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Energy Conversions
• Life on Earth is solar-powered by autotrophs
Photosynthesis
Chapter 10
Pg. 184 – 205
Plants
• Plants collect light and transform it to chemical
energy, and obtain atoms from the environment to
produce organic molecules needed for growth and
survival.
• Structure
– Leaves: absorb sunlight
– Stomates: pores on underside of
leaves; gas (CO2 and O2) and
water exchange
– Roots: absorb water and nutrients
– Chloroplasts: contain pigments, perform photosynthesis
– Autotrophs make their own food and have no need to
consume other organisms.
– They are the ultimate source of organic compounds.
– Known as producers.
– Photoautotrophs use photosynthesis to make their own
food from light and CO2. This occurs in the chloroplasts of
leaf cells.
• Heterotrophs live on compounds produced by other
organisms.
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–
–
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Known as consumers.
Includes animals, fungi, and many prokaryotes.
Depend on photosynthesis for food and oxygen.
Heterotrophs use cell respiration to get energy.
Chloroplasts
• Specific sites of photosynthesis in plant cells
• Located in the green parts of plants, but most
abundant in the mesophyll cells
• Structure
– Double-membrane
– Fluid-filled area called the stroma
– Vast network of interconnected membranous sacs
called the thylakoids
• Inner area of the thylakoid is the thylakoid space
– Chlorophyll is located in the thylakoid
membranes
• It is a light-absorbing pigment that drives
photosynthesis and is responsible for plants’
green color
Plant Pigments
• Light, or electromagnetic energy, is absorbed by plant
pigments in photosynthesis.
• Two major types
– Chlorophyll: chlorophyll a and chlorophyll b absorb all wavelengths of
light in the red, blue, and violet range, but not much green.
– Carotenoids: they absorb light in the blue, green, and violet range, but
not much yellow, orange, or reds.
• Carotenoids include xanthophyll and phycobilins (in red algae).
• Chlorophyll b, carotenoids, and phycobilins are antenna
pigments because they capture light in wavelengths other
than those captured by chlorophyll a. They absorb photons of
light and pass energy to chlorophyll a.
• The central atom in chlorophyll is magnesium.
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Photosynthesis
• Reaction:
6CO2 + 6H2O + light energy  C6H12O6 + 6O2
• It is the reverse of cell respiration.
• All O2 that you breathe is provided by this process when
water is split.
– The H2O molecule is split to provide electrons. These electrons go on
to reduce carbon dioxide to sugar.
• Two stages occur:
– Light reactions: occur in the thylakoid, sunlight is required, and light is
converted to chemical energy (ATP and NADPH).
– Calvin cycle: occur in the stroma, sunlight not required, and chemical
energy in ATP and NADPH are used to convert CO2 into sugar
Reduction in Photosynthesis
• Splitting water releases electrons and
hydrogen ions (H+).
• These bond to the carbon dioxide, reducing it
to a sugar.
• Electrons increase the potential energy of the
molecule – ΔG is positive, making the reaction
endergonic.
• Light provide extra free energy.
Oxidation/Reduction Reactions
(Redox Reactions)
Oxidation
Reduction
• Removes electrons from an
atom
• Results in a positive charge
• Adds electrons to an atom
• Results in a negative charge
Think: OIL RIG
Oxidation Is Loss
(of electrons)
Reduction is Gain
(of electrons)
(reducing the charge)
Light Reaction
• Occurs in the thylakoid membranes
• Solar energy  chemical energy
• Net products: NADPH (stores electrons), ATP,
oxygen
• Main events:
– Light is absorbed by chlorophyll and drives the transfer
of electrons from water to NADP+, forming NADPH.
– Water is split and O2 is released.
– ATP is generated, using chemiosmosis to power the
addition of a phosphate group to ADP, a process called
photophosphorylation.
Light
• Light = electromagnetic energy made up of
photons
• Substances that absorb light are called pigments
– different pigments absorb different
wavelengths.
• An absorption spectrum shows which
wavelengths of light a particular pigment
absorbs.
• The action spectrum for photosynthesis graphs
the effectiveness of different wavelengths of light
in driving photosynthesis.
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Photosystems
• Photons are absorbed by groups of pigment molecules in the
thylakoids, called photosystems:
– Light-harvesting complex: many chlorophyll and carotenoid molecules
that allow the complex to gather light effectively.
• When photons are absorbed, one of chlorophyll’s electrons is raised to an
orbital of higher potential energy – it is said to be in the “excited” state.
– Reaction center: consists of two chlorophyll a molecules, which donate
the electrons to the primary electron acceptor.
• This is the first step of the light reaction, a conversion of light energy to
chemical energy, and what makes photoautotrophs the producers of the
natural world.
• Thylakoids contain two photosystems, named photosystem I
and photosystem II.
Linear (Noncyclic) Electron Flow
• Overview
– Predominant route
– Sunlight energizes electrons, which drives the
synthesis of ATP as they are passed from
photosystem II to photosystem I.
– They are again excited in photosystem I and
transferred to NADP+, forming NADPH.
– NADPH and ATP are used in the Calvin cycle to
make sugar from CO2.
Noncyclic Electron Flow
• 1) Photosystem II absorbs light, an electron is excited and lost, and
chlorophyll now has an “electron hole”.
• 2) An enzyme splits a water molecule into 2 H+, two electrons, and
an oxygen atom. The oxygen combines with another oxygen
molecule, forming atmospheric O2.
• 3) The original excited electron passes from the primary electron
acceptor of photosystem II to photosystem I through an electron
transport chain (similar to that in cellular respiration). ATP is made
through chemiosmosis.
