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Chapter 7
Photosynthesis: Using Light
to Make Food
PowerPoint® Lectures for
Campbell Essential Biology, Fifth Edition, and
Campbell Essential Biology with Physiology,
Fourth Edition
– Eric J. Simon, Jean L. Dickey, and Jane B. Reece
Lectures by Edward J. Zalisko
© 2013 Pearson Education, Inc.
Biology and Society:
Biofuels
•  Wood has historically been the main fuel used to
produce
–  heat and
–  light.
© 2013 Pearson Education, Inc.
Figure 7.0
Biology and Society:
Biofuels
•  Industrialized societies replaced wood with fossil
fuels including
–  coal,
–  gas, and
–  oil.
•  To limit the damaging effects of fossil fuels,
researchers are investigating the use of biomass
(living material) as efficient and renewable energy
sources.
© 2013 Pearson Education, Inc.
Biology and Society:
Biofuels
•  There are several types of biofuels.
–  Bioethanol is a type of alcohol produced by the
fermentation of glucose made from starches in
crops such as grains, sugar beets, and sugar cane.
–  Bioethanol may be used
–  directly as a fuel source in specially designed
vehicles or
–  as a gasoline additive.
© 2013 Pearson Education, Inc.
Biology and Society:
Biofuels
–  Cellulosic ethanol is a type of bioethanol made
from cellulose in nonedible plant material such as
wood or grass.
–  Biodiesel is made from plant oils or recycled frying
oil.
© 2013 Pearson Education, Inc.
THE BASICS OF PHOTOSYNTHESIS
•  Photosynthesis
–  is used by plants, algae (protists), and some
bacteria,
–  transforms light energy into chemical energy, and
–  uses carbon dioxide and water as starting
materials.
© 2013 Pearson Education, Inc.
THE BASICS OF PHOTOSYNTHESIS
•  The chemical energy produced via photosynthesis
is stored in the bonds of sugar molecules.
•  Organisms that use photosynthesis are
–  photosynthetic autotrophs and
–  the producers for most ecosystems.
© 2013 Pearson Education, Inc.
Figure 7.1
PHOTOSYNTHETIC AUTOTROPHS
Photosynthetic Protists
(aquatic)
Photosynthetic Bacteria
(aquatic)
LM
Plants
(mostly on land)
Forest plants
Kelp, a large, multicellular alga
Micrograph of cyanobacteria
Chloroplasts: Sites of Photosynthesis
•  Chloroplasts are
–  the site of photosynthesis and
–  found mostly in the interior cells of leaves.
•  Inside chloroplasts are interconnected, membranous
sacs called thylakoids, which are suspended in a
thick fluid called stroma.
•  Thylakoids are concentrated in stacks called grana.
© 2013 Pearson Education, Inc.
Chloroplasts: Sites of Photosynthesis
•  The green color of chloroplasts is from
chlorophyll, a light-absorbing pigment that plays a
central role in converting solar energy to chemical
energy.
•  Stomata are tiny pores in leaves where
–  carbon dioxide enters and
–  oxygen exits.
© 2013 Pearson Education, Inc.
Figure 7.2-1
Photosynthetic
cells
Vein
CO2
O2
Stomata
Leaf cross section
Figure 7.2-2
Chloroplast
Photosynthetic
cells
Vein
O2
Stomata
LM
CO2
Leaf cross section
Interior cell
Figure 7.2-3
Chloroplast
Photosynthetic
cells
Inner and outer
membranes
Vein
Stroma
O2
Granum
Stomata
Leaf cross section
Interior cell
Colorized TEM
LM
CO2
Thylakoid
space
Figure 7.2a
Vein
(transports
Photosynthetic water and
cells
nutrients)
CO2
O2
Stomata
Leaf cross section
The Simplified Equation for Photosynthesis
•  In the overall equation for photosynthesis, notice
that the reactants of photosynthesis are the waste
products of cellular respiration.
© 2013 Pearson Education, Inc.
Figure 7-UN01
Light
energy
6 CO2
Carbon
dioxide
C6H12O6
6 H2O
Water
Photosynthesis
Glucose
6 O2
Oxygen gas
The Simplified Equation for Photosynthesis
•  In photosynthesis,
–  sunlight provides the energy,
–  electrons are boosted “uphill” and added to carbon
dioxide, and
–  sugar is produced.
© 2013 Pearson Education, Inc.
The Simplified Equation for Photosynthesis
•  During photosynthesis, water is split into
–  hydrogen and
–  oxygen.
•  Hydrogen is transferred along with electrons and
added to carbon dioxide to produce sugar.
•  Oxygen escapes through stomata into the
atmosphere.
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A Photosynthesis Road Map
•  Photosynthesis occurs in two multistep stages:
1.  the light reactions convert solar energy to
chemical energy and
2.  the Calvin cycle uses the products of the light
reactions to make sugar from carbon dioxide.
