1. Using Light to Make Food
Ch. 7 PHOTOSYNTHESIS
Using Light to Make Food

2. Autotrophs Are the Producers of The Biosphere
Autotrophs make their own food
Photoautotrophs use the energy of light and chloroplasts to produce organic molecules
Most plants, algae and other protists, some prokaryotes (cyanobacteria)
Copyright © 2009 Pearson Education, Inc.

3. Figure 7.1A Forest plants.

4. Chloroplast Outer and inner membranes Thylakoid Intermembrane space
Stroma
Granum
Thylakoid
space
Figure 7.2 The location and structure of chloroplasts.

5. Visible Radiation Drives the Light Reactions
Sunlight is a type of electromagnetic energy (radiation)
Visible light is a small part of the EM spectrum
Light travels in waves and is particulate
One wavelength = distance between the crests of two adjacent waves (shorter λ’s have more energy)
Photons are discrete packets of light energy (they contain a fixed quantity of light energy)

6. Increasing energy 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1 m 103 m Micro-
Gamma
rays
Micro-
waves
Radio
waves
X-rays UV
Infrared
Visible light
Figure 7.6A The electromagnetic spectrum and the wavelengths of visible light. (A
380
400
500
600
700
750
Wavelength (nm)
650 nm

7. Visible Radiation Drives the Light Reactions
Pigments
Are proteins
Absorb specific wavelengths of light, transmit others
The color of a pigment is the color of light most reflected or transmitted by that pigment
Ex. chlorophyll reflects/transmits green and appears green
Various pigments are built into the thylakoid membrane of the chloroplast
Each type of pigment absorbs certain wavelengths of light because it is able to absorb the

8. Visible Radiation Drives the Light Reactions
Chloroplasts contain several different pigments and all absorb light of different wavelengths
Chlorophyll a: absorbs blue violet and red light and reflects green
Chlorophyll b: absorbs blue and orange and reflects yellow-green
The carotenoids: absorb mainly blue-green light and reflect yellow and orange

9. Photosynthesis is a process that converts solar energy to chemical energy
Plants use water and atmospheric carbon dioxide to produce a simple sugar and release oxygen
Light
energy 6
CO2
+ 6
H2O
C6H12O6
+ 6 O2
Carbon dioxide
Water
Glucose
Oxygen gas
Photosynthesis

10. Photosynthesis Occurs in Chloroplasts in Plant Cells
Leaves have:
Stomata = pores that allow CO2 to enter and O2 to exit
Veins transport water & nutrients absorbed by roots
Chloroplasts = site of photosynthesis
Contain the green pigment chlorophyll
Vein
CO2 O2
Stoma
Figure 7.2 The location and structure of chloroplasts.
Chloroplasts

11. Photosynthesis is a Redox Process, as is Cellular Respiration
A loss of electrons = oxidation
A gain of electrons = reduction
Electrons are lost and gained in the form of hydrogen
During redox reactions of photosynthesis, H2O donates electrons and is oxidized, CO2 accepts electrons and is reduced
Reduction
6 CO H2O
C6H12O O2
Oxidation

12. H2O CO2 Chloroplast Light NADP+ ADP  P LIGHT REACTIONS CALVIN CYCLE
(in stroma)
(in thylakoids)
ATP
Electrons
NADPH O2
Sugar

13. The “Photo” in Photosynthesis:
THE LIGHT REACTIONS

14. Photosystems Capture Solar Power in the Light Reactions
A photosystem is a functional unit that captures sunlight and helps convert it to the chemical energy of ATP & NADPH
A Photosystem has a light-harvesting complex surrounding a reaction center
chl. a and a primary e- acceptor are in the reaction center
Light-harvesting
complexes
Reaction
center e–
Figure 7.7B Light-excited chlorophyll embedded in a photosystem: Its electron is transferred to a primary electron acceptor before it returns to ground state.
Pigment
molecules
Pair of
Chlorophyll a molecules

15. Photosystems Capture Solar Power in the Light Reactions
Two photosystems exist: photosystem I and photosystem II
Each photosystem has a characteristic reaction center
Photosystem II: functions first; the chl. a of this photosystem is called P680 (it best absorbs light at 680nm or red)
Photosystem I: functions next; the chl. a of this photosystem is called P700 (it best absorbs light at 700nm, also red)

