Photosynthesis

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Photosynthesis
Life is woven out of air by light-Jacob
Moleschott
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You should understand

• Ecological and social contexts
• How scientists came to understand photosynthesis
• The basic inputs and outputs of the two main
  process involved in photosynthesis
• Some of the details of the light reactions and
  the Calvin cycle
• Alternative methods of photosynthesis

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Sources of matter and energy for living organisms

• Where do you get your
• energy and matter?

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Sources of matter and energy for living organisms

• Where do you get your
• energy and matter?

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Sources of matter and energy for living organisms

• Where do you get your
• energy and matter?

Inputs
Outputs
Process
Energy
Metabolic Pathway
Food
Biomass (carbon-based)
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Sources of matter and energy for living organisms

• Where do green plants get their energy and
  matter?

Inputs
Outputs
Process
Energy
Metabolic Pathway
Biomass (carbon-based)
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Sources of matter and energy for living organisms

• Where do green plants get their energy and
  matter?

Inputs
Outputs
Process
Energy
Metabolic Pathway
Food from soil?
Biomass (carbon-based)
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Where do plants get their energy and matter?

• In the mid-1700s, Van Helmont grew willows in
  pots for five years
• The willow seedlings increased in biomass by 74.4
  kg
• The soil decreased in mass by 57 grams

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Where do plants get their energy and matter?
Inputs
Outputs
Energy
Food from soil
Metabolic Pathway
Biomass (carbon-based)
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Where do plants get their energy and matter?

• In the late-1700s, Joseph Priestly noticed that
  flames burning in sealed containers with plants
  outlasted those burning without plants.
• Jan Ingenhousz later found that this only
  occurred in the presence of sunlight.
• Ingenhousz suggested that plants might be taking
  in CO2 from the air, internalizing the carbon,
  and releasing the oxygen gas.

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Where do plants get their energy and matter?
Inputs
Outputs
CO2
Energy
Metabolic Pathway
Light
Biomass (carbon-based)
O2
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Where do plants get their energy and matter?

• Where does the oxygen really come from?
• Ruben and Kamen gave water with heavy isotopes of
  oxygen to green plants.
• CO2 2 H218O ? CH2O 18O2

Light
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Where do plants get their energy and matter?
Inputs
Outputs
CO2
Energy
Metabolic Pathway
Light
Biomass
H2O
O2
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The current understanding.

• Photosynthesis is the conversion of light energy
  to chemical energy the production of
  carbohydrates from carbon dioxide and water by
  plants, algae and some bacteria.

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The current understanding.

• Photosynthesis is the conversion of light energy
  to chemical energy the production of
  carbohydrates from carbon dioxide and water by
  plants, algae and some bacteria.

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The current understanding.

• Photosynthesis is the conversion of light energy
  to chemical energy the production of
  carbohydrates from carbon dioxide and water by
  plants, algae and some bacteria.

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The current understanding.
Light energy
Photosynthesis
6 CO2 6 H2O ? C6H12O6 6 O2
Carbon dioxide
Water
Oxygen
Glucose
Low energy
High energy
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The process of photosynthesis
Inputs
Outputs
CO2
ATP
Carbohydrates
Photosynthesis
Light
Biomass (carbon-based)
H2O
O2
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Light energy
Photosynthesis
6 CO2 6 H2O ? C6H12O6 6 O2
Carbon dioxide
Water
Oxygen
Glucose
Low energy
High energy
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Ecological importance

• Photosynthesis produces more than 250 metric tons
  of carbon annually
• It is the main route in which energy enters the
  biosphere
• Virtually all organisms on Earth are directly or
  indirectly dependent on this energy and matter

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Social importance
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The process of photosynthesis
Inputs
Outputs
CO2
ATP
Carbohydrates
Photosynthesis
Light
Biomass (carbon-based)
H2O
Energy transduction reactions (Light reactions)
O2
Carbon fixation reactions (Calvin cycle)
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The energy transduction reactions
Inputs
Outputs
Energy ATP NADPH
Light
Light reactions
H2O
O2
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The connection between energy transduction and
carbon fixation reactions
Inputs
Outputs
Light reactions
O2
Light
H2O
ATP and NADPH
Calvin cycle
Carbohydrates
CO2
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The connection between energy transduction and
carbon fixation reactions
Inputs
Outputs
Light
Electricity
Product
Raw materials
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The light reactions
Inputs
Outputs
Light reactions
Light
O2
H2O

• Chloroplasts are in all green plant parts (green
  because of chlorophyll), mostly in mesophyll
  cells
• Leaves are most important photosynthetic organ
• Light reactions take place in the thylakoid
  membranes
• Light comes from the sun, water from the vascular
  tissue, oxygen leaves through stomata

