Light Independent Reactions (9) Flashcards
Q: What do the light independent reactions (Calvin Cycle) do with NADPH and CO2?
A: The light independent reactions (Calvin Cycle) oxidize NADPH to reduce CO2, ultimately synthesizing glucose.
Q: What are light independent reactions also known as, and where do they take place?
A: Light independent reactions are sometimes called “Dark Reactions,” though darkness is not a requirement. They take place in the stroma of the chloroplast.
Q: What is the Calvin Cycle, and what does it consist of?
A: The Calvin Cycle (also called the Calvin-Benson Cycle) is the combination of the 3 phases of the light independent reactions (dark reactions).
Q: What is the first step of the Calvin Cycle?
A: The enzyme RuBisCo combines RuBP (ribulose bisphosphate) and CO2 to make two 3-phosphoglycerate (3PG) molecules, and this process occurs three times.
Q: What happens during the second step of the Calvin Cycle?
A: 3PG is phosphorylated by 3-phosphoglycerate kinase using one ATP, resulting in the formation of 1,3-bisphosphoglycerate (BPG). This step occurs six times.
Q: What happens during the third step of the Calvin Cycle?
A: BPG is reduced to glyceraldehyde-3-phosphate (G3P) by the enzyme glyceraldehyde-3-phosphate dehydrogenase, with the oxidation of 1 NADPH to 1 NADP+. This step occurs six times.
Q: What happens during the fourth step of the Calvin Cycle?
A: At this point, 6 G3P molecules have been produced. 1 G3P is removed from the cycle to be used for glucose synthesis, while the remaining 5 G3P molecules are used to regenerate RuBP.
Q: What happens during the fifth step of the Calvin Cycle?
A: In this step, 5 G3P molecules and 3 ATP are consumed to synthesize the 3 RuBP molecules required to complete another turn of the Calvin Cycle.
Q: How is glucose synthesized from G3P in the Calvin Cycle?
A: The synthesis of glucose from G3P is similar to the process in humans but proceeds in the opposite direction of glycolysis. G3P molecules are used to build glucose and other sugars, eventually forming glucose.
Q: What are the reactants and products of one full turn of the Calvin Cycle?
A:
Reactants: 3 CO2 + 9 ATP + 6 NADPH
Products: 1 G3P + 9 ADP + 8 Pi + 6 NADP+
Q: What is photorespiration?
A:
Photorespiration is the oxygenation of RuBP that occurs at high O2 levels and high temperatures. In this process, RuBP is no longer usable by the Calvin Cycle, leading to a loss of energy efficiency in the chloroplast.
Q: Why does photorespiration occur?
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A:
Photorespiration occurs because O2 and CO2 both compete to bind to the active site of RuBisCo. At higher temperatures, RuBisCo has more difficulty differentiating between the two, leading to the oxygenation of RuBP instead of its carboxylation
Q: What type of photosynthesis do most plants perform?
A:
Most plants perform C3 photosynthesis, where the first step of carbon fixation creates a 3-carbon molecule, 3-phosphoglycerate (3PG). This is the most common type of photosynthesis, particularly in plants found in temperate climates.
Q: Where do C3 plants perform their light and dark reactions?
A:
C3 plants perform both the light and dark reactions in their mesophyll chloroplasts. However, this exposes RuBisCo to both CO2 and O2, which increases the risk of photorespiration.
Q: How do C4 plants separate the light and dark reactions?
A:
C4 plants separate the light and dark reactions by using mesophyll cells for the light reactions and CO2 capture. The captured CO2 is then passed to the bundle sheath cells where the dark reactions (Calvin Cycle) take place.
Q: How do C4 plants capture CO2?
A:
C4 plants capture CO2 by synthesizing malate, a 4-carbon compound, in their mesophyll cells. The malate is then transported to the bundle sheath cells, where it is decarboxylated to release CO2 for use by RuBisCo in the Calvin Cycle.
Q: Why do C4 plants use malate as a CO2 carrier, despite the energy cost?
A:
C4 plants use malate as a CO2 carrier because it helps them avoid photorespiration. By shuttling CO2 into the bundle sheath cells, where RuBisCo is isolated from O2, the plant reduces the chances of RuBisCo binding with oxygen, making it more efficient at carbon fixation despite the energy cost.
Q: What is the key feature of CAM plants, and how do they differ from C4 plants?
A:
CAM plants, like cacti and succulents, separate CO2 collection from the light and dark reactions temporally, meaning they collect CO2 at night and perform the light reactions during the day. This is different from C4 plants, which spatially separate the light and dark reactions by using different cells for each. CAM plants do this to conserve water in arid environments, as they can close their stomata during the day to reduce water loss.
Q: How do CAM plants collect and store CO2?
A:
CAM plants open their stomata at night to collect CO2 when the risk of water loss is lower. The CO2 is then converted into malate (a 4-carbon molecule) and stored in vacuoles. During the day, when the stomata are closed to conserve water, the malate is transported out of the vacuoles and decarboxylated to release CO2 for use in the Calvin Cycle. This process allows CAM plants to carry out photosynthesis while minimizing water loss.
