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Chapter 10
Photosynthesis
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: The Process That Feeds the Biosphere
• Photosynthesis is the process that converts solar
energy into chemical energy
• Directly or indirectly, photosynthesis nourishes
almost the entire living world
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Autotrophs sustain themselves without eating
anything derived from other organisms
• Autotrophs are the producers of the biosphere,
producing organic molecules from CO2 and other
inorganic molecules
• Almost all plants are photoautotrophs, using the
energy of sunlight to make organic molecules from
water and carbon dioxide
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Photosynthesis occurs in plants, algae, certain
other protists, and some prokaryotes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-2
Plants
Unicellular protist 10 µm
Purple sulfur
bacteria
Multicellular algae
Cyanobacteria
40 µm
1.5 µm
• Heterotrophs obtain their organic material from
other organisms
• Heterotrophs are the consumers of the biosphere
• Almost all heterotrophs, including humans,
depend on photoautotrophs for food and oxygen
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Two Stages of Photosynthesis
1. Light Reactions
- solar energy is captured and transformed
into chemical energy
- ‘photo’ of photosynthesis
2. Calvin Cycle
- the chemical energy is used to make organic
molecules of food
- ‘synthesis’ of photosynthesis
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Concept 10.1: Photosynthesis converts light energy
to the chemical energy of food
A. Chloroplasts: The Sites of Photosynthesis in
Plants
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A. Chloroplasts: The Sites of Photosynthesis in
Plants
1. Leaves are the major locations of
photosynthesis
2. Their green color is from chlorophyll, the
green pigment within chloroplasts
a. Light energy absorbed by chlorophyll drives
the synthesis of organic molecules in the
chloroplast
3. Through microscopic pores called stomata,
CO2 enters the leaf and O2 exits
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
4. Chloroplasts are found mainly in cells of the
mesophyll, the interior tissue of the leaf
a. A typical mesophyll cell has 30-40
chloroplasts
5. The chlorophyll is in the membranes of
thylakoids (connected sacs in the chloroplast);
thylakoids may be stacked in columns called
grana
6. Chloroplasts also contain stroma, a dense
fluid
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-3
Leaf cross section
Vein
Mesophyll
Stomata
CO2 O2
Mesophyll cell
Chloroplast
5 µm
Outer
membrane
Thylakoid
Thylakoid
Stroma Granum
space
Intermembrane
space
Inner
membrane
1 µm
7. Photosynthesis can be summarized as the
following equation:
6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
8. The Splitting of Water
a. Chloroplasts split water into hydrogen and
oxygen
b. incorporating the electrons of hydrogen into
sugar molecules
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-4
Products:
12 H2O
6 CO2
Reactants:
C6H12O6
6 H2O
6 O2
9. Photosynthesis as a Redox Process
a. Photosynthesis is a redox process in which
water is oxidized and carbon dioxide is
reduced
b. Electrons are transferred along with the
hydrogen ions from the water to CO2,
reducing it to sugar
c. Electrons increase in potential energy –
requires energy
d. Endergonic reaction – energy provided by
sunlight
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
10. Cellular Respiration
a. Energy is released from sugar
b. Electrons lose energy as they fall down the
electron transport chain toward
electronegative oxygen
c. Mitochondria harness that energy to make
ATP
d. Exergonic reaction – electrons lose potential
energy
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
B. The Two Stages of Photosynthesis: A Preview
1. Photosynthesis consists of the light reactions
(the photo part) and Calvin cycle (the
synthesis part)
2. The light reactions (in the thylakoids) split
water, release O2, produce ATP, and form
NADPH
3. The Calvin cycle (in the stroma) forms sugar
from CO2, using ATP and NADPH
a. The Calvin cycle begins with carbon fixation,
incorporating CO2 into organic molecules
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-5_1
H2O
Light
LIGHT
REACTIONS
Chloroplast
LE 10-5_2
H2O
Light
LIGHT
REACTIONS
ATP
NADPH
Chloroplast
O2
LE 10-5_3
H2O
CO2
Light
NADP+
ADP
+ Pi
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH
Chloroplast
O2
[CH2O]
(sugar)
Concept 10.2: The light reactions convert solar
