PHOTOSYNTHESIS IB Biology HL E. McIntyre

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PHOTOSYNTHESISIB Biology HLE. McIntyre
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Simple Photosynthesis Overview

• Simplified Chemical summary
• 6CO2 6H2O energy (sun) ? C6H12O6 6O2

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Properties of Light

• Electromagnetic Radiation and the Visible Light
  Spectrum
• Englemans experiment showing which wavelength of
  visible light is best for photosynthesis

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Structure of a Leaf

• Look at the various cells in the cross section of
  the leaf. In which cells does photosynthesis take
  place?
• Take this test...

Palisade means to surround with a wall in
order to fortify
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Stoma

• This structure allows for the plant to exchange
  gasses with its environment. What gasses??

• Stoma
• Guard cells

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Chloroplast structure

• http//Animation Show first 20 sec for
  chloroplast anatomy (link 2)

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Micrograph of Chloroplast
Label your diagram!
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take a quiz!
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• http//indycc1.agri.huji.ac.il/zacha/chloroplast.
  jpg

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Photosynthesis An Overview of the Light and
Dark Reactions

• Occurs in Photoautotrophs (organisms that can
  make their own using energy from the sun).
• Photosynthesis takes place in the chloroplasts.
• Photosynthesis includes two processes

http//simple animation

• LIGHT REACTIONS
• Requires sunlight
• Occurs in the granna of chloroplasts
• Produces ATP and NADPH (used to power the Calvin
  cycle)

• DARK REACTIONS
• (a misnomeraka Calvin cycle)
• Doesnt require sunlight (happens 24/7).
• Occurs in the stroma of chloroplasts
• Produces PGAL (which can later be used to make
  glucose)

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Photosystems

• Photosystems are arrangements pigment-protein
  complexes. They contain chlorophyll and other
  accessory pigments packed into thylakoids.
• Many prokaryotes have only one photosystem,
  Photosystem I. Eukaryotes have Photosystem I plus
  Photosystem II.
• Photosystem I was the first to evolve.

• Photosystem I uses chlorophyll a, in the form
  referred to as P700. It absorbs light up to 700
  nm. Photosystem II uses a form of chlorophyll a
  known as P680. It absorbs light up to 680 nm.

Graphic http//kvhs.nbed.nb.ca/gallant/biology/ph
otosystem.jpg
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Photosystems

• The accessory pigments (chlorophyll b,
  carotenoids , and xanthophylls) play an indirect
  role in the formation of glucose through
  photosynthesis. These pigments provide
  chlorophyll a with the energy that they have
  captured from the sun. These pigments capture
  varying wavelengths of light and thus allow the
  plant to receive sun energy across a greater
  spectrum. Accessory pigments absorb energy that
  chlorophyll a does not absorb.
• Some carotenoids play a role in energy absorption
  rather than in photosynthesis. They absorb light
  to prevent damage to chlorophyll. The energy is
  lost as heat.
• Why do leaves of deciduous trees turn pretty
  colors in autumn?

Image http//www.thrivingnow.com/for/photos/image
_med/82/
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A Closer Look a Photosystems
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The Chlorophyll Molecule
How does the chlorophyll molecule stay in the
correct orientation when embedded in the
thylakoid membrane?
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Light Absorption by Various Pigments

• http//www.uic.edu/classes/bios/bios100/lecturesf0
  4am/lect10.htm

• Why do most photosynthetic organisms look green?

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more detail
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Phosphorylation

• Phosphorylation The chemical addition of a
  phosphate group (phosphorous and oxygen) to a
  compound. i.e. adding Pi to ADP to get ATP
• Photophosphorylation is addition of a phosphate
  using the suns energy!
• There are two types of photophosphorylation
  cyclic and non-cyclic.

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Cyclic Photophosphorylation

• Cyclic photophosphorylation probably occurs in
  plants when there is too little NADP available
  (more on this later).
• Cyclic photophosphorylation is also seen in
  certain photosynthetic bacteria. Note that the
  bacteria have no chloroplasts. All structures are
  embedded in the membrane. The proton gradient is
  created between the cell membrane and the
  capsule.

