2 Photorespiration occurs when the CO2 levels inside a leaf become low Photorespiration occurs when the CO2 levels inside a leaf become low. This happens on hot dry daysOn hot dry days the plant is forced to close its stomata to prevent excess water loss.The plant continues fix CO2 when its stomata are closed, the CO2 will get used up and the O2 ratio in the leaf will increase relative to CO2 concentrations.
3 When the CO2 levels inside the leaf drop to around 50 ppm, Rubisco starts to combine O2 with RuBP instead of CO2The net result of this is that instead of producing 2 3C PGA molecules, only one molecule of PGA is produced and a toxic 2C molecule called phosphoglycolateis produced.
5 phosphoglycolateThe plant must get rid of the phosphoglycolate since it is highly toxic.It converts the molecule to glycolic acid.The glycolic acid is then transported to the peroxisome and there converted to glycine.
7 The serine is then used to make other organic molecules. All these conversions cost the plant energy and results in the net loss of CO2 from the plantTo prevent this process, two specialized biochemical additions have been evolved in the plant world: C4 and CAM metabolism.
10 The C4 pathway is designed to efficiently fix CO2 at low concentrations and plants that use this pathway are known as C4 plants.These plants fix CO2 into a four carbon compound (C4) called oxaloacetate. This occurs in cells called mesophyll cells.
11 1. CO2 is fixed to a three-carbon compound called phosphoenolpyruvate to produce the four-carbon compound oxaloacetate. The enzyme catalyzing this reaction, PEP carboxylase, fixes CO2 very efficiently so the C4 plants don't need to to have their stomata open as much. The oxaloacetate is then converted to another four-carbon compound called malate in a step requiring the reducing power of NADPH
12 2. The malate then exits the mesophyll cells and enters the chloroplasts of specialized cells called bundle sheath cells.Here the four-carbon malate is decarboxylated to produce CO2, a three-carbon compound called pyruvate, and NADPH.The CO2 combines with ribulose bisphosphate and goes through the Calvin cycle.
13 3.The pyruvate re-enters the mesophyll cells, reacts with ATP, and is converted back to phosphoenolpyruvate, the starting compound of the C4 cycle.
15 CAM plants live in very dry condition and, unlike other plants, open their stomata to fix CO2 only at night.Like C4 plants, the use PEP carboxylase to fix CO2, forming oxaloacetate.The oxaloacetate is converted to malate which is stored in cell vacuoles. During the day when the stomata are closed, CO2 is removed from the stored malate and enters the Calvin cycle
17 Differences between calvin (C3) and C4 Temp CAbsence of malateOne carboxylation reactionCO2 is the substrateUsual leaf structuresTemp CPresence of malate2 carboxylation reactionsHCO3 is the substrateClosed stomata to reduce water loss and concentrating CO2 in the bundle sheet cellsAdditional ATP is required
19 Comparison between C3, C4, and CAM productG3PDay &nightMalateNight onlyAnatomyNo bundle sheet cellBundle sheet cellNo of stomataPhotorespirationUp to 40%Not detectableSpeciesWheat, rice, potatoSugar canePineapple, vanilla, cacti
20 Factors affecting photorespiration O2: CO2RatioIf Cells Have Low O2 but Higher CO2, Normal photosynthesis i.e. Calvin Cycle DominatesC4Plants Have Little Photorespiration because They Carry the CO2to the bundle Sheath Cells and can Build up High [CO2]
21 Calvin Cycle Reactions always Favored over Photorespiration If Cells Have Higher O2and Lower CO2, Photorespiration DominatesTemperaturePhotorespiration Increases with Temperature
22 Regulation of Carbon Dioxide Fixation Plant cells have chloroplasts that carry outphotosynthesis: CO glucosePlant cells also have mitochondria and carry out glycolysis, TCA, and oxidative phosphorylation: Glucose CO2To prevent futile cycling of carbohydrate, cells must regulate the activities of key Calvin cycle enzymes .
23 These enzymes respond indirectly to light activation. light energy is available the Calvin cycle proceeds.If no light available, no fixation of CO2occurAmong the key changes that regulate Calvin cycle versus respiration are:Environment Factors: Light intensity, temperature, & availability of H2O, CO2, O2Cellular factors: cell state of key metabolites (NADPH, ATP, inhibitors, reducing power, etc )