• 4) The energy from the transfer of electrons down the electron
transport chain is used to pump protons into the thylakoid space,
creating a build up of H+. H+ ions diffuse through ATP synthase,
generating ATP to be used in the Calvin cycle.
• 5) The electron from photosystem II ends up
in the reaction center of photosystem I, which
has just lost an electron due to light energy.
– Keep in mind that the ultimate source of electrons
is water.
• 6) The excited electrons are passed to another
electron transport chain, which transmits
them to NADP+, which is reduced to NADPH.
The high-energy electrons of NADPH are now
available for use in the Calvin cycle.
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Cyclic Electron Flow
• This type of electron flow uses PS I only.
• It uses a short circuit of linear electron flow by
cycling the excited electrons back to their original
starting point in PS I.
• Creates equal amounts of ATP and NADPH but the
Calvin cycle requires more ATP than NADPH.
• Electrons are sometimes rerouted back to the
electron transport chain from photosystem I to
produce more ATP.
• Uses chemiosmosis to produce ATP, but does not
produce NADPH, or oxygen, and water isn’t used.
Chemiosmosis
• Chloroplasts and mitochondria generate ATP by
chemiosmosis.
• Basic steps:
– Electron transport chain uses flow of electrons to
pump H+ across thylakoid membrane.
– A proton-motive force is created within the thylakoid
space that can be utilized by ATP synthase to
phosphorylate ADP to ATP. It is generated in 3 places:
• H+ from water
• H+ pumped across the membrane by cytochrome complex
• Removal of a H+ from the stroma when NADP+ is reduced to
NADPH.
DARK REACTION (CALVIN CYCLE)
Overview
• Occurs in the stroma of the chloroplast
• CO2 from air is incorporated into organic
molecules in carbon fixation
• Uses fixed carbon, NADPH, and ATP from the
light reactions to form new sugars
• Actual product is glyceraldehyde 3-phosphate
(G3P), which then forms glucose and other
sugars
– 3CO + 9ATP + 6NADPH  G3P + 9ADP + 9P +
6NADP
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Calvin Cycle
The Long Version
• 1) Three CO2 molecules are
attached to three molecules
of the 5-carbon sugar
ribulose biphosphate
(RuBP).
– These reactions are catalyzed
by the enzyme rubisco
The Short Version
• 1) Carbon Fixation
– 3 CO2 + RuBP
• RuBP = 5-carbon sugar
ribulose bisphosphate
• Catalyzed by the enzyme
rubisco (RuBP carboxylase)
• They produce an unstable
product that immediately
splits into two 3-carbon
compounds called 3phosphogycerate.
– At this point, carbon has been
fixed – the incorporation of
CO2 into an organic compound.
The Long Version
• 5) Finally, RuBP is
regenerated as the 5
G3P are reworked into
three of the starting
molecules, with the
expenditure of 3 ATP
molecules.
The Long Version
• 2) The 3-phosphoglycerate
molecules are
phosphorylated to become
1,3-bisphosphoglycerate
• 3) 6 NADPH reduce those
molecules to six
glyceraldehyde 3-phosphate
(G3P).
• 4) One G3P leaves the cycle
to be used by the plant cell.
Two G3P can combine to
form glucose, which is
typically listed as the final
photosynthetic product.
The Short Version
• 2)Reduction
– Use 6 ATP and 6 NADPH to
produce 1 net G3P
• G3P – glyceraldehyde 3
phosphate
The Short Version
• 3) Regeneration
– Use 3 ATP to regenerate RuBP
Endergonic Reaction
• In the Calvin cycle, the formation of one net
G3P requires the following:
– 9 ATP are consumed
Accounting
• The accounting is complicated:
– 3 turns of Calvin cycle = 1 G3P
– 3 CO2  1 G3P (3C)
• Replenished by the light reactions
– 6 NADPH
• Also replenished by light reactions
• One of the 6 G3P molecules produced is a net
gain, and will be used for biosynthesis or the
energy needs of the cell.
– 6 turns of Calvin cycle = 1 C6H12O6 (6C)
– 6 CO2  1 C6H12O6 (6C)
– 18 ATP + 12 NADPH  1 C6H12O6
– Any ATP left over from light reactions will be used
elsewhere by the cell
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Photosynthesis Summary
• Light reactions
– Produced ATP, NADPH, and O2
– Consumed H2O
• Calvin cycle
– Produced G3P (sugar)
– Regenerated ADP and NADP
– Consumed CO2
Evolution of Alternative Mechanisms
• Problem with photosynthesis and C3 plants:
– Stomata allow CO2 to enter and H2O to exit.
– On hot, dry days, C3 plants produce less sugar because
declining CO2 starves the Calvin cycle – the plant must
keep stomata closed to conserve water, thereby
reducing CO2 uptake.
– Additionally, rubisco can bind O2 in the place of CO2,
causing oxidation/breakdown of RuBP, resulting in a
loss of energy and carbon for the plant
(photorespiration). This process can drain away up to
50% of the carbon fixed by the Calvin cycle.
ALTERNATIVES
Adaptations to Arid Climates
• Metabolic adaptations reduce
photorespiration:
– C4 plants: the two steps of photosynthesis occur in
different locations, which reduces
photorespiration and increases sugar production.
• Physically separated
– CAM plants: keep stomata closed during the day
to prevent excessive water loss; stomata open and
CO2 is absorbed at night; the Calvin cycle occurs in
the early morning once the stomata are closed.
• Temporally separated
Photosynthetic Pathways
C3
Carbon fixation
and Calvin
cycle are
together
Rubisco
C4
Carbon fixation
and Calvin
cycle are in
different cells
PEP
carboxylase
CAM
Carbon fixation
and Calvin
cycle occur at
different times
Organic acid
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