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Cyclic Photophosphorylation

• A single photosystem is involved.
• A photon of light strikes a pigment molecule in
  the P700 antenna system.
• The energy eventually reaches a molecule of P700
  (specialized chlorophyll a - the reaction
  centre). This electron is ejected from the
  photosystem.
• The energized electron leaves P700 and is passed
  to an acceptor molecule Ferrodoxin (fd).
• The electron is then passed through the
  cytochrome b6f complex. This complex pumps
  protons (H) into the space between bacteriums
  cell membrane and capsule (or in the case of
  plants, inside the thylakoid). This creates a
  proton gradient.
• Protons can only cross back through the membrane
  via ATP synthase. ATP synthase uses the energy
  flow of protons (proton motive force) to make ATP
  (Phosphorylaion).

Animation 1 Development of Proton Motive Force
(proton gradient) via chemiosmosis
Animation 3 ATP synthase
Animation 2 Formation of ATP from Proton motive
force
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Cyclic Photophosphorylation

• The electron is then passed through the
  plastocyanin (pC).
• The electron is passed back to the reaction
  centre.
• The electrons energy is gradually lost during
  this process.
• The de-energized electron returns to the
  chlorophyll a molecule to be energized again.
• We call this process cyclic photophosphorylation
  because electrons return to the photosystem and
  are then again energized. The process is a cycle!
• The energy released during this electron
  transport generates a proton gradient which is
  used to produce ATP.
• Animation (non) cyclic photophosphorylation
  animation

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Light Reactions and Non-Cyclic Photophosphorylatio
n
Non-cyclic photophosphorylation
Hmmmm Try to interpret this diagram in laymens
terms.
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Light Reactions and Non-Cyclic
Photophosphorylation

• Happens in PLANTS. Two photosystems are involved.
• A photon hits Photosystem II (PS II or P680).
  This energy is relayed to the reaction centre via
  accessory pigments. A high energy electron is
  emitted.
• meanwhile, an enzyme in PS II (enzyme Z) splits
  water. The oxygen is released as a byproduct.
  Electrons from water are used to replace those
  lost by PS II.

• The electron excited in PS II then travels to
  plastoquinone (Q), then to the b6f complex
  (proton pump).

Proton pump
Fd
PC
Q
NADP Reductase
Animation (non) cyclic photophosphorylation
animation
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Light Reactions and Non-Cyclic
Photophosphorylation

• The proton pump uses this energy to pump protons
  across the thylakoid membrane, from the stroma
  into the thylakoid space. These protons can only
  exit the thylakoid via ATP synthase. The flow of
  protons (proton motive force) through ATP
  synthase is used to make ATP. ATP production in
  this manner is called Chemiosmosis.

Proton pump
Fd
PC
Q
NADP Reductase
Animation (non) cyclic photophosphorylation
animation
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..Non-Cyclic Photophosphorylation

• The electron then goes to plastocyanin (PC) and
  then to PS I.
• Remember, the electron has lost energy
  becausethe proton pump used it up! Its now
  de-energized!
• A photon hits PS I (P 700). Energy is passed
  from accessory pigments to reaction centre which
  ejects a high energy electron.

• The de-energized electron replaces the electron
  lost from PS I.

Proton pump
Fd
PC
Q
NADP Reductase
Animation (non) cyclic photophosphorylation
animation
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Non-Cyclic Photophosphorylation

• The electron is then passed to ferrodoxin (Fd)
  and then to NADP reductase, which uses the newly
  energized electron to reduce NADP to NADPH.
• The ATP and NADPH produced during non-cyclic
  photophosphorylation go to the Calvin cycle to
  provide energy and raw materials to make SUGAR!

Proton pump
Fd
PC
Q
NADP Reductase
Animation (non) cyclic photophosphorylation
animation
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NON-cyclic photo-phosphorylation
Non-cyclic photophosphorylation
Does this make sense now?
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Watch the animation, then answer this
questionWhere do the protons come from that go
through ATP synthase?
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• Cyclic vs. non-cyclic photophosphorylation in
  plants.

• Cyclic photophosphorylation occurs less commonly
  in plants than noncyclic photophosphorylation
  does. Examine the two diagrams below. What are
  the similarities and differences?

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Examine the formula that summarizes
photosynthesis
sunlight

• CO2 H2O C6H12O6 O2

• You should know
• Where the O2 byproduct comes from
• Infer
• Where the carbon in glucose comes from
• Where the hydrogen in glucose comes from
• Where the oxygen in glucose comes from

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The Calvin Cycle

• In Photosynthesis, ATP and NADPH are produced in
  photophosphorylation, aka the Light Reactions.
  This happens in the thylakoid but notice that the
  products are actually produced in the stroma.
  This sets up the next series of reactions, the
  Calvin cycle which happens completely in the
  stroma. This is where sugars are manufactured.
  Melvin Calvin discovered this cycle in 1940.

