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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


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.


  • 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!


  • 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.


  • 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.