C 4 Pathway A Mechanism for Tropical Plants

C 4 Pathway: A Mechanism for Tropical Plants to Accelerate Photosynthesis • CO 2 is concentrated in the chloroplasts of bundle-sheath cells (major site of photosynthesis) by shuttling a four-carbon (C 4) compound, malate, from mesophyll cells that are in contact with air • Decarboxylation of the C 4 compound in the bundle-sheath cell maintains a high local concentration of CO 2 to support the Calvin cycle (this effectively acts as an ATP-driven “pumping” of CO 2 into the bundle-sheath cells) • This system can increase the concentration of CO 2 in bundle-sheath cells by a factor of 20, thus accelerating the carboxylase activity of rubisco & effectively outcompeting the oxygenase (photorespiration) reaction in tropical plants

Questions for Group Work 1. How do the intrinsic properties of membranes contribute to the high efficiency of photosynthesis? 2. Excited state molecules can display the unusual property of being both easily oxidized and easily reduced. Why? (Hint: Consider the energy diagram of an excited state molecule). 3. State at least five factors that contribute to the efficiency of the bacterial photoreaction center. 4. Classify the types of reactions occurring in each step of the carboxylation reaction of rubisco:

Questions for Individual Work 1. What is the pathway of electron transport in oxygenic photosynthesis? 2. What role do Photosystem I and Photosystem II play in oxygenic photosynthesis? 3. In what region of the chloroplast do the light and dark reactions of photosynthesis take place? 4. How are the light and dark reactions coupled and/or regulated?