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 without using organic molecules derived from any other living thing
Photoautotrophs : Autotrophs that use the energy of light to produce organic molecules
Most plants, algae and other protists, and some prokaryotes are photoautotrophs (cyanobacteria)
The ability to photosynthesize is directly related to the structure of chloroplasts
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3. Figure 7.1A Forest plants.

4. Visible Radiation Drives the Light Reactions
Sunlight is a type of electromagnetic energy (radiation)
Visible light is a small part of the electromagnetic spectrum
Light exhibits the properties of both waves and particles
One wavelength = distance between the crests of two adjacent waves (shorter λ’s have greater energy)
Light behaves as discrete packets of energy called photons (photons contain a fixed quantity of light energy)

5. 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 wavelength of 650 nm is illustrated.)
380
400
500
600
700
750
Wavelength (nm)
650 nm

6. Visible Radiation Drives the Light Reactions
Pigments are proteins that absorb specific wavelengths of light and transmit others
Various pigments are built into the thylakoid membrane
We see the color of the wavelengths that are transmitted (not absorbed)
Ex. chlorophyll transmits green

7. 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

8. 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

9. Photosynthesis Occurs in Chloroplasts in Plant Cells
Leaves contain:
Stomata = tiny pores in the leaf; allow CO2 to enter and O2 to exit
Veins deliver water & nutrients absorbed by roots
Chlorophyll= a light absorbing pigment
responsible for the green color of plants
located on thylakoid membrane
Vein
CO2 O2
Stoma
Figure 7.2 The location and structure of chloroplasts.
Chloroplasts

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

11. Photosynthesis is a Redox Process, as is Cellular Respiration
Photosynthesis is a redox (oxidation-reduction) process
A loss of electrons = oxidation
A gain of electrons = reduction
(OIL RIG)
These electrons are lost and gained in the form of hydrogen
In photosynthesis water loses electrons and CO2 gains electrons

12. Photosynthesis is a Redox Process, as is Cellular Respiration
Reduction
6 CO H2O
C6H12O O2
Oxidation
Figure 7.4A Photosynthesis (uses light energy).

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

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

15. Photosystems Capture Solar Power in the Light Reactions
A photosystem is a functional unit that captures sunlight and converts it to chemical energy (ATP & NADPH)
A Photosystem consists of a light-harvesting complex surrounding a 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

16. Photosystems Capture Solar Power in the Light Reactions
Two types of photosystems exist: photosystem I and photosystem II
Each type of 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)

17. Photosystems Capture Solar Power in the Light Reactions
When light strikes a photosystem, energy is captured by pigments and passed from pigment to pigment within the photosystem
From a pigment, energy is passed to chl. a in the reaction center where it excites electrons of chl. a
An excited e- from chl. a is transferred to a primary electron acceptor
This solar-powered transfer of an electron from the reaction center chl. a to the primary electron acceptor is the first step of the light reactions
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
Figure 7.8A Electron flow in the light reactions of photosynthesis: Both photosystems and the electron transport chain that connects them are located in the thylakoid membrane. The energy from light drives electrons from water to NADPH.
Thylakoid
space 3
H2O 2  1 O2
+ 2 H+

18. The Light Reactions
The primary e- acceptor passes electrons to an electron transport chain (ETC)
The ETC is a bridge between photosystems II and I. The ETC also generates ATP.
To fill the electron void of chl. a, it oxidizes H2O (takes electrons from water)
In this is the step, water is oxidized and oxygen is released

19. Electron transport chain
The Light Reactions
Electron transport chain
Provides energy for
synthesis of
by chemiosmosis
NADP+ + H+
NADPH
Photon
Photon
Photosystem I
Photosystem II
ATP 6
Stroma 1
Primary
acceptor
Primary
acceptor 2 e– e–
Thylakoid
mem-
brane 4 5
P700
P680
Figure 7.8A Electron flow in the light reactions of photosynthesis: Both photosystems and the electron transport chain that connects them are located in the thylakoid membrane. The energy from light drives electrons from water to NADPH.
Thylakoid
space 3
H2O 2  1 O2
+ 2 H+

20. The Light Reactions
As the second protein of the ETC accepts electrons it pumps H+ into the thylakoid space.
Pumping H+ into the thylakoid space generates a proton (H+) gradient
Protons (H+) flow from the thylakoid space to the stroma, (down their gradient) through an enzyme called ATP synthase.
ATP synthase phosphorylates ADP (forming ATP) using the energy from the flow of H+ down their gradient
This is an energy-coupling process where the exergonic flow of H+ powers the endergonic phosphorylation of ADP in a mechanism called: chemiosmosis

21. The Light Reactions ATP is 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

22. Two Photosystems Connected by an ETC Generate ATP and NADPH
Electrons moving down the ETC are passed to P700 of Photosystem I, and ultimately to NADP+ by the enzyme NADP+ reductase
This step produces NADPH in the stroma

23. Electron transport chain
Stroma (low H+
concentration) H+
Light
Light H+
ADP + P
ATP H+
NADP+ + H+
NADPH H+
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+
Photosystem II
Electron
transport
chain
Photosystem I H+
ATP synthase H+
Thylakoid space
(high H+ concentration)

24. THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS

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 then use 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:
1. Carbon Fixation- Atmospheric carbon (as CO2) is
incorporated into a molecule of ribulose bisphosphate
(RuBP) using 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
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27. Step Regeneration of RuBP G3P G3P
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
Step Release of one
molecule of G3P 3 4 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
Step Regeneration of RuBP 4
G3P
G3P 3
Glucose
and other
compounds
Output: 1 P
G3P

28. PHOTOSYNTHESIS REVIEWED AND EXTENDED
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29. 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

30. 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, but no sugar or ATP are produced
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31. EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants
Some plants have evolved a means of carbon fixation that saves water during photosynthesis
C4 plants partially shut stomata when hot and dry to conserve water
This reduces CO2 levels, so contain PEP carboxylase to bind CO2 at low levels (carbon fixation);
This allows plant to continue making sugar by photosynthesis
Copyright © 2009 Pearson Education, Inc.

32. Another adaptation to hot and dry environments has evolved
EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants
Another adaptation to hot and dry environments has evolved
CAM 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)
It is released into the Calvin cycle during the day
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33. 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

34. Separation of Photosynthetic Pigments in Chloroplasts

35. Photon (fluorescence) Chlorophyll molecule
Excited state e–
Heat
Photon
Photon
(fluorescence)
Ground state
Figure 7.7A A solution of chlorophyll glowing red when illuminated (left); a diagram of an isolated, light-excited chlorophyll molecule that releases a photon of red light (right).
Chlorophyll
molecule