Photosynthetic Pathways - OpenStax CNX [PDF]

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Photosynthetic Pathways

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Robert Bear David Rintoul This work is produced by OpenStax-CNX and licensed under the Creative Commons Attribution License 4.0„

You can't have a light without a dark to stick it in

." Arlo Guthrie, American musician

1 In a previous module, you learned about photosynthesis, the mechanism plants use to convert solar energy into chemical energy. The light energy captured is used to make ATP and NADPH, which is then used to reduce carbon from a simple form (CO2 ) into a more complex form (sugars). The rst step of the Calvin cycle is the xation of carbon dioxide to RuBP, and the plants that only use this mechanism of carbon xation are called C3 plants. About 85% of the plant species on the planet are C3 plants; some examples are rice, wheat, soybeans and all trees. The process of photosynthesis has a theoretical eciency of 30% (i.e., the maximum amount of chemical energy output would be only 30% of the solar energy input), but in reality the eciency is much lower. It is only about 3% on cloudy days. Why is so much solar energy lost? There are a number of factors contributing to this energy loss, and one metabolic pathway that contributes to this low eciency is photorespiration. During photorespiration, the key photosynthetic enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase oxygenase) uses O2 as a substrate instead of CO2 . This process uses up a considerable amount of energy without making sugars (Figure 1). When a plant has its stomata open (CO2 is diusing in while O2 and water are diusing out), photorespiration is minimized because Rubisco has a higher anity for CO2 than

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for O2 when air temperatures are below 30 C (86 F). However, when a plant closes its stomata during times of water stress and O2 from photosynthesis builds up inside the cell, the rate of photorespiration increases because O2 is now more abundant inside the mesophyll. So, there is a tradeo. Plants can leave the stomata open and risk drying out, or they can close the stomata, thereby reducing the uptake of CO2 , and decreasing the eciency of photosynthesis. In addition, Rubisco has a higher anity for O2 when temperatures increase, which means that C3 plants use more energy (ATP) for photorespiration at higher temperatures.

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Figure 1: A comparison of photorespiration and carbon xation in C3 plants. During photorespiration, O2 is bound to RuBP and forms phosphoglycolate (PG) and Phosphoglycerate (PGA), PG then undergoes an number energy requiring reactions releasing CO2. Work by Eva Horne.

Evolutionarily speaking, why is photorespiration still around? One hypothesis is that it is evolutionary baggage from a time when the atmosphere had a lower O2 concentration than it does today.

In other

words, when Rubisco rst evolved millions of years ago, the O2 concentration was so low that excluding O2 from its binding site had little or no inuence on the eciency of photosynthesis. The modern Rubisco retains some of its ancestral anity for O2 , which leads to the energy costs associated with photorespiration. However, plant cell physiologists are discovering that there might be some metabolic benets associated with photorespiration, which would help explain why this seemingly wasteful pathway is still found in plants. Adding to the dilemma is the fact that when plant geneticists knock out Rubisco's ability to x O2 , Rubisco also loses its ability to x CO2 .

It is possible that the active site of this enzyme cannot be

engineered, by articial or natural selection, so that it exclusively binds CO2 and not O2 .

2 C4 plant and CAM Pathways as a Means of Reducing Photorespiration The C4 and CAM pathways for xing CO2 are two adaptations that improve the eciency of photosynthesis, by ensuring that Rubisco encounters high CO2 concentrations and thus reduces photorespiration. These two photosynthetic adaptations for xing CO2 have evolved independently a number of times in species that evolved from wet and dry, but typically warm climates. Why have these mechanisms evolved independently so many times? Plants that minimize photorespiration may have a signicant competitive advantage, because a considerable amount of energy (in the form of ATP) is lost in plants during photorespiration. In many environments, plants that use solar energy more eciently should out-compete those which are less ecient.

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2.1 C4 Pathway

In C4 plants, the light-dependent reactions and the Calvin cycle are physically separated, with the lightdependent reactions occurring in the mesophyll cells and the Calvin cycle occurring in special cells that surround the veins in the leaves. These cells are called bundle-sheath cells. How does this work? Atmospheric CO2 is xed in the mesophyll cells as a simple 4-carbon organic acid (malate) by an enzyme that has no anity for O2 .

Malate is then transported to the bundle-sheath cells.