energy to the chemical energy of ATP and NADPH
A. Chloroplasts are solar-powered chemical
factories
1. Their thylakoids transform light energy into
the chemical energy of ATP and NADPH
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B. The Nature of Sunlight
1. Light is a form of electromagnetic energy, also
called electromagnetic radiation
a. Like other electromagnetic energy, light
travels in rhythmic waves
b. Wavelength = distance between crests of
waves
c. Wavelength determines the type of
electromagnetic energy
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
2. Light also behaves as though it consists of
discrete particles, called photons
3. The electromagnetic spectrum is the entire
range of electromagnetic energy, or radiation
4. Visible light consists of colors we can see,
including wavelengths that drive
photosynthesis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-6
10–5 nm 10–3 nm
Gamma
rays
103 nm
1 nm
X-rays
106 nm
Infrared
UV
1m
(109 nm)
Microwaves
103 m
Radio
waves
Visible light
380
450
500
Shorter wavelength
Higher energy
550
600
650
700
750 nm
Longer wavelength
Lower energy
C. Photosynthetic Pigments: The Light Receptors
1. Pigments are substances that absorb visible
light
a. Different pigments absorb different
wavelengths
b. Wavelengths that are not absorbed are
reflected or transmitted
c. Leaves appear green because chlorophyll
reflects and transmits green light
Animation: Light and Pigments
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-7
Light
Reflected
light
Chloroplast
Absorbed
light
Granum
Transmitted
light
2. A spectrophotometer measures a pigment’s
ability to absorb various wavelengths
a. This machine sends light through pigments
and measures the fraction of light transmitted
at each wavelength
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-8a
White
light
Refracting
prism
Chlorophyll
solution
Photoelectric
tube
Galvanometer
0
Slit moves to
pass light
of selected
wavelength
Green
light
100
The high transmittance
(low absorption)
reading indicates that
chlorophyll absorbs
very little green light.
LE 10-8b
White
light
Refracting
prism
Chlorophyll
solution
Photoelectric
tube
0
Slit moves to
pass light
of selected
wavelength
Blue
light
100
The low transmittance
(high absorption)
reading indicates that
chlorophyll absorbs
most blue light.
3. An absorption spectrum is a graph plotting a
pigment’s light absorption versus wavelength
a. The absorption spectrum of chlorophyll a
suggests that violet-blue and red light work
best for photosynthesis
4. An action spectrum profiles the relative
effectiveness of different wavelengths of
radiation in driving a process
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-9a
Absorption of light by
chloroplast pigments
Chlorophyll a
Chlorophyll b
Carotenoids
400
500
600
Wavelength of light (nm)
Absorption spectra
700
Rate of photosynthesis (measured
by O2 release)
LE 10-9b
Action spectrum
LE 10-9c
Aerobic bacteria
Filament
of algae
400
500
Engelmann’s experiment
600
700
5. Chlorophyll a is the main photosynthetic
pigment
a. Accessory pigments, such as chlorophyll b,
broaden the spectrum used for photosynthesis
b. Accessory pigments called carotenoids
absorb excessive light that would damage
chlorophyll
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-10
CH3
CHO
in chlorophyll a
in chlorophyll b
Porphyrin ring:
light-absorbing
“head” of
molecule; note
magnesium atom
at center
Hydrocarbon tail:
interacts with
hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts; H atoms not
shown
D. Excitation of Chlorophyll by Light
1. When a pigment absorbs light, one of the
molecule’s electrons is elevated to an orbital
where it has more potential energy
2. The electron has moved from its ground state
to an excited state, which is unstable
3. When excited electrons fall back to the ground
state, photons are given off, an afterglow
called fluorescence
4. If illuminated, an isolated solution of
chlorophyll will fluoresce, giving off light and
heat
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-11
e–
Excited
state
Heat
Photon
Chlorophyll
molecule
Photon
(fluorescence)
Ground
state
Excitation of isolated chlorophyll molecule
Fluorescence
E. A Photosystem: A Reaction Center Associated
with Light-Harvesting Complexes
1. A photosystem consists of a reaction
center surrounded by light-harvesting
complexes
a. The light-harvesting complexes (pigment
molecules bound to proteins) funnel the
energy of photons to the reaction center
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
b. A primary electron acceptor in the
reaction center accepts an excited electron
