Photosynthesis

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Photosynthesis

• Chapter 10 AP Biology

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Some Basic Terms to Know

• These terms refer to plants and other organisms
  and their places in the biosphere
• Autotrophs and Heterotrophs
• Photoautotrophs plants
• Producers and Consumers
• Decomposers

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Remember

• Plants are NOT the ONLY photoautotrophs
• Certain bacteria
• Certain protists (algae)

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Chloroplasts

• Where photosynthesis happens
• Chloroplasts contain Chlorophyll
• The photosynthetic pigment
• Chloroplasts are found primarily in LEAVES
• Primarily in the MESOPHYLL (middle) of the leaf

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Chloroplast Structure

• Double Membrane
• Stacks of thylakoids
• Stack granum (pl. grana)
• Thylakoids are where the chlorphyll is found
• Fluid surrounding thylakoids stroma
• Click HERE for an animation showing chloroplast
  structure

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Chlorophyll

• Found embedded in the membranes of the thylakoids
• Absorbs light energy which is then used by the
  plant to make sugar

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The Photosynthetic Equation

• 6CO2 6H2O Light Energy ? C6H12O6 6O2
• Carbon Dioxide Water Light Energy ? Glucose
  Oxygen

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The Photosynthetic Reactions

• It takes two sets of chemical reactions (two
  biochemical pathways) to make a sugar molecule
• The two sets of reactions are called
• Light Reactions and Dark Reactions OR
• Light Dependent Reactions and Light Independent
  Reactions OR
• Light Reactions and Calvin Cycle

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Light Reactions vs Dark Reactions

• Fundamentally, Light Reactions MUST have light in
  order to proceed
• Dark reactions can occur with or without light
  being present, HOWEVER
• Dark reactions ARE dependent on the PRODUCTS of
  the LIGHT reactions in order to occur.

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Basics about Light and Chlorophyll

• Behaves both as a wave and a particle
• When light is behaving as a particle a photon
• This is a fixed quantity of energy
• Chlorophyll is best at absorbing red and blue
  light in the visible light portion of the
  electromagnetic spectrum
• Chlorophyll does NOT absorb green light
  REFLECTS it. Hence, leaves appear green.

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Types of Chlorophyll and other Pigments

• Chlorophyll a
• Only pigment that DIRECTLY participates in the
  light reactions
• Blue green in color
• Accessory pigments
• Absorb light and transfer its energy TO
  chlorophyll a
• Chlorophyll b
• Yellow green in color
• Carotenoids
• Yellow/orange in color
• Also serve in PHOTOPROTECTION
• Absorb and dissipate excessive light energy that
  might damage chloropyll

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Molecules and absorption of light energy

• When a molecule absorbs a photon, one of the
  molecules electrons is elevated to an orbital
  where it has MORE potential energy.
• Normal orbital ground state Higher orbital
  excited state
• Particular compounds absorb photons at very
  particular wavelengths
• Chlorophyll absorbs photons at wavelengths of
  680 nm and 700 nm
• Once a molecules electron is excited to a higher
  energy level, it will normally fall back down to
  its original level unless something catches the
  electron and holds it at the elevated state.

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Absorption of Light Energy by Chlorophyll a -
Diagram
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The Light Reactions Begin With Photosystems

• Photosystems
• Complexes containing chlorophyll, proteins and
  other pigments and organic molecules
• Embedded in thylakoid membrane
• Photosystems contain a special region called the
  antenna complex
• Light gathering structure
• Cluster of chlorophyll a and other pigment
  molecules
• Acts like a catchers mitt to catch photons
  from the sun
• Energy from these photons is bounced around in
  the antenna complex until it lands on a
  particular chlorphyll a molecule

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Photosystem Antenna Complex Diagram
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Antenna Complex The Reaction Center

• The particular chlorophyll a molecule upon which
  lights energy ultimately lands is found in a
  particular region of the antenna complex called
  the reaction center.
• Also in the reaction center is a compound called
  the primary electron acceptor

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Excited Electrons and the Primary Electron
Acceptor

• Energy landing on the chlorophyll a in the
  reaction center elevates the electrons of this
  chlorophyll a to a higher energy level.
• Before these electrons can lose this energy and
  return to their original energy level, the
  electrons are caught by the primary electron
  acceptor which keeps them at this elevated state
• POTENTIAL ENERGY provided by LIGHT is now STORED
  in CHEMICAL FORM.

