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PHOTOSYNTHESIS: ACQUIRING
ENERGY FROM THE SUN
CHAPTER 6
AN OVERVIEW OF PHOTOSYNTHESIS
• Most of the energy used by almost all living cells
ultimately comes from the sun.
• Plants, algae, and some bacteria capture the sunlight
energy by a process called photosynthesis.
• Only about 1% of the available energy in sunlight is
captured.
Cross-section of leaf
Cuticle
Epidermis
Mesophyll
Vascular
bundle
Stoma
Bundle
Vacuole Nucleus
sheath
Cell wall
Thylakoid
Inner membrane
Outer membrane
Granum
Stroma
Chloroplast
Chloroplasts
Mesophyll cell
AN OVERVIEW OF PHOTOSYNTHESIS
• The leaf cells
of plants
contain
chloroplasts.
• The chloroplast
contains
internal
membranes
called
thylakoids.
• The thylakoids are stacked together in columns called
grana.
• The stroma is a semi liquid substance that surrounds
the thylakoids.
AN OVERVIEW OF PHOTOSYNTHESIS
• The photosystem is the starting point of
photosynthesis.
• It is a network of pigments in the membrane of the
thylakoid.
• The primary pigment of a photosystem is
chlorophyll.
• The pigments act as an antenna to capture
energy from sunlight.
• Individual chlorophyll pigments pass the
captured energy between them.
AN OVERVIEW OF PHOTOSYNTHESIS
•
Photosynthesis takes place in three stages:
1. Capturing energy from sunlight.
2. Using the captured energy to produce ATP
and NADPH.
3. Using the ATP and NADPH to make
carbohydrates from CO2 in the atmosphere.
AN OVERVIEW OF PHOTOSYNTHESIS
• The process of photosynthesis is divided into
two types of reactions.
• Light-dependent reactions take place only in the
presence of light and produce ATP and NADPH.
• Light-independent reactions do not need light to
occur and result in the formation of organic
molecules.
• More commonly known as the Calvin cycle.
AN OVERVIEW OF PHOTOSYNTHESIS
• The overall reaction for photosynthesis may be
summarized by this simple equation:
6 CO2 + 6 H2O
carbon
dioxide
water
+
Light
energy
C6H12O6
glucose
+ 6 O2
oxygen
HOW PLANTS CAPTURE ENERGY FROM
SUNLIGHT
• Light is comprised of packets of energy called
photons.
• Sunlight has photons of varying energy levels.
• The possible range of energy levels is represented
by an electromagnetic spectrum.
• Human eyes only perceive photons of intermediate
energy levels.
• This range of the spectrum is known as visible
light.
Increasing energy of photon
Increasing wave length
0.001 nm
1 nm
1,000 nm
1 cm
10 nm
0.01 cm
1m
100 m
Radioactive X-ray
Light People
Microwave FM radio
Radar ovens
elements machines bulbs
AM radio
X UV
Gamma rays rays light Infrared Microwaves
Radio waves
Visible light
400 nm 430 nm
500 nm 560 nm 600 nm 650 nm
740 nm
HOW PLANTS CAPTURE ENERGY FROM
SUNLIGHT
• Pigments are molecules that absorb light
energy.
• The main pigment in plants is chlorophyll.
• Chlorophyll absorbs light at the ends of the
visible spectrum, mainly blue and red light.
HOW PLANTS CAPTURE ENERGY FROM
SUNLIGHT
• Plants also contain other pigments, called
accessory pigments, that absorb light levels
that chlorophyll does not.
• These pigments give color to flowers, fruits,
and vegetables.
• They are present in leaves too, but are
masked by chlorophyll until the fall when the
chlorophyll is broken down.
WHY ARE PLANTS GREEN?
1. All wave lengths except
500- to600-nanometer wave lengths
(green) absorbed by leaf
4. Brain perceives
“green”
2. Green reflected
by leaf
3. Green absorbed
by eye pigment
ABSORPTION SPECTRA OF
CHLOROPHYLLS AND CAROTENOIDS
Relative light absorption
CarotenoidsChlorophyll bChlorophyll a
400
450
500
550 600 650
Wavelength (nm)
700
ORGANIZING PIGMENTS INTO
PHOTOSYSTEMS
• In plants, the light-dependent reactions occur
within a complex of proteins and pigments called
a photosystem.
