1. 8 Photosynthesis CAMPBELL BIOLOGY IN FOCUS
Urry • Cain • Wasserman • Minorsky • Reece
8 Photosynthesis
Kathleen Fitzpatrick and Nicole Tunbridge, Simon Fraser University Modified by James R. Jabbur, Houston Community College
© 2016 Pearson Education, Inc

2. Overview: The Process That Feeds the Biosphere
Photosynthesis is the process that converts solar energy into chemical energy
Autotrophs sustain themselves without eating anything derived from others. Autotrophs are the biosphere’s producers
Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from H2O and CO2
Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes…
BioFlix: Photosynthesis

3. Some 450 million year old chef’s
(a) Plants
(c) Unicellular protist
Figure 10.2 Photoautotrophs
10 µm
(e) Purple sulfur
bacteria
1.5 µm
(b) Multicellular alga
(d) Cyanobacteria
40 µm

4. Heterotrophs obtain their organic material from other organisms
Heterotrophs obtain their organic material from other organisms. Heterotrophs are the biosphere’s consumers
Almost all heterotrophs (including humans, we are chemoheterotrophs) depend on photoautotrophs for food and O2
Respect mother Earth!

5. Concept 8.1: Photosynthesis converts light energy to the chemical energy of food
Leaves are the major location of photosynthesis
Their green color is from chlorophyll, the green pigment within chloroplasts
Chloroplasts are structurally similar to and most likely evolved from photosynthetic bacteria
Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast

6. Chloroplasts: The Sites of Photosynthesis in Plants
CO2 enters and O2 exits the leaf through microscopic pores caled stomata (Gr. “mouth”)
Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf
A typical mesophyll cell has 30–40 chloroplasts
The chlorophyll is in the connected membranes of thylakoids, which are stacked in columns called grana, surrounded by intra-organellar liquid called stroma

7. Leaf cross section
Vein
Mesophyll
Stomata
CO2 O2
Chloroplast
Mesophyll cell
Outer
membrane
Thylakoid
Figure 10.3 Zooming in on the location of photosynthesis in a plant
Intermembrane
space
5 µm
Stroma
Granum
Thylakoid
space
Inner
membrane
1 µm

8. Tracking Atoms Through Photosynthesis
Photosynthesis can be summarized as the following equation:
Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules, which derive their carbon and oxygen from CO2
Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced
6 CO2 + 6 H2O + Light energy  C6H12O6 + 6 O2

9. Where do the atoms go? Reactants: 6 CO2 12 H2O Products: C6H12O6 6 H2O
Figure 10.4 Tracking atoms through photosynthesis
All carbon atoms making sugar come from the atmosphere (air).

10. The Two Stages of Photosynthesis: A Preview
The “light reactions” take place in the thylakoids:
Split H2O and abstract the electrons
Release the remaining O2 from split H2O
Reduce NADP+ to NADPH using electrons and H+ from split H2O
Generate ATP by photophosphorylation of ADP
The “dark reactions” take place in the stroma:
The Calvin cycle forms sugar from CO2, using ATP and NADPH for energy to complete the process.
This process is called carbon fixation, incorporating CO2 into organic molecules

11. i Light NADP+ ADP Light Reactions Chloroplast H2O + P
Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle
Chloroplast

12. i Light NADP+ ADP Light Reactions ATP NADPH Chloroplast O2 H2O + P
Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle
NADPH
Chloroplast O2

13. i CO2 Light NADP+ ADP Dark Reactions Of Calvin Cycle Light Reactions
H2O
CO2
Light
NADP+
ADP + P i
Dark Reactions
Of Calvin
Cycle
Light
Reactions
ATP
Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle
NADPH
Chloroplast O2

14. i CO2 Light NADP+ ADP Dark Reactions Of Calvin Cycle Light Reactions
H2O
CO2
Light
NADP+
ADP + P i
Dark Reactions
Of Calvin
Cycle
Light
Reactions
ATP
Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle
NADPH
Chloroplast
[CH2O]
(sugar) O2

15. Concept 8.1: The “light reactions” convert solar energy to the chemical energy of ATP and NADPH
Chloroplast thylakoids transform light energy into the chemical energy of ATP and NADPH
Light is a form of electromagnetic energy, also called electromagnetic radiation, that travels in rhythmic waves
The wavelength is the distance between crests of waves and determines the type of electromagnetic energy
The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation. Visible light consists of specific wavelengths that produce specific colors we see
Light also behaves as though it consists of discrete particles, called photons.