© 2013 Pearson Education, Inc.
A Photosynthesis Road Map
•  The initial incorporation of carbon from the
atmosphere into organic compounds is called
carbon fixation.
–  This lowers the amount of carbon in the air.
–  Deforestation reduces the ability of the biosphere
to absorb carbon by reducing the amount of
photosynthetic plant life.
BioFlix Animation: Photosynthesis
© 2013 Pearson Education, Inc.
Figure 7.3-2
H 2O
Chloroplast
CO2
Light
NADP+
ADP
+ P
Light
reactions
Calvin
cycle
ATP
– –
NADPH
O2
Sugar
THE LIGHT REACTIONS: CONVERTING
SOLAR ENERGY TO CHEMICAL ENERGY
•  Chloroplasts
–  are chemical factories powered by the sun and
–  convert sunlight into chemical energy.
© 2013 Pearson Education, Inc.
Figure 7-UN02
Light
CO2
H2O
NADP+
Light
reactions
ADP
+ P
Calvin
cycle
ATP
– –
NADPH
O2
Sugar
The Nature of Sunlight
•  Sunlight is a type of energy called radiation, or
electromagnetic energy.
•  The distance between the crests of two adjacent
waves is called a wavelength.
•  The full range of radiation is called the
electromagnetic spectrum.
© 2013 Pearson Education, Inc.
Figure 7.4
Increasing wavelength
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 7.5
Light
Reflected
light
Chloroplast
Absorbed
light
Transmitted
light (detected
by your eye)
The Process of Science:
What Colors of Light Drive Photosynthesis?
•  Observation: In 1883, German biologist Theodor
Engelmann saw that certain bacteria tend to cluster
in areas with higher oxygen concentrations.
•  Question: Could this information determine which
wavelengths of light work best for photosynthesis?
•  Hypothesis: Oxygen-seeking bacteria will
congregate near regions of algae performing the
most photosynthesis.
© 2013 Pearson Education, Inc.
The Process of Science:
What Colors of Light Drive Photosynthesis?
•  Experiment: Engelmann
–  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.
© 2013 Pearson Education, Inc.
The Process of Science:
What Colors of Light Drive Photosynthesis?
•  Results: Bacteria
–  mostly congregated around algae exposed to redorange and blue-violet light and
–  rarely moved to areas of green light.
•  Conclusion: Chloroplasts absorb light mainly in
the blue-violet and red-orange part of the
spectrum.
Animation: Light and Pigments
© 2013 Pearson Education, Inc.
Figure 7.6
Light
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.
2.  Chlorophyll b
–  absorbs mainly blue and orange light and
–  participates indirectly in the light reactions.
© 2013 Pearson Education, Inc.
Chloroplast Pigments
•  Carotenoids
–  absorb mainly blue-green light,
–  participate indirectly in the light reactions, and
–  absorb and dissipate excessive light energy that
might damage chlorophyll.
•  The spectacular colors of fall foliage are due partly
to the yellow-orange light reflected from
carotenoids.
© 2013 Pearson Education, Inc.
Figure 7.7
How Photosystems Harvest Light Energy
•  Light behaves as photons, a fixed quantity of light
energy.
•  Chlorophyll molecules absorb photons.
–  Electrons in the pigment gain energy.
–  As the electrons fall back to their ground state,
energy is released as heat or light.
© 2013 Pearson Education, Inc.
Figure 7.8a
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
How Photosystems Harvest Light Energy
•  In the thylakoid membrane, chlorophyll molecules
are organized with other molecules into
photosystems.
•  A photosystem is a cluster of a few hundred
pigment molecules that function as a lightgathering antenna.
© 2013 Pearson Education, Inc.
How Photosystems Harvest Light Energy
•  The reaction center of the photosystem consists
of chlorophyll a molecules that sit next to another
molecule called a primary electron acceptor,
which traps the light-excited electron from
chlorophyll a.
•  Another team of molecules built into the thylakoid
membrane then uses that trapped energy to make
–  ATP and
–  NADPH.
© 2013 Pearson Education, Inc.
Figure 7.9
Chloroplast
Cluster of pigment
molecules
Photon
e–
Electron
transfer
Primary
electron
acceptor
Reactioncenter
chlorophyll a
Pigment
molecules
Thylakoid membrane
Transfer
of energy
Photosystem
Reaction
center
Figure 7.9b
Photon
e–
Electron
transfer
Primary
electron
acceptor
Reactioncenter
chlorophyll a
Pigment
molecules
Transfer
of energy
Photosystem
Reaction
center
How the Light Reactions Generate ATP and
NADPH
•  Two types of photosystems cooperate in the light
reactions:
1.  the water-splitting photosystem and
2.  the NADPH-producing photosystem.
© 2013 Pearson Education, Inc.