16. Photosystems Capture Solar Power in the Light Reactions
Light energy is absorbed by pigments of PS II and passed from pigment to pigment within the photosystem
The energy passes to chl. a, exciting its electrons
An excited e- from chl. a is transferred to the primary electron acceptor
An enzyme oxidizes water and gives the electrons to chl. a
This step releases oxygen
Electron transport chain
Provides energy for
synthesis of
by chemiosmosis
NADP+ + H+
NADPH
Photon
Photon
Photosystem II
ATP
Photosystem I 6
Stroma 1
Primary
acceptor
Primary
acceptor 2 e– e–
Thylakoid
mem-
brane 4 5
P700
P680 e–
Thylakoid
space e– e– 3
H2O H+ 2  1 O2
+ 2

17. The Electron Transport Chain
Each photoexcited e- passes from the primary e- acceptor of PS II to PS I via an electron transport chain (ETC)
The exergonic “fall” of electrons down the ETC will help generate a H+ gradient used to generate ATP.
The ETC is a bridge between photosystems II and I

18. The Electron Transport Chain
NADP+ + H+
NADPH
Photon
Photon
Photosystem I
Photosystem II 6
Stroma 1
Primary
acceptor
Primary
acceptor 2 e– e–
Thylakoid
mem-
brane 4 5
P700
P680
Thylakoid
space 3
H2O 2  1 O2
+ 2 H+

19. The Electron Transport Chain
The ETC accepts electrons from PS II
Electrons are passed along ETC and the 2nd protein complex pumps H+ into the thylakoid space
This generates a proton (H+) gradient
H+ flows from the thylakoid space to the stroma, down its gradeint, through an ATP synthase.
ATP synthase phosphorylates ADP forming ATP
This is an energy-coupling process called chemiosmosis, where the exergonic flow of H+ powers the endergonic phosphorylation of ADP

20. Electron transport chain
Stroma (low H+
concentration) H+ H+
ADP + P
ATP H+
NADP+ + H+
NADPH
PS II H+
PS I
H2O H+ H+ H+
Figure 7.9 The production of ATP by chemiosmosis in photosynthesis. 2  1 O2
+ 2 H+ H+ H+ H+ H+ H+
Electron
transport
chain H+
ATP synthase H+
Thylakoid space (high H+ concentration)

21. The Light Reactions
Electrons moving down the ETC are passed to chl. a of PS I
NADP+ reductase transfers electrons from PS I to NADP+ forming NADPH

22. Electron transport chain
Stroma (low H+
concentration) H+ H+
ADP + P
ATP H+
NADP+ + H+
NADPH
PS II H+
PS I
H2O H+ H+ H+
Figure 7.9 The production of ATP by chemiosmosis in photosynthesis. 2  1 O2
+ 2 H+ H+ H+ H+ H+ H+
Electron
transport
chain H+
ATP synthase H+
Thylakoid space (high H+ concentration)

23. The Light Reactions
As a result of the light reactions, ATP and NADPH are produced (in the stroma)
Photophosphorylation is the process of generating ATP from ADP & phosphate by means of a proton-motive force generated by the thylakoid membrane in the light reactions of photosynthesis

24. THE CALVIN CYCLE: CONVERTING CO2 TO SUGAR

25. ATP and NADPH Power Sugar Synthesis in the Calvin Cycle
The Calvin cycle makes sugar within a chloroplast
Atmospheric CO2, ATP, and NADPH are required to produce sugar
Using these three ingredients, a three-carbon sugar called glyceraldehyde-3-phosphate (G3P) is produced
A plant cell may G3P to make glucose and other organic molecules
CO2
ATP
NADPH
Input
CALVIN
CYCLE
Figure 7.10A An overview of the Calvin cycle.
Output:
G3P

26. ATP and NADPH Power Sugar Synthesis in the Calvin Cycle
The Calvin Cycle Has Three Phases:
Carbon Fixation- Atmospheric carbon (CO2) is
incorporated into a molecule of ribulose bisphosphate (RuBP)
by the enzyme rubisco
3 molecules of CO2 are required to make 1 molecule of G3P
2. Reduction Phase – NADPH reduces 3PGA to G3P
3. Regeneration of Starting Material – RuBP is
regenerated and the cycle starts again
To synthesize one G3P molecule, the Calvin cycle consumes nine ATP and six NADPH molecules, which were provided by the light reactions.
Copyright © 2009 Pearson Education, Inc.