Inside the bundle sheath, malate

is oxidized to a 3-C organic acid, and in the process, 1 molecule of CO2 is produced from every malate molecule (Figure 2). The CO2 is then xed by Rubisco into sugars, via the Calvin cycle, exactly as in C3 photosynthesis. There is an additional cost of two ATPs associated with moving the three-carbon ferry molecule from the bundle sheath cell back to the mesophyll to pick up another molecule of atmospheric CO2 . Since the spatial separation in bundle-sheath cells minimizes O2 concentrations in the locations where Rubisco is located, photorespiration is minimized (Figure 3). This arrangement of cells reduces photorespiration and increases the eciency of photosynthesis for C4 plants. In addition, C4 plants require about half as much water as a C3 plant. The reason C4 plants require less water is due to the fact that the physical shape of the stomata and leaf structure of C4 plants helps reduce water loss by developing a large CO2 concentration gradient between the outside of the leaf (400 ppm) and the mesophyll cells (10 ppm). concentration gradient reduces water loss via transpiration through the stomata.

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The large CO2

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Figure 2: Cross section of a C3 and C4 plant leaf. Work by Eva Horne

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Figure 3: The spatial separation of Carbon xation and the Calvin cycle in C4 plants. Work by Eva Horne

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The C4 pathway is used in about 3% of all vascular plants; some examples are crabgrass, sugarcane and corn. C4 plants are common in habitats that are hot, but are less abundant in areas that are cooler, because the enzyme that xes the CO2 in the mesophyll is less ecient at lower temperature. One hypothesis for the abundance of C4 plants in hot habitats is that the benets of reduced photorespiration and water loss exceeds the ATP cost of moving the the CO2 from the mesophyll cell to bundle-sheath cell. 2.2 CAM

Many plants such as cacti and pineapples, which are adapted to arid environments, use a dierent energy and water saving pathway called crassulacean acid metabolism (CAM). This name comes from the family of plants (Crassulaceae) in which scientists rst discovered the pathway.

Instead of separating the light-

dependent reactions and the use of CO2 in the Calvin cycle spatially, CAM plants separate these processes temporally (Figure 4).

At night, CAM plants open their stomata, and an enzyme in the mesophyll cells

x the CO2 as an organic acid and store the organic acid in vacuoles until morning. During the day the light-dependent reactions supply the ATP and NADPH necessary for the Calvin cycle to function, and the CO2 is released from those organic acids and used to make sugars. Plant species using CAM photosynthesis are the most water ecient of all; the stomata are only open at night when humidity is typically higher and the temperatures are much cooler (which serves to lower the diusive gradient driving water loss from leaves).

The CAM pathway is primarily an adaptation to water-limited environments; the fact that this

pathway also stops photorespiration is an added benet.

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Figure 4: Temporal separation of Carbon xation and the Calvin cycle in CAM plants. Work by Eva Horne

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Overall, C3 , C4 and CAM plants all use the Calvin cycle to make sugars from CO2 . However, the various ways in which plants x CO2 varies with the advantages and disadvantages associated with the mechanism and the habitats where plants can be found (Table 1). As humans continue to burn fossil fuels, CO2 levels in the atmosphere will continue to increase. This human alteration of the environment has sparked the development of a number of interesting questions. What inuence will increasing CO2 have on the distributions of C3 , C4 and CAM plants? What inuence will increasing CO2 have on agricultural production? Is it possible that an increase in agricultural production by additional CO2 in the atmosphere could oset or mitigate the decrease in agricultural production caused by climate change?

Cost

C3 plant

C4 plant

Photorespiration

ATP

CAM Plant

cost

associated

with

xing

twice.

Carbon xation

is

less

carbon

ecient

Reduced xed

amount

carbon,

of

stomata

only open at night

under

cold conditions. Benets

Carbon xation without

Reduced

using ATP

tion and ability to x

tion and reduced water

Carbon under high tem-

loss

peratures

photorespira-

and

Reduced

photorespira-

reduced

water loss Separation

light-

None, all of these reac-

Spatial, these two sets of

Temporal,

reactions

tions occur in the same

reactions occur in dier-

sets of reactions occur at

and carbon xation

cells

ent cells

dierent times of day

Habitat

Cool and moist

Hot, not in cold environ-

Hot and dry, large tem-

ments (see cost.)

perature dierential be-

dependent

of

these

tween night and day

Table 1:

Characteristics of C3 , C4 and CAM methods of xing CO2

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