Photosynthesis Quest Number 3 Monkemeier CP Biology 2011

 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.

 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.

 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 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.

 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.

 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.

 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.

C3 Plants C4 PlantsCAM Plants StomataOpen at Night Closed during Day Partially closed during day and partially closed at night Closed during day and Open at night Carbon fixationCarbon 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 LossHas 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