Download Photosynthesis

8
Photosynthesis: Energy from
Sunlight
8 Photosynthesis: Energy from Sunlight
• 8.1 What Is Photosynthesis?
• 8.2 How Does Photosynthesis Convert Light
Energy into Chemical Energy?
• 8.3 How Is Chemical Energy Used to
Synthesize Carbohydrates?
• 8.4 How Do Plants Adapt to the
Inefficiencies of Photosynthesis?
• 8.5 How Is Photosynthesis Connected to
Other Metabolic Pathways in Plants?
8.1 What Is Photosynthesis?
Photosynthesis: “synthesis from light”
The broad outline:
• Plants take in CO2 and release water
and O2
• Light is required
6CO2  6H 2O  C6 H12O6  6O2
Figure 8.1 The Ingredients for Photosynthesis
8.1 What Is Photosynthesis?
Ruben and Kamen determined the
source of O2 released during
photosynthesis by using radioisotope
tracers.
6CO2  12H 2O  C6 H12O6  6O2  6H 2O
Figure 8.2 Water Is the Source of the Oxygen Produced by Photosynthesis
8.1 What Is Photosynthesis?
Two pathways:
Light reactions: light energy converted to
chemical energy (in ATP and NADPH + H+)
Light-independent reactions: use the ATP
and NADPH + H+ plus CO2 to produce
sugars
Figure 8.3 An Overview of Photosynthesis
8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
Light is a form of electromagnetic
radiation, which comes in discrete
packets called photons and behave as
particles.
Light also behaves as if propagated as
waves.
Energy of a photon is inversely
proportional to its wavelength.
8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
When a photon meets a molecule it can be:
• Scattered or reflected
• Transmitted or pass through the molecule
• Absorbed—the molecule acquires the
energy of the photon. The molecule goes
from ground state to excited state.
Figure 8.4 Exciting a Molecule (A)
Figure 8.4 Exciting a Molecule (B)
Energy from the photon boosts an electron into another shell
8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
Photons can have a wide range of
wavelengths and energy levels.
Molecules that absorb specific
wavelengths in the visible range of the
spectrum are called pigments.
Figure 8.5 The Electromagnetic Spectrum
8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
Absorption spectrum: plot of
wavelengths absorbed by a pigment
Action spectrum: plot of biological
activity as a function of the wavelengths
of light the organism is exposed to
Figure 8.6 Absorption and Action Spectra (Part 1)
Figure 8.6 Absorption and Action Spectra (Part 2)
8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
Several types of pigments absorb light
energy used in photosynthesis:
• Chlorophylls: a and b
• Accessory pigments: absorb in red
and blue regions, transfer the energy to
chlorophylls—carotenoids,
phycobilins
Figure 8.7 The Molecular Structure of Chlorophyll (Part 1)
Figure 8.7 The Molecular Structure of Chlorophyll (Part 2)
8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
When a pigment returns to ground state,
some of the energy may be given off as
heat, some may be given off as
fluorescence.
Fluorescence has longer wavelengths
and less energy than the absorbed light
energy. No work is done.
8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
If pigment can pass the energy to another
molecule, there’s no fluorescence.
The energy can be passed to a reaction
center where it is converted to chemical
energy.
8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
Pigments are arranged in antenna
systems.
Pigments are packed together on thylakoid
membrane proteins.
Excitation energy is passed from pigments
that absorb short wavelengths to those that
absorb longer wavelengths, and ends up in
the reaction center pigment.
Figure 8.8 Energy Transfer and Electron Transport
8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
The reaction center molecule is
chlorophyll a.
The excited chlorophyll (Chl*) is a
reducing agent (electron donor).

