Chapter 10 - The Calvin Cycle Flashcards
What does the Calvin cycle do?
Uses the chemical energy of ATP and NADPH to reduce CO2 to sugar
- The Calvin cycle regenerates its starting material after molecules enter and leave the cycle
- The cycle is anabolic, building carbohydrates from smaller molecules and consuming energy (spends ATP and consumes NADPH as reducing power for adding high energy electrons)
How does carbon enter and leave the Calvin cycle?
Carbon enters the Calvin cycle in the form of CO2 and leaves in the form of a 3 carbon sugar (glyceraldehyde 3-phosphate; G3P)
How many times must the cycle take place for the net synthesis of one molecule of G3P?
The cycle must take place three times, fixing three molecules of CO2 - one per turn of the cycle
What are the three phases of the Calvin cycle?
Phase 1: carbon fixation - the cycle incorporates each CO2 molecule, one at a time, by attaching it to a five carbon sugar named ribulose bisphosphate (RuBP); This first step is catalyzed by rubisco (the most abundant protein in chloroplasts and is also thought to be the most abundant protein on Earth); the product of this reaction is a six carbon intermediate that is short lived because it is so energetically unstable that it immediately splits in half, forming two molecules of 3-phosphoglycerate (for each CO2 fixed).
Phase 2: reduction - each molecule of 3-phoshoglycerate receives an additional phosphate group from ATP, becoming 1,3-bisphosphoglycerate; next, a pair of electrons donated from NADPH reduces 1,3-bisphosphoglycerate, becoming G3P (a 3 carbon sugar)
*For every three molecules of CO2 that enter the cycle, there are six molecules of G3P formed; but only one molecule of this three carbon sugar can be counted as a net gain of carbohydrate because the other five molecules must be recycled to regenerate the three molecules of RuBP required to complete the cycle
Phase 3: regeneration of the CO2 acceptor - the five molecules of G3P are rearranged into three molecules of RuBP. To accomplish this, the cycle spends three more molecules of ATP. The RuBP is now prepared to receive CO2 again, and the cycle continues.
For the net synthesis of one G3P molecule, how many ATP and NADPH molecules does the Calvin cycle consume?
For the net synthesis of one G3P molecule, the Calvin cycle consumes a total of nine molecules of ATP and six molecules of NADPH. The light reactions regenerate the ATP and NADPH so…
*To synthesize one glucose molecule, the Calvin cycle uses 6 molecules of CO2, 18 molecules of ATP, and 12 molecules of NADPH.
What is rubisco?
The Calvin cycle enzyme that adds CO2 to rebulose biphosphate
On hot, dry days, how to plants react?
On hot, dry days, plants close the stomata, which conserves H2O, but limits photosynthesis; the closing of the stomata reduces access to CO2 and causes O2 to build up; these conditions favor photorespiration
What are C3 plants?
In most plants, the initial fixation of CO2, via rubisco, forms a 3 carbon compound, 3-phosphoglycerate.
- When their stomata partially close on hot, dry days, C3 plants produce less sugar because the declining level of CO2 in the leaf starves the Calvin cycle
- Rice, wheat, and soybeans are C3 plants.
What is photorespiration?
A metabolic pathway that consumes oxygen and ATP, releases CO2, and decreases photosynthetic output; generally occurs on hot, dry days when stomata close and the O2/CO2 ratio in the leaf increases, favoring the binding of O2 rather than Co2 by rubisco.
Rubisco adds O2 instead of CO2, producing a two carbon compound.
Photorespiration occurs in the light and consumes O2 and organic fuel and releases CO2 without producing ATP or sugar.
- Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle
- In many plants, photorespiration is a problem, because on a hot, dry day, it can drain as much as 50% of the carbon fixed by the Calvin cycle
Is photorespiration an evolutionary relic?
Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2
What are C4 plants?
Plants that prefer the Calvin cycle with an alternate mode of carbon fixation that forms a four carbon compound as its first produce; (sugarcane and corn)
In C4 plants, there are two types of photosynthetic cells:
- Bundle sheath cells
2 Mesophyll cells
What are bundle sheath cells?
Arranged into tightly packed sheaths around the veins of the leaf
What are mesophyll cells?
Loosely packed between the bundle sheath and the leaf surface
What is the 3 step process for sugar production in C4 plants?
- The first step is carried out by an enzyme present only in mesophyll cells called PEP carboxylase; this enzyme adds CO2 to phosphoenolpyruvate (PEP), forming the four carbon producet oxaloacetate. PEP carboxylase has a much higher affinity for CO2 than does rubisco and no affinity for O2 (it can fix CO2 even when CO2 concentrations are low)
- After the C4 plant fixes carbon from CO2, the mesophyll cells export their four carbon products to bundle sheath cells through the plasmodesmata
- Within the bundle sheath cells, the four carbon compounds release CO2, which is reassimilated into organic material by rubisco and the Calvin cycle. The same reaction regenerates pyruvate, which is transported to mesophyll cells. There, ATP is used to convert pyruvate to PEP, allowing the reaction cycle to continue
What are CAM plants?
Plants that utilize a mode of carbon fixation called crassulacean acid metabolism, or CAM.
- CAM plants open their stomata at night, incorporating CO2 into organic acids. The mesophyll cells of CAM plants store the organic acids they make during the night in their vacuoles until morning, when the stomata close. During the day, when the light reactions can supply ATP and NADPH for the Calvin cycle, CO2 is released from the organic acids made the night before to become incorporated into sugar in the chloroplasts.
- Pineapple, some succulents and cacti*
