Photosynthesis

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Photosynthesis

• Historical background
• Overview of the reaction
• Leaf anatomy
• Chloroplast anatomy
• Details of the light reactions
• Details of the dark reactions
• Variations of photosynthesis
• Factors affecting the rate
• Cyclic photophosphorylation
• Summary of energy flow

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Photosynthesis- historical background

• Historical background
• Johan Baptist van Helmont 1648
• Discovered that mass of plant during growth did
  NOT come from the soil
• Willow grown in pot for 5 years plant added 75
  kg but only 57 grams of soil less.
• Joseph Priestley 1772
• Discovered that plants restored bad air
  produced by a burning candle or a living mouse

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Photosynthesis- historical background

• Antoin-Laurent Lavoiser 1772
• Identified the component of Joseph Priestleys
  experiments and gave it the name oxygen. He
  also thought of air as a mixture of different
  gases
• Jan Ingenhousz Jean Senebier 1779
• Determined that plants need light in order to
  refresh bad air

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Photosynthesis- historical background

• N. Theodore de Saussure 1804
• Plants take in CO2 from the air
• Plants take in nitrogen compounds from the soil
• T. Engelmann 1880s
• Elegant experiment to show that photosynthesis
  worked better with red/ orange and blue/indigo
  wavelengths of light

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Photosynthesis- overview of the
reaction

• Photo synthesis
• Using light to build molecules (anabolic rxns)
• Basic reaction
• 6CO2 12H2O ? C6H12O6 6O2 6H2O
• Broken down into 3 different sections
• Capture of light energy from the sun
• Using the energy to form ATP and NADPH
• Using ATP and NADPH to produce organic
  energy-rich compounds
• Why is there 12 H2O and 6 H2O ?????

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Photosynthesis- anatomy of the leaf

• Typical leaf structure
• Cuticle
• Upper Epidermis
• Palisade layer
• Spongy layer MESOPHYLL layer
• Stomata w/ guard cells
• Lower Epidermis
• Cuticle

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Photosynthesis- anatomy of the leaf
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Photosynthesis- anatomy of the leaf

• Only layers that have chloroplasts are the
  mesophyll cells in the palisade and spongy layers
• Cuticle is to restrict water loss
• Epidermis is for protection
• Stomata are controlled by guard cells and
  regulate the gas exchange with the ambient air

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Photosynthesis- chloroplast anatomy

• Chloroplast structure
• Chloroplasts contain the entire mechanism to run
  all of the photosynthetic reactions.
• Double-membrane bound organelles inside
  photosynthetic eukaryotic cells
• Inside are hollow pancake-like structures made of
  thylakoid membranes
• Pancakes exist in stacks called grana/granum

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Photosynthesis- chloroplast anatomy

• Semi-liquid material surrounding grana is called
  the stroma
• Attached to the thylakoid membranes are a series
  of photosynthetic pigments that are grouped
  together photosystem
• Pigments capture photons of light and transfer
  the energy within the photosystem to carrier
  proteins so ATP/NADPH can be formed.

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Photosynthesis- chloroplast anatomy
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Photosynthesis- details of the light
reactions

• Light exists as packets of energy called
  photons and has particle AND wave properties.
• The waves of light have different wavelengths
  depending on their color
• (long wavelengths red short violet)
• The shorter the wavelength the more energy the
  wave has (higher frequency)

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Photosynthesis- details of the light
reactions
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Photosynthesis- details of the light
reactions

• Light interacts with pigments by absorbing and
  reflecting certain wavelengths depending on the
  pigment
• Red pigments will reflect red, blue will reflect
  blue.
• The other wavelengths are absorbed more or less,
  depending on the material and shade of color.

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Photosynthesis- details of the light
reactions

• Chlorophyll a is a pigment found in all
  photosynthetic organisms. It is green.
• Chlorophyll b is a pigment found in some
  photosynthetic organisms too. It is green.

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Photosynthesis- details of the light
reactions
Chlorophyll a solid Chlorophyll b dotted
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Photosynthesis- details of the light
reactions

• Some accessory pigments exist in certain
  photosynthetic organisms too (i.e. xanthophylls,
  carotenoids, etc.)
• What would be the advantage of having these
  pigments as well as chlorophyll a and b?