Chl  A  Chl  A
*

8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
A is the first in a chain of electron carriers
on the thylakoid membrane—electron
transport, a series of redox reactions.
The final electron acceptor is NADP+


NADP  e  NADPH  H

8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
Two systems of electron transport:
Noncyclic electron transport—produces
NADPH + H+ and ATP
Cyclic electron transport—produces ATP
only
8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
Noncyclic electron transport: light
energy is used to oxidize water → O2,
H+, and electrons
Chl+ is a strong oxidizing agent. It takes
electrons from water, splitting the water
molecule.
8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
Two photosystems required in noncyclic
electron transport.
Each photosystem consists of several
chlorophyll and accessory pigment
molecules.
The photosystems complement each
other, must be constantly absorbing light
energy to power noncyclic electron
transport.
Figure 8.9 Noncyclic Electron Transport Uses Two Photosystems
8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
Photosystem I
• Light energy reduces NADP+ to NADPH +
H+
• Reaction center is chlorophyll a molecule
P700—absorbs in the 700nm range
8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
Photosystem II
• Light energy oxidizes water → O2, H+,
and electrons
• Reaction center is chlorophyll a
molecule P680—absorbs at 680nm
8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
The Z scheme models describes
noncyclic electron transport.
The reactions in the electron transport
chain are coupled to a proton pump that
results in the chemiosmotic formation of
ATP.
8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
Cyclic electron transport: an electron
from an excited chlorophyll molecule
cycles back to the same chlorophyll
molecule.
A series of exergonic redox reactions, the
released energy creates a proton
gradient that is used to synthesize ATP.
Figure 8.10 Cyclic Electron Transport Traps Light Energy as ATP
8.2 How Does Photosynthesis Convert Light Energy into
Chemical Energy?
Photophosphorylation: light-driven
production of ATP—a chemiosmotic
mechanism.
Electron transport is coupled to the
transport of H+ across the thylakoid
membrane—from the stroma into the
lumen.
Figure 8.11 Chloroplasts Form ATP Chemiosmotically (Part 1)
Figure 8.11 Chloroplasts Form ATP Chemiosmotically (Part 2)
8.3 How Is Chemical Energy Used to Synthesize Carbohydrates?
CO2 fixation—CO2 is reduced to
carbohydrates.
Enzymes in the stroma use the energy in
ATP and NADPH to reduce CO2.
Because the ATP and NADPH are not
“stockpiled” these light-independent
reactions must also take place in the
light.
8.3 How Is Chemical Energy Used to Synthesize Carbohydrates?
Calvin and Benson used the 14C
radioisotope to determine the sequence
of reactions in CO2 fixation.
They exposed Chlorella to 14CO2, then
extracted the organic compounds and
separated them by paper
chromatography.
Figure 8.12 Tracing the Pathway of CO2 (Part 1)
Figure 8.12 Tracing the Pathway of CO2 (Part 2)
8.3 How Is Chemical Energy Used to Synthesize Carbohydrates?
3-second exposure of the Chlorella to
14CO revealed that the first compound
2
to be formed is 3PG, a 3-carbon sugar
phosphate.
8.3 How Is Chemical Energy Used to Synthesize Carbohydrates?
The pathway of CO2 fixation is the Calvin
cycle.
CO2 is first added to a 5-C RuBP; the 6-C
compound immediately breaks down
into two molecules of 3PG.
The enzyme is rubisco—the most
abundant protein in the world.
Figure 8.13 The Calvin Cycle (Part 1)
Figure 8.13 The Calvin Cycle (Part 2)
Figure 8.14 RuBP Is the Carbon Dioxide Acceptor
8.3 How Is Chemical Energy Used to Synthesize Carbohydrates?
The Calvin cycle consists of 3 processes:
• Fixation of CO2
