6.6. Photorespiration: Putting the Brakes on Photosynthesis

Many plants have trouble carrying out C₃ photosynthesis when the weather is hot. A cross section of a leaf here shows how it responds to hot, arid weather:

As temperatures increase in hot, arid conditions, plants partially close their leaf openings, called stomata (singular, stoma), to conserve water. As a result, you can see above that CO₂ and O₂ are not able to enter and exit the leaves through these openings. The concentration of CO₂ in the leaves falls, while the concentration of O₂ in the leaves rises. Under these conditions rubisco, the enzyme that carries out the first step of the Calvin cycle, engages in photorespiration, where the enzyme incorporates O₂, not CO₂, into the cycle and when this occurs, CO₂ is ultimately released as a by-product. Photorespiration thus short-circuits the successful performance of the Calvin cycle.

C₄ Photosynthesis

Some plants are able to adapt to climates with higher temperatures by performing C₄ photosynthesis. In this process, plants such as sugarcane, corn, and many grasses are able to fix carbon using different types of cells and chemical reactions within their leaves, thereby avoiding a reduction in photosynthesis due to higher temperatures.

A cross section of a leaf from a C₄ plant is shown in figure 6.11. Examining it, you can see how these plants solve the problem of photorespiration. In the enlargement, you see two cell types: The green cell is a mesophyll cell and the tan cell is a bundle-sheath cell. In the mesophyll cell, CO₂ combines with a three-carbon molecule instead of RuBP as it did in figure 6.10, producing a four-carbon molecule, oxaloacetate (hence the name, C₄ photosynthesis), rather than the three-carbon molecule phosphoglycerate you saw in figure 6.10. C₄ plants carry out this process in the mesophyll cells of their leaves, using a different enzyme. The oxaloacetate is then converted to malate, which is transferred to the bundle-sheath cells of the leaf. In the tan bundle-sheath cell, malate is broken down to regenerate CO₂, which enters the Calvin cycle you are familiar with from figure 6.10, and sugars are synthesized. Why go to all this trouble? Because the bundle-sheath cells are impermeable to CO₂ and so the concentration of CO₂ increases within them, so much that the rate of photorespiration is substantially lowered.

Figure 6.11. Carbon fixation in C₄ plants.

This process is called the C₄ pathway because the first molecule formed in the pathway is a four-carbon sugar, oxaloacetate. This molecule is converted into malate that is transported into bundle- sheath cells. Once there, malate undergoes a chemical reaction producing carbon dioxide. The carbon dioxide is trapped in the bundle-sheath cell, where it enters the Calvin cycle.

A second strategy to decrease photorespiration is used by many succulent (water-storing) plants such as cacti and pineapples. This mode of initial carbon fixation is called crassulacean acid metabolism (CAM) after the plant family Crassulaceae in which it was first discovered. In these plants, the stomata open during the night when it’s cooler, and close during the day. CAM plants initially fix CO₂ into organic compounds at night, using the C₄ pathway. These organic compounds accumulate at night and are subsequently broken down during the following day, releasing CO₂. These high levels of CO₂ drive the Calvin cycle and decrease photorespiration. To understand how photosynthesis differs in CAM plants and C₄ plants, examine figure 6.12. In C₄ plants (on the left), the C₄ pathway occurs in mesophyll cells, while the Calvin cycle occurs in bundle-sheath cells. In CAM plants (on the right), the C₄ pathway and the Calvin cycle occur in the same cell, a mesophyll cell, but they occur at different times of the day, the C₄ cycle at night and the Calvin cycle during the day.

Figure 6.12. Comparing carbon fixation in C₄ and CAM plants.

Both C₄ and CAM plants utilize the C₄ and C₃ pathways. In C₄ plants, the pathways are separated spatially; the C₄ pathway takes place in the mesophyll cells and the C₃ pathway (the Calvin cycle) in the bundle-sheath cells. In CAM plants, the two pathways occur in mesophyll cells but are separated temporally; the C₄ pathway is utilized at night and the C₃ pathway during the day.

Key Learning Outcome 6.6. Photorespiration occurs due to a buildup of oxygen within photosynthetic cells. C₄ plants get around photorespiration by synthesizing sugars in bundle-sheath cells, and CAM plants delay the light-independent reactions until night, when stomata are open.

Today’s Biology

Cold-Tolerant C₄ Photosynthesis

Corn (Zea mays), one of humanity's most important agricultural crops, is highly productive when grown at warm temperatures. However, its commercial use in northern areas is severely limited by its much poorer performance at low temperatures. Much of corn's high productivity results from its use of the C₄ photosynthetic pathway, which has the highest efficiency of photosynthesis known. However, much of this efficiency is lost below 20^o C. At 5^o C, 80% of photosynthesis is lost.

In C₄ species like corn, sugarcane, sorghum, and switchgrass, sensitivity to low temperatures appears to depend on the sensitivity of key C₄ photosynthetic enzymes, particularly the Calvin cycle enzyme catalyzing the final stage illustrated in figure 6.11. This enzyme has the imposing name pyruvate orthophosphate dikinase and is abbreviated PPDK. PPDK, which appears to be the rate-limiting step in corn C₄ photosynthesis, is very sensitive to low temperature, with little activity remaining when temperatures fall below 10^o C.

