, but can confuse students when energy is also thought of as discrete packets called photons. The dual nature of light, which exhibits the properties of waves and particles, may need to be discussed further, if students are to do more than just accept definitions.
2. The authors note that sunlight is a type of radiation. Many students think of radiation as a result of radioactive decay, a serious threat to health. The diverse types of radiation and the varying energy associated with each might need to be explained.
Teaching Tips
Consider bringing a prism to class and demonstrating the spectrum of light. Depending on what you have available, it can be a dramatic reinforcement.
380
400
500
600
700
750
Wavelength (nm)
The shorter the wavelength, the greater the energy.
650 nm 16

17. Animation: Light and Pigments
What pigments and wavelengths are responsible for photosynthesis?
Pigments = compounds that absorb light
Chloroplasts contain 4 major pigments:
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
provide photoprotection
Animation: Light and Pigments

18. 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 18

19. What happens when a pigment absorbs light?
When pigments absorb photons:
potential energy of electrons increases; sending electrons to unstable state
As electrons drop back down to ‘ground state’, they release excess energy as heat
Excited state
Heat
Ground state
Photon of light
Photon (fluorescence)
Chlorophyll molecule
What if this energy could be harnessed?
Student Misconceptions and Concerns
Even at the college level, students struggle to understand why we perceive certain colors. The authors discuss the specific absorption and reflection of certain wavelengths of light, noting which colors are absorbed and which are reflected (and thus available for our eyes to detect). Consider spending time to make sure that your students understand how photosynthetic pigments absorb and reflect certain wavelengths.
Teaching Tips
The authors discuss a phenomenon that most students have noticed: dark surfaces heat up faster in the sun than do lighter-colored surfaces. This is an opportunity to demonstrate to your students the various depths of scientific explanations and help them appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. In high school, they may have learned about light energy and the fact that dark surfaces absorb more of this energy than lighter surfaces. Now, in college, they are learning that at the atomic level, darker surfaces absorb the energy of more photons, exciting more electrons, which then fall back to a lower state, releasing more heat.
© 2012 Pearson Education, Inc. 19

20. Photosynthesis in Two Acts
Light energy 6
CO2 6
H2O
C6H12O6 6 O2
Carbon dioxide
Water
Oxygen gas
Photosynthesis
Glucose
Act 1 - The Light Reactions
GOAL: Transform light energy into chemical energy of ATP and NADPH
Harvest electrons from H2O, forming O2
Use these excited electrons to generate ATP
Then transfer these electrons to NADP+, forming NAPDH.
Light energy
H2O + ADP + P + NADP O2 + ATP + NADPH

21. Light Reactions Take Place in the Thylakoids
Cast of characters
2 Photosystems:
Photosystem II (P680)
Reacts with light to oxidize H2O
Photosystem I (P700)
Reacts with light to transfer electrons to NADP+
Student Misconceptions and Concerns
Even at the college level, students struggle to understand why we perceive certain colors. The authors discuss the specific absorption and reflection of certain wavelengths of light, noting which colors are absorbed and which are reflected (and thus available for our eyes to detect). Consider spending time to make sure that your students understand how photosynthetic pigments absorb and reflect certain wavelengths.
Teaching Tips
The authors discuss a phenomenon that most students have noticed: dark surfaces heat up faster in the sun than do lighter-colored surfaces. This is an opportunity to demonstrate to your students the various depths of scientific explanations and help them appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. In high school, they may have learned about light energy and the fact that dark surfaces absorb more of this energy than lighter surfaces. Now, in college, they are learning that at the atomic level, darker surfaces absorb the energy of more photons, exciting more electrons, which then fall back to a lower state, releasing more heat.
© 2012 Pearson Education, Inc. 21