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Photosystem Antenna Complex Diagram Reaction
Center
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The LIGHT REACTIONS Step by Step

• There are TWO possible paths electrons can take
  in the light reactions
• Noncyclic electron flow
• The predominant path
• The one we will focus on NOW
• Cyclic electron flow
• Occurs less often
• Well focus on this one LATER

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The LIGHT REACTIONS Step by Step

• AlsoRemember that electrons are going to be
  MOVING in these reactions.
• Just like in cell respiration, we depend on REDOX
  (oxidation-reduction) reactions to pass electrons
  from one compound to another
• As electrons move from one compound to the next
  they are moving from a compound that is less
  electronegative to one that is more
  electronegative

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Light Reactions Non Cyclic Electron Flow Step
1

• Photon strikes antenna complex of Photosystem II
  (P680)
• Energy is bounced on molecules in antenna complex
  until they land on chlorophyll a in the reaction
  center
• Electrons of reaction center chlorophyll a are
  excited to a higher energy level and CAUGHT by
  the primary electron acceptor

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Light Reactions Non Cyclic Electron Flow Step
2

• An electron hole is left behind in the
  chlorophyll a in the reaction center of
  Photosystem II
• An enzyme extracts electrons from water found in
  the chloroplast (stroma) and supplies these
  electrons to the chlorophyll a molecule so that
  it can work again.
• This enzyme splits a water molecule into TWO H
  ions and an oxygen atom that will combine with
  another oxygen atom to form O2.
• THIS IS WHY WATER IS NEED FOR PHOTOSYNTHESIS
• THIS IS WHERE THE OXYGEN GAS COMES FROM IN
  PHOTOSYNTHESIS

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Light Reactions Non Cyclic Electron Flow Step
3

• Electrons pass from the primary electron acceptor
  of Photosystem II to Photosystem I by way of an
  electron transport chain
• This is VERY much like the ETC found in the
  mitochondrion
• As electrons fall down the chain their energy is
  used to PUMP PROTONS from the stroma INTO the
  thylakoid space
• This creates a PROTON GRADIENT
• HIGH inside the thylakoid LOW outside in the
  stroma
• ATP is made as protons flow WITH their
  concentration gradient THROUGH a molecule of ATP
  synthase
• ATP is created as ADP and Pi are combined at ATP
  Synthase

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Light Reactions Non Cyclic Electron Flow Step
3, cont.

• Remember ATP synthesis using a proton gradient
  and ATP synthase is called CHEMIOSMOSIS
• Using Light Energy to drive this type of ATP
  synthesis is called PHOTOPHOSPHORYLATION
• Remember substrate level phosphorylation and
  oxidative phosphorlyation? This is the same type
  of thing.
• The ATP generated by this method will be sent on
  to provide energy to the DARK REACTIONS to
  provide energy for making glucose.

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Light Reactions Non Cyclic Electron Flow Step
4

• When electrons reach the bottom of the ETC, they
  are passed to chlorophyll a of the reaction
  center of photosystem I (P700)
• Here, these electrons fill an electron hole left
  in this reaction center when electrons of this
  reaction centers chlorophyll a are excited by
  their own photon of light.
• Just as in Photosystem II, the electrons here are
  also caught and passed on by the primary electron
  acceptor of photosystem I. This, again, leaves an
  electron hole to fill.

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Light Reactions Non Cyclic Electron Flow Step
5

• Electrons from Photosystem I are passed from its
  primary electron acceptor to a compound called
  NADP Reductase
• This enzyme passes electron to a coenzyme called
  NADP
• NADP will become NADPH when it picks up the
  electrons. (reduced form of NADP)
• NADPH takes the electrons (and their ENERGY!) to
  the Calvin Cycle (Dark Reactions) where this
  energy will be used to make sugar.

