Lecture 12: Photosynthesis - PDF

LECTURE 12 PHOTOSYNTHESI S ……….Continuatio n2 OTHER TYPES OF PHOTOSYNTHESIS The majority of land plants carry out photosynthesis as described and called C3 plants. C3 plants use the enzyme RuBP carboxylase to fix CO2 to RuBP in mesophyll cells. The first deteted molecule f...

LECTURE 12 PHOTOSYNTHESI S ……….Continuatio n2 OTHER TYPES OF PHOTOSYNTHESIS The majority of land plants carry out photosynthesis as described and called C3 plants. C3 plants use the enzyme RuBP carboxylase to fix CO2 to RuBP in mesophyll cells. The first deteted molecule following fixation is the 3-carbon molecule 3PG (3- phophoglycerate). RuBP RuBP + CO2Carboxylase (2) 3PG  Leaves have small openings called stomata through which water can leave and carbon dioxide can enter. If the weather is hot and dry, stomata close, conserving water. When the stomata remains closed the concentration of CO2 decreases in leaves, while O2 concentration as a by-product of photosynthesis increases.  When O2 rises in C3 plants, RuBP carboxylase combines it with ribulose biphosphate (RuBP) instead of CO2. The result is one molecule of 3PG and eventual release of CO2. This is called photorespiration because in the presence of light (photo), oxygen is taken up and carbon dioxide is released (respiration). An adaptation called C4 photosynthesis enables some plants to avoid photorespiration.  Photorespiration reduces the photosynthetic rate by 25% in C3 plants. It reduces the amount carbon dioxide needed to be converted into sugar. The oxygenation reaction of RuBisCO is a wasteful process because 3- phosphoglycerate is created at a reduced rate and higher metabolic cost. Photorespiration also incurs a direct cost of one ATP and one NADPH.  C4 Photosynthesis The C4 pathway was elucidated by Australian scientist in the mid 1960s using sugar cane. Experiments conducted indicated that the first product of photosynthesis in some plant is malate instead of 3- phoshoglycerate. The enzymes involved in carbon fixation in these plants are divided between two cell types called the split mesophyll cells and bundle sheath cells. They form what called the Kranz anatomy. Kranz Anatomy  Studies have shown that there are three variants of the C4 cycles in plants one of which involves NAD-malic enzyme type. This is found in plants such as maize, sugarcane and sorghum and involves four steps. 1. Carbon assimilation (in mesophyll) 2. Transport to bundle sheath cells 3. Decarboxylations (removal of carbondioxide from a compound). 4. Transport and regeneration in mesophyll.  The initial carboxylation reaction is catalysed by photophoenolpyruvate (PEP) carboxylase also called PEPCase. The enzyme catalyses the reaction between phosphoenol pyruvate (PEP) and hydrogen carbonate (HCO3-) to produce oxaloacetate (QAA) according to the following equation; PEP + HCO3- oxaloacetate (QAA) + Pi Secondly, oxaloacetate is then converted into malate using the reducing power of NADPH according the following equation;  OAA + NADPH + H+ malate + NADP+. The above reaction catalysed by NADP- dependent malate dehydrogenase (NADP-MD). Thirdly, the malate produced is this reaction is transported to bundle sheath cells where it is decarboxylated to produce pyruvate and carbon dioxide; Malate + NADP+ pyruvate + NADPH + CO2 This reaction is catalysed by NADP-Malic enzyme (NADP-ME).  Fourthly, the carbon dioxide produced is used by Rubisco for C3- type carboxylation while the pyruvate is transported back to mesophyll cells where it is converted into phosphoenol pyruvate; Pyruvate + Pi + ATP PEP + AMP + PPi The PEP thus produced is again used in the PEP carboxylase enzyme reaction.  Facts about C4 cycles Many of the C4 pathways enzymes are regulated by light (active in light). C4 plants show partitioning in space such that carbondioxide fixation occurs in mesophyll cells the Calvin cycle occurs in the bundle sheath. C4 pathways allows more efficient carbon fixation at higher temperatures than those at which C3 plants carry out photosynthesis. Phosphoenol pyruvate carboxylase binds CO2 at very low concentration. Hence, stomata can be partially closed without significantly affecting the reaction that catalyses.  