Photosynthesis
Lecture Objectives
1. Intro and Overview
2. Light Dependent Reactions
3. Calvin Cycle
PHOTOSYNTHESIS use solar energy by autotrophs
to manufacture food.
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The equation for Photosynthesis
Chloroplast
Figure 7.2b
Thylakoid
space
Granum
LM
Stroma
Inner and outer
membranes
Colorized TEM
Interior cell
A Photosynthesis Road Map
 Photosynthesis occurs in two multistep stages:
1. light reactions “Photo”
 light is captured
 Converts solar energy to chemical energy
 Occurs in the thylakoid
2. Calvin cycle “Synthesis”
 uses products of the light reactions to make sugar from carbon dioxide
 Occurs in the stroma
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Figure 7.3-1
H2O
Chloroplast
Light
Light
reactions
ATP
– –
NADPH
O2
Figure 7-UN05
Chloroplast
Light
CO2
H2O
Stack of
thylakoids
NADP
Light
reactions
ADP
P
Stroma
Calvin
cycle
ATP
– –
NADPH
O2
Sugar
Sugar used for
• cellular respiration
• cellulose
• starch
• other organic compounds
The Nature of Sunlight
 Light energy
 Kinetic energy
 Made up of energy packets called photons
 Can do work
 Photons measured in waves
 distance between the crests of two adjacent waves is called a wavelength.
 The full range of radiation is called the electromagnetic spectrum.
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Increasing wavelength
Figure 7.4
10–5 nm 10–3 nm 1 nm
Gamma
rays
X-rays
103 nm 106 nm
Infrared
UV
103 m
1m
Microwaves
Radio
waves
Visible light
380 400
500
600
Wavelength (nm)
Wavelength 
580
nm
700
750
Figure 7.5a
Light
Reflected
light
Chloroplast
Absorbed
light
Transmitted
light (detected
by your eye)
The Process of Science:
What Colors of Light Drive Photosynthesis?
 Hypothesis: Oxygen-seeking bacteria will congregate near regions of
algae performing the most photosynthesis.
 Experiment: Engelmann (1883)
 laid a string of freshwater algal cells in a drop of water on a microscope slide,
 added oxygen-sensitive bacteria to the drop, and
 used a prism to create a spectrum of light shining on the slide.
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Light
Figure 7.6
Prism
Number of bacteria
Microscope slide
Bacteria
Bacteria
Algal cells
400
500
600
Wavelength of light (nm)
700
Chloroplast Pigments
 Chloroplasts contain several pigments:
1. Chlorophyll a

absorbs mainly blue-violet and red light and

participates directly in the light reactions
o Absorb photons (light energy)

Electrons become excited (gain energy)
o Fall to ground state  energy released
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Figure 7.8
Excited state
Absorption of a
photon excites
an electron.
e–
The electron
falls to its
ground state.
Heat
Light
Light
(fluorescence)
Photon
Chlorophyll
molecule
(a) Absorption of a photon
Ground
state
(b) Fluorescence of a glow stick
Photosystems Harvest Light Energy
 Photosystem
 cluster of a few hundred pigment molecules that function as a light-gathering
antenna.
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Figure 7.9b
Photon
e–
Electron
transfer
Primary
electron
acceptor
Reactioncenter
chlorophyll a
Pigment
molecules
Transfer
of energy
Photosystem
Reaction
center
Figure 7.10-1
Primary
electron
acceptor
2e –
Light
1
Reactioncenter
chlorophyll
H2O
2e –
2 H 
1
2
O2
Water-splitting
photosystem
Figure 7.10-2
Primary
electron
acceptor
Energy
to make ATP
2
2e –
Light
1
Reactioncenter
chlorophyll
H2O
2e –
2 H 
1
2
O2
Water-splitting
photosystem
Figure 7.10-3
Primary
electron
acceptor
Primary
electron
acceptor
Energy
to make ATP
2e –
2e–
3
2
– –
NADPH
2e –
Light
Light
Reactioncenter
chlorophyll
1
Reactioncenter
chlorophyll
H2O
2e –
2 H 
1
2
O2
NADP
Water-splitting
photosystem
NADPH-producing
photosystem
Figure 7.12
e–
ATP
e–
e–
– –
NADPH
e–
e–
e–
e–
Water-splitting
photosystem
NADPH-producing
photosystem
THE CALVIN CYCLE: MAKING SUGAR FROM CARBON
DIOXIDE
 The Calvin cycle
 functions like a sugar factory within a chloroplast and
 regenerates the starting material with each turn.
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Figure 7-UN03
Light
CO2
H2O
NADP
Light
reactions
ADP
+ P
Calvin
cycle
ATP
– –
NADPH
O2
Sugar
Evolution Connection:
Solar-Driven Evolution
 C3 plants
 Use CO2 directly from air
 C4 plants
 stomata barely open during day
 hot and dry weather
 Extra steps to produce enzyme  CO2 sticky tape
 sugarcane, corn
 CAM plants
 are adapted to very dry climates and
 open their stomata only at night to conserve water
 Cacti, pineapple & other fleshy plants
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