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

2. To make sugar and oxygen gas from carbon dioxide and water
Photosynthesis is the process by which certain organisms use light energy
To make sugar and oxygen gas from carbon dioxide and water
Light
energy
PHOTOSYNTHESIS 6
CO2 +
H2O
Carbon dioxide
Water
C6H12O6 O2
Glucose
Oxygen gas

3. As the human demand for energy grows
Fossil fuel supplies are dwindling
Energy plantations
Are being planted to serve as a renewable energy source

4. AN OVERVIEW OF PHOTOSYNTHESIS
7.1 Autotrophs are the producers of the biosphere
Plants are autotrophs
Producing their own food and sustaining themselves without eating other organisms

5. Plants, algae, and some bacteria are photoautotrophs
Plants, algae, and some bacteria are photoautotrophs
Producers of food consumed by virtually all organisms
Figure 7.1A–D

6. 7.2 Photosynthesis occurs in chloroplasts
In plants, photosynthesis
Occurs primarily in the leaves, in the chloroplasts, which contain stroma, and stacks of thylakoids called grana
Leaf Cross Section
Leaf
Mesophyll Cell
Mesophyll
Vein
Stoma
CO2 O2
Chloroplast
Grana
Stroma
TEM 9,750 
Granum
Thylakoid
space
Outer
membrane
Inner
Intermembrane
LM 2,600 
Figure 7.2

7. 7.3 Plants produce O2 gas by splitting water
The O2 liberated by photosynthesis
Is made from the oxygen in water
Reactants:
Products:
6 CO2
12 H2O
C6H12O6
6 H2O
6 O2
Labeled
Experiment 1
Experiment 2
Not
labeled +
Figure 7.3A–C

8. 7.4 Photosynthesis is a redox process, as is cellular respiration
7.4 Photosynthesis is a redox process, as is cellular respiration
In photosynthesis
H2O is oxidized and CO2 is reduced
Reduction
Oxidation
6 O2
6 H2O
6 CO2 
C6H12O6
Figure 7.4A, B

9. 7.5 Overview: Photosynthesis occurs in two stages linked by ATP and NADPH
The complete process of photosynthesis consists of two linked sets of reactions
The light reactions and the Calvin cycle

10. The Calvin cycle assembles sugar molecules from CO2
The light reactions
Convert light energy to chemical energy and produce O2
The Calvin cycle assembles sugar molecules from CO2
Using ATP and NADPH from the light reactions
Figure 7.5
Light
CO2
H2O
Chloroplast
LIGHT
REACTIONS
(in thylakoids)
CALVIN
CYCLE
(in stroma)
NADP+
ADP + P
ATP
NADPH O
Sugar
Electrons

11. THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY
7.6 Visible radiation drives the light reactions
Certain wavelengths of visible light, absorbed by pigments
Drive the light reactions of photosynthesis
Increasing energy
10–5 nm
10–3 nm
1 nm
103 nm
106 nm
1 m
103 m
Gamma
rays
X-rays UV
Infrared
Micro-
waves
Radio
Visible light
400
500
600
700
750
650 nm
Wavelength (nm)
Transmitted
light
Absorbed
Reflected
Light
Chloroplast
380
Figure 7.6A, B

12. Absorption Spectra of Photosynthetic Pigments
Chapter 5
Section 2 Photosynthesis
Absorption Spectra of Photosynthetic Pigments

13. 7.7 Photosystems capture solar power
7.7 Photosystems capture solar power
Thylakoid membranes contain multiple photosystems
That absorb light energy, which excites electrons
Figure 7.7A

14. Chlorophyll a molecule
Each photosystem consists of
Light-harvesting complexes of pigments
A reaction center with a primary electron acceptor that receives excited electrons from a reaction-center chlorophyll
Energy of electron
Photon
Excited state
Heat
(fluorescence)
Ground state
Chlorophyll
molecule e–
Photosystem
Light-harvesting
complexes
Reaction
center
Primary electron
acceptor
To electron
transport chain
Pigment molecules
Chlorophyll a molecule
Transfer of energy
Thylakoid membrane
Figure 7.7B, C

15. 7.8 In the light reactions, electron transport chains generate ATP and NADPH
Two connected photosystems absorb photons of light
And transfer the energy to chlorophyll P680 and P700

16. Electron transport chain
The excited electrons
Are passed from the primary electron acceptor to electron transport chains
Thylakoid
space
Photon
Stroma
Thylakoid membrane 1
Photosystem I
Photon
P700 e– 5
Photosystem II e–
P680 2 +
NADP+ H+
NADPH 6
ATP
Electron transport chain
Provides energy for synthesis of
by chemiosmosis 4
H2O 1 2
+ 2 O2 H+ 3
Figure 7.8A

17. Electrons shuttle from photosystem II to I
Providing energy to make ATP
Electrons from photosystem I
Reduce NADP+ to NADPH
Figure 7.8B e–
ATP
Mill
makes
Photon
Photosystem II
Photosystem I
NADPH

