Chapter 6
Where It Starts –
Photosynthesis
Albia Dugger • Miami Dade College
6.1 Biofuels
• Coal, petroleum, and natural gas are fossil fuels – the
remains of ancient forests, a limited resource
• Biofuels such as oils, gases, or alcohols are made from
organic matter that is not fossilized – a renewable resource
• In the United States, biofuels are produced mainly from food
crops such as corn, soybeans, and sugarcane
• Researchers are looking for ways to use non-food-plant
matter such as switchgrass and agricultural wastes
Biofuels Research
Autotrophs and Heterotrophs
• Autotrophs harvest energy directly from the environment,
and obtain carbon from inorganic molecules
• Plants and most other autotrophs make their own food by
photosynthesis, a process which uses the energy of sunlight
to assemble carbohydrates from carbon dioxide and water
• Animals and other heterotrophs get energy and carbon by
breaking down organic molecules assembled by other
organisms
6.2 Sunlight as an Energy Source
• Energy flow through nearly all ecosystems on Earth begins
when photosynthesizers intercept energy from the sun
• Photosynthetic organisms use pigments to capture the energy
of sunlight and convert it to chemical energy – the energy
stored in chemical bonds
Properties of Light
• Visible light is part of an electromagnetic spectrum of energy
radiating from the sun
• Travels in waves
• Organized into photons
• Wavelength
• The distance between the crests of two successive waves
of light (nm)
• Shorter wavelength have greater energy
Electromagnetic Spectrum
of Radiant Energy
shortest wavelengths
(highest energy)
gamma
rays
x-rays
range of heat
range of most radiation escaping from
reaching Earth’s surface Earth’s surface
visible light
near-infrared
ultraviolet
radiation
radiation
400 nm
500 nm
longest wavelengths
(lowest energy)
radiation
microwaves radio waves
infrared
600 nm
700 nm
Wavelength and Energy
Pigments: The Rainbow Catchers
• Different wavelengths form colors of the rainbow
• Photosynthesis uses wavelengths of 380-750 nm
• Pigment
• An organic molecule that selectively absorbs light of
specific wavelengths
• Chlorophyll a
• The most common photosynthetic pigment
• Absorbs violet and red light (appears green)
Photosynthetic Pigments
• Collectively, chlorophyll and accessory pigments absorb most
wavelengths of visible light
• Certain electrons in pigment molecules absorb photons of
light energy, boosting electrons to a higher energy level
• Energy is captured and used for photosynthesis
Some Photosynthetic Pigments
Take-Home Message:
How do photosynthesizers absorb light?
• Energy radiating from the sun travels through space in waves
and is organized as packets called photons
• The spectrum of radiant energy from the sun includes visible
light; humans perceive different wavelengths of visible light as
different colors; the shorter the wavelength, the greater the
energy
• Pigments absorb light at specific wavelengths; photosynthetic
species use pigments such as chlorophyll a to harvest the
energy of light for photosynthesis
6.3 Exploring the Rainbow
• Photosynthetic pigments work together to harvest light of
different wavelengths
• Engelmann identified colors of light that drive photosynthesis
(violet and red) by using a prism to divide light into colors –
algae using these wavelengths gave off the most oxygen
Photosynthesis and
Wavelengths of Light
bacteria
alga
400 nm
500 nm
600 nm
Wavelength
700 nm
ANIMATED FIGURE: T. Englemann's
experiment
Absorption Spectra
• Most photosynthetic organisms use a combination of
pigments to drive photosynthesis
• An absorption spectrum shows which wavelengths each
pigment absorbs best
• Organisms in different environments use different pigments
Absorption Spectra
chlorophyll b
phycoerythrobilin
phycocyanobilin
Light absorption
β-carotene
400 nm
chlorophyll a
500 nm
600 nm
Wavelength
700 nm
Take-Home Message: Why do cells use more
than one photosynthetic pigment?