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Light Reactions BOTTOM LINE

• The light reactions use LIGHT power to generate
  ATP and NADPH
• ATP and NADPH provide CHEMICAL ENERGYand REDUCING
  POWER to the GLUCOSE-MAKING reactions of the
  CALVIN CYCLE.
• WITHOUT THE CHEMICAL ENERGY (ATP/NADPH) PROVIDED
  BY the LIGHT REACTIONS, the DARK REACTIONS CANT
  OCCUR.

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Light Reactions and CYCLIC Electron Flow

• The flow of electrons in noncyclic electron flow
  of the light reactions is basically a straight
  line from point A (photosystem II) to NADP
  (NADPH).
• However, flow of the electrons in this way does
  NOT produce enough ATP to fuel the needs of the
  dark reactions
• THIS IS WHERE CYCLIC FLOW COMES IN.

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Cyclic Electron Flow

• In cyclic electron flow, electrons do not pass
  through Photosystem I and go on to NADP
  reductase.
• Instead, electrons continually loop (or cycle)
  backwards to go through the electron transport
  chain again and again.
• This results in extra protons being pumped and
  more ATP being made via chemiosmosis.
• This makes sure that adequate ATP is made for the
  dark reactions.

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Cyclic Electron Flow - Diagram
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The Dark Reactions OR The Calvin Cycle

• The Calvin Cycle uses the ATP and NADPH created
  in the LIGHT REACTIONS to convert CO2 to SUGAR
• The Calvin Cycle is the reason why photosynthesis
  requires the input of CARBON DIOXIDE.

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Calvin Cycle

• Carbon enters the Calvin Cycle in the form of CO2
• Carbon leaves the Calvin Cycle in the form of
  sugar
• Note the sugar made DIRECTLY by the carbon
  cycle is NOT glucose
• G3P is the sugar made by the Calvin Cycle
• The Calvin Cycle must use 3CO2s to make ONE G3P
  molecule
• THREE turns ONE G3P

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Three Phases of the Calvin Cycle

• PHASE I CARBON FIXATION
• In this phase, CO2 from the atmosphere is fixed
  or incorporated into an organic compound that the
  cycle can then convert into sugar.
• Think of the CO2 molecule as a little butterfly
  fluttering all over the place. The Calvin Cycle
  needs to grab this butterfly and nail it down
  so that it cant fly away. Only then can it use
  the CO2 to make sugar.

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Three Phases of the Calvin Cycle

• PHASE I CARBON FIXATION
• The molecule that acts as the hammer to nail down
  the CO2 butterfly is called RUBISCO
• RUBISCO is an ENZYME
• Rubisco nails CO2 to a compound called Ribulose
  Bisphosphate or RuBP for short.
• RuBP is a fairly large sugar molecule containing
  5 carbons
• Adding CO2 to RuBP results in the molecule being
  split and 2 molecules of something called
  3-phosphoglycerate being created. We really
  dont care much about the name of this compound
• What we care about is that now CO2 is in a form
  that the Calvin Cycle can work with to make G3P
  (sugar)

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Three Phases of the Calvin Cycle

• PHASE II REDUCTION
• In this phase, all that chemical energy that we
  transformed from Light in the Light Reactions is
  now USED to store energy in what will become the
  G3P (sugar) molecules
• Phosphate groups are transferred FROM ATP to the
  3-phophoglycerate molecules this stores energy
  in the molecules
• NADPH then add their electrons to the molecules,
  further storing energy in them, and making the
  desired G3P
• Remember, G3P is not Glucose, but is used by the
  plant to make Glucose.

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Three Phases of the Calvin Cycle

• PHASE III REGENERATION OF THE CO2 ACCEPTOR
• In other words, we need to get our RuBP back, so
  we can do the whole thing over again
• In the reduction phase, 6 G3P molecules were
  made.
• Only ONE of these is sent out of the Calvin Cycle
  to make Glucose
• The other 5 go back into the cycle to make RuBP
  so that more CO2 can be fixed and the cycle
  repeated.
• More ATP (obtained from the light reactions) is
  required to regenerate this RuBP from the G3P
  molecules

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BOTTOM LINE for Calvin Cycle

• For the Net synthesis of 1G3P molecule the Calvin
  Cycle consumes a total of
• 3 CO2 molecules
• 6 NADPH molecules
• 9 ATP molecules
• 6 for making G3P
• 3 for regeneration of RuBP from G3P
• CO2 comes from the atmosphere through the stomata
• NADPH and ATP come from the Light Reactions

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Problems with Photosynthesis

• The pathways of photosynthesis you have just
  learned are collectively called C3
• This is regular photosynthesis
• This method of photosynthesis works great EXCEPT
  it is not so great in times of drought.