18. Photosystem II regains electrons by splitting water
Releasing O2

19. 7.9 Chemiosmosis powers ATP synthesis in the light reactions
The electron transport chain
Pumps H+ into the thylakoid space

20. The diffusion of H+ back across the membrane through ATP synthase
Powers the phosphorylation of ADP to produce ATP (photophosphorylation)
Chloroplast
Stroma (low H+ concentration)
Light
NADP+ + H+
NADPH
ATP P
ADP +
Thylakoid
membrane
H2O 1 2 O2
Photosystem II
Electron
transport chain
Photosystem I
ATP synthase
Thylakoid space
(high H+ concentration)
Figure 7.9

21. 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
Consists of carbon fixation, reduction, release of G3P, and regeneration of RuBP
Figure 7.10A
Input
CO2
ATP
NADPH
CALVIN
CYCLE
G3P
Output:

22. Using carbon from CO2, electrons from NADPH, and energy from ATP
The cycle constructs G3P, which is used to build glucose and other organic molecules 3 P
CO2
Step Carbon fixation. An enzyme
called rubisco combines CO2 with a five-carbon sugar called ribulose bisphosphate (abbreviated RuBP). The unstable product splits into two molecules of the three-carbon organic acid, 3-phosphoglyceric acid (3-PGA). For three CO2 entering, six 3-PGA result. 1
Input:
In a reaction
catalyzed by rubisco,
CO2 is added to RuBP. 6
RuBP
3-PGA 3
G3P 6 P 2
Step Reduction. Two che-
mical reactions (indicated by the two blue arrows) consume energy from six molecules of ATP and oxidize six molecules of NADPH. Six molecules of 3-PGA are reduced, producing six molecules of the energy-rich three-carbon sugar, G3P
ATP
ADP +
NADPH
NADP+ 3
3 ADP
ATP 4
Step Regeneration of RuBP. A series of chemical reactions uses energy from ATP to rearrange the atoms in the five G3P molecules (15 carbons total), forming three RuBP molecules (15 carbons).These can start another turn of the cycle. 5 P
G3P
CALVIN
CYCLE 3
Step Release of one molecule of G3P. Five of the G3Ps from step 2 remain in the cycle. The single molecule of G3P you see leaving the cycle is the net product of photosynthesis. A plant cell uses two G3P molecules to make one molecule of glucose.
Output: 1 P
G3P
Glucose
and other
compounds
Figure 7.10B

23. PHOTOSYNTHESIS REVIEWED AND EXTENDED
7.11 Review: Photosynthesis uses light energy to make food molecules
Light
H2O
CO2
NADP+
Photosystem II
Photosystem I
Electron
transport
chains
ADP P +
RUBP
CALVIN
CYCLE
(in stroma)
3-PGA
Stroma
G3P
NADPH
ATP O2
LIGHT REACTIONS
CALVIN CYCLE
Sugars
Cellular
respiration
Cellulose
Starch
Other organic
compounds
Thylakoid
membranes
Chloroplast
Figure 7.11

24. 7.12 C4 and CAM plants have special adaptations that save water
In C3 plants a drop in CO2 and rise in O2 when stomata close on hot dry days
Divert the Calvin cycle to photorespiration

25. C4 plants first fix CO2 into a four-carbon compound
That provides CO2 to the Calvin cycle
Figure 7.12 (left half)
Sugarcane
C4 plant
CALVIN
CYCLE
3-C sugar
CO2
4-C compound
Mesophyll cell
Bundle-sheath cell

26. CAM plants open their stomata at night
Making a four-carbon compound used as a CO2 source during the day
CO2
Figure 7.12 (right half)
CAM plant
Day
CALVIN
CYCLE
3-C sugar
CO2
4-C compound
Night
Pineapple

27. PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH’S ATMOSPHERE CONNECTION
7.13 Photosynthesis moderates global warming
Greenhouses used to grow plants
Trap solar radiation, raising the temperature inside
Figure 7.13A

28. Excess CO2 in the atmosphere Is contributing to global warming
Sunlight
ATMOSPHERE
Some heat
energy escapes
into space
Radiant heat
trapped by CO2
and other gases
Figure 7.13B

29. Photosynthesis, which removes CO2 from the atmosphere
Moderates this warming

30. 7.14 Mario Molina talks about Earth’s protective ozone layer
TALKING ABOUT SCIENCE
7.14 Mario Molina talks about Earth’s protective ozone layer
Figure 7.14A

31. Solar radiation converts O2 high in the atmosphere to ozone (O3)
Which shields organisms on the Earth’s surface from the damaging UV radiation

32. But international restrictions on CFC use are allowing recovery
Industrial chemicals called CFCs have caused dangerous thinning of the ozone layer
But international restrictions on CFC use are allowing recovery
Southern tip of
South America
Antarctica
Figure 7.14B