PACKET #31 CHAPTER #10 Photorespiration

Introduction Plants that use the Calvin Cycle to fix carbon, in the molecule sugar, are called C 3 plants. During photorespiration, C 3 plants consume oxygen and generate CO 2 but do not produce ATP  Conserving water and preventing passage of CO 2 into the leaf Photorespiration is said to be an evolutionary relic and lowers the efficiency of photosynthesis by removing carbon molecules from the Calvin Cycle.  Photorespiration reduces the rate of photosynthesis.

Introduction II Photorespiration reduces the rate of photosynthesis in plants in three ways  Carbon dioxide is released  Energy (ATP) is diverted from photosynthetic reactions to photorespiratory reactions.  Competition between oxygen and carbon dioxide reduces the efficiency of the important photosynthetic enzyme ribulose- bisphosphate (RuBP) carboxylase.

Photorespiration in C 3 Plants On dry, hot days in the presence of light C 3 plants close their stomata. This causes the plant to use of oxygen, similar to aerobic cellular respiration, and produce CO 2 and H 2 O.

C 4 and CAM Plants

C4 & CAM Plants Introduction C 4 and CAM plants have devised mechanisms that prevent/reduce the impact of photorespiration.

C 4 Plants

Photorespiration & C 4 Plants Photorespiration is negligible in C 4 plants because the concentration of carbon dioxide is always high in the bundle sheath cells.

Photorespiration & C 4 Plants II In C 4 plants, during hot dry periods, oxygen molecules in the chloroplast bind with Rubisco and prevent carbon fixation. RuBP and oxygen combine, and the intermediates in the Calvin Cycle degrade to form CO 2 and water.

Pathway in C 4 Plants

C 4 Plants Developed a technique to reduce the occurrence of photorespiration PEP carboxylase binds CO 2  Even at low concentrations CO 2 is fixed into oxaloacetate Oxaloacetate is converted to malate  Malate moves into a bundle sheath cell while CO 2 is removed CO 2 enters the Calvin Cycle

CAM Plants

CAM is an acronym for crassulacean acid metabolism. Examples  Succulent plants {family Crassulaceae}  Family Cactaceae  Family Lilaceae  Family Orchidaceae  Many others in 25 families. Plants exhibit a pathway similar to C 4 plants and allow them to live in highly xeric conditions.

CAM Plants PEP carboxylase fixes carbon at night in the mesophyll cells  Stomata are open at night  Minimizes water loss and allows the entry of CO 2 Calvin Cycle occurs during the daytime

Homework Assignment What are some of the similarities, and differences, between C 4 and CAM plants?