from chlorophyll a
2. Solar-powered transfer of an electron from
a chlorophyll a molecule to the primary
electron acceptor is the first step of the
light reactions
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-12
Thylakoid
Photosystem
Photon
Thylakoid membrane
Light-harvesting
complexes
Reaction
center
STROMA
Primary electron
acceptor
e–
Transfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
3. There are two types of photosystems in the
thylakoid membrane
a. Photosystem II functions first (the numbers
reflect order of discovery) and is best at
absorbing a wavelength of 680 nm
b. Photosystem I is best at absorbing a
wavelength of 700 nm
c. The two photosystems work together to use
light energy to generate ATP and NADPH
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
F. Noncyclic Electron Flow
1. During the light reactions, there are two
possible routes for electron flow: cyclic and
noncyclic
2. Noncyclic electron flow, the primary pathway,
involves both photosystems and produces
ATP and NADPH
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-13_1
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Energy of electrons
e–
Light
P680
Photosystem II
(PS II)
LE 10-13_2
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Energy of electrons
Primary
acceptor
2
H+
1/ 2
+
O2
Light
H2O
e–
e–
e–
P680
Photosystem II
(PS II)
LE 10-13_3
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Energy of electrons
Pq
2 H+
+
1/ 2 O 2
Light
H2O
e–
Cytochrome
complex
Pc
e–
e–
P680
ATP
Photosystem II
(PS II)
LE 10-13_4
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
e–
Energy of electrons
Pq
2
H+
1/ 2
+
O2
Light
H2O
e–
Cytochrome
complex
Pc
e–
e–
P700
P680
Light
ATP
Photosystem II
(PS II)
Photosystem I
(PS I)
LE 10-13_5
H2 O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
e–
Pq
Energy of electrons
2
H+
e–
H2O
Cytochrome
complex
+
1/2 O2
Light
Fd
e–
e–
NADP+
reductase
Pc
e–
e–
NADPH
+ H+
P700
P680
Light
ATP
Photosystem II
(PS II)
NADP+
+ 2 H+
Photosystem I
(PS I)
LE 10-14
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Photosystem II
Photosystem I
G. Cyclic Electron Flow
1. Cyclic electron flow uses only photosystem I
and produces only ATP
2. Cyclic electron flow generates surplus ATP,
satisfying the higher demand in the Calvin
cycle
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LE 10-15
Primary
acceptor
Primary
acceptor
Fd
Fd
NADP+
Pq
NADP+
reductase
Cytochrome
complex
NADPH
Pc
Photosystem I
Photosystem II
ATP
H. A Comparison of Chemiosmosis in Chloroplasts
and Mitochondria
1. Chemiosmosis
a. an energy-coupling mechanism that
uses energy stored in the form of H+ ion
gradient across a membrane to drive
cellular work, such as the synthesis of ATP
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H. A Comparison of 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 ATP; chloroplasts transform light
energy into the chemical energy of ATP
• The spatial organization of chemiosmosis
differs in chloroplasts and mitochondria
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-16
Mitochondrion
Chloroplast
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
H+
Intermembrane
space
Membrane
Lower [H+]
Thylakoid
space
Electron
transport
chain
ATP
synthase
Key
Higher [H+]
Diffusion
Stroma
Matrix
ADP + P i
ATP
H+
• Water is split by photosystem II on the side
of the membrane facing the thylakoid space
• The diffusion of H+ from the thylakoid space
back to the stroma powers ATP synthase
• ATP and NADPH are produced on the side
facing the stroma, where the Calvin cycle
takes place
Animation: Calvin Cycle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-17
H2 O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
STROMA
(Low H+ concentration)
O2
[CH2O] (sugar)
Cytochrome
complex
Photosystem II
Light
2
Photosystem I
Light
NADP+
reductase
H+
NADP+ + 2H+
Fd
NADPH + H+
Pq
H2O
THYLAKOID SPACE
(High H+ concentration)
1/2
Pc
O2
+2 H+
2 H+
To
Calvin
cycle
Thylakoid
membrane
STROMA
(Low H+ concentration)
ATP
synthase
ADP
+
Pi
ATP
H+
Concept 10.3: The Calvin cycle uses ATP and
NADPH to convert CO2 to sugar
A. The Calvin cycle, like the citric acid cycle,
regenerates its starting material after
molecules enter and leave the cycle
1. Calvin cycle – anabolic
2. Citric acid cycle – catabolic, oxidizing
glucose, releasing energy
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Concept 10.3: The Calvin cycle uses ATP and
NADPH to convert CO2 to sugar
B. The cycle builds sugar from smaller
molecules by using ATP and the reducing
power of electrons carried by NADPH
