Alternative Pathways: C3, C4 and CAM

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Alternative Pathways C3, C4 and CAM

• Photosynthesis Quest Number 3
• Monkemeier
• CP Biology 2011

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Carbon Fixation

• The incorporation of carbon dioxide into organic
  compounds is known as carbon fixation.
• Carbon fixation occurs during the light
  independent reactions aka Calvin Cycle or Dark
  Reactions.

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Calvin Cycle Reminder of Steps!

• Carbon dioxide diffuses into the stroma of the
  chloroplast.
• The enzyme, RibuloseBiphosphatase (aka Rubisco)
  enables Carbon Dioxide to combine with a 5-carbon
  sugar known as Ribulose biphosphate.
• This six carbon compound is UNSTABLE and splits
  into two 3-Carbon sugars. 3-PGA
• The two 3-Carbon sugars then get rearranged and
  form G3P
• One of the G3P will go on to make glucose
• The other G3P will go to regenerate the 5-Carbon
  sugar known as RibuloseBiphosphatase.

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C3!

• The Calvin Cycle just described is known as the
  C3 pathway because the first stable organic
  molecule formed is a three carbon sugar!
• Examples of C3 plants are grass, oak trees,
  maple trees, rose bushes, etc.

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C3 Plants
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C3 Plants

• C3 plants keep their stomata OPEN during the day
  when the sun is out and CLOSED at night when sun
  is not shining.
• The carbon dioxide can get IN and Oxygen can get
  OUT during the day when the stomata are open.
• ON hot, dry, days, water loss (transpiration) is
  a PROBLEM. C3 Plants have decreased levels of
  photosynthesis due to water loss!

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Photorespiration

• When the stomata are closed, carbon dioxide can
  not enter the leaf and oxygen can not leave the
  leaf.
• Oxygen competes with carbon dioxide in The Calvin
  Cycle.
• When too much oxygen is inside the leaf, it will
  bind with RUBISCO (aka ribulose biphosphatase)
  and shut down the Calvin Cycle. This is known as
  photorespiration.

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Photorespiration

• When oxygen combines with the enzyme of the
  Calvin Cycle (Rubisco, aka ribulose
  biphosphatase), it then combines with the
  5-Carbon sugar and the combination breaks down.
• The Calvin Cycle shuts down because the
  5-Carbon sugar is not available to make a sugar
  and it breaks down into water and carbon dioxide.

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Alternative Pathways

• Plants have had to evolve in different climates.
• In HOT and DRY climates, plants have evolved
  alternative pathways.
• Plants fight transpiration, photorespiration and
  the need for carbon dioxide to enter the leaf.

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C4 Plants

• Corn, sugar cane and crab grass.
• Help fight transpiration by keeping their stomata
  partially closed during the day and partially
  closed at night.
• Fix carbon dioxide into a temporary storage
  molecule that has four carbons!
• It is called a C4 pathway because the first
  stable compound that carbon dioxide is fixed into
  is a four carbon compound.
• The four carbon compound is transported to
  specialized cells where Calvin Cycle is taking
  place!
• The four carbon compound releases carbon dioxide
  to run Calvin Cycle.

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C4 Plants
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CAM Plants

• Jade plants, succulent plants, pineapple,
• Keep stomata CLOSED during the day and OPEN at
  night.
• Store carbon dioxide as an organic acid.
• The organic acid then releases carbon dioxide
  directly to the Calvin Cycle.
• CAM plants grow very slowly, but they require
  less water than C3 or C4 plants.

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CAM Plants
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Comparison
C3 Plants C4 Plants CAM Plants
Stomata Open at Night Closed during Day Partially closed during day and partially closed at night Closed during day and Open at night
Carbon fixation Carbon dioxide is fixed into a three carbon compound that is stable Carbon dioxide is temporarily stored as a 4 carbon stable compound Carbon dioxide is temporarily stored as an organic acid.
Water Loss Has trouble with water loss due to transpiration. Trouble with photorespiration Less water loss than C3 plants. Photorespiration is not a problem Grow very slowly and no problems with photorespiration