Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates Flashcards
In most plants, initial fixation of carbon occurs via rubisco, the Calvin cycle enzyme that adds to ribulose bisphosphate. Such plants are called
C3 plants because the first organic product of carbon fixation is a three-carbon compound, 3-phosphoglycerate
When their stomata partially close on hot, dry days, C3 plants produce less sugar because the
declining level of in the leaf starves the Calvin cycle
In addition, rubisco is capable of binding O2 in place of CO2. As CO2 becomes scarce within the air spaces of the leaf and O2 builds up, rubisco adds
O2 to the Calvin cycle instead of CO2.
The product splits, and a two-carbon compound leaves the
chloroplast.
Peroxisomes and mitochondria within the plant cell rearrange and split this compound, releasing .
CO2
The process is called ____________________ because it occurs in the light (photo) and consumes O2 while producing CO2 (respiration).
photorespiration
However, unlike normal cellular respiration, photorespiration uses
ATP rather than generating it.
And unlike photosynthesis, photorespiration produces no
sugar.
In fact, photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle and releasing
CO2 that would otherwise be fixed
According to one hypothesis, photorespiration is evolutionary baggage—a metabolic relic from a much earlier time when the atmosphere had less
O2 and more CO2 than it does today.
The hypothesis suggests that modern rubisco retains some of its chance affinity for O2, which is now so concentrated in the atmosphere that a certain amount of
photorespiration is inevitable.
photorespiration drains away as much as ____ of the carbon fixed by the Calvin cycle.
50%
In some plant species, alternate modes of carbon fixation have evolved that minimize photorespiration and optimize the
Calvin cycle—even in hot, arid climates.
The two most important of these photosynthetic adaptations are
C4 photosynthesis and crassulacean acid metabolism (CAM).
are so named because they preface the Calvin cycle with an alternate mode of carbon fixation that forms a four-carbon compound as its first product.
C4 Plants
The anatomy of a C4 leaf is correlated with the mechanism of
C4 photosynthesis
In C4 plants, there are two distinct types of photosynthetic cells:
bundle-sheath cells and mesophyll cells.
are arranged into tightly packed sheaths around the veins of the leaf
Bundle-sheath cells
Between the bundle sheath and the leaf surface are the more loosely arranged_______________, which, in leaves, are closely associated and never more than two to three cells away from the bundle-sheath cells.
mesophyll cells
The Calvin cycle is confined to the chloroplasts of the
bundle-sheath cells.
However, the Calvin cycle is preceded by incorporation of CO2 into organic compounds in the
mesophyll cells.
The first step is carried out by an enzyme present only in mesophyll cells called
PEP carboxylase
- After the is fixed in the mesophyll cells, the four-carbon product are exported to
bundle-sheath cells through plasmodesmata
- Within the bundle-sheath cells, the four-carbon compounds release , which is re-fixed into organic material by
rubisco and the Calvin cycle.
In effect, the mesophyll cells of a C4 plant pump CO2 into the bundle-sheath cells, keeping the CO2 concentration in those cells high enough for rubisco to bind CO2 rather than
O2.
The cyclic series of reactions involving PEP carboxylase and the regeneration of PEP can be thought of as an ATP-powered pump that concentrates
CO2.
In this way, C4 photosynthesis spends ATP energy to minimize photorespiration and enhance
sugar production.
Which type of plant would stand to gain more from increasing CO2 levels? Recall that in C3 plants, the binding of O2 rather than CO2 by rubisco leads to
photorespiration, lowering the efficiency of photosynthesis.
C4 plants overcome this problem by concentrating CO2 in the bundle-sheath cells at the cost of
ATP.
Rising CO2 levels should benefit C3 plants by lowering the amount of
photorespiration that occurs.
C4 photosynthesis is considered more efficient than C3 photosynthesis because it uses
less water and resources
A second photosynthetic adaptation to arid conditions has evolved in many
succulent (water-storing) plants, numerous cacti, pineapples, and representatives of several other plant families
Closing stomata during the day helps desert plants conserve water, but it also prevents
CO2 from entering the leaves
During the night, when their stomata are open, these plants take up and incorporate it into a variety of organic acids. This mode of carbon fixation is called _________________________________ after the plant family Crassulaceae, the succulents in which the process was first discovered.
crassulacean acid metabolism, or CAM,
The mesophyll cells of ______________ store the organic acids they make during the night in their vacuoles until morning, when the stomata close.
CAM plants