• A combination of pigments allows a photosynthetic organism
to most efficiently capture the particular range of light
wavelengths that reaches the habitat in which it evolved
6.4 Overview of Photosynthesis
• In plants and other photosynthetic eukaryotes, photosynthesis
occurs in chloroplasts
• Photosynthesis occurs in two stages
Two Stages of Photosynthesis
• Light-dependent reactions (noncyclic pathway)
• First stage of photosynthesis
• Light energy is transferred to ATP and NADPH
• Water molecules are split, releasing O2
• Light-independent reactions
• Second stage of photosynthesis
• Energy in ATP and NADPH drives synthesis of glucose
and other carbohydrates from CO2 and water
Summary: Photosynthesis
6CO2 + 6H2O → light energy → C6H12O6 + 6O2
The Chloroplast
• Chloroplast
• An organelle that specializes in photosynthesis in plants
and many protists
• Thylakoid membrane
• Folded membrane that make up thylakoids
• Contains clusters of light-harvesting pigments that absorb
photons of different energies and convert light energy into
chemical energy (first stage of photosynthesis)
The Chloroplast
• Stroma
• A semifluid matrix surrounded by the two outer
membranes of the chloroplast
• Sugars are built in the stroma (second stage of
photosynthesis)
two outer membranes
of chloroplast
stroma
part of thylakoid
membrane system:
thylakoid
compartment,
cutaway view
Figure 6-5b p105
INTERACTION: Structure of a chloroplast
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Take-Home Message: Where do the
reactions of photosynthesis take place?
• In the first stage of photosynthesis, light energy drives the
formation of ATP and NADPH, and oxygen is released; in
eukaryotic cells, these light-dependent reactions occur at the
thylakoid membrane of chloroplasts
• The second stage of photosynthesis, the light-independent
reactions, occur in the stroma of chloroplasts; ATP and
NADPH drive the synthesis of carbohydrates
3D ANIMATION: Photosynthesis Bio
Experience 3D
6.5 Light-Dependent Reactions
• Light-dependent reactions convert light energy to the energy
of chemical bonds
• Photons boost electrons in pigments to higher energy levels
• Light-harvesting complexes absorb the energy
• Electrons are released from special pairs of chlorophyll a
molecules in photosystems
• Electrons may be used in noncyclic or cyclic pathways of ATP
formation
Figure 6-7 p106
The Thylakoid Membrane
photosystem
light-harvesting complex
The Noncyclic Pathway
• Photosystems (type II and type I) contain “special pairs” of
chlorophyll a molecules that eject electrons
• Electrons lost from photosystem II are replaced by photolysis
of water molecules – the process by which light energy
breaks down a water molecule into hydrogen and oxygen
• Electrons lost from a photosystem enter an electron transfer
chain (ETC) in the thylakoid membrane
The Noncyclic Pathway
• In the ETC, electron energy is used to build up a H+ gradient
across the membrane
• H+ flows through ATP synthase, which attaches a phosphate
group to ADP
• ATP is formed in the stroma by chemiosmosis, or electron
transfer phosphorylation
The Noncyclic Pathway
• Electrons from the first electron transfer chain (from
photosystem II) are accepted by photosystem I
• Electrons ejected from photosystem I enter a different
electron transfer chain in which the coenzyme NADP+ accepts
the electrons and H+, forming NADPH
• ATP and NADPH are the energy products of light-dependent
reactions in the noncyclic pathway
Noncyclic Pathway of Photosynthesis
H+
light energy
electron transfer chain
light energy
to second stage
of reactions
ADP + Pi
ATP
synthase
photosystem II
photosystem I
thylakoid
compartment
stroma
The Cyclic Pathway
• When NADPH accumulates in the stroma, the noncyclic
pathway stalls
• A cyclic pathway runs in type I photosystems to make ATP;
electrons are cycled back to photosystem I and NADPH does
not form
What happens during the
light-dependent reactions of photosynthesis?