Figure 7.10-1
Primary
electron
acceptor
2e –
Light
1
Reactioncenter
chlorophyll
H 2O
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
H 2O
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
H 2O
2e –
2 H+ +
1
2
O2
NADP+
Water-splitting
photosystem
NADPH-producing
photosystem
How the Light Reactions Generate ATP and
NADPH
•  The light reactions are located in the thylakoid
membrane.
•  An electron transport chain
–  connects the two photosystems and
–  releases energy that the chloroplast uses to make
ATP.
© 2013 Pearson Education, Inc.
Figure 7.11
To Calvin cycle
Light
Light
– –
NADPH
NADP+
H+
Stroma
Thylakoid
membrane
Photosystem
Electron transport
chain
Photosystem
ADP + P
Electron flow
–
2e
H+
H+
H+
1
2
Thylakoid
membrane
ATP
ATP
synthase
Inside thylakoid
H 2O
H+
O2
H+
H+
Figure 7.11a
H+
Stroma
Thylakoid
membrane
Photosystem
Electron transport
chain
Inside thylakoid
2e
H 2O
H+
1
2
Thylakoid
membrane
–
O2
Figure 7.11b
To Calvin cycle
Light
–
–
H+
NADPH
NADP+
H+
Electron transport
chain
Photosystem
ATP
ADP + P
ATP
synthase
Electron flow
H+
H+
H+
H+
H+
Figure 7.12
e–
ATP
e–
e–
– –
NADPH
e–
e–
Photon
e–
Photon
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.
Blast Animation: Photosynthesis: Light-Independent Reactions
© 2013 Pearson Education, Inc.
Figure 7-UN03
Light
CO2
H 2O
NADP+
Light
reactions
ADP
+ P
Calvin
cycle
ATP
– –
NADPH
O2
Sugar
Figure 7.13-1
CO2 (from air)
1
P
RuBP sugar
Three-carbon molecule
P
P
Calvin
cycle
Figure 7.13-2
CO2 (from air)
1
P
RuBP sugar
Three-carbon molecule
ATP
P
P
ADP + P
Calvin
cycle
– –
NADPH
NADP+
P
G3P sugar
2
Figure 7.13-3
CO2 (from air)
1
P
RuBP sugar
Three-carbon molecule
ATP
P
P
ADP + P
Calvin
cycle
– –
NADPH
NADP+
G3P sugar
3
P
P
G3P sugar
P
G3P sugar
2
Glucose
(and other
organic
compounds)
Figure 7.13-4
CO2 (from air)
1
P
RuBP sugar
Three-carbon molecule
4
ATP
P
P
ADP + P
Calvin
cycle
ADP + P
– –
NADPH
ATP
NADP+
G3P sugar
3
P
P
G3P sugar
P
G3P sugar
2
Glucose
(and other
organic
compounds)
Evolution Connection:
Solar-Driven Evolution
•  C3 plants
–  use CO2 directly from the air and
–  are very common and widely distributed.
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Evolution Connection:
Solar-Driven Evolution
•  C4 plants
–  close their stomata to save water during hot and
dry weather and
–  can still carry out photosynthesis.
•  CAM plants
–  are adapted to very dry climates and
–  open their stomata only at night to conserve water.
© 2013 Pearson Education, Inc.
Figure 7.14
ALTERNATIVE PHOTOSYNTHETIC PATHWAYS
C4 Pathway
(example: sugarcane)
Cell
type 1
Cell
type 2
CAM Pathway
(example: pineapple)
CO2
CO2
Four-carbon
compound
Four-carbon
compound
CO2
CO2
Calvin
cycle
Calvin
cycle
Sugar
Sugar
C4 plant
CAM plant
Night
Day
Figure 7.14a
C4 Pathway
(example: sugarcane)
Cell
type 1
Cell
type 2
CAM Pathway
(example: pineapple)
CO2
CO2
Four-carbon
compound
Four-carbon
compound
CO2
CO2
Calvin
cycle
Calvin
cycle
Sugar
Sugar
C4 plant
CAM plant
Night
Day
Figure 7.14b
C4 Pathway
(example: sugarcane)
Figure 7.14c
CAM Pathway
(example: pineapple)
Figure 7-UN04
Light energy
6 CO2
Carbon
dioxide
6 H2O
Water
Photosynthesis
C6H12O6
6 O2
Glucose
Oxygen
gas
Figure 7-UN05
Chloroplast
Light
CO2
H 2O
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
Figure 7-UN06
–
ADP
e–
acceptor
ATP
e
2e –
acceptor
2e –
– –
NADPH
2e –
Photon
Photon
Chlorophyll
H 2O
Chlorophyll
2e –
+
2H +
1 O
2 2
NADP+
Water-splitting
photosystem
NADPH-producing
photosystem
Figure 7-UN07
CO2
ADP
ATP
Calvin
cycle
– –
NADPH
G3P
P
P
NADP+
Glucose and
other compounds
(such as cellulose
and starch)