22. 7.7 Photosystems capture solar energy
Pair of chlorophyll a molecules surrounded by pigments and various enzyme
Capture solar energy to excite electrons in chlorophyll a
Light
Light-harvesting complexes
Reaction-center complex
Primary electron acceptor
Thylakoid membrane
Student Misconceptions and Concerns
Even at the college level, students struggle to understand why we perceive certain colors. The authors discuss the specific absorption and reflection of certain wavelengths of light, noting which colors are absorbed and which are reflected (and thus available for our eyes to detect). Consider spending time to make sure that your students understand how photosynthetic pigments absorb and reflect certain wavelengths.
Teaching Tips
The authors discuss a phenomenon that most students have noticed: dark surfaces heat up faster in the sun than do lighter-colored surfaces. This is an opportunity to demonstrate to your students the various depths of scientific explanations and help them appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. In high school, they may have learned about light energy and the fact that dark surfaces absorb more of this energy than lighter surfaces. Now, in college, they are learning that at the atomic level, darker surfaces absorb the energy of more photons, exciting more electrons, which then fall back to a lower state, releasing more heat.
Pigment molecules
Pair of chlorophyll a molecules
Transfer of energy 22

23. 7.7 Two Photosystems Cooperate in the Light Reactions
Photosystem II (P680) - oxidizes H2O
Photosystem I (P700) - reduces NADP+
Electron transport chain Provides energy for synthesis of ATP by chemiosmosis
NADP 
Light H
NADPH
Light
Photosystem I 6
Photosystem II
Stroma 1
Primary acceptor
Primary acceptor
Student Misconceptions and Concerns
Even at the college level, students struggle to understand why we perceive certain colors. The authors discuss the specific absorption and reflection of certain wavelengths of light, noting which colors are absorbed and which are reflected (and thus available for our eyes to detect). Consider spending time to make sure that your students understand how photosynthetic pigments absorb and reflect certain wavelengths.
Teaching Tips
The authors discuss a phenomenon that most students have noticed: dark surfaces heat up faster in the sun than do lighter-colored surfaces. This is an opportunity to demonstrate to your students the various depths of scientific explanations and help them appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. In high school, they may have learned about light energy and the fact that dark surfaces absorb more of this energy than lighter surfaces. Now, in college, they are learning that at the atomic level, darker surfaces absorb the energy of more photons, exciting more electrons, which then fall back to a lower state, releasing more heat. 2
Thylakoid membrane 4 5
P680
P700
Thylakoid space 3
H2O 2 1 O2 H  2 23

24. Cast of Characters, Cont’d
Electron Transport Chain
Complex of proteins that transfer electrons between PSII and PSI
And actively transport H+ from stroma into thylakoid space
ATP synthase
Enzyme responsible for ATP synthesis

25. 7.8 Two photosystems connected by an electron transport chain generate ATP and NADPH
Light
Stroma
Photosystem II
Thylakoid space
Thylakoid membrane
Primary acceptor
P680
P700
Photosystem I
NADP 
NADPH
Electron transport chain Provides energy for synthesis of ATP by chemiosmosis 2 1
H2O 3 4 5 6 H O2 
Figure 7.8A Electron flow in the light reactions: light energy driving electrons from water to NADPH
Between the two photosystems, the electrons
move down an electron transport chain and
provide energy for the synthesis of ATP.
Electrons are
removed from water,
passed from PS II to PSI and
accepted by NADP+, reducing it to NADPH. 25

26. ATP NADPH Mill makes ATP Photosystem II Photosystem I Photon Photon
Figure 7.8B
ATP
NADPH
Mill makes ATP
Photon
Figure 7.8B A mechanical analogy of the light reactions
Photon
Photosystem II
Photosystem I 26

27. 7.9 Chemiosmosis powers ATP synthesis
Chemiosmosis (chemiosmostic theory)
Peter Mitchell
ATP is generated because the electron transport chain produces a concentration gradient of hydrogen ions (H+) across a membrane
In the Light Reactions:
Light energy is used to allow ETC to pump H+ into the thylakoid space
the resulting concentration gradient drives H+ back through ATP synthase, producing ATP.
Photophosphorylation = using light energy to drive H+ pumps and ATP synthesis by chemiosmosis
Same general process is used to produce ATP in cellular respiration, expect oxidation of glucose is used to generate H+ gradient
Teaching Tips
Module 7.9 notes the similarities between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts. If your students have not already read or discussed chemiosmosis in mitochondria, consider assigning Modules 6.6 and 6.10 to show the similarities of these processes. (As noted in Module 7.2, the thylakoid space is analogous to the intermembrane space of mitochondria.)
© 2012 Pearson Education, Inc. 27

28. 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 28

30. 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 30