Chapter 3 - Photosynthesis: The Details

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Chapter 3 - PhotosynthesisThe Details
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Photosynthesis

• Using the sun to make useful forms of energy
• Sunlight plays a much larger role in our
  sustenance than we may expect all the food we
  eat and all the fossil fuel we use is a product
  of photosynthesis, which is the process that
  converts energy in sunlight to chemical forms of
  energy that can be used by biological systems

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How does this occur

• Various forms of radiation surround us, from the
  sun and other sources.
• Some are visible and some are invisible.

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Wave model of light

• Electromagnetic radiation travels at
  300000000m/s
• Frequencies of visible radiation (light) are
  perceived as different colours

We can remember the visible spectrum with
ROYGBIV!
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• Frequencies of visible radiation (light) are
  perceived as different colours.

• Highest frequency, smallest wavelength violet

• Lowest frequency, largest wavelength red

• All frequencies and wavelengths white

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Properties of Light

• http//wps.prenhall.com/esm_krogh_biology_3/0,8750
  ,1135943-,00.html
• http//www.sumanasinc.com/webcontent/animations/co
  ntent/harvestinglight.html

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Photon model of light

• Light travels in energy packets called photons
• Photons travel at 300000000m/s
• The amount of energy in a photon depends on the
  frequency of the light. The higher the frequency
  of light, the more energy the photon carries
• Light can transmitted reflected or absorbed
• Air, water .mirrorplants

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How Does a Plant Capture Light?

• Light can be
• transmitted (light passes through an object.

• Reflected (light bounces off object)

• Absorbed (light goes into object)

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How Does a Plant Capture Light?

• Plants have chlorophyll PIGMENTS (molecules that
  can absorb specific wavelengths of light)
• Plant leaves appear green. Therefore, what
  colours must the chlorophyll pigments absorb?
  reflect?

Everything but Green
GREEN
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Chlorophyll Pigments

• There are 2 types of cholorophyll
• Yellow - green
• Blue green
• Found in the highly folded plant organelle

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Chlorophyll Absorption

• Whatever chlorophyll a does not absorb,
  chlorophyll b will absorb they are good
  complements to each other.
• As you can see, the region between 500-600 nm is
  not absorbed very well, thus plants appear green.
• As the amount of chlorophyll in autumn decays,
  the green colour fades and is replaced with
  oranges and reds of carotenoids (other plant
  pigments).

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• Plants are the only photosynthetic organisms to
  have leaves (and not all plants have leaves). A
  leaf may be viewed as a solar collector crammed
  full of photosynthetic cells.

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Photosynthesis in plants

• Light energy is used to transform carbon dioxide
  and water to energy rich food molecules composed
  of glucose monomers
• There are 2 stages in this process

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Photosynthesis

• Divided into two steps

• The Light Reactions
• Noncyclic electron flow
• The Calvin Cycle
• Cyclic electron flow

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The Light Reactions

• Divided into three steps

• Photoexcitation
• Electron Transport Chain
• 3. Chemiosmosis

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The Light Reactions

• Photosystems are embedded in the thylakoid
  membrane.
• They contain chlorophyll and accessory pigments
  that are associated with proteins.
• A photosystem consists of an antenna complex and
  a reaction centre.

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The Light Reactions

• The antenna complex absorbs a photon and
  transfers energy to the reaction centre.
• The reaction centre contains chlorophyll a, whose
  electrons absorb energy and begin photosynthesis.

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The Light Reactions

• Photosystem II (P680)
• Two photons strike photosystem II and excite 2
  electrons from chlorophyll P680.
• The excited electrons are captured by a primary
  electron acceptor and are then transferred to
  plastoquinone (PQ) and the ETC.

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The Light Reactions

• Photosystem II (P680)
• In the ETC, the 2 electrons pass through a proton
  pump (Q cycle).
• The Q cycle transports 4 protons from the stroma
  into the thylakoid lumen to create a proton
  gradient.

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The Light Reactions

• Photosystem II (P680)
• The electrochemical gradient drives the
  photophosphorylation of ADP to ATP.
• 1 ATP forms for every 4 protons that pass through
  ATPase from the thylakoid lumen into the stroma.

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The Light Reactions

• Photosystem II (P680)
• A Z protein splits water into 2 protons, 2
  electrons and 1 oxygen atom.
• The electrons replace those lost from chlorophyll
  P680.
• The protons remain in the thylakoid space to add
  to the proton gradient.
• Oxygen leaves as a byproduct.

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The Light Reactions

• Photosystem I (P700)
• Two photons strike photosystem I and excite 2
  electrons from chlorophyll P700 (replaced by
  electrons from P680).
• These electrons pass through another ETC.
• The enzyme NADP reductase uses the 2 electrons
  and a proton from the stroma to reduce 1 NADP to
  1 NADPH.

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The Calvin Cycle

• Occurs in the stroma of chloroplasts.
• Cyclical reactions similar to the Krebs Cycle.
• Divided into three phases

1. Carbon Fixation
2. Reduction Reactions
3. Regeneration of RuBP

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The Calvin Cycle

• Phase 1 Carbon Fixation
• 3 CO2 are added to RuBP to form 3 unstable
  6-carbon intermediates.
• The intermediates split into six 3-carbon
  molecules called PGA.
• These reactions are catalyzed by rubisco.

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The Calvin Cycle

• Phase 2 Reduction Reactions
• 6 PGAs are phosphorylated by 6 ATPs to form 6
  molecules of 1, 3-BPG.
• 6 NADPH molecules reduce the six 1,3-BPG to 6 G3P
  or PGAL.
• One molecule of G3P exits the cycle as a final
  product.

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The Calvin Cycle

• Phase 3 Regeneration of RuBP
• 3 ATP are used to rearrange the remaining 5 G3P
  into 3 molecules of RuBP.
• The cycle continues with the RuBP fixing more
  CO2.

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To Produce One G3P

• 3 RuBP 3 CO2 9 ATP 6 NADPH 5 H2O ? 9 ADP
  8 Pi 6 NADP G3P 3 RuBP