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The Calvin cycle
Inputs
Outputs
ATP
ATP and NADPH
Calvin cycle
Carbohydrate
CO2
Plant biomass

• Calvin cycle occurs in the stroma of the
  chloroplasts
• ATP and NADPH come from the light reactions
• CO2 enters the stroma through stomata
• Products are transported as needed

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The rest of the story
Inputs
Outputs
Light reactions
O2
Light
H2O
ATP and NADPH
Calvin cycle
Carbohydrates
CO2
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The rest of the story
Inputs
Outputs
O2
Light
H2O
ATP and NADPH
Calvin cycle
Carbohydrates
CO2
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The light reactions
Inputs
Outputs
Energy ATP NADPH
Light
H2O
O2
Process
The light reactions convert solar energy to
chemical energy. To understand how this works,
it is necessary to understand light!
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The light reactions

• The nature of light
• Light is a form of energy
• Light travels in waves
• Light also behaves like a particle (photon is a
  discrete packet of light energy)
• There is an inverse relationship between
  wavelength and energy

Process
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The light reactions

• Photosynthetic pigments and light
• When matter meets light it may be reflected or
  absorbed
• Pigments are light-absorbing molecules
• Plants have several kinds of pigments, each with
  a different absorption spectra
• In general, photosynthesis is activated by blue
  and red light
• Green light is reflected

Process
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The light reactions

• Photosynthetic pigments and light
• When matter meets light it may be reflected or
  absorbed
• Pigments are light-absorbing molecules
• Plants have several kinds of pigments, each with
  a different absorption spectra
• In general, photosynthesis is activated by blue
  and red light
• Green light is reflected

What happens when these pigments absorb light?
Process
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The light reactions
Process

• Brief orientation
• Photosystems are embeded in the thylakoid
  membranes (contain a reaction center surrounded
  by pigments)
• Two types of pigments clumps are found in the
  thylakoid membranes (P 680, associated with PSII
  and P700, associated with PSI)
• Differences are based on the excitation of the
  chlorophyll a in the reaction center

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The light reactions-Linear electron flow

• When struck by light of the appropriate
  wavelength, an electron in a pigment is excited.

Process
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The light reactions-Linear electron flow

• When struck by light of the appropriate
  wavelength, an electron in a pigment is excited.
• The energy from that electron can be passed
  around to other pigment molecules.

Process
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The light reactions-Linear electron flow

• When struck by light of the appropriate
  wavelength, an electron in a pigment is excited.
• The energy from that electron can be passed
  around to other pigment molecules.
• Eventually, the energy is transferred to a
  special chlorophyll a in the P 680 reaction
  center.

Process
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The light reactions-Linear electron flow

• When struck by light of the appropriate
  wavelength, an electron in a pigment is excited.
• The energy from that electron can be passed
  around to other pigment molecules.
• Eventually, the energy is transferred to a
  special chlorophyll a in the P 680 reaction
  center.
• Its excited electron is transferred to the
  carrier.

Process
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The light reactions-Linear electron flow

• When struck by light of the appropriate
  wavelength, an electron in a pigment is excited.
• The energy from that electron can be passed
  around to other pigment molecules.
• Eventually, the energy is transferred to a
  special chlorophyll a in the P 680 reaction
  center.
• Its excited electron is transferred to the
  carrier.
• This loss of an electron encourages the splitting
  of water, oxygen is released.

Process
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The light reactions-Linear electron flow

• Excited electron is passed from the electron
  acceptor of PSII to reaction of center of PSI via
  an ETC

Process
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The light reactions-Linear electron flow

• Excited electron is passed from the electron
  acceptor of PSII to reaction of center of PSI via
  an ETC
• The fall of the electron lowers the energy state,
  a proton gradient is produced and so is ATP

Process
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The light reactions-Linear electron flow

• Excited electron is passed from the electron
  acceptor of PSII to reaction of center of PSI via
  an ETC
• The fall of the electron lowers the energy state,
  a proton gradient is produced and so is ATP
• Electrons in PSI are excited, eventually the
  pigments in the reaction center are excited

Process
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The light reactions-Linear electron flow

• Excited electron is passed from the electron
  acceptor of PSII to reaction of center of PSI via
  an ETC
• The fall of the electron lowers the energy state,
  a proton gradient is produced and so is ATP
• Electrons in PSI are excited, eventually the
  pigments in the reaction center are excited
• Electrons are picked up by the primary electron
  acceptor from PSI (the void is filled by new
  electrons coming from PSII)

Process
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The light reactions-Linear electron flow

