Mini Quiz

1
Mini Quiz

• Diagram a summary of the light reactions
• Include the reactants and products and the
  location of each step.

2
Review

• Light Reactions
• At Thylakoid
• Uses H20, Light energy
• Produces NADPH, ATP and O2
• Calvin Cycle
• In Stroma
• Uses CO2, ATP and NADPH
• Produces PGAl/GP3, to be made into glucose

3
Calvin Cycle

• Uses ATP and NADPH to convert CO2 to sugars
• Phase 1 Carbon fixation CO2 from the air is
  incorporated into a 3 carbon sugar by an enzyme
  Rubisco
• Phase 2 Reduction NADPH and ATP reduce molecule
  to a 3 carbon sugar which can be made into
  glucose. (PGAL)

4
Calvin Cycle From CO2 to C6H12O6

• CO2 has very little stored energy (fully
  oxidized)
• C6H12O6 contains a lot of chemical energy
  (reduced)
• Reduction of CO2 to C6H12O6 proceeds in many
  small uphill (requiring energy) steps
• Each step is catalyzed by a specific enzyme
• Using energy stored in ATP and NADPH

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Very important enzyme
6 CARBONS
3 CARBONS x2
5 CARBONS
3 CARBONSPGAL or GP3 (used to make glucose)
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Calvin Cycle

• PGAL or GP3 (same compound)
• End product of the Calvin Cycle
• Energy rich sugar
• 3 carbon compound
• C3 photosynthesis
• PGAL can be used to make glucose, and is an
  intermediate in making other biologically
  important molecules (proteins, lipids etc.)

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Rubisco

• The enzyme that fixes carbon from the atmosphere
• Possibly the most important enzyme in the
  worldable to make organic molecules (required
  for life) out of air
• Definitely the most abundant enzyme

8
Photosynthesis summary

• Making organic molecules from light energy
• Carbon Water Energy ? Glucose Oxygen
• Dioxide
• CO2 H2O Light Energy ? C6H12O6 O2

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Photosynthesis Summary

• Light Reactions
• Produced ATP
• Produced NADPH
• Consumed H2O
• Produced O2 as a waste product
• Calvin Cycle
• Consumed CO2
• Produced PGAL
• Regenerated ADP
• Regenerated NADP

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CO2 H2O Light Energy ? C6H12O6 O2

• Where did the CO2 come from?
• Where did the CO2 go?
• Where did the H2O come from?
• Where did the H2O go?
• Where did the energy come from?
• Where did the O2 come from?
• Where will the O2 go?
• What else is involved that is not listed in this
  equation?

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Why is photosynthesis so important?Why
important for you?
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Review How do plants get CO2 for photosynthesis?

• Stomates are able to open and close
• When there is a lot of water, they are open!
• Less water closed

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Photosynthesis in different climates

• Plants have to balance taking in CO2 and giving
  off O2 with also losing water through open
  stomatestranspiration
• This is especially a problem in hot and arid
  climates
• When water levels are low, stomates close
• This decreases glucose production, Why?
• O2 builds up
• CO2 is depleted

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Photorespiration

• Rubisco can also bind to O2
• O2 is an alternative substrate
• Rubisco can add O2 to the Calvin cycle instead of
  CO2
• O2 will break down the first carbon compound in
  the Calvin Cycle and release Carbon dioxide, like
  respiration
• This is called photorespiration (light
  respiration)
• Oxidizes carbon compounds in Calvin Cycle
• Uses energy!
• Makes no ATP!
• Slows down photosynthesis!
• BAD!!

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Photorespiration

• What kind of plants have the most
  photorespiration occurring?
• In these plants, photorespiration can drain away
  as much as 50 of carbon fixed by the Calvin
  cycle.

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C4 Plants

• Alternative Modes for photosynthesis
• C4 plants incorporate CO2 into a four carbon
  compound BEFORE beginning the Calvin cycle.
• C4 plants have two phases, in two different cell
  types/different compartments.

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C4 plants

• Outer cells mesophyll cells
• Inner cells Bundle sheath cells

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C4 plants

• Outer
• New step happens here
• Light reactions and carbon fixation
• Pumps C4/CO2 compound to inner cells
• Keeps O2 away from inner cells
• Inner Same old Calvin Cycle

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C4 Plants

• PEP carboxylase enzyme
• Higher affinity for CO2 than O2 (better than
  Rubisco)
• Fixes CO2 into a four carbon compound
• Sends four carbon compound to bundle sheath cells
• Rubisco can now work efficiently on regenerated
  CO2 in these cells

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CAM plants

• Different adaptation to hot, dry climates
• Succulents, some cacti, pineapple
• Close stomates during the day
• Open stomates at night
• At night, carbon is fixed in storage compounds
• Organic acids malic acid, isocitric acid (C4)
• In day, release CO2 from storage compounds to
  the Calvin Cycle
• Increases CO2 in cells
• NADPH and ATP are made during the day from light
  reactions

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C4 vs. CAM

• Adaptations for hot, dry climates
• Solves CO2/O2 gas exchange vs. H2O loss challenge
• Minimizes photorespiration
• Increases efficiency of sugar production!
• C4 plants separate 2 steps of carbon fixation
  anatomically
• 2 different cells
• CAM plants separate 2 steps of carbon fixation
  temporally
• 2 different times

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