Review of Act 1of photosynthesis: the light reactions

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Review of Act 1of photosynthesis the light
reactions

• ____ energy is converted to ____ energy
• Light chemical
• What are the products?
• NADPH, ATP (both used in the Calvin cycle)
  oxygen
• Why not stop after light reactions?
• NADPH ATP arent stable energy can not be
  stored or transported in these forms
• carbon compounds are needed for organisms to grow
  

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Act 2 of photosynthesisCalvin Cycle

• General equation for photosynthesis
• 6CO2 6H2O Sunlight ? C6H12O6 6O2
• Note This is for the light reactions and the
  Calvin cycle combined
• The mission
• Store chemical energy in the form of 3-carbon
  sugars made from CO2
• http//www.khanacademy.org/video/photosynthesis---
  calvin-cycle?playlistBiology

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Calvin Cycle, a.ka. dark reactions

• light independent reactions
• The Calvin cycle does not operate at night
• requires ATP and NADPH, which are produced in
  the light reactions
• several of the Calvin cycle enzymes are
  activated by light
• little CO2 is available when the stomates are
  closed at night.

http//simple.wikipedia.org/wiki/Stomates
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The Players

• RuBP (Ribulose bisphosphate) - a 5-carbon sugar
• CO2 - Carbon Dioxide
• Rubisco (Ribulose bisphosphate carboxylase-oxygena
  se) - an enzyme that allows CO2 to attach to RuBP
• PGA- (3-Phosphoglyceric acid)- 3-carbon
  carboxylic acid
• ATP (Adenosine triphosphate) - the main energy
  source
• DPG (1,3-Disphosphoglycerate)- 3-carbon molecule
  that has a phosphate group on both ends
• NADPH- (Nicotinamide adenine dinucleotide
  phosphate) product of light reaction
• PGAL OR G3P- (Glyceraldehyde-3-phosphate)- the
  simplest sugar known, food for plants

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The Scene

• These reactions occur in the stroma of
  chloroplasts

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Scene 1 Carbon Fixation
http//simple.wikipedia.org/wiki/Stomates

• CO2 comes into the stroma of the chloroplast via
  the stomata of the leaves. 
• Rubisco catalyzes the bonding of CO2 to RuBP to
  create an unstable 6-carbon molecule that
  instantly splits into two 3-carbon molecules of
  PGA
• CO2 RuBP -------gt unstable molecule-------gt
  2 PGA

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Scene 2 Reduction

• ATP phosphorylates (adds a phosphate group to)
  each PGA molecule and creates DPG.
• This in turn results in the loss of the terminal
  phosphate group from ATP thus making ADP
• NADPH reduces DPG which causes the phosphate
  group to break off once again.  The molecule then
  picks up a proton (H) from the medium to become
  PGAL. 
• NADPH is oxidized (loses an electron) by this
  process and becomes NADP.

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Scene 3 Regeneration

• For every six molecules of PGAL created, five
  molecules continue on to phase 3 which leave one
  to be used to make glucose. 
• ATP is once again needed.  However, this time it
  phosphorylates PGAL to regenerate RuBP after some
  rearrangement.
• http//www.science.smith.edu/departments/Biology/B
  io231/calvin.html

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Accounting

• For every 6 CO2s IN, there is 1 molecule of
  glucose OUT.
• 2 PGAL (3C) molecules combine and leave the
  Calvin Cycle, where they are linked to form
  glucose
• 10 PGAL (3C) molecules are rearranged to form
  RuBP
• For every glucose made, 18 ATP and 12 NADPH are
  used.

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Why bother with all this work??!

• Photosynthesis is very "costly" to the cell,
  requiring a lot of energy from the sun as well as
  a cast of molecules that make the needed energy
  and rearrange the chemical bonds needed to make
  sugar.
• The payoff - when sugar is broken down, it yields
  even more chemical energy needed to do all other
  cellular reactions in cells - growth,
  reproduction, metabolism.... This is the subject
  of the next chapter, Cellular Respiration.

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What affects photosynthesis rates?

• Light intensity
• Temperature
• CO2 concentration

www.ksphotography.com.au/
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How does light intensity affect the rate of
photosynthesis?

• The rate of photosynthesis levels off. The light
  reactions are saturated.
• Photoinhibition can occur when plants are exposed
  to full light where some of the extra energy gets
  passed to oxygen molecules and hydrogen peroxide
  forms which damages chloroplasts.

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How does temperature affect the rate of
photosynthesis?

• The rate of the photosynthesis reaction speeds up
  as higher temperatures provide more energy,
  however if the temperature gets too high, the
  proteins become denatured.

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How does carbon dioxide concentration affect the
rate of photosynthesis?

• An increase in CO2 increases the rate of
  photosynthesis to a maximum point, after which
  the rate levels off.
• What effect will climate change have???

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Whats photorespiration?

• O2 can have an inhibitory effect upon
  photosynthesis
• In the presence of elevated O2 levels,
  photosynthesis rates are lower due to competition
  between O2 and CO2 on the Rubisco enzyme

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Photorespiration cont.

• Recall, the "normal" reaction to start the Calvin
  cycle has CO2 joined with RUBP to form 2
  molecules of 3PGA.
• In the process called photorespiration, O2
  replaces CO2 in a non-productive, wasteful
  reaction
• It is believed that photorespiration in plants
  has increased over geologic time and is the
  result of increasing levels of O2 in the
  atmosphere--the byproduct of photosynthetic
  organisms themselves.
• The appearance of C4-type plants appears to be an
  evolutionary mechanism by which photorespiration
  is suppressed.

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C3vs. C4 vs. CAM

• C3 Photosynthesis C3 plants
• Called C3 because the CO2 is first incorporated
  into a 3-carbon compound.
• Stomata are open during the day.
• Uses RUBISCO to fix CO2.
• ?Photosynthesis takes place throughout the
  interior of the leaf.
• Adaptive Value more efficient than C4 and CAM
  plants under cool and moist conditions and under
  normal light because it requires less machinery
  (fewer enzymes and no specialized anatomy).
• 95 of all plants are C3.

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C4 Photosynthesis C4 plants

• Called C4 because the CO2 is first incorporated
  into a 4-carbon compound.
• Stomata open close during the day.
• Uses PEP Carboxylase in the uptake of CO2, then
  it "delivers" the CO2 directly to RUBISCO
• Photosynthesis takes place in inner cells of leaf
  (called the bundle sheath)
• Adaptive Value
• Photosynthesizes faster than C3 plants under high
  light intensity and high temperatures because the
  CO2 is delivered directly to RUBISCO, not
  allowing it to grab oxygen and undergo
  photorespiration.
• Is more efficient because PEP Carboxylase brings
  in CO2 faster and so does not need to keep
  stomata open as much for the same amount of CO2
  gain for photosynthesis.
• Examples sugarcane, corn, and many of our summer
  annual plants.

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CAM Photosynthesis CAM Plants(Crassulacean
Acid Metabolism)

• ? The CO2 is stored in the form of an acid before
  use in the Calvin Cycle.
• ? Stomata open only at night (when evaporation
  rates are usually lower).
• ? CO2 is converted to an acid and stored during
  the night. During the day, the acid is broken
  down and the CO2 is released to RUBISCO for
  photosynthesis
• ? Adaptive Value
• More efficient with water than C3 plants due to
  opening stomata at night (no sunlight, lower
  temperatures, lower wind speeds, etc.).
• When conditions are extremely arid, CAM plants
  can just leave their stomata closed night and
  day.
• Examples succulents such as cactuses and agaves
  and also some orchids and bromeliads

www.livefortheoutdoors.com/