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Alternative Pathways: C3, C4 and CAM. Photosynthesis Quest Number 3 Monkemeier CP Biology 2011. Carbon Fixation. The incorporation of carbon dioxide into organic compounds is known as carbon fixation.

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alternative pathways c3 c4 and cam

Alternative Pathways: C3, C4 and CAM

Photosynthesis Quest Number 3

Monkemeier

CP Biology 2011

carbon fixation
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.
calvin cycle reminder of steps
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 Ribulosebiphosphate.
  • 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.
slide4
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.
c3 plants1
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!
photorespiration
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.
photorespiration1
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.
alternative pathways
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.
c4 plants
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.
cam plants
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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