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. • Carbon fixation occurs during the light independent reactions aka Calvin Cycle or Dark Reactions.
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.
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 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 • 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.
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 • 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 • 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 • 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.