ORGANIZING PIGMENTS INTO
PHOTOSYSTEMS
Electron
• Light energy is first captured
acceptor
Reaction
by any one of the chlorophyll Photon
center
e–
pigments.
chlorophyll
e–
• The energy is passed along
to other pigments until it
reaches the reaction center
chlorophyll molecule.
Chlorophyll
• The reaction center then molecules
releases an excited electron,
Photosystem
which is then transferred to
an electron acceptor.
• The excited electron that is lost is then replaced by an
electron donor.
Electro
donor
ORGANIZING PIGMENTS INTO
PHOTOSYSTEMS
• The light-dependent reactions in plants and
algae use two photosystems.
• Photosystem II
• Captures a photon of light and releases an
excited electron to the electron transport system
(ETS).
• The ETS then produces ATP.
ORGANIZING PIGMENTS INTO
PHOTOSYSTEMS
Energy of electrons
• Photosystem I
• Absorbs another photon of light and releases an
excited electron to another ETS.
• The ETS produces NADPH.
• The electron from photosystem II replaces the
electron from the reaction center.
Excited
reaction
center
Excited
reaction
center
4
e–
e–
Photon
Reaction
center
P680
1
Photosystem II
Reaction
center
3
H+
Proton gradient
Water-splitting formed for ATP
synthesis
enzyme
–
e 2H2O
4H+ + O2
Electron transport system
5
NADP+ + H+
e–
ATP
2
e–
NADPH
Photon
P700
Photosystem I
Electron transport system
HOW PHOTOSYSTEMS CONVERT
LIGHT TO CHEMICAL ENERGY
• Plants produce both ATP and NADPH by
noncyclic photophosphorylation.
• The excited electrons flow through both photosystems
and end up in NADPH.
• High energy electrons generated by photosystem II
are used to make ATP and are then passed along to
photosystem I to drive the production of NADPH.
THE PHOTOSYNTHETIC ELECTRON
TRANSPORT SYSTEM
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Stroma
Photon
Calvin
cycle
Antenna
complex
Thylakoid
membrane
ATP
Photon
H+ + NADP+
NADP
H+
NADP
e–
e–
e–
e–
Light-dependent
reactions
2H2O
Thylakoid s pace
Proton
gradient
H+
Water-splitting
enzyme
O2
H+
4 H+
Photosystem II
H+
Electron transport system
H+
Photosystem I
H+
Thylakoid
space
Electron transport system
HOW PHOTOSYSTEMS CONVERT LIGHT
TO CHEMICAL ENERGY
• As a result of the proton pump of the ETS, a
large concentration of protons builds up in the
thylakoid space.
• The thylakoid membrane is impermeable to protons.
• Protons can only re-enter the stroma by traveling
through a protein channel called ATP synthase.
• As the protons follow their concentration gradient
and pass through ATP synthase channels, ADP is
phosphorylated into ATP.
• This process is called chemiosmosis.
CHEMIOSMOSIS IN A CHLOROPLAST
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Stroma
Calvin
cycle
H+
Photon
ADP
ATP
H+
H+
H+
ATP
NADPH
Light-dependent
reactions
Thylakoid space
e–
e–
2H2O
H+
Thylakoid
O2
space
4 H+
Photosystem II
H+
H+
Electron transport system
ATP synthase
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BUILDING NEW MOLECULES
• Cells use the products of the light-dependent
reactions to build organic molecules.
• ATP is needed to drive endergonic reactions.
• NADPH is needed to provide reducing power in the
form of hydrogens.
BUILDING NEW MOLECULES
• The synthesis of new molecules employs the
light-independent, or Calvin cycle, reactions.
• These reactions are also known as C3
photosynthesis.
BUILDING NEW MOLECULES
• The Calvin cycle reactions occur in 3 stages
• 1. Carbon fixation
• Carbon from CO2 in the air is attached to an organic
molecule, RuBP.
• 2. Making sugars
• The carbons are shuffled about through a series of
reactions to make sugars.
