1. Photosynthesis
Chapter 9

2. Photosynthesis: The Big Picture
Source of BOTH matter and energy for most living organisms
Captures light energy from the sun and converts it into chemical energy
Synthesized organic molecules from inorganic molecule
BOTTOM LINE: Makes FOOD

3. Definitions Autotroph:
Organisms that make their own food (energy-rich organic molecules) from simple, inorganic molecules
Photoautotroph:
Organisms that make their own food through photosynthesis; obtain energy from the sun
Type of autotroph
Heterotroph:
Get carbon and energy by eating autotrophs or one another

4. Photoautotrophs
Capture sunlight energy and use it to carry out photosynthesis
Plants
Some bacteria
cyanobacteria
Many protistans
algae
Plants
Algae (spirogyra)
Cyanobacteria
Algea (Kelp)

5. Linked Processes Photosynthesis Aerobic Respiration
Energy-storing pathway
Releases oxygen
Requires carbon dioxide
Aerobic Respiration
Energy-releasing pathway
Requires oxygen
Releases carbon dioxide

6. Photosynthesis Equation
LIGHT ENERGY
6H2O + 6CO2
6O2 + C6H12O6
Water
Carbon Dioxide
Oxygen
Glucose
In-text figure Page 115

7. Chloroplast Structure
Inner and outer membranes
Stroma
Granum
(Grana)
Thylakoid
Figure 7.3d, Page 116

8. Two Stages of Photosynthesis
Light dependent reactions
Converts light energy into chemical energy (ADP ATP)
Gathers e- and H+ from water (NADP+  NADPH)
Occurs in thylakoid membranes
Light independent reactions (Calvin-Benson Cycle)
Reduces CO2 to synthesize glucose using energy and hydrogens (i.e. ATP and NADPH) generated in the light dependent reaction
Occurs in Stroma
Notice that these reactions do not create NADH, but rather NADPH

9. Two Stages of Photosynthesis
sunlight
water uptake
carbon dioxide uptake
ATP
LIGHT-DEPENDENT REACTIONS
ADP + Pi
LIGHT-INDEPENDENT REACTIONS
NADPH
NADP+ P
glucose
oxygen release
new water
In-text figure Page 117

10. Electromagnetic Spectrum
Shortest Gamma rays
wavelength X-rays
UV radiation
Visible light
Infrared radiation
Microwaves
Longest Radio waves
wavelength

11. Visible Light Electromagnetic energy with a wavelength of 308-750nm
Light energy is organized into packets called photons
The shorter the wavelength the greater the energy carried by the photons

12. Properties of Light
White light (from the sun) contains all of the wavelengths of light
When light hits matter, it can be reflected (transmitted) or absorbed
White substances reflect all light
Black substances absorb all light

13. Pigments A substance that absorbs light
We see the color that is transmitted by pigment
The absorbed color disappears into pigment

14. Plant pigments Plant use a variety of pigments during photosynthesis:
Chlorophylls a and b
Carotenoids
Anthocyanins
Phycobilins
The main photosynthetic pigment is Chlorophyll a

15. Wavelength absorption (%) Wavelength (nanometers)
Chlorophylls
Chlorophyll a absorbs red and blue light, and reflects green light (what we see)
Note: The colors that are absorbed are used for photosynthesis
Wavelength absorption (%)
chlorophyll a
chlorophyll b
Figure 7.7 Page 120
Figure 7.6a Page 119
Wavelength (nanometers)

16. Effect of Light on Pigments
What happens when light hits pigments?
The color disappears, but the energy does not
Absorbing photons of light excites electrons (e-), thus adding potential energy
Ground state: normal pigment
Excited state: pigment absorbing light (e- excited)
Photon of light: e- e-
Atom in pigment:
Ground state
Atom in pigment:
Excited state

17. Photosystems
In thylakoid membrane, pigments are organized in clusters called photosystems
These clusters contain several hundred pigment molecules
Two types of photosystems
Photosystem I = P700 (absorbs light at 700nm)
Photosystem II = P680 (absorbs light at 680nm)

18. Reaction Center Chlorophyll
One of the pigments in each photosystem is known as the reaction center chlorophyll (RCC)
if any pigment within the photosystem gets hit by a photon, the energy is transferred to the RCC
The RCC will then transfer its excited e- into an electron transport chain

19. Pigments in a Photosystem
reaction center
Figure 7.11 Page 122

20. Light Dependent Reactions
Location: the thylakoid membranes
Function: to generate ATP (energy!) and NADPH (reducing power!) that will be used in the light independent reaction
Two processes:
Non-cyclic electron flow
Generates ATP and NADPH
Cyclic electron flow
Generates only ATP

21. Noncyclic Electron Flow
Two-step pathway for light absorption and electron excitation
Uses two photosystems: type I and type II
Produces ATP and NADPH
Involves photolysis - splitting of water

22. Machinery of Noncyclic Electron Flow
H2O
second electron transfer chain
photolysis e– e–
ATP SYNTHASE
first electron transfer chain
NADP+
NADPH
ATP
PHOTOSYSTEM II
PHOTOSYSTEM I
ADP + Pi
Figure 7.13a Page 123

23. Steps of Non-cyclic electron flow
Photosystem II gets hit by a photon; electron of RCC gets excited
The excited (high energy) e- gets picked up by an electron carrier and taken into an electron transfer chain (ETC)
The excited e- provides energy to pump protons (H+) into the thylakoid (tiny space)
Through chemiosmosis, ATP is generated

24. Chemiosmotic Model of ATP Formation
Electrical and H+ concentration gradients are created between thylakoid compartment and stroma
H+ flow down gradients into stroma through ATP synthase
The energy driven by the flow of H+ powers the formation of ATP from ADP and Pi

