1. LE 10-3 Leaf cross section Vein Mesophyll Stomata CO2 O2
Mesophyll cell
Chloroplast
5 µm
Outer
membrane
Thylakoid
Stroma
Granum
Thylakoid
space
Intermembrane
space
Inner membrane
1 µm

2. Chloroplasts are organelles that are the site of photosynthesis
Leaves are the major locations of photosynthesis
Their green color is from chlorophyll, the green pigment within chloroplasts
Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast
Through microscopic pores called stomata, CO2 enters the leaf and O2 exits

3. Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf
The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana
Chloroplasts also contain stroma, a dense fluid

4. 6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O
Photosynthesis can be summarized as the following equation:
6 CO H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O
Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules
Photosynthesis is a redox process in which water is oxidized and carbon dioxide is reduced

5. LE 10-5_3 H2O CO2 Light NADP+ ADP + CALVIN CYCLE LIGHT REACTIONS ATP
NADPH
Chloroplast
[CH2O]
(sugar) O2

6. The Two Stages of Photosynthesis: A Preview
Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)
The light reactions (in the thylakoids) split water, release O2, produce ATP, and form NADPH
The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH
The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules

7. LE 10-7 Light Reflected light Chloroplast Absorbed Granum light
Transmitted
light

8. Photosynthetic Pigments: The Light Receptors
Pigments are substances that absorb visible light
Different pigments absorb different wavelengths
Wavelengths that are not absorbed are reflected or transmitted
Leaves appear green because chlorophyll reflects and transmits green light

9. Chlorophyll a is the main photosynthetic pigment
Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis
Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll

10. (INTERIOR OF THYLAKOID)
LE 10-12
Thylakoid
Photosystem
STROMA
Photon
Light-harvesting
complexes
Reaction
center
Primary electron
acceptor
Thylakoid membrane e–
Transfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)

11. A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes
A photosystem consists of a reaction center surrounded by light-harvesting complexes
The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center

12. A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a
Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions

13. There are two types of photosystems in the thylakoid membrane
Photosystem II functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm
Photosystem I is best at absorbing a wavelength of 700 nm
The two photosystems work together to use light energy to generate ATP and NADPH

14. Non Cyclic LE 10-13_5 ADP Electron Transport chain Primary acceptor
H2O
CO2
Light
NADP+
ADP
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH O2
Electron
Transport
chain
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
Electron transport chain Fd Pq e– e–
H2O e– e–
NADP+
Cytochrome
complex
2 H+
NADP+
reductase
+ 2 H+ +
1/2 O2
NADPH Pc e–
+ H+
Energy of electrons e–
P700
Light
P680
Light
ATP
Photosystem I
(PS I)
Photosystem II
(PS II)

15. NADPH Mill makes ATP Photosystem II Photosystem I LE 10-14 e– ATP e–
Photon e–
Photon
Photosystem II
Photosystem I

16. Noncyclic Electron Flow
During the light reactions, there are two possible routes for electron flow: cyclic and noncyclic
Noncyclic electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH

17. Primary acceptor Primary Fd acceptor Fd NADP+ Pq NADP+ reductase
LE 10-15
Primary
acceptor
Primary
acceptor Fd Fd
NADP+ Pq
NADP+
reductase
Cytochrome
complex
NADPH Pc
Photosystem I
ATP
Photosystem II

18. Cyclic Electron Flow
Cyclic electron flow uses only photosystem I and produces only ATP
Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle

19. LE 10-16 Mitochondrion Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST
Diffusion
Intermembrane
space
Thylakoid
space
Electron
transport
chain
Membrane
Key
ATP
synthase
Matrix
Stroma
Higher [H+]
Lower [H+]
ADP + P i
ATP H+

20. A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy
Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP
The spatial organization of chemiosmosis differs in chloroplasts and mitochondria

21. LE 10-17 STROMA (Low H+ concentration) Cytochrome complex
H2O
CO2
Light
NADP+
ADP
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH
STROMA
(Low H+ concentration) O2
[CH2O] (sugar)
Cytochrome
complex
Photosystem II
Photosystem I
Light
NADP+
reductase
Light
2 H+ Fd
NADP+ + 2H+
NADPH
+ H+ Pq Pc
H2O
THYLAKOID SPACE
(High H+ concentration)
1/2 O2
+2 H+
2 H+ To
Calvin
cycle
Thylakoid
membrane
ATP
synthase
STROMA
(Low H+ concentration)
ADP +
ATP P i H+

22. Water is split by photosystem II on the side of the membrane facing the thylakoid space
The diffusion of H+ from the thylakoid space back to the stroma powers ATP synthase
ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place

23. The Calvin cycle has three phases:
Carbon fixation (catalyzed by rubisco)
Reduction
Regeneration of the CO2 acceptor (RuBP)

24. LE 10-18_3 Input 3 (Entering one at a time) CO2
H2O
CO2
Input
Light 3
(Entering one
at a time)
NADP+
ADP
CO2
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
Phase 1: Carbon fixation
NADPH
Rubisco O2
[CH2O] (sugar) 3 P P
Short-lived
intermediate 3 P P 6 P
Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate 6
ATP
6 ADP
3 ADP
CALVIN
CYCLE 3 6 P P
ATP
1,3-Bisphosphoglycerate 6
NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+ 6 P i 5 P
G3P 6 P
Glyceraldehyde-3-phosphate
(G3P)
Phase 2:
Reduction 1 P
G3P
(a sugar)
Glucose and
other organic
compounds
Output

25. The Calvin cycle uses ATP and NADPH to convert CO2 to sugar
The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle
The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH
Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)
For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2

26. Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates
Dehydration is a problem for plants, sometimes requiring tradeoffs with other metabolic processes, especially photosynthesis
On hot, dry days, plants close stomata, which conserves water but also limits photosynthesis
The closing of stomata reduces access to CO2 and causes O2 to build up
These conditions favor a seemingly wasteful process called photorespiration

27. Photorespiration: An Evolutionary Relic?
In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound
In photorespiration, rubisco adds O2 to the Calvin cycle instead of CO2
Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar

28. C4 leaf anatomy and pathway
Mesophyll
cell
Mesophyll cell
CO2
Photosynthetic
cells of C4 plant
leaf
PEP carboxylase
Bundle-
sheath
cell
The C4 pathway
Oxaloacetate (4 C)
PEP (3 C)
Vein
(vascular tissue)
ADP
Malate (4 C)
ATP
C4 leaf anatomy
Pyruvate (3 C)
Bundle- sheath
cell
Stoma
CO2
CALVIN
CYCLE
Sugar
Vascular
tissue

29. C4 Plants
C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells
These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

30. CAM Plants
CAM plants open their stomata at night, incorporating CO2 into organic acids
Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle

31. LE 10-20 Sugarcane Pineapple C4 CAM CO2 CO2 Mesophyll cell
CO2 incorporated
into four-carbon
organic acids
(carbon fixation)
Night
Organic acid
Organic acid
Bundle-
sheath
cell
CO2
CO2
Day
Organic acids
release CO2 to
Calvin cycle
CALVIN
CYCLE
CALVIN
CYCLE
Sugar
Sugar
Spatial separation of steps
Temporal separation of steps

32. The Importance of Photosynthesis: A Review
The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds
Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells
In addition to food production, photosynthesis produces the oxygen in our atmosphere