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Photosynthesis- details of the light
reactions
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Photosynthesis- details of the light
reactions
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Photosynthesis- details of the light
reactions
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Photosynthesis- details of the light
reactions

• Photoelectric effect-
• Light strikes metal atoms and causes them to lose
  an e- photoelectric effect.
• This happens with chlorophyll. A Mg atom at the
  center of chlorophyll accept the energy from
  light and loses an electron. This electron is
  going to be used in an Electron Transport system
  to release energy.
• The Mg atoms lost e- is returned by the one from
  water. H2O ? e- H O2

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Photosynthesis- details of the light
reactions

• Once electron is lost, it is transported through
  electron carriers while going through enzyme
  complexes that pump H ions across membrane into
  the thylakoid space.
• As these H need to cross back over they are
  aided by ATP synthase to run the rxn
• ATP synthase
• ADP Pi energy ? ATP

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Photosynthesis- details of the light
reactions
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Photosynthesis- details of the light
reactions
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Photosynthesis- details of the light
reactions
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Photosynthesis- details of the light
reactions

• Summary of Light-Dependent reactions
• Light photons strike pigment molecules of
  photosystem II and transfer energy to rxn ctr
• Electron is lost from rxn ctr (chl. a) to
  electron transport system
• Electron transport system releases energy to pump
  H to cavity of thylakoids for chemiosmosis of
  ATP
• 
  ATP synthase
• ADP Pi energy ? ATP

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Photosynthesis- details of the light
reactions

• Summary of Light rxns (cont.)
• Electron lost from photosystem II is replaced by
  H2O (in most plants) releasing H and O2
• Electron from E.T. system is passed to
  photosystem I and light energy is used to boost
  its energy further
• Electron enters another E.T. system to release
  energy to drive reaction
• NADP reductase
• NADP H ? NADPH

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Photosynthesis- details of the dark
reactions

• Light-Independent reactions (Calvin-Benson Cycle)
• This cycle takes CO2 through a cycle of reactions
  in order to produce glucose and other sugars
• 3 CO2 molecules combine with three ribulose 1,5
  biphosphate molecules RuBP to form 3 unstable
  6-carbon molecules. This is the most important
  reaction in the light independent reactions
• Called carbon fixation
• Uses rubisco, the most famous and abundant
  enzyme in the world.

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Photosynthesis- details of the dark
reactions
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Photosynthesis- details of the dark
reactions

• The 3-6C molecules split into six 3 C molecules,
  which will be rearranged until they form
  glyceraldehyde 3-phosphate (G3P). These
  arrangements require the ATP and the NADPH
  produced in the light reactions
• Of these 6 G3P molecules that are formed, five
  are used to reconstruct RuBP.

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Photosynthesis- details of the dark
reactions

• The other G3P is thrown into the cytoplasm where
  it goes through a reversal of glycolysis to form
  glucose 6 phosphate and fructose 6 phosphate
• These two will eventually go through a chemical
  rxn to form sucrose (gl fr) which is the
  disaccharide that plants transport through their
  tissues (phloem)

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Photosynthesis- details of the dark
reactions

• Since the end-product is a 3 Carbon molecule,
  this is called C3 photosynthesis.

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Photosynthesis- details of the dark
reactions
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Photosynthesis- variations of
photosynthesis

• Photorespiration
• Rubisco can catalyze two different reactions
• Whether it catalyzes one reaction or the other,
  depends on the quantity of O2 / CO2
• If CO2 level higher than O2, then-
• RuBP CO2 ? PGA (phosphoglyceric acid)
• If O2 level higher than CO2, then-
• RuBP O2 ? PGA glycolate
• Glycolate enters mitochondria and is broken down
  to release CO2
• Why is photorespiration a problem?

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Photosynthesis- variations of
photosynthesis

• When photorespiration is likely
• Hot, dry conditions stomates must be shut
• C4 pathway
• Enzymes catalyze C fixation even if CO2 at low
  levels
• Stable compound formed is oxaloacetic acid
• (a 4 carbon compound) hence C4
• Examples include corn, sorghum, crabgrass,
  sugarcane, etc.

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Photosynthesis - C3 C4 contrasting anatomy
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Photosynthesis- variations of
photosynthesis

• CAM plants
• Crassulacean Acid Metabolism
• Crassulacea group of plants
• At night,
• CAM plants take in CO2 through their open stomata
  (they tend to have reduced numbers of them).
• The CO2 joins with PEP to form the 4-carbon
  oxaloacetic acid.
• This is converted to 4-carbon malic acid that
  accumulates during the night in the central
  vacuole of the cells.

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Photosynthesis- variations of
photosynthesis

• In the morning,
• the stomata close (thus conserving moisture as
  well as reducing the inward diffusion of oxygen).
  
• The accumulated malic acid leaves the vacuole and
  is broken down to release CO2.
• The CO2 is taken up into the Calvin (C3) cycle.
• These adaptations also enable their owners to
  thrive in conditions of
• high daytime temperatures
• intense sunlight
• low soil moisture.
• Some examples of CAM plants
• cacti
• pineapple

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Photosynthesis - factors affecting the rate
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Photosynthesis - factors affecting the rate
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Photosynthesis - cyclic photophosphorylation
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Photosynthesis - summary of energy flow
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Photosynthesis - C3 C4 contrasting anatomy
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Photosynthesis