25. Chemiosmotic Model for ATP Formation
H+ is shunted across membrane by some components of
the first electron transfer chain
Gradients propel H+ through ATP synthases;
ATP forms by phosphate-group transfer
Photolysis in the thylakoid compartment splits water
H2O e–
acceptor
ATP SYNTHASE
ATP
ADP + Pi
PHOTOSYSTEM II
Figure 7.15 Page 124

26. Non-cyclic electron flow: Photolysis
While Photosystem II gets hit by light, etc., water is split:
H2O  ½ O2 + 2H+ + 2e-
This process is called photolysis
The H+ are pumped into the thylakoid to create the proton gradient
The e- replace the excited e- that was taken away from the RCC

27. Non-Cyclic Electron Flow: The saga continues
Photosystem I gets excited at the same time as photosystem II
Its excited e- gets taken into a second electron transfer chain that attaches the excited e- and the leftover H+ to NADP+ to make NADPH:
NADP+ + H+ + e-  NADPH

28. Non-cyclic electron flow
The “electron hole” in photosystem I is then filled with the used up, low energy e- from photosystem II
Now everything is back to normal, and we can start all over again

29. Energy Changes in Non-cyclic electron flow
second
transfer
chain e–
NADPH
first e–
transfer
chain
Potential to transfer energy (volts) e– e–
(Photosystem I)
(Photosystem II)
H2O
1/2O2 + 2H+
Figure 7.13b Page 123

30. Non-cyclic electron flow: Summary
After two excited photosystems, two ETCs and the splitting of water, both ATP and NADPH are generated!!!

31. Cyclic electron flow
The light independent reactions require more ATP than NADPH
Cyclic electron flow is like a short cut to making extra ATP
Involves only Photosystem I

32. Cyclic electron flow Photosystem I gets excited
Excited e- is carried into the first ETC; energy goes to pump H+ into thylakoid compartment
Chemiosmosis powers formation of ATP
The same e- (now low energy) replaces itself in the “electron hole” in Photosystem I

33. second electron transfer chain first electron transfer chain
Cyclic electron flow
H2O
second electron transfer chain
photolysis e– e–
ATP SYNTHASE
first electron transfer chain
NADP+
NADPH
ATP
PHOTOSYSTEM II
PHOTOSYSTEM I
ADP + Pi
Figure 7.13a Page 123

34. Light dependent reactions: Summary
Non-cyclic electron flow
Generates ATP and NADPH
Uses both photosystems (P680 and P700) and both electron transport chains
Involves photolysis
Cyclic electron flow
Generates ATP only
Uses only P700 and only one ETC

35. Light-Independent Reactions
Synthesis part of photosynthesis
Can proceed in the dark
Take place in the stroma
Also called Calvin-Benson cycle, or Calvin Cycle, or Dark Reactions

36. Calvin- Benson Cycle Three Phases: Carbon Fixation Reduction
Regeneration of RUBP

37. Calvin-Benson Cycle: Carbon Fixation
Capturing atmospheric (gaseous) CO2 by attaching it to RuBP, a 5-carbon organic molecule
This process forms two 3-carbon molecules
The enzyme that catalyzes this process is called Rubisco

38. Calvin Benson Cycle: Reduction
The captured CO2 has very little energy and no hydrogens
In order to make sugar, energy and hydrogens need to be added to the molecules formed by Carbon fixation
ATP and NADPH (made in the light dependent reactions) break down to form ADP and NADP+ and, in the process, transfer energy and hydrogens to the 3-carbon compounds formed by carbon fixation, resulting in sugar formation

39. Calvin-Benson Cycle: Regeneration
Some of the sugar created by reduction leaves the Calvin cycle, and is used to build up glucose and other organic molecules
The rest of the sugar is used to remake (regenerate) RuBP
This process requires ATP (which was made in the light dependent reactions)

40. Calvin-Benson Cycle: Summary
The cycle proceeds 6 times to form each molecule of glucose
In the process, ATP and NADPH is used up
6CO2 are converted into C6H12O6 - glucose

41. The C3 Pathway
In Calvin-Benson cycle, as described, the first stable intermediate is a three-carbon PGA
Because the first intermediate has three carbons, the pathway is called the C3 pathway

42. Photorespiration in C3 Plants
On hot, dry days stomata (holes in the leaf) close to prevent evaporation of water
As a result, within the leaf oxygen levels rise, and Carbon dioxide levels drop
Rubisco attaches RuBP to oxygen instead of carbon dioxide
Results in a VERY wasteful process known as Photorespiration – uses up ATP without generating sugar

43. C4 and CAM Plants
To avoid photorespiration, plants that live in hot, dry climates evolved mechanisms to separate carbon fixation from the Calvin Cycle
The CO2 that enters the Calvin cycle is derived from the breakdown of previously synthesized organic acids
In this way, the enzyme that catalyzes the reaction that attaches CO2 to RuBP is not exposed to atmospheric oxygen

44. C4 and CAM Plants
C4 plants (grasses) do carbon fixation in a different location (cell type) than the Calvin cycle
CAM plants (succulents and Cacti) do carbon fixation at a different time (night) that the Calvin cycle (day)

45. Summary of Photosynthesis
light
6O2
12H2O
CALVIN-BENSON CYCLE
C6H12O6
(phosphorylated glucose)
NADPH
NADP+
ATP
ADP + Pi
PGA
PGAL
RuBP P
6CO2
end product (e.g., sucrose, starch, cellulose)
LIGHT-DEPENDENT REACTIONS
6H2O
LIGHT-INDEPENDENT REACTIONS
Figure 7.21 Page 129