• Excited electron is passed from the electron
  acceptor of PSII to reaction of center of PSI via
  an ETC
• The fall of the electron lowers the energy state,
  a proton gradient is produced and so is ATP
• Electrons in PSI are excited, eventually the
  pigments in the reaction center are excited
• Electrons are picked up by the primary electron
  acceptor from PSI (the void is filled by new
  electrons coming from PSII)
• Electrons are passed down another ETC to NADP

Process
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The light reactions-Linear electron flow
Process
The bottom line Solar energy is used to make
ATP and NADPH
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The light reactions-Linear electron flow
Process
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The light reactions-Cyclic electron flow
Process

• Uses only PSI
• Electrons are excited flow to PSI primary
  acceptor, down through the cytochrome complex
• ATP is generated (NADPH is not)
• Electrons return to PSI

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The carbon Calvin cycle
Inputs
Outputs
Light reactions
O2
Light
H2O
ATP
ATP and NADPH
Carbohydrate
CO2

• Building carbs (glucose) from CO2
• This process uses ATP and NADPH produced in the
  energy trans reactions

Plant biomass
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The Calvin cycle

• Note that a 3-C sugar is produced
• It takes three turns of the Calivin cycle to make
  on 3-C sugar

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The Calvin cycle

• Phase 1
• Carbon fixation-one CO2 molecule is fixed at a
  time by Rubisco
• The product of the reaction is an unstable 6-C
  molecule
• It immediately splits to two 3-C molecules per CO2

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The Calvin cycle

• Phase 2
• Each 3-C molecule is phosphorylated and a pair
  of electrons are donated from NADPH
• One of the the 3-C molecule exits (it can then be
  converted to glucose)
• The remaining five stay in the cycle

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The Calvin cycle

• Phase 3
• Regeneration of the CO2 acceptor
• Skeletons of the five 3-C molecules are
  rearranged to three 5-C molecules
• Costs 3 ATP

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The Calvin cycle

• The Calvin cycle includes
• Carbon fixation
• Reduction
• Regeneration of the starting compound
• It costs 9 ATP and 6 NADPH to make one 3-C sugar

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Summary
Inputs
Outputs
O2
Light
H2O
ATP and NADPH
Calvin cycle
Carbohydrates
CO2
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Photorespiration

• In hot and dry climates, plants face a great
  challenge

Hunger
Thirst

• Close stomata to reduce water loss
• Reduces CO2 concentration in the leaf tissues
• Increases O2 concentration

• Open stomata to take in more CO2
• Water evaporates

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Photorespiration

• In hot and dry climates, plants face a great
  challenge

Hunger
Thirst

• Close stomata to reduce water loss
• Reduces CO2 concentration in the leaf tissues
• Increases O2 concentration

• Open stomata to take in more CO2
• Water evaporates

• This causes photorespiration
• Rubisco binds with O2
• In the process of breaking down the molecule
  formed, cell releases CO2 and uses ATP
• Relic of low O2 atmosphere

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Photorespiration

• In hot and dry climates, plants face a great
  challenge

Hunger
Thirst

• Close stomata to reduce water loss
• Reduces CO2 concentration in the leaf tissues
• Increases O2 concentration

• Open stomata to take in more CO2
• Water evaporates

Alternative pathways have evolved to reduce
photorespiration

• This causes photorespiration
• Rubisco binds with O2
• In the process of breaking down the molecule
  formed, cell releases CO2 and uses ATP
• Relic of low O2 atmosphere

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C 4 photosynthesis

• The carbon fixation and the Calvin are separated
  in space
• C4 plants have two types of photosynthetic cells
• bundle sheath cells
• Mesophyll cells

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C 4 photosynthesis

• Step 1 PEP carboxylase fixes CO2 in the
  mesophyll cells
• Step 2 4-C molecule is shipped to the bundle
  sheath cells
• Step 3 4-C molecules releases CO2 which is fixed
  by Rubisco and the Calvin cycle begins
• Requires ATP to regenerated PEP carboxylase,
  but there is still an advantage in dry, hot
  climates

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CAM Photosynthesis

• CAM plants open their stomata at night and close
  them during the day
• Take in CO2 at night, incorporate it into an acid
  (stored in the vacuole until day)
• During the day the CO2 is released and the Calvin
  cycle begins

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Photosynthesis transforms light energy to
chemical energy
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You should understand

• Ecological and social contexts
• How scientists came to understand photosynthesis
• The basic inputs and outputs of the two main
  process involved in photosynthesis
• Some of the details of the light reactions and
  the Calvin cycle
• Alternative methods of photosynthesis

63
Life is woven out of air by light-Jacob
Moleschott