One relative of corn recently has been shown to be strikingly different. Chinese silver grass (Miscanthus giganteus) is a perennial grass that uses the same C₄ pathway as corn. However, in marked contrast to corn, it produces efficiently at temperatures as low as 5^o C. With its greater tolerance of low temperatures, this species thrives at chilling temperatures, with individual stalks growing as high as 13 feet! Similar temperatures severely limit C₄ photosynthesis in its relative.

What is the cause of Miscanthus's tolerance of cold? At low temperatures, when amounts of PPDK fall in corn, PPDK activity actually rises in Miscanthus. Researchers are currently examining the Miscanthus PPDK gene to better understand the cold-tolerance it confers. If these early results are confirmed, genetic engineers can explore the possibility of replacing the corn PPDK gene with the Miscanthus version, in the hope of greatly extending the northern range of corn, a key agricultural crop.

Inquiry & Analysis

Does Iron Limit the Growth of Ocean Phytoplankton?

Phytoplankton are microscopic organisms that live in the oceans, carrying out much of the earth's photosynthesis. The photo below is of Chaetoceros, a phytoplankton. Decades ago, scientists noticed "dead zones” in the ocean where little photosynthesis occurred. Looking more closely, they found that phytoplankton collected from these waters are not able to efficiently fix CO₂ into carbohydrates. In an attempt to understand why not, the scientists hypothesized that lack of iron (needed by the ETS) was the problem, and predicted that fertilizing these ocean waters with iron could trigger an explosively rapid growth of phytoplankton.

To test this idea, they carried out a field experiment, seeding large areas of phytoplankton-poor ocean waters with iron crystals to see if this triggered phytoplankton growth. Other similarly phytoplankton-poor areas of ocean were not seeded with iron and served as controls.

In one such experiment, the results of which are presented in the graph to the right, a 72-km² grid of phytoplankton-deficient ocean water was seeded with iron crystals and a tracer substance in three successive treatments, indicated with arrows on the x axis of the graph (on days 0, 3, and 7). The multiple seedings were carried out to reduce the effect of the iron crystals dissipating over time. A smaller control grid, 24 km², was seeded with just the tracer substance.

To assess the numbers of phytoplankton organisms carrying out photosynthesis in the ocean water, investigators did not actually count organisms. Instead, they estimated the amount of chlorophyll a in water samples as an easier-to-measure index. An index is a parameter that accurately reflects the quantity of another less-easily-measured parameter. In this instance, the level of chlorophyll a, easily measured by monitoring the wavelengths of light absorbed by a liquid sample, is a suitable index of phytoplankton, as this pigment is found nowhere else in the ocean other than within phytoplankton.

Chlorophyll a measurements were made periodically on both test and control grids for 14 days.

The results are plotted on the graph. Red points indicate chlorophyll a concentrations in iron-seeded waters; blue points indicate chlorophyll a levels in the control grid waters that were not seeded.

1. Applying Concepts

a. Variable. In the graph above, which is the dependent variable?

b. Index. What does the increase in levels of chlorophyll a say about numbers of phytoplankton?

c. Control. What substance is lacking in the waters sampled in the blue-dot plots?

2. Interpreting Data

a. What happened to the levels of chlorophyll a in the test areas of the ocean (red dots)?

b. What happened to the levels of chlorophyll a in the control areas (blue dots)?

c. Comparing the red line to the blue line, about how many times more numerous are phytoplankton in iron-seeded waters on the three days of seeding?

3. Making Inferences

a. What general statement can be made regarding the effect of seeding phytoplankton-poor regions of the ocean with iron?

b. Why did chlorophyll a levels drop by day 14?

4. Drawing Conclusions Do these results support the claim that lack of iron is limiting the growth of phytoplankton, and thus of photosynthesis, in certain areas of the oceans?

5. Further Analysis Based on this experiment, what would be a potential drawback of using this method of seeding with iron to increase levels of ocean photosynthesis?

Test Your Understanding

1. The energy that is used by almost all living things on our planet comes from the sun. It is captured by plants, algae, and some bacteria through the process of

a. thylakoid.

b. chloroplasts.

c. photosynthesis.

d. the Calvin cycle.

2. Plants capture sunlight

a. through photorespiration.

b. with molecules called pigments that absorb photons and use their energy.

c. with the light-independent reactions.

d. with the electron transport system.

3. Visible light occupies what part of the electromagnetic spectrum?

a. the entire spectrum

b. the upper half of the spectrum (with longer wavelengths)

c. a small portion in the middle of the spectrum

d. the lower half of the spectrum (with shorter wavelengths)

4. The colors of light that are absorbed by chlorophyll are

a. red and blue.

b. green and yellow.

c. infrared and ultraviolet.

d. All colors are equally effective.

5. Once a plant has initially captured the energy of a photon,

a. a series of reactions occurs in thylakoid membranes of the cell.

b. the energy is transferred through several steps into a molecule of ATP.

c. a water molecule is broken down, releasing oxygen.

d. All of the above.

6. Plants use two photosystems to capture energy used to produce ATP and NADPH. The electrons used in these photosystems

a. recycle through the system constantly, with energy added from the photons.

b. recycle through the system several times and then are lost due to entropy.

c. only go through the system once; they are obtained by splitting a water molecule.

d. only go through the system once; they are obtained from the photon.

7. During photosynthesis, ATP molecules are generated by

a. the Calvin cycle.

b. chemiosmosis.

c. the splitting of a water molecule.

d. photons of light being absorbed by chlorophyll molecules.