1. Carbon enters the cycle as CO2 and leaves
as a sugar named glyceraldehyde-3-phospate
(G3P)
2. For net synthesis of one G3P, the cycle must
take place three times, fixing three molecules
of CO2
a. consumes 9 ATP and 6 NADPH
(regenerated by light reactions)
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C. The Calvin cycle has three phases:
1. Carbon fixation (catalyzed by rubisco)
2. Reduction
3. Regeneration of the CO2 acceptor (RuBP)
Play
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-18_1
H2 O
CO2
Input
Light
(Entering one
CO2 at a time)
3
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
Short-lived
intermediate
P
P
6
3-Phosphoglycerate
3 P
P
Ribulose bisphosphate
(RuBP)
6
6 ADP
CALVIN
CYCLE
ATP
LE 10-18_2
H2O
CO2
Input
Light
(Entering one
CO2 at a time)
3
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
P
Short-lived
intermediate
3 P
P
6
P
3-Phosphoglycerate
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
CALVIN
CYCLE
6 P
P
1,3-Bisphosphoglycerate
6 NADPH
6 NADP+
6 Pi
6
P
Glyceraldehyde-3-phosphate
(G3P)
1
P
G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
LE 10-18_3
H2O
CO2
Input
Light
(Entering one
CO2 at a time)
3
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
P
Short-lived
intermediate
3 P
P
6
P
3-Phosphoglycerate
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
3 ADP
3
CALVIN
CYCLE
6 P
ATP
P
1,3-Bisphosphoglycerate
6 NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+
6 Pi
P
5
G3P
6
P
Glyceraldehyde-3-phosphate
(G3P)
1
P
G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
Concept 10.4: Alternative mechanisms of carbon
fixation have evolved in hot, arid climates
A. Dehydration is a problem for plants,
sometimes requiring tradeoffs with other
metabolic processes, especially
photosynthesis
1. On hot, dry days, plants close stomata,
which conserves water but also limits
photosynthesis
2. The closing of stomata reduces access to
CO2 and causes O2 to build up
3. These conditions favor a seemingly wasteful
process called photorespiration
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B. Photorespiration: An Evolutionary Relic?
1. In most plants (C3 plants), initial fixation of
CO2, via rubisco, forms a three-carbon
compound
2. In photorespiration, rubisco adds O2 to the
Calvin cycle instead of CO2
3. Photorespiration consumes O2 and organic
fuel and releases CO2 without producing ATP
or sugar
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Photorespiration may be an evolutionary relic
because rubisco first evolved at a time when
the atmosphere had far less O2 and more CO2
4. In many plants, photorespiration is a problem
because on a hot, dry day it can drain as much
as 50% of the carbon fixed by the Calvin cycle
5. rice, wheat, soybeans, and many other plants
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C. C4 Plants
1. C4 plants minimize the cost of
photorespiration by incorporating CO2 into
four-carbon compounds in mesophyll cells
2. These four-carbon compounds are exported to
bundle-sheath cells, where they release CO2
that is then used in the Calvin cycle
3. sugarcane, corn
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-19
Photosynthetic
cells of C4 plant
leaf
Mesophyll
cell
PEP carboxylase
Mesophyll cell
CO2
Bundlesheath
cell
The C4 pathway
Oxaloacetate (4 C) PEP (3 C)
Vein
(vascular tissue)
ADP
Malate (4 C)
ATP
C4 leaf anatomy
Stoma
Bundlesheath
cell
Pyruvate (3 C)
CO2
CALVIN
CYCLE
Sugar
Vascular
tissue
D. CAM Plants
1. CAM plants open their stomata at night,
incorporating CO2 into organic acids
2. Stomata close during the day, and CO2 is
released from organic acids and used in the
Calvin cycle
3. Different from C4 plants because this happens
in the same mesophyll cell just at different
times of day
4. Succulent (water-storing) plants – jade, cacti,
pineapple
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-20
Sugarcane
Pineapple
CAM
C4
CO2
Mesophyll
cell
Organic acid
Bundlesheath
cell
CO2
CO2 incorporated
into four-carbon Organic acid
organic acids
(carbon fixation)
CO2
CALVIN
CYCLE
Sugar
Spatial separation of steps
CO2
Organic acids
release CO2 to
Calvin cycle
Night
Day
CALVIN
CYCLE
Sugar
Temporal separation of steps
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
• In addition to food production, photosynthesis
produces the oxygen in our atmosphere
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-21
Light reactions
Calvin cycle
H2O
CO2
Light
NADP+
ADP
+ Pi
RuBP
Photosystem II
Electron transport
chain
Photosystem I
ATP
NADPH
3-Phosphoglycerate
G3P
Starch
(storage)
Amino acids
Fatty acids
Chloroplast
O2
Sucrose (export)