Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates

Question 1

In most plants, initial fixation of carbon occurs via rubisco, the Calvin cycle enzyme that adds to ribulose bisphosphate. Such plants are called

Answer 1

C3 plants because the first organic product of carbon fixation is a three-carbon compound, 3-phosphoglycerate

Question 2

When their stomata partially close on hot, dry days, C3 plants produce less sugar because the

Answer 2

declining level of in the leaf starves the Calvin cycle

Question 3

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

Answer 3

O2 to the Calvin cycle instead of CO2.

Question 4

The product splits, and a two-carbon compound leaves the

Answer 4

chloroplast.

Question 5

Peroxisomes and mitochondria within the plant cell rearrange and split this compound, releasing .

Answer 5

CO2

Question 6

The process is called ____________________ because it occurs in the light (photo) and consumes O2 while producing CO2 (respiration).

Answer 6

photorespiration

Question 7

However, unlike normal cellular respiration, photorespiration uses

Answer 7

ATP rather than generating it.

Question 8

And unlike photosynthesis, photorespiration produces no

Answer 8

sugar.

Question 9

In fact, photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle and releasing

Answer 9

CO2 that would otherwise be fixed

Question 10

According to one hypothesis, photorespiration is evolutionary baggage—a metabolic relic from a much earlier time when the atmosphere had less

Answer 10

O2 and more CO2 than it does today.

Question 11

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

Answer 11

photorespiration is inevitable.

Question 12

photorespiration drains away as much as ____ of the carbon fixed by the Calvin cycle.

Answer 12

50%

Question 13

In some plant species, alternate modes of carbon fixation have evolved that minimize photorespiration and optimize the

Answer 13

Calvin cycle—even in hot, arid climates.

Question 14

The two most important of these photosynthetic adaptations are

Answer 14

C4 photosynthesis and crassulacean acid metabolism (CAM).

Question 15

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.

Answer 15

C4 Plants

Question 16

The anatomy of a C4 leaf is correlated with the mechanism of

Answer 16

C4 photosynthesis

Question 17

In C4 plants, there are two distinct types of photosynthetic cells:

Answer 17

bundle-sheath cells and mesophyll cells.

Question 18

are arranged into tightly packed sheaths around the veins of the leaf

Answer 18

Bundle-sheath cells

Question 19

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.

Answer 19

mesophyll cells

Question 20

The Calvin cycle is confined to the chloroplasts of the

Answer 20

bundle-sheath cells.

Question 21

However, the Calvin cycle is preceded by incorporation of CO2 into organic compounds in the

Answer 21

mesophyll cells.

Question 22

The first step is carried out by an enzyme present only in mesophyll cells called

Answer 22

PEP carboxylase

Question 23

2. After the is fixed in the mesophyll cells, the four-carbon product are exported to

Answer 23

bundle-sheath cells through plasmodesmata

Question 24

3. Within the bundle-sheath cells, the four-carbon compounds release , which is re-fixed into organic material by

Answer 24

rubisco and the Calvin cycle.

Question 25

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

Answer 25

O2.

Question 26

The cyclic series of reactions involving PEP carboxylase and the regeneration of PEP can be thought of as an ATP-powered pump that concentrates

Answer 26

CO2.

Question 27

In this way, C4 photosynthesis spends ATP energy to minimize photorespiration and enhance

Answer 27

sugar production.

Question 28

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

Answer 28

photorespiration, lowering the efficiency of photosynthesis.

Question 29

C4 plants overcome this problem by concentrating CO2 in the bundle-sheath cells at the cost of

Answer 29

ATP.

Question 30

Rising CO2 levels should benefit C3 plants by lowering the amount of

Answer 30

photorespiration that occurs.

Question 31

C4 photosynthesis is considered more efficient than C3 photosynthesis because it uses

Answer 31

less water and resources

Question 32

A second photosynthetic adaptation to arid conditions has evolved in many

Answer 32

succulent (water-storing) plants, numerous cacti, pineapples, and representatives of several other plant families

Question 33

Closing stomata during the day helps desert plants conserve water, but it also prevents

Answer 33

CO2 from entering the leaves

Question 34

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.

Answer 34

crassulacean acid metabolism, or CAM,

Question 35

The mesophyll cells of ______________ store the organic acids they make during the night in their vacuoles until morning, when the stomata close.

Answer 35

CAM plants