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The Calvin Cycle

• The end product of photosysnthesis isnt really
  glucose its PGAL (phosphoglyceraldehyde). PGAL
  (AKA G3P) can be used to manufacture glucose, or
  other sugars, fatty acids or amino acids.
• The Calvin Cycle has three phases
• 1st phase Carbon Fixation
• 2nd phase Reduction
• 3rd phase Regeneration of the Carbon acceptor
  molecule (RuBP)

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1st Phase Carbon Fixation 1. Three five-carbon
sugar molecules called ribulose bisphosphate, or
RuBP, are the acceptors that bind 3 CO2
molecules (dissolved in the stroma). This
reaction is catalyzed by the enzyme rubisco. 2.
Three unstable 6-C molecules are produced (not
shown) which quickly break down to give six
molecules of the three-carbon phosphoglyceric
acid (PGA).
The Calvin Cycle
3 x CO2
1
2
6 x PGA (3-C)
3 x RuBP (5-C)
Rubisco
Phosphate carbon
Animation Calvin cycle
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2nd Phase Reduction 3. The six PGA molecules
are phosphorylated to six 1,3 BPG (1,3
bisphosphoglycerate) as each PGA accepts a high
energy P from ATP. 1,3 BPG is reduced to PGAL
(phosphoglyceraldehyde), a three-carbon sugar.
This phosphate bond is then broken and hydrogen
is added from NADPH. 4. Six molecules of PGAL are
produced. However, only one of the six molecules
exits the cycle as an output (to make sugar,
etc.) while...
The Calvin Cycle
3 x CO2
1
2
6 x PGA (3-C)
3 x RuBP (5-C)
6 x ATP
Rubisco
6 x ADP
3
6 x 1,3 BPG
6 x NADPH
6 x NADP
6 x Pi
6 x PGAL (3-C)
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Animation Calvin cycle
NOTE PGAL is also referred to as G3P
1 x PGAL (3-C)
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• 3rd Phase Regeneration of the Carbon acceptor
  molecule (RuBP)
• 5. ...the remaining five enter a complex process
  that regenerates more RuBP to continue the
  cycle....
• 6. In this process, ATP is used to convert the
  five PGALs to three RuBPs.
• 7. Summary...
• 9 ATP used
• 6 NADPH used
• 1 PGAL produced
• RuBP regenerated

The Calvin Cycle
3 x CO2
1
2
6 x PGA (3-C)
3 x RuBP (5-C)
6 x ATP
Rubisco
6 x ADP
3
3 x ADP
3 x ATP
6 x 1,3 BPG
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6 x NADPH
6 x NADP
6 x Pi
5 x PGAL (3 C)
6 x PGAL (3-C)
5
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Animation Calvin cycle
1 x PGAL (3-C)
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Photosynthetic Rate

• Photosynthetic rate is often measured as the rate
  of CO2 absorption per unit area of the leaf.
  (mmolCO2/m2/s)

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How does Irradiance Affect Rate of Photosynthesis?

• Light-compensation point the point on a
  light-response curve at which
• photosynthetic CO2 uptake respiratory CO2
  evolution
• Light saturation point the irradiance level at
  which the carbon fixation levels reach a maximum
  rate.

• http//www.marietta.edu/spilatrs/biol103/photolab
  /compexpl.html

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How does Irradiance affects Rate of
Photosynthesis?

• How does irradiance initially affect rate of CO2
  uptake?
• As irradiance increases, CO2 uptake increases in
  a linear fashion.
• Describe CO2 absorption in absence of light.
  Explain.
• It is negative. Plant PRODUCES CO2 due to cell
  respiration.
• What is the significance of the light saturation
  point?

• What is the significance of the light saturation
  point?
• the maximum irradiance that can be used by the
  plant. Not enough enzymes to take advantage of
  increased light intensities.
• Explain the significance of the flat portion of
  the curve.

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How Temperature affects Rate of Photosynthesis

• Temperature affects enzyme efficacy. Enzymes will
  work within an optimal temperature range. They
  can become denatured if the temperature is
  outside this range.
• How does temperature affect photosynthetic rate?
  Explain.

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Interpret the graph!
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Overview of light dependent reactions