16. the electromagnetic spectrum
(109 nm)
10–5 nm
10–3 nm
1 nm
103 nm
106 nm
103 m
Gamma
rays
Micro-
waves
Radio
waves
X-rays UV
Infrared
Visible light
Figure 10.6 The electromagnetic spectrum
380
450
500
550
600
650
700
750 nm
Shorter wavelength
Longer wavelength
Higher energy
Lower energy

17. Photosynthetic Pigments: The Light Receptors
Pigments are substances that absorb visible light at specific wavelengths
Wavelengths that are not absorbed are reflected or transmitted (thus, we see them)
Leaves appear green because chlorophyll reflects and transmits green light
Animation: Light and Pigments

18. Why Leaves are Green: Interaction of Light With Chloroplasts
Figure 8.7 Why leaves are green: interaction of light with chloroplasts

19. Chlorophyll a is the main photosynthetic pigment
Absorption spectra
Chlorophyll b
Absorption of light by
chloroplast pigments
Carotenoids
400
500
600
700
Wavelength of light (nm)
An absorption spectrum is a graph that plots a pigment’s light absorption versus wavelength
Chlorophyll a is the main photosynthetic pigment
Chlorophyll b broadens the spectrum used for photosynthesis
Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll
(measured by O2 release)
Rate of photosynthesis
Figure 10.9 Which wavelengths of light are most effective in driving photosynthesis?
(b) Action spectrum
Aerobic bacteria
Filament
of alga
(c) Engelmann’s
experiment
400
500
600
700

20. interacts with hydrophobic regions of proteins inside
CH3
in chlorophyll a
CHO
in chlorophyll b
Porphyrin ring:
light-absorbing
“head” of molecule
(note magnesium
atom at center)
Figure Structure of chlorophyll molecules in chloroplasts of plants
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
Chloroplasts
(H atoms not shown)

21. Experiment: Which Wavelengths of Light are Most Effective in Driving Photosynthesis?
An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process
Theodor Englemann exposed different segments of a filamentous alga to different wavelengths of light
Areas receiving wavelengths favorable to photosynthesis produced excess oxygen, indicating growth
Figure Inquiry: Which wavelengths of light are most effective in driving photosynthesis? (part 2: action spectrum)

22. Excitation of Chlorophyll by Light
When chlorophyll (or any pigment) absorbs light, it goes from a ground state to an excited state, which is unstable
When excited electrons fall back to the ground state, photons are given off (energy released)
Albert Einstein received the Nobel Prize for his work in elucidating The Photoelectric Effect

23. A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes
A photosystem consists of light-harvesting complexes (pigment molecules bound to proteins) which surround and funnel the energy of photons to the reaction-center complex (a type of protein complex)
Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions (next slide)

24. (INTERIOR OF THYLAKOID)
Photosystem
STROMA
Photon
Primary
electron
acceptor
Light-harvesting
complexes
Reaction-center
complex e–
Thylakoid membrane
Figure How a photosystem harvests light
Transfer
of energy
Special pair of
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)

25. Figure 8.12-2 The structure and function of a photosystem (part 2: structure)

26. 2 Types of Photosystems
There are two types of photosystems in the thylakoid membrane
Photosystem II (PS II) functions first and is best at absorbing a wavelength of 680 nm. The reaction-center chlorophyll a of PS II is called P680
Photosystem I (PS I) is best at absorbing a wavelength of 700 nm. The reaction-center chlorophyll a of PS I is called P700

