#pepcase

[tenth-sentence]

Although the discovery of these genes suggests that carbon is assimilated through the C4 pathway, diatoms also possess bicarbonate transporters and carbonic anhydrases that may function to elevate the concentration of CO2 at the active site of rubisco.

"Plant Physiology and Development" int'l 6e - Taiz, L., Zeiger, E., Møller, I.M., Murphy, A.

[whats-in-a-sentence]

In this anatomical context, the transport of CO2 from the external atmosphere to the bundle sheath cells proceeds through five successive stages (see Figure 8.11 and Table 8.4):

Fixation of the HCO3- in phosphoenolpyruvate by PEPCase (see Table 8.4, reaction 1). The reaction product, oxaloacetate, is subsequently reduced to malate by NADP-malate dehydrogenase in the mesophyll chloroplasts (see Table 8.4, reaction 2) or converted to aspartate by transamination with glutamate in the cytosol (see Table 8.4, reaction 3).

Transport of the four-carbon acids (malate or aspartate) to bundle sheath cells that surround the vascular bundles.

Decarboxylation of the four-carbon acids and generation of CO2, which is then reduced to carbohydrate via the Calvin-Benson cycle. Prior to this reaction, an aspartate aminotransferase catalyzes the conversion of aspartate back to oxaloacetate in some C4 plants (see Table 8.4, reaction 3). Different types of C4 plants make use of different decarboxylases to release CO2 for the effective suppression of the oxygenase reaction of rubisco (see Table 8.4, reactions 4a, 4b, and 5).

Transport of the three-carbon backbone (pyruvate or alanine) formed by the decarboxylation step back to the mesophyll cells.

Regeneration of phosphoenolpyruvate, the HCO3- acceptor. ATP and inorganic phosphate convert pyruvate to phosphoenolpyruvate, releasing AMP and pyrophosphate (see Table 8.4, reaction 7). Two molecules of ATP are consumed in the conversion of pyruvate to phosphoenolpyruvate: one in the reaction catalyzed by pyruvate-phosphate dikinase (see Table 8.4, reaction 7) and another in the transformation of AMP to ADP catalyzed by adenylate kinase (see Table 8.4, reaction 8). When alanine is the three-carbon compound exported by sheath cells, the formation of pyruvate by alanine aminotransferase precedes phosphorylation by pyruvate-phosphate dikinase (see Table 8.4, reaction 6).

"Plant Physiology and Development" int'l 6e - Taiz, L., Zeiger, E., Møller, I.M., Murphy, A.

[simplifiedbiology-blog]

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Photosynthesis MCQs 2

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Question #1: A plant in your garden avoids photorespiratory losses, has improved water use efficiency, shows high rates of photosynthesis at high temperatures and has improved efficiency of nitrogen utilisation. In which of the following groups would you assign this plant?

C4 plants CAM Nitrogen fixer C3 plants

Question #2: Which of the following is a false statement ?

PEP acts as first acceptor of CO2 in both CAM plants and C4 plants. The plants undergoing crassulacean acid metabolism do not undergo C3 cycle. In CAM plants there is separation of initial carboxylation and Calvin cycle in time instead of space in C4 plants. all of the above

Question #3: How many ATP and NADPH2 molecules are used for fixing one molecule of CO2 ?

Three and two respectively Three each Two each Two and three respectively

Question #4: C4 plants are more expensive in terms of ATP required for CO2 fixation, the additional number of ATP required by C4 plants in comparison to C3 plants is :

6 ATP 12 ATP 18 ATP 24 ATP

Question #5: Consider the following processes : 1. O2 evolution 2. CO2 fixation 3. NADPH formation 4. HMP pathway Which of these do not occur in thylakoids ?

1, 2, 4 2, 3, 4 3 & 4 2 & 4

Question #6: Solarisation is a process in which ?

Chlorophyll is found Sugars are formed with the help of solar energy Destruction of chlorophyll and ultimate death of living organism None of the above

Question #7: During light reaction of photosynthesis, which of the following phenomenon is observed during cyclic phosphorylation as well as non-cyclic phosphorylation ?

Formation of ATP Formation of NADPH2 Release of O2 Photorespiration

Question #8: In case of C4-plants, which enzyme fixes the CO2 released during decarboxylation of malate :

Rubisco Malate dehydrogenase PEPcase None of these

Question #9: Photosynthesis consists of essentially two biological reaction systems, one followed by the other, the second of these systems does which of the following?

Traps light energy Synthesizes starch Fixes CO2 Works only in the presence of light

Question #10: The first step for initiation of photosynthesis will be

Photolysis of water Excitement of chlorophyll molecules due to absorption of light ATP formation Glucose formation

Question #11: Inhibition of photosynthesis in high concentration of oxygen is mainly due to

Distribution of RuBP carboxylase Inactivation of RuBP carboxylase Non-synthesis of RuBP carboxylase RuBP carboxylase acting as oxygenase

Question #12: Select the wrong statement :

One Calvin cycle needed to form one glucose molecule Reduction of one molecule of CO2 to carbohydrate requires a minimum of 4 quanta of light Reduction of NADP+ to NADPH takes place during Calvin’s cycle All are wrong

Question #13: Choose the incorrect statement :

CO2 fixation in C3 plants reaches saturation at much lower light intensities than C4 plants Optimum temperature for C4 photosynthesis is higher as compared to C3 photosynthesis C3 plants are able to use water more efficiently than C4 plants Under ideal conditions, the rate of photosynthesis in C4 plants is 2-3 times higher than that of C3 plants

Question #14: Oxygenic photosynthesis occurs in :

Chlorobium Chromatium Oscillatoria Rhodospirillum

Question #15: With reference to factors affecting the rate of photosynthesis, which of the following statements is not correct ?

Increasing atmospheric CO2 concentration up to 0.05% can enhance CO2 fixation rate C3 plants respond to higher temperatures with enhanced photosynthesis while C4 plants have much lower temperature optimum Tomato is a greenhouse crop which can be grown in CO2 - enriched atmosphere for higher yield Light saturation for CO2 fixation occurs at 10% of full sunlight

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[whats-in-a-sentence]

In the C4 cycle, the enzyme phosphoenolpyruvate carboxylase (PEPCase), rather than rubisco, catalyzes the initial carboxylation in mesophyll cells close to the external atmosphere (Table 8.4, reaction 1). (...) In the NADP-malic enzyme (NADP-ME) type of C4 photosynthesis, malate enters the chloroplast of bundle sheath cells where it is decarboxylated by NADP-ME (see Table 8.4, reaction 4a). (...) The decarboxylation of oxaloacetate in both cases takes place in the mitochondria of bundle sheath cells via NAD-ME (see Table 8.4, reaction 4b) and via PEPCK (see Table 8.4, reaction 5). (...) At night, cytosolic PEPCase fixes atmospheric and respiratory CO2 into oxaloacetate using phosphoenolpyruvate formed via the glycolytic breakdown of stored carbohydrates (see Table 8.4, reaction 1). A cytosolic NADP-malate dehydrogenase converts the oxaloacetate to malate, which is stored in the acidic solution of vacuoles for the remainder of the night (see Table 8.4, reaction 2). During the day, the stored malate exits the vacuole for decarboxylation by mechanisms similar to those in C4 plants – that is, by a cytosolic NADP-ME or mitochondrial NAD-ME (see Table 8.4, reactions 4a and 4b).

"Plant Physiology and Development" int'l 6e - Taiz, L., Zeiger, E., Møller, I.M., Murphy, A.