Photorespiration is very low, so high oxygen concentration has no significant effect at higher temperatures in the C4 plants. The C4 pathways is mostly found among tropical grasses (growing in warm and sunny environments). C4 plants can not compete with C3 plants in moist, colder and less sunny environments. Less than 1% of plant species use C4 photosynthesis.  Crassulacean Acid Metabolism (CAM) The Crussutaceae is a family of flowering succulent (water)-containing plants that live in warm, dry regions of the world. CAM represents partitioning in time in terms of occurence of carbon fixation and Calvin cycle. During the night, CAM plants use PEPCase to fix some CO2 forming C4 molecules which are stored in large vacuoles in mesophyll cells.  During the day, C4 molecules (malate) release CO2 to the Calvin cycle when NADPH and ATP are available from the light reactions. The primary advantage of this partitioning has to do with the conservation of water. CAM plants open their stomata only at night and therefore only at that time does atmospheric CO2 enter the plant.  During the day, the stomata close, this conserves water but CO2 cannot enter the plant. Photosynthesis in CAM plants is therefore minimal because a limited amount of CO2 is fixed at night but helps CAM plants to live under stressful conditions.  Environmental Factors that affect photosynthesis 1. Light intensity and photo- inhibition Light is very essential for photosynthesis, however it has certain effects which needs to be considered. At low light, light is a limiting but the photosystem apparatus can also become saturated.  In order to maintain the flow of electrons through the photosystems, they must be supplied with NADP+ , which is regenerated by the activity of the C3 cycle. If too much light energy hits the photosystems, they can begin to react with substrates other than the normal electron carriers.  These reactions can generate very damaging substances such as hydrogen peroxide, singlet oxygen radicals and superoxide. Under high light intensity, the core protein of PSII will begin to breakdown reducing the capacity for photosynthesis. Conditions that reduce the activity of the C3 cycle can make the PS apparatus more susceptible to damage by light. 2. Changes in photo-system ratios Plants can vary the ratio of PSII to PSI in order to optimise photosynthesis efficiently. When plants grow under the canopy of shade, the light shifts to the far red. Under this condition, plants will increase the ratio of PSII. Could explain you why this is the case? 3. Photosynthetic rate versus light The amount of light where photosynthesis activity balances respiratory activity is the light compensation point. The point where photosynthesis rate can no longer increase despite an increase in light intensity is called the light saturation point. This indicates that the C3 reactions or CO2 are limiting. 4. Photosynthesis and Carbon dioxide Concentration The concentration of carbon dioxide where photosynthetic CO2 uptake balances respiratory CO2 release is called CO2 compensation point. The carbon dioxide concentration at which the rate of photosynthesis does not increase despite and increase in photosynthesis is called CO2 saturation point. 5. The effect of temperature on photosynthesis The C4 reactions are insensitive to temperature, while the C3 reactions are highly sensitive to temperature. Photosynthesis in C3 plants becomes less efficient at higher temperature due to increasing photorespiration. C4 plants generally grow better than C3 plant in dry, arid climate while C3 grow better than C4 plants in cool, moist climate.  C3 Pathway versus C4 C3 Pathway Pathway C4 Pathway The primary acceptor of The primary acceptor of CO2 is ribulose CO2 is phosphoenol biphosphate (RUBP) a 5 pyruvate, a 3 carbon carbon compound compound. The first stable product is The first stable compound phosphoglycerate. is oxaloacetatic acid It occurs only in the It occurs in both the mesophyll cells of the mesophyll and bundle leaves sheath cells of the leaves. It is slower process of It is a faster process of carbon fixation and carbon fixation and photorespiration losses photorespiration losses are high are low. Photosynthesis is affected Photorespiration does not END OF LECTURE! THANK YOU.