27. 2 Types of Electron Flow
During the light reactions, there are two possible routes for electron flow: linear or cyclic
Linear electron flow (the primary pathway) involves both photosystems and produces ATP and NADPH using light energy
Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH

28. Electron transport chain
Linear Electron Flow
Electron
transport
chain
Primary
acceptor
Primary
acceptor 4 7
Electron transport chain Fd Pq e– 2 e– 8 e– e–
NADP+
+ H+
H2O
Cytochrome
complex
2 H+
NADP+
reductase + 3
1/2 O2
NADPH Pc e– e–
P700 5
P680
Light 1
Light 6 6
Figure How linear electron flow during the light reactions generates ATP and NADPH
ATP
Pigment
molecules
Photosystem I
(PS I)
Photosystem II
(PS II)

29. Summary: Linear Electron Flow
(1) A photon hits a pigment and its energy is passed among pigment molecules until it excites P680
(2) An excited electron from P680 is transferred to the primary electron acceptor
(3) P680+ (P680 that is missing an electron) is a very strong oxidizing agent
H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680
O2 is released as a by-product of this reaction
(4) Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I
(5) Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane
Diffusion of H+ (protons) across the membrane drives ATP synthesis
(6) In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor
P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain
(7) Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)
(8) The electrons are then transferred to NADP+ and reduce it to NADPH
The electrons of NADPH are available for the reactions of the Calvin cycle

30. ATP NADPH Mill makes ATP Photosystem II Photosystem I e– e– e– e– e–
Photon
Figure A mechanical analogy for the light reactions e–
Photon
Photosystem II
Photosystem I

31. Cyclic Electron Flow (not discussed in book)
Primary
acceptor
Primary
acceptor Fd Fd
NADP+
+ H+ Pq
NADP+
reductase
Cytochrome
complex
NADPH Pc
Figure Cyclic electron flow
Photosystem I
Photosystem II
ATP
Cyclic electron flow generates surplus ATP.
Cyclic electron flow evolved before linear electron flow.

32. Comparison: 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 metabolic energy (ATP), whereas chloroplasts transform light energy into ATP
The spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows great similarities (next slide)

33. Mitochondrion Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST
Diffusion
Intermembrane
space
Thylakoid
space
Electron
transport
chain
Inner
membrane
Thylakoid
membrane
Figure Comparison of chemiosmosis in mitochondria and chloroplasts
ATP
synthase
Matrix
Stroma
Key
ADP + P i
ATP
Higher [H+] H+
Lower [H+]

34. (low H+ concentration) (high H+ concentration) (low H+ concentration)
STROMA
(low H+ concentration)
Cytochrome
complex
Photosystem II
Photosystem I
4 H+
Light
NADP+
reductase
Light Fd 3
NADP+ + H+ Pq
NADPH e– Pc e– 2
H2O 1
1/2 O2
THYLAKOID SPACE
(high H+ concentration)
+2 H+
4 H+ To
Calvin
Cycle
Thylakoid
membrane
Figure The light reactions and chemiosmosis: the organization of the thylakoid membrane
ATP
synthase
STROMA
(low H+ concentration)
ADP +
ATP P i H+
ATP and NADPH are produced on the side facing the stroma, where it is used to make sugar (Calvin cycle happens)

35. Concept 8.3: The Calvin cycle (C3 plants) 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 (glyceraldehyde-3-phosphate) from CO2 by using ATP and the reducing power of electrons carried by NADPH
For the net synthesis of 1 glyceraldehyde-3-phosphate, the cycle must take place three times, fixing 3 molecules of CO2
The Calvin cycle has three phases:
Carbon fixation (catalyzed by the enzyme rubisco)
Reduction (pump energy into the sugar product – glyceraldehyde-3-phosphate)
Regeneration of the CO2 acceptor (Ribulose Bis-Phosphate)

36. Phase 1: Carbon fixation Ribulose bisphosphate
Input 3
(Entering one
at a time)
CO2
Phase 1: Carbon fixation
Rubisco 3 P P
Short-lived
intermediate 3 P P 6 P
Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate
Calvin
Cycle
Figure The Calvin cycle

