1. CHAPTER 7 Photosynthesis: Using Light to Make Food
Modules 7.6 – 7.14

2. 7.6 Visible radiation drives the light reactions
THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY
7.6 Visible radiation drives the light reactions
Certain wavelengths of visible light drive the light reactions of photosynthesis
Gamma rays
Micro- waves
Radio
waves
X-rays UV
Infrared
Visible light
Wavelength (nm)
Figure 7.6A

3. Reflected light Light Chloroplast Absorbed light Transmitted light
Figure 7.6B

4. 7.7 Photosystems capture solar power
Each of the many light-harvesting photosystems consists of:
an “antenna” of chlorophyll and other pigment molecules that absorb light
a primary electron acceptor that receives excited electrons from the reaction-center chlorophyll

5. Primary electron acceptor
PHOTOSYSTEM
Photon
Reaction center
Pigment molecules of antenna
Figure 7.7C

6. Fluorescence of isolated chlorophyll in solution
Heat
Photon (fluorescence)
Photon
Chlorophyll molecule
Figure 7.7A

7. Primary electron acceptor
Excitation of chlorophyll in a chloroplast
Primary electron acceptor
Other compounds
Photon
Chlorophyll molecule
Figure 7.7B

8. 7.8 In the light reactions, electron transport chains generate ATP, NADPH, and O2
Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons
The excited electrons are passed from the primary electron acceptor to electron transport chains
Their energy ends up in ATP and NADPH

9. Where do the electrons come from that keep the light reactions running?
In photosystem I, electrons from the bottom of the cascade pass into its P700 chlorophyll

10. Photosystem II regains electrons by splitting water, leaving O2 gas as a by-product
Primary electron acceptor
Electron transport
Primary electron acceptor
Electron transport chain
Photons
Energy for synthesis of
PHOTOSYSTEM I
PHOTOSYSTEM II
by chemiosmosis
Figure 7.8

11. 7.9 Chemiosmosis powers ATP synthesis in the light reactions
The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane
The flow of H+ back through the membrane is harnessed by ATP synthase to make ATP
In the stroma, the H+ ions combine with NADP+ to form NADPH

12. ELECTRON TRANSPORT CHAIN
The production of ATP by chemiosmosis in photosynthesis
Thylakoid compartment (high H+)
Light
Light
Thylakoid membrane
Antenna molecules
Stroma (low H+)
ELECTRON TRANSPORT CHAIN
PHOTOSYSTEM II
PHOTOSYSTEM I
ATP SYNTHASE
Figure 7.9

13. 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle
THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS
7.10 ATP and NADPH power sugar synthesis in the Calvin cycle
The Calvin cycle occurs in the chloroplast’s stroma
This is where carbon fixation takes place and sugar is manufactured
INPUT
CALVIN CYCLE
Figure 7.10A
OUTPUT:

14. The Calvin cycle constructs G3P using
carbon from atmospheric CO2
electrons and H+ from NADPH
energy from ATP
Energy-rich sugar is then converted into glucose

15. Details of the Calvin cycle
INPUT: 3
In a reaction catalyzed by rubisco, 3 molecules of CO2 are fixed.
CO2
Step Carbon fixation. 1 1 3 P P 6 P
RuBP
3-PGA 6
ATP
3 ADP
Step Energy consumption and redox. 2
6 ADP + P 3
ATP
CALVIN CYCLE 2 6 4
NADPH
6 NADP+
Step Release of one molecule of G3P. 3 5 P 6 P
G3P
G3P 3
Step Regeneration of RuBP. 4
Glucose and other compounds
OUTPUT: 1 P
G3P
Figure 7.10B

16. 7.11 Review: Photosynthesis uses light energy to make food molecules
PHOTOSYNTHESIS REVIEWED AND EXTENDED
7.11 Review: Photosynthesis uses light energy to make food molecules
A summary of the chemical processes of photo-synthesis
Chloroplast
Light
Photosystem II Electron transport chains Photosystem I
CALVIN CYCLE
Stroma
Electrons
Cellular respiration
Cellulose
Starch
Other organic compounds
LIGHT REACTIONS
CALVIN CYCLE
Figure 7.11

17. Many plants make more sugar than they need
The excess is stored in roots, tuber, and fruits
These are a major source of food for animals

18. 7.12 C4 and CAM plants have special adaptations that save water
Most plants are C3 plants, which take CO2 directly from the air and use it in the Calvin cycle
In these types of plants, stomata on the leaf surface close when the weather is hot
This causes a drop in CO2 and an increase in O2 in the leaf
Photorespiration may then occur

19. Photorespiration in a C3 plant
CALVIN CYCLE
2-C compound
Figure 7.12A

20. Some plants have special adaptations that enable them to save water
Special cells in C4 plants—corn and sugarcane—incorporate CO2 into a four-carbon molecule
This molecule can then donate CO2 to the Calvin cycle
4-C compound
CALVIN CYCLE
3-C sugar
Figure 7.12B

21. The CAM plants—pineapples, most cacti, and succulents—employ a different mechanism
They open their stomata at night and make a four-carbon compound
It is used as a CO2 source by the same cell during the day
4-C compound
Night
Day
CALVIN CYCLE
3-C sugar
Figure 7.12C

22. PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH’S ATMOSPHERE
7.13 Human activity is causing global warming; photosynthesis moderates it
Due to the increased burning of fossil fuels, atmospheric CO2 is increasing
CO2 warms Earth’s surface by trapping heat in the atmosphere
This is called the greenhouse effect

23. Sunlight
ATMOSPHERE
Radiant heat trapped by CO2 and other gases
Figure 7.13A & B

24. Because photosynthesis removes CO2 from the atmosphere, it moderates the greenhouse effect
Unfortunately, deforestation may cause a decline in global photosynthesis

25. 7.14 Talking About Science: Mario Molina talks about Earth’s protective ozone layer
Mario Molino received a Nobel Prize in 1995 for his work on the ozone layer
His research focuses on how certain pollutants (greenhouse gases) damage that layer
Figure 7.14A

26. The O2 in the atmosphere results from photosynthesis
Solar radiation converts O2 high in the atmosphere to ozone (O3)
Ozone shields organisms on the Earth’s surface from the damaging effects of UV radiation

27. International restrictions on these chemicals are allowing recovery
Industrial chemicals called CFCs have hastened ozone breakdown, causing dangerous thinning of the ozone layer
International restrictions on these chemicals are allowing recovery
Sunlight
Southern tip of South America
Antarctica
Figure 7.14B