Photorespiration

When weather is hot and dry, guard cells decrease the size of stomata, also decreasing the concentration of carbon dioxide. • Oxygen (O2)starts to compete with CO2 for rubisco’s active site • RRubisco adds O2 to RuBP forming one PGA molecule (3C) and one glycolate molecule (2C) • GGlycolate leaves the Calvin Cycle and is partially converted to CO2 • PPhotorespiration consumes O2 and revolves CO2 while generating no ATP • It is believed that Rubiso’s inability to distinguish between CO2 and O2 is an evolutionary remnant of an earlier mechanism, better suited to an early Earth whose atmosphere was rich in CO2 and poor in O2 Photorespiration

THE PROBLEM • It occurs when the stoma close in an effort to save water on high light, low water availability days (like some hot dry summer days around here). • Oxygen concentration goes up in the closed off leaf and competes with the dwindling CO2 for the active site on the enzyme “Rubisco”

3. This short circuits the Calvin Cycle and no glucose is made. 4. Why is “Rubisco” so dumb that it can’t tell CO2 from O2? It evolved when oxygen level in the atmosphere was very low; before photosynthesis raised oxygen levels in the atmosphere. Plants are in a way a victim of their own success.

THE SOLUTIONS: • Ignore It: C3 Plants (So called because the first compound CO2 is fixed in is a 3-carbon PGA) can survive around here because the days promoting photorespiration are few enough that the plant will not starve.

Solutions 2. C4 SOLUTION (A solution of location): • a. C4 plants have evolved a way to survive in areas that have a significant number of Photorespiration causing days in the growing season; too may for a C3 to survive. • b.Their leaf and chlorophyll anatomy are different. “Kranz (German for wreath) Anatomy”

i.I. No Palisade cells, only spongy mesophyll cells. ii.. Bundle sheath cells have chloroplasts but without thylakoids (thus can only do dark reactions).

Alternative Carbon Fixation Reactions • There are some alternative methods for carbon fixation, e.g. C4 photosynthesis: • 3C sugar + CO2 _ 4C sugar • CO2 is fixed into a 4 carbon sugar molecule • Prevalent in tropical grasses such as sugar cane and maize • C4 photosynthesis is typically found in plants that grow at higher temperatures and under high light intensity

c.In the spongy mesophyll cells a 3-carbon compound (PEP) picks up a CO2 to form a 4-carbon compound (hence the name C4). The enzyme that fixes the CO2 to the PEP in the spongy cells has a particularly high affinity for CO2 – PEP Carboxylase. Unlike Rubisco, this enzyme does not react with O2 at all.

d.This 4-carbon compound is shuttled into the bundle sheath cells where it releases the CO2. The reformed 3-carbon PEP goes back out into the spongy cells and picks up another CO2. Calvin Cycle then takes place in the bundle sheath cells. The object is to get CO2 to the Rubisco in a protected place so that it doesn’t compete with the O2.

e.So efficient is this CO2 “shuttle” that the Bundle Sheath cells have 10 to 60 times to CO2 compared to the spongy mesophyll cells. The concentration favors the productive reaction with Rubisco.

F. Bundle Sheath cell chloroplasts have no thylakoids because they are specialized to do Light Independent Reactions. The spongy mesophyll cells do all the light dependent reactions the plant needs.

g. If this is so good why don’t all the plants do it? Because it takes about 30 ATP to make a molecule of glucose by this extra pathway compared to the 18 ATP per glucose in plain old C3. So a cost benefit analysis would show if only happens where it is a matter of survival.

C4 Plants • C4 plants minimize respiration even in hot, dry climates • C4 plants have a 4C stable intermediate oxaloacetate • C4 plants carry out the first stages of carbon fixation in the mesophyll cells • CO2 is fixed into oxaloacetate, which is converted to malate • Malate is transferred to the bundle sheath tissue. CO2 is released and used in the normal Calvin cycle • This is used to increase the CO2 concentration within the bundle sheath tissue to prevent photorespiration • Ex: sugar cane, corn, sorghum

C4 Pathway Occurs in the cytoplasm, not in the chloroplast

CAM Plants • Water- storing plants (known as succulents) such as cacti and pineapples • Open their stomata at night and close them during the day • When stoma open at night CO2 is incorporated into C4 organic acids by the enzyme PEP carboxylase • These organic acids are stored in vacuoles until the morning • The stoma close and the organic acids release CO2 that enter the C3 Calvin cycle to be fixed into carbohydrates

CAM Solution(A solution of time): • a.CAM plants have the same problem and live in areas so dry even C4 plants would die.

CAM SOLUTION • b.CAM plants grow in “Xeric” areas like deserts. They are all about conserving water. • i.Their body structure is often very different (have a low s/v ratio). • ii.Their leaves are minimized in terms of surface area and do not do photosynthesis (often form spines or thorns). • iii.The stem is the main photosynthetic structure. iv. Stomata are located in deep pits to minimize evaporation.

CAM Solutions • C. AND they open their stomata at night so they can get CO2 and minimize the loss of water. • D. During the night they fix CO2 that comes into the plant in a 4-carbon compound and store it in vacuoles. • E. During the day the stomata close, to save water and the plant uses the 4-carbon compound made last night just like the C4 plant does.

C4 utilizes a spatial separation of steps. The two cycles, carbon fixation and Calvin Cycle occur in different types of cells. The carbon fixation step occurs in the mesophyll cell, where CO2 is incorporated into 4-carbon organic acids. The CO2 is then released into the bundle-sheath cell into the Calvin Cycle. CO2 is taken in by the PEP carboxylase (an enzyme that has greater affinity for CO2 than rubisco and no affinity for O2), and released into the bundle-sheath cells next to the mesophyll cells.

CAM utilizes a temporal separation of steps The two cycles occur in the same cells but at different times, with carbon fixation during the night, when CO2 is taken up, and changed into organic acids. These organic acids are saved in vacuoles in the mesophyll cells until morning, when the stomata close to conserve water in dry regions. In the morning, the light reactions allow photosynthesis to occur- providing NADPH and ATP so that the Calvin Cycle can occur. This releases CO2 from the organic acids made during the night, to be combined with other elements within the chloroplast to form sugar.

Fun Facts!! • All three plant types (C3, C4, and CAM) make glucose by Calvin Cycle. The difference is how the CO2 gets to Rubisco. • a.C3 use CO2 directly from the air • b.C4 and Cam first combine CO2 with PEP making a 4-carbon compound. • The C4 and CAM solutions are similar chemically but the main difference is: • a.C4 plants solve the problem by location of the reactions. • b.CAM plants solve the problem by the timing of the reactions

More FUN! 3. FAMOUS PLANTS THAT ARE • a.C3: Wheat, oats, rice, regular grass, most deciduous trees (oak, maple, elm) • b.C4: Corn, crabgrass, Sugar Cane, Bermuda grass • c.CAM: Cactus, pineapple, vanilla, orchids

SUMMARY OF CO2 FIXATION

Photorespiration