37. Figure 10.18 The Calvin cycle
Input 3
(Entering one
at a time)
CO2
Phase 1: Carbon fixation
Rubisco 3 P P
Short-lived
intermediate 3 P P 6 P
Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate 6
ATP
6 ADP
Calvin
Cycle 6 P P
1,3-Bisphosphoglycerate 6
NADPH
6 NADP+ 6 P i
Figure The Calvin cycle 6 P
Glyceraldehyde-3-phosphate
(G3P)
Phase 2:
Reduction 1 P
Glucose and
other organic
compounds
Output
G3P
(a sugar)

38. Figure 10.18 The Calvin cycle
Input 3
(Entering one
at a time)
CO2
Phase 1: Carbon fixation
Rubisco 3 P P
Short-lived
intermediate 3 P P 6 P
Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate 6
ATP
6 ADP
3 ADP
Calvin
Cycle 6 3 P P
ATP
1,3-Bisphosphoglycerate 6
NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+ 6 P i
Figure The Calvin cycle 5 P
G3P 6 P
Glyceraldehyde-3-phosphate
(G3P)
Phase 2:
Reduction 1 P
Glucose and
other organic
compounds
Output
G3P
(a sugar)

39. fixation of 3 molecules of CO2, via rubisco, forms 1 three-carbon
Called C3 plants because initial
fixation of 3 molecules of CO2,
via rubisco, forms 1 three-carbon
glyceraldehyde-3-phosphate
sugar molecule
3 CO2
Carbon fixation
3  5C
6  3C
Calvin
Cycle
Regeneration of
CO2 acceptor
5  3C
Reduction
1 G3P (3C)

40. Alternative mechanisms of carbon fixation have evolved in hot, arid climates
When in a hot and dry climate, dehydration is a big problem for plants
To conserve H2O, plants close their stomata, limiting photosynthesis by reducing access to CO2 and causing O2 to build up inside plant cells
These conditions favor photorespiration, where rubisco adds O2 instead of CO2 in the Calvin cycle, consuming O2 and organic fuel, releasing CO2 without producing ATP or sugar
Why? The process of photorespiration limits damaging products of light reactions (charged radicals) that build up in the absence of the Calvin cycle

41. Alternative 1: C4 Plants
C4 plants minimize the cost of photorespiration by initially incorporating CO2, creating a four-carbon compound inside mesophyll cells
This step requires the enzyme PEP carboxylase, which has a higher affinity for CO2 than rubisco; it can fix CO2 even at low CO2 concentrations
These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

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

43. Alternative 2: CAM Plants
Some plants use crassulacean acid metabolism (CAM) to fix carbon
CAM plants open their stomata at night (when it is cooler), incorporating CO2 into organic acids
During the day, the stomata close and CO2 is released from organic acids to be used in the Calvin cycle

44. Spatial separation of steps Temporal separation of steps
Sugarcane
Pineapple C4
CAM
CO2
CO2
Mesophyll
cell 1
CO2 incorporated
into four-carbon
organic acids
(carbon fixation)
Night
Organic acid
Organic acid
Figure C4 and CAM photosynthesis compared
Bundle-
sheath
cell
CO2
CO2
Day 2
Organic acids
release CO2 to
Calvin cycle
Calvin
Cycle
Calvin
Cycle
Sugar
Sugar
Spatial separation of steps
Temporal separation of steps

45. 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
Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits
In addition to food production, photosynthesis produces the O2 in our atmosphere

46. A Review of Photosynthesis
Figure 8.19 A review of photosynthesis

47. Make Connections: The Working Cell
Figure 8.20 Make connections: the working cell

49. Electron transport chain
H2O
CO2
Light
NADP+
ADP + P i
Light
Reactions:
Photosystem II
Electron transport chain
Photosystem I
RuBP
3-Phosphoglycerate
Calvin
Cycle
ATP
G3P
Starch
(storage)
Figure A review of photosynthesis
NADPH
Chloroplast O2
Sucrose (export)