27. Step Carbon fixation Input: 3 CO2 Rubisco 3 P P 6 P RuBP 3-PGA CALVIN1
Input: 3
CO2
Rubisco 1 3 P P 6 P
RuBP
3-PGA
CALVIN
CYCLE
Figure 7.10B Details of the Calvin cycle, which takes place in the stroma of a chloroplast.

28. Step Carbon fixation Input: 3 CO2 Rubisco Step Reduction 3 P P 6 P1
Input: 3
CO2
Rubisco 1
Step Reduction 2 3 P P 6 P
RuBP
3-PGA 6
ATP
6 ADP + P
CALVIN
CYCLE 2 6
NADPH 6
NADP+
Figure 7.10B Details of the Calvin cycle, which takes place in the stroma of a chloroplast. 6 P
G3P

29. Step Carbon fixation Input: 3 CO2 Rubisco Step Reduction 3 P P 6 P1
Input: 3
CO2
Rubisco 1
Step Reduction 2 3 P P 6 P
RuBP
3-PGA 6
ATP
6 ADP + P
CALVIN
CYCLE 2 6
NADPH 6
NADP+
Figure 7.10B Details of the Calvin cycle, which takes place in the stroma of a chloroplast. 5 P 6 P
G3P
G3P
Glucose
and other
compounds
Output: 1 P
G3P

30. Step Regeneration of RuBP 6 NADP+
Step Carbon fixation 1
Input: 3
CO2
Rubisco 1
Step Reduction 2 3 P P 6 P
RuBP
3-PGA 6
ATP
3 ADP
6 ADP + P 3
ATP
CALVIN
CYCLE 3 2 6
NADPH
Step Regeneration of RuBP 3 6
NADP+
Figure 7.10B Details of the Calvin cycle, which takes place in the stroma of a chloroplast. 5 P 6 P
G3P
G3P
Glucose
and other
compounds
Output: 1 P
G3P

31. Overview of Photosynthesis

32. PHOTOSYNTHESIS REVIEWED AND EXTENDED
Copyright © 2009 Pearson Education, Inc.

33. H2O CO2 Chloroplast Light NADP+ ADP + P RuBP CALVIN CYCLE Electron
Photosystem II
RuBP
CALVIN
CYCLE
Electron
transport
chains
3-PGA
(in stroma)
Thylakoid
membranes
Photosystem I
ATP
Stroma
Figure 7.11 A summary of the chemical processes of photosynthesis.
NADPH
G3P
Cellular
respiration
Cellulose
Starch O2
Sugars
Other organic
compounds
LIGHT REACTIONS
CALVIN CYCLE

35. EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants
In hot climates, plant stomata close to reduce water loss so oxygen builds up
Rubisco adds oxygen instead of carbon dioxide to RuBP in a process called photorespiration
Photorespiration uses oxygen, produces CO2; sugar and ATP are not produced
Copyright © 2009 Pearson Education, Inc.

36. EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants
C4 plants
Have a unique leaf anatomy
First stable compound is a 4C compound
C4 plants partially shut stomata when hot and dry to conserve water
Have PEP carboxylase to bind CO2 at low levels (carbon fixation)
Allows plant to maintain an adequate concentration of carbon to feed into the Calvin Cycle to continue making sugar
Copyright © 2009 Pearson Education, Inc.

38. EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants
CAM plants open their stomata at night thus admitting CO2 in w/o loss of H2O
CO2 enters, and is fixed into a four-carbon compound, (carbon fixation)
Carbon is released into the Calvin cycle during the day

39. CALVIN CYCLE CALVIN CYCLE
Mesophyll
cell
CO2
CO2
Night
4-C compound
4-C compound
CO2
CO2
CALVIN
CYCLE
CALVIN
CYCLE
Figure 7.12 Comparison of photosynthesis in C4 and CAM plants: In both pathways, CO2 is first incorporated into a four-carbon compound, which then provides CO2 to the Calvin cycle.
Bundle-
sheath
cell
3-C sugar
3-C sugar
Day
C4 plant
CAM plant

40. Separation of Photosynthetic Pigments in Chloroplasts