Take-Home Message:
• In light-dependent reactions, chlorophylls and other pigments
in thylakoid membrane transfer light energy to photosystems
• Photosystems eject electrons that enter electron transfer
chains in the membrane; electron flow through ETCs sets up
hydrogen ion gradients that drive ATP formation
• In the noncyclic pathway, oxygen is released and electrons
end up in NADPH
• A cyclic pathway involving only photosystem I allows the cell
to continue making ATP when the noncyclic pathway is not
running; NADPH does not form; O2 is not released
3D ANIMATION: Photophosphorylation
ANIMATED FIGURE: Noncyclic pathway of
electron flow
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6.6 Energy Flow in Photosynthesis
• Energy flow in the light-dependent reactions is an example of
how organisms harvest energy from their environment
Photophosphorylation
• Photophosphorylation is a light-driven reaction that
attaches a phosphate group to a molecule
• In noncyclic photophosphorylation, electrons move from water
to photosystem II, to photosystem I, to NADPH
• In cyclic photophosphorylation, electrons cycle within
photosystem I
Excited
P700
energy
Excited
P680
P700
(photosystem I)
P680
(photosystem II)
light energy
light energy
Energy flow in the noncyclic reactions of photosynthesis
Stepped Art
Figure 6-9a p108
energy
Excited
P700
P700
(photosystem I)
light energy
Energy flow in the cyclic reactions of
photosynthesis
Stepped Art
Figure 6-9b p108
How does energy flow
during the reactions of photosynthesis?
Take-Home Message:
• Light provides energy inputs that keep electrons flowing
through electron transfer chains
• Energy lost by electrons as they flow through the chains sets
up a hydrogen ion gradient that drives the synthesis of ATP
alone, or ATP and NADPH
6.7 Light-Independent Reactions
• The cyclic, light-independent reactions of the Calvin-Benson
cycle are the “synthesis” part of photosynthesis
• Calvin-Benson cycle
• Enzyme-mediated reactions that build sugars in the
stroma of chloroplasts
Carbon Fixation
• Carbon fixation
• Extraction of carbon atoms from inorganic sources
(atmosphere) and incorporating them into an organic
molecule
• Builds glucose from CO2
• Uses bond energy of molecules formed in light-dependent
reactions (ATP, NADPH)
The Calvin-Benson Cycle
• The enzyme rubisco attaches CO2 to RuBP
• Forms two 3-carbon PGA molecules
• PGAL is formed
• PGAs receive a phosphate group from ATP, and hydrogen
and electrons from NADPH
• Two PGAL combine to form a 6-carbon sugar
• Rubisco is regenerated
1
4
Calvin–
Benson
Cycle
2
other molecules
3
glucose
Stepped Art
Figure 6-10 p109
ANIMATED FIGURE: Photosynthesis
overview
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Take-Home Message: What happens in light-
independent reactions of photosynthesis?
• Light-independent reactions of photosynthesis run on the
bond energy of ATP and energy of electrons donated by
NADPH; both formed in the light-dependent reactions
• Collectively called the Calvin–Benson cycle, these carbonfixing reaction use hydrogen (from NADPH), and carbon and
oxygen (from CO2) to build sugars
6.8 Adaptations:
Different Carbon-Fixing Pathways
• Environments differ, and so do details of photosynthesis:
• C3 plants
• C4 plants
• CAM plants
Stomata
• Stomata
• Small openings through the waxy cuticle covering
epidermal surfaces of leaves and green stems
• Allow CO2 in and O2 out
• Close on dry days to minimize water loss
C3 Plants
• C3 plants
• Plants that use only the Calvin–Benson cycle to fix carbon
• Forms 3-carbon PGA in mesophyll cells
• Used by most plants, but inefficient in dry weather when
stomata are closed
• Example: barley
Photorespiration
• When stomata are closed, CO2 needed for light-independent
reactions can’t enter, O2 produced by light-dependent
reactions can’t leave
• Photorespiration
• At high O2 levels, rubisco attaches to oxygen instead of
carbon
• CO2 is produced rather than fixed
A C3 Plant: Barley
palisade
mesophyll cell
spongy
mesophyll cell
mesophyll cell
CO2
O2
glycolate
RuBP
Calvin–
Benson
PGA
Cycle
ATP
NADPH
B On dry days, stomata close and oxygen accumulates
inside leaves. The excess causes rubisco to attach oxygen
instead of carbon to RuBP. This is photorespiration, and it
makes sugar production inefficient in C3 plants.