• 3. Reforming RuBP
• The remaining molecules are used to reform RuBP.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1
2
3
CO2
3
P
3
RuBP
(Starting material)
P
6
3-phosphoglycerate
6
3-phosphoglycerate
6
3
RuBP
(Starting material)
ATP
6 NADPH
3
P
P
Glyceraldehyde
3-phosphate
6
P
1
ATP
5
Glyceraldehyde
3-phosphate
Glyceraldehyde
3-phosphate
Glucose
The Calvin cycle begins when a carbon
atom from a CO2 molecule is added to
a five-carbon molecule (the starting
material). The resulting six-carbon
molecule is unstable and immediately
splits into three-carbon molecules.
(Three “turns” of the cycle are indicated
here with three molecules of CO2
entering the cycle.)
Then, through a series of reactions,
energy from ATP and hydrogens from
NADPH (the products of the lightdependent reactions) are added to the
three-carbon molecules. The nowreduced three-carbon molecules either
combine to make glucose or are used to
make other molecules.
Most of the reduced three-carbon
molecules are used to regenerate the
five-carbon starting material, thus
completing the cycle.
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BUILDING NEW MOLECULES
• The Calvin cycle must “turn” 6 times in order to
form a new glucose molecule.
• Only one carbon is added from CO2 per turn.
• The Calvin cycle also recycles reactants needed
for the light-dependent reactions.
• It returns ADP so that it is available for chemiosmosis
in photosystem II.
• It returns NADP+ back to the ETS of photosystem I.
REACTIONS OF THE CALVIN CYCLE
Stroma of chloroplast
3
CO2
Calvin
cycle
P
AT
P
NADPH
Light-dependent
reactions
Thylakoid space
• Rubisco is thought
to be the most
abundant protein
on earth
6
3-phosphoglycerate
Rubisco
3
RuBP
6 ATP
Carbon fixation
6 ADP
3 ADP
P
3 ATP
Reforming
RuBP
P
6
1,3-bisphosphoglycera
2 Pi
6 NADPH
Making sugars
6 NADP
6 Pi
P
P
5
Glyceraldehyde 3-phosphate
6
Glyceraldehyde 3-phosphate
P
1
Glyceraldehyde
3-phosphate
Glucose and
other sugars
PHOTORESPIRATION: PUTTING THE
BRAKES ON PHOTOSYNTHESIS
• Many plants have trouble carrying out C3
photosynthesis when it is hot.
• Plants close openings in their leaves, called stomata
(singular, stoma), in order to prevent water loss.
• The closed stoma also prevent gas exchange.
• O2 levels build up inside the leaves while the
concentrations of CO2 fall.
• The enzyme rubisco fixes oxygen instead of carbon.
• This process is called photorespiration and shortcircuits the Calvin cycle and photosynthesis.
PLANT RESPONSE IN HOT, ARID WEATHER
Leaf
epidermis
Heat
Under hot, arid
conditions, leaves
lose water by
evaporation through
openings in the
leaves called stomata.
H2O
H2O
Stoma
The stomata close
to conserve water
but as a result, O2
builds up inside the
leaves, and CO2
cannot enter the
leaves. This leads
to photorespiration.
O2
CO2
O2
CO2
CARBON FIXATION IN C4 PLANTS
CO2
• Some plants have adapted to
hot climates by performing C4
photosynthesis.
• These C4 plants include sugarcane,
corn, and many grasses.
• They fix carbon using different
types of cells and reactions than C3
plants and do not run out of CO2
even in hot weather.
• CO2 becomes trapped in cells
called bundle-sheath cells.
Phosphoenol- Oxalopyruvate (PEP) acetate
PPi +
AMP
Mesophyll
cell
Pi +
ATP
Pyruvate
Malate
Pyruvate
Malate
CO2
Calvin
cycle
Glucose
Bundlesheath
cell
PHOTORESPIRATION: PUTTING THE
BRAKES ON PHOTOSYNTHESIS
• Another strategy to avoid a reduction in
photosynthesis in hot weather occurs in many
succulent (water-storing) plants, such as cacti
and pineapples.
• These plants undergo crassulacean acid
metabolism (CAM).
• Photosynthesis occurs via the C4 pathway at night
and the C3 pathway during the day.
COMPARING CARBON FIXATION IN C4
AND CAM PLANTS
CO2
CO2
Mesophyll
cell
C4
pathway
CO2
Bundlesheath
cell
C4
pathway
Night
COMesophyll
2
cell
Calvin
cycle
Calvin
cycle
Glucose
Glucose
C4 plants
CAM plants
Day