• Reduction of 3PG to G3P
• Regeneration of RuBP
8.3 How Is Chemical Energy Used to Synthesize Carbohydrates?
G3P: glyceraldehyde 3-phosphate
Most is recycled into RuBP
The rest is converted to starch and
sucrose
8.3 How Is Chemical Energy Used to Synthesize Carbohydrates?
Covalent bonds in carbohydrates
produced in the Calvin cycle represent
the total energy yield of photosynthesis.
This energy is used by the autotrophs
themselves, and by heterotrophs—other
organisms that cannot photosynthesize.
8.3 How Is Chemical Energy Used to Synthesize Carbohydrates?
The Calvin cycle is stimulated by light:
• Proton pumping from stroma into
thylakoids increases the pH which
favors the activation of rubisco.
• Electron flow from photosystem I
reduces disulfide bonds to activate
Calvin cycle enzymes.
Figure 8.15 The Photochemical Reactions Stimulate the Calvin Cycle
8.4 How Do Plants Adapt to the Inefficiencies of Photosynthesis?
Rubisco is an oxygenase as well as a
carboxylase.
It can add O2 to RuBP instead of CO2,
reduces amount of CO2 fixed, and limits
plant growth.
Products: 3PG and phosphoglycolate
8.4 How Do Plants Adapt to the Inefficiencies of Photosynthesis?
Photorespiration:
The phosphoglycolate forms glycolate—
moves into peroxisomes—converted to
glycine.
Glycine diffuses into mitochondria, 2
glycines are converted into glycerate +
CO2
Figure 8.16 Organelles of Photorespiration
8.4 How Do Plants Adapt to the Inefficiencies of Photosynthesis?
Rubisco has 10 times more affinity for
CO2.
In the leaf, if O2 concentration is high,
photorespiration occurs. If CO2
concentration is high, CO2 is fixed.
Photorespiration is more likely at high
temperatures, such as hot days when
stomata are closed.
8.4 How Do Plants Adapt to the Inefficiencies of Photosynthesis?
C3 plants: first product of CO2 fixation is
3PG. Palisade cells in the mesophyll
have abundant rubisco.
C4 plants: first product of CO2 fixation is
oxaloacetate, a 4-C compound.
Mesophyll cells contain PEP
carboxylase.
Figure 8.17 Leaf Anatomy of C3 and C4 Plants
8.4 How Do Plants Adapt to the Inefficiencies of Photosynthesis?
C4 plants—corn, sugar cane, tropical
grasses—can keep stomata closed on
hot days, but photorespiration does not
occur.
In mesophyll cells, CO2 is accepted by
PEP (phosphoenolpyruvate) to form
oxaloacetate.
PEP carboxylase has no affinity for O2.
8.4 How Do Plants Adapt to the Inefficiencies of Photosynthesis?
Oxaloacetate diffuses to bundle sheath
cells which have abundant rubisco.
The oxaloacetate is decarboxylated (PEP
returns to mesophyll cells), CO2 enters
the Calvin cycle.
Figure 8.18 The Anatomy and Biochemistry of C4 Carbon Fixation (A)
Figure 8.18 The Anatomy and Biochemistry of C4 Carbon Fixation (B)
8.4 How Do Plants Adapt to the Inefficiencies of Photosynthesis?
CAM plants—crassulacean acid
metabolism
Fix CO2 with PEP carboxylase—at
night—stomata can open with less water
loss. Oxaloacetate is converted to malic
acid.
Day—malic acid goes to chloroplasts and
is decarboxylated—CO2 enters the
Calvin cycle.
8.5 How Is Photosynthesis Connected to Other Metabolic
Pathways in Plants?
Photosynthesis and respiration are
closely linked by the Calvin cycle.
Glycolysis in the cytosol, respiration in
the mitochondria, and photosynthesis in
the chloroplasts can occur
simultaneously.
Figure 8.19 Metabolic Interactions in a Plant Cell (Part 1)
Figure 8.19 Metabolic Interactions in a Plant Cell (Part 2)
8.5 How Is Photosynthesis Connected to Other Metabolic
Pathways in Plants?
Photosynthesis results in only 5 percent
of total sunlight energy being
transformed to the energy of chemical
bonds.
Understanding the inefficiencies of
photosynthesis may be important as
climate change drives changes in
photosynthetic activity of plants.
Figure 8.20 Energy Losses During Photosynthesis