sugars
Figure 6-11b p110
C4 Plants
• C4 plants
• Plants that have an additional set of reactions for sugar
production on dry days when stomata are closed;
compensates for inefficiency of rubisco
• Forms 4-carbon oxaloacetate in mesophyll cells, then
bundle-sheath cells make sugar
• Examples: Corn, switchgrass, bamboo
A C4 Plant: Millet
mesophyll cell
bundle-sheath cell
B C4 plants.
Oxygen also builds
up inside leaves
when stomata
close during
photosynthesis.
An additional
pathway in these
plants keeps the
CO2 concentration
high enough in
bundle-sheath
cells to prevent
photorespiration.
CO2 from inside plant
mesophyll cell
oxaloacetate
bundle-sheath cell
C4
Cycle
CO2
PGA
RuBP
Calvin–
Benson
Cycle
sugars
Figure 6-12b p110
CAM Plants
• CAM plants (Crassulacean Acid Metabolism)
• Plants with an alternative carbon-fixing pathway that
allows them to conserve water in climates where days are
hot
• Forms 4-carbon oxaloacetate at night, which is later
broken down to CO2 for sugar production
• Example: succulents, cactuses
A CAM Plant: Jade Plant
mesophyll cell
CO2 from outside plant
oxaloacetate C4
Cycle
night
day
CO2
PGA
RuBP
Calvin–
Benson
Cycle
sugars
Figure 6-13a p111
ANIMATED FIGURE: Carbon-fixing
adaptations
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Take-Home Message:
How do carbon-fixing reactions vary?
• When stomata are closed, oxygen builds up inside leaves of
C3 plants; rubisco then can attach oxygen (instead of carbon
dioxide) to RuBP; photorespiration reduces the efficiency of
sugar production, so it can limit the plant’s growth
• Plants adapted to dry conditions limit photorespiration by
fixing carbon twice: C4 plants separate the two sets of
reactions in space; CAM plants separate them in time
Biofuels Revisited
• The first cells on Earth were chemoautotrophs that extracted
energy and carbon from inorganic molecules in the
environment, such as hydrogen sulfide and methane
• The evolution of photosynthesis dramatically and permanently
changed Earth’s atmosphere
• Photoautotrophs use photosynthesis to make food from CO2
and water, releasing O2 into the atmosphere
Earth’s Early Atmosphere
Earth With an Oxygen Atmosphere
Effects of Atmospheric Oxygen
• Selection pressure on evolution of life
• Oxygen radicals
• Development of ATP-forming reactions
• Aerobic respiration
• Formation of ozone (O3) layer
• Protection from UV radiation
The Atmospheric Carbon Cycle
• Photosynthesis removes carbon dioxide from the atmosphere,
and locks carbon atoms in organic compounds
• Aerobic organisms break down organic compounds for
energy, and release CO2 into the atmosphere
• Since photosynthesis evolved, these two processes have
constituted a more or less balanced cycle of the biosphere
• Today, Earth’s atmosphere is out of balance – the level of
CO2 is increasing, mainly as a result of human activity
Fossil Fuels
• When we burn fossil fuels, carbon that has been locked for
hundreds of millions of years is released back into the
atmosphere, mainly as carbon dioxide
• Today, we release about 28 billion tons of carbon dioxide into
the atmosphere each year, more than ten times the amount
we released in the year 1900
• Increased atmospheric CO2 contributes to global warming
and disrupts natural biological systems
Fossil Fuel Emissions
Renewable Energy Sources
• Biofuels are a renewable source of energy
• The carbon in plant matter comes from atmospheric CO2,
fixed by photosynthesis
• Making biofuel production economically feasible is a high
priority for today’s energy researchers