Following are certain statements regarding C2, C3, C4 and CAM carbon metabolism in plants. A. In C3 cycle, one molecule of 3-phosphoglycerate formed during carboxylation phase is utilized for the biosynthesis of sugars, fatty acid and amino acids. B. During C2 cycle glycine is transported from peroxisome to mitochondria and glycerate is transported from peroxisome to chloroplast. C. The concentration of CO2 in bundle sheath of C4 plants is several fold lower than the external atmosphere. D. The stomata of CAM plants open at night. Which one of the following combination of statements is correct?
B and D
Plant Carbon Metabolism Statement Analysis
Let's carefully analyze each statement regarding C2, C3, C4, and CAM carbon metabolism in plants.
Statement A: C3 cycle and 3-Phosphoglycerate Utilization
Statement A says: In C3 cycle, one molecule of 3-phosphoglycerate formed during carboxylation phase is utilized for the biosynthesis of sugars, fatty acid and amino acids.
In the C3 cycle, CO2 is fixed to Ribulose-1,5-bisphosphate (RuBP) by the enzyme RuBisCO, producing two molecules of 3-phosphoglycerate (3-PGA). These 3-PGA molecules are then reduced to triose phosphates (like glyceraldehyde-3-phosphate, G3P) using ATP and NADPH produced during the light reactions. Triose phosphates are the direct precursors for the synthesis of sugars (like sucrose and starch), fatty acids, and amino acids. However, most of the triose phosphate produced is used to regenerate RuBP, allowing the cycle to continue. Typically, out of 6 molecules of triose phosphate produced from 3 molecules of CO2 fixation, 5 are used for RuBP regeneration, and only 1 net molecule is available for export or biosynthesis. The statement mentions 3-phosphoglycerate directly being utilized for biosynthesis, which is not the molecule typically exported or directly used as a building block; it is the triose phosphate derived from 3-PGA. While 3-PGA is a precursor, the statement is not entirely accurate in how the net product is utilized for biosynthesis.
Therefore, statement A is considered incorrect.
Statement B: C2 Cycle Transport
Statement B says: During C2 cycle glycine is transported from peroxisome to mitochondria and glycerate is transported from peroxisome to chloroplast.
The C2 cycle, also known as photorespiration, involves the metabolism of phosphoglycolate (produced by RuBisCO's oxygenase activity) across three organelles: chloroplasts, peroxisomes, and mitochondria. The key transport steps are:
- Glycolate is transported from chloroplast to peroxisome.
- Glycine is transported from peroxisome to mitochondrion.
- Serine is transported from mitochondrion to peroxisome.
- Glycerate is transported from peroxisome to chloroplast.
The statement accurately describes two of these transport steps: glycine moving from peroxisome to mitochondria and glycerate moving from peroxisome to chloroplast.
Therefore, statement B is considered correct.
Statement C: CO2 Concentration in C4 Bundle Sheath
Statement C says: The concentration of CO2 in bundle sheath of C4 plants is several fold lower than the external atmosphere.
C4 plants have a mechanism to concentrate CO2 in the bundle sheath cells where the C3 cycle (Calvin cycle) operates. CO2 is initially fixed in mesophyll cells by PEP carboxylase into a four-carbon compound (like oxaloacetate), which is then converted to malate or aspartate. These four-carbon compounds are transported to the bundle sheath cells, where they are decarboxylated, releasing CO2. This process elevates the CO2 concentration in the bundle sheath cells to a level significantly higher (several fold) than the external atmospheric CO2 concentration. This high CO2 level helps maximize RuBisCO's carboxylase activity and minimize photorespiration.
Therefore, statement C is considered incorrect.
Statement D: Stomata of CAM Plants
Statement D says: The stomata of CAM plants open at night.
CAM (Crassulacean Acid Metabolism) plants, such as succulents and cacti, have adapted to arid environments by performing gas exchange at night. They open their stomata during the cool, humid night to take in CO2, which is then fixed into organic acids (like malic acid) and stored in vacuoles. During the day, the stomata close to conserve water, and the stored organic acids are broken down to release CO2 internally, which is then used in the C3 cycle during daylight hours when ATP and NADPH are available from the light reactions.
Therefore, statement D is considered correct.
Identifying the Correct Combination
Based on the analysis:
- Statement A: Incorrect
- Statement B: Correct
- Statement C: Incorrect
- Statement D: Correct
The combination of correct statements is B and D.
Important Questions from Photosynthesis
Which one of the following reactions takes place during the reduction phase of the Calvin - Benson cycle?
The following statements were made with the assumption that the concentration of 3- phosphoglycerate is high inside chloroplasts of an actively photosynthesizing leaf.
A. There will be high concentration of triose phosphate in the chloroplast.
B. The activity of ADP-glucose pyrophosphorylase will be inhibited.
C. The carbon flow will be diverted from sucrose to starch.
D. Starch synthesis will be inhibited and carbon flow will be more towards sucrose synthesis.
Which one of the following combinations of above statements is correct?
The table below shows photosynthetic type, temperature and sunlight intensity levels.
Photosynthetic Type Temperature Sunlight Intensity A. C3 Plant i High P. High B. C4 plant ii Moderate Q. Moderate iii Low R Low Which of the following correctly matches the plant photosynthetic type with the temperature and sunlight conditions in which photosynthetic rate per unit leaf area is maximum for that plant?
Calvin-Benson cycle is divided into three phases, namely carboxylation, reduction and regeneration. The following statements are related to the three phases of Calvin-Benson cycle:
A. The product of light reaction, ATP and NADPH is utilized in the carboxylation phase.
B. Six molecules of 3-phosphoglycerate is converted into six molecules of glyceraldehyde 3-phosphate in the reduction phase.
C. The action of aldolase enzyme for the production of fructose 1, 6-bisphosphate takes place in reduction phase.
D. Formation of seven carbon compound, sedoheptulose-7- phosphate takes place in the regeneration phase.
Which one of the following combinations is correct?
Which one of the following metabolites formed during Calvin-Benson cycle in chloroplast is involved in starch biosynthesis and can also be transported to cytosol?