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What happens in drought

• Remember CO2 enters the leaf through stomata
• Water vapor also exits leaves through stomata
  TRANSPIRATION
• On a hot dry day, plants will CLOSE their stomata
  (using GUARD CELLS) to prevent water loss
• Trouble is, they also close off the only means
  they have to take in CO2
• This means photosynthesis is slowed or stops in
  times of drought in order to prevent dehydration.

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Another problem with closing of stomata

• Not only can no CO2 get in, but Oxygen cannot
  leave
• Oxygen builds up at higher than normal
  concentrations in the plants tissues
• Rubisco is stupid
• It is NOT very good at telling CO2 from O2
• This means that if O2 is present in high
  concentrations in the plants tissues, Rubisco
  will start nailing or fixing O2 to RuBP.
• THIS IS CALLED PHOTORESPIRATION and it is BAD
  WASTEFUL
• Wastes Carbon compounds NOTHING useful is made
  AND Carbon compounds are lost from the cell.

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Some plants have evolved ways to overcome the
problems with drought

• C4 plants and CAM plants have evolved different
  mechanisms to deal with drought conditions
• Both differ from C3 plants in the way the Calvin
  Cycle is handled

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C4 Plants

• This alternative form of the Calvin Cycle depends
  on a different leaf anatomy from C3 plants
• Bundle Sheath Cells very tightly packed sheaths
  not found in C3 plants
• Isolate the Calvin Cycle from other tissues
• Located next to mesophyll cells, near vein of
  leaf

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C4 Plants

• Also involved is a new enzyme not found in C3
  Plants
• PEP Carboxylase
• NOT STUPID
• Can tell the difference between Oxygen and CO2
  very well
• Has a much stronger affinity for CO2 than rubisco
  does

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C4 Heres how it works

• CO2 enters the leaf as normal, but instead of
  meeting up with rubisco in the Calvin Cycle, it
  is caught by PEP Carboxylase and is THEN sent
  to the BUNDLE SHEATH CELLS
• The Bundle sheath cells are the HIGHLY PROTECTED
  home of the Calvin Cycle and its very silly
  rubisco enzyme

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C4 Heres how it works

• Since the Calvin cycle and rubisco are so well
  protected in the bundle sheath cells, they will
  NOT come in contact with oxygen no matter how
  much of it accumulates in the leaf
• Hence, the Calvin Cycle in C4 plants can avoid
  the hazard of photorespiration in drought because
  of
• PEP Carboxylase AND
• The physical separation of rubisco (and the
  Calvin Cycle) from the rest of the plant tissue
  that might contain an excess of Oxygen gas in
  drought conditions.

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CAM Plants

• CAM Plants avoid the perils of photorespiration
  by separating the Calvin Cycle from oxygen
  temporally (in time).

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CAM Plants How they work

• CAM Plants close stomata during the day when
  drought conditions are at their worst.
• CAM Plants open their stomata at night when water
  loss will be at a minimum
• This is the opposite of most plants.
• Most plants open stomata during the day because
  this is when energy is available to run the
  Calvin Cycle
• Because this is when LIGHT reactions are able to
  work.

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CAM Plants How they work

• CO2 enters the leaf at night
• Because there is no light, the plant cannot run
  the Calvin Cycle because the light reactions
  cannot make ATP or NADPH
• Howeverin a CAM Plant,
• CO2 is converted to a form known collectively as
  organic acids
• These organic acids are stored overnight and then
  the CO2 is extracted from them during the day
• During the day, when the light reactions can
  work, this CO2 can be used by the Calvin Cycle
  and sugar is made

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Photosynthesis Overview - Diagram