Photosynthesis

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Photosynthesis
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How do plants grow?
Van Helmont - 1648
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Joseph Priestley
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Priestley 1771 plants restore good quality
to air
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Jan Ingenhousz 1796 plants only restore good
quality to air in the presence of light
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Water is source of oxygen released during
photosynthesis

• Van Niel was studying the activities of
  photosynthetic bacteria - he found that purple
  sulfur bacteria reduce carbon to carbohydrates
  but do not release oxygen instead the purple
  sulfur bacteria use hydrogen sulfide in their
  photosynthesis - so for them the reaction is as
  follows
• CO2 2H2S light energy gt(CH2O) H2O 2S
• Van Niel then generalized this to the following
  reaction for all photosynthetic activity
• CO2 2H2A light energygt(CH2O) H2O 2A

C.B. van Niel 1930s
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Photosynthesis has two separate reactions

• Experiments by F.F. Blackman in 1905 demonstrated
  that photosynthesis has two stages or steps - one
  is a light-dependent stage and the other is a
  light-independent stage - due to changes in the
  effectiveness of the light-independent stage with
  increases in temperature, Blackman concluded that
  this stage was controlled by enzymes

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The role of pigments

• A pigment is any substance that absorbs visible
  light - most absorb only certain wavelengths and
  reflect or transmit the wavelengths they don't
  absorb
• Chlorophyll absorbs light primarily in the
  violet, blue and red wavelengths and reflects
  green wavelengths, and thus appears green
• Absorption spectrum - the light absorption
  pattern of a pigment
• Action spectrum - the relative effectiveness of
  different wavelengths for a specific
  light-requiring process
• Chlorophyll is implicated as the principle
  pigment in photosynthesis because its absorption
  spectrum is the same as the action spectrum for
  photosynthesis

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The Photosynthetic Pigments

• Chlorophyll a - found in all photosynthetic
  eukaryotes and cyanobacteria - essential for
  photosynthesis in these organisms
• chlorophyll b - found in vascular plants,
  bryophytes, green algae and euglenoid algae - it
  is an accessory pigment - a pigment that serves
  to broaden the range of light that can be used in
  photosynthesis - the energy the accessory pigment
  absorbs is transmitted to chlorophyll a
• carotenoids - red, orange or yellow fat-soluble
  accessory pigments found in all chloroplasts and
  cyanobacteria - caroteniods are embedded in
  thylakoids as are chlorophylls - two types -
  carotenes and xanthophylls (xanthophylls have
  oxygen in their structure, carotenes don't)

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When pigments absorb light, electrons are
temporarily boosted to a higher energy level

• One of three things may happen to that energy
• 1. the energy may be dissipated as heat
• 2. the energy may be re-emitted almost instantly
  as light of a longer wavelength - this is called
  fluorescence
• 3. the energy may be captured by the formation
  of a chemical bond - as in photosynthesis

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Overview of Photosynthesis
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The Photosystems

• The chlorophylls and other pigments are embedded
  in thylakoids in discrete units called
  photosystems
• Each photosystem has 250 to 400 pigment molecules
  in two closely linked components - the reaction
  center-protein complex and the antenna protein
  complex
• All pigments in the photosystem are capable of
  absorbing photons of light, but only one pair of
  those in the reaction center-protein complex can
  actually use the energy in a photochemical
  reaction
• The other pigments in the antenna protein complex
  act like antenna to gather light and transfer
  that energy to the photochemically active pigments

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The Photosystems

• There are two different kinds of photosystems
• Photosystem I - has chlorophyll a - has an
  optimum absorption peak of 700 nanometers of
  light - the chlorophyll a is called P700 because
  of this activity
• Photosystem II - has special chlorophyll a active
  at 680 nanometers - the P680 chlorophyll a
• In general the two photosystems work together
  simultaneously and continuously - however,
  photosystem I can work independently

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Overview of Photosynthesis
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Calvin Cycle - details

• The Calvin cycle begins when CO2 enters the cycle
  and is joined to RuBP this forms a 6 carbon
  compound which immediately splits into two 3
  carbon compounds (the 6 carbon intermediate has
  never been isolated) - the 3 carbon compound is
  3-phosphoglycerate (PGA)
• Because each PGA has three carbons, this is
  sometimes also called the C3 pathway
• Each full turn of the Calvin cycle begins with
  entry of a CO2 molecule and ends when RuBP is
  regenerated - it takes 6 full turns of the Calvin
  cycle to generate a 6 carbon sugar such as
  glucose
• Although we usually report glucose as the product
  of photosynthesis, the cell usually produces
  either sucrose or starch as its storage products
• At night, sucrose is produced from the starch and
  it is transported from the chloroplast to the
  rest of the cell

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The full Calvin Cycle equation

• 6CO2 12NADPH 12H 18ATP gt
• C6H12O6 (GLUCOSE) 12NADP 18ADP 18 Pi
  6H2O

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The C4 Pathway

• In some plants the first carbon compound produced
  through the light-independent reactions is not
  the 3 carbon PGA, but rather is a 4 carbon
  molecule oxaloacetate - plants that use this
  pathway are called C4 plants
• Leaves of C4 plants typically have very orderly
  arrangement of mesophyll around a layer of bundle
  sheath cells

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Electron micrograph with C4 pathway shown
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Why use C4 pathway?

• A problem with C3 is that for all C3 plants,
  photosynthesis is always accompanied by
  photorespiration which consumes and releases CO2
  in the presence of light - it wastes carbon fixed
  by photosynthesis - up to 50 of carbon fixed in
  photosynthesis may be used in photorespiration in
  C3 plants as fixed carbon is reoxidized to CO2
• Photorespiration is nearly absent in C4 plants -
  so greatly increases their efficiency - this is
  because a high CO2 low O2 concentration limits
  photorespiration - C4 plants essentially pump CO2
  into bundle sheath cells (or the products of its
  reduction) thus maintaining high CO2
  concentration in cells where Calvin cycle will
  occur
• Thus net photosynthetic rates are higher for C4
  plants (corn, sorgham, sugarcane) than in C3
  relatives (wheat, rice, rye, oats)

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Why use C4 pathway?

• C4 plants evolved in tropics and are well adapted
  to life at high temperature, high light intensity
  and dry conditions - optimal temperature for C4
  photosynthesis is much higher than for C3 -
  efficient use of CO2 allows C4 plants to keep
  stomata closed longer and thus they lose less
  water during photosynthesis than do C3 plants
• C4 monocots do especially well at high
  temperature
• C4 dicots do especially well in dry conditions

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Crassulacean Acid Metabolism

• Crassulacean Acid Metabolism (CAM) has evolved
  independently in many plant families including
  the stoneworts (Crassulaceae) and cacti
  (Cactaceae)
• Plants which carry out CAM have ability to fix
  CO2 in the dark (night) via the activity of PEP
  carboxylase - malic acid (malate) so formed is
  stored in the cell's vacuole - during the light
  (day) the malic acid is decarboxylated and CO2 is
  transferred to RuBP in Calvin cycle within the
  same cell
• so CAM plants, like C4 plants, use both C4 and C3
  pathways, but CAM plants separate the cycles
  temporally and C4 plants separate them spatially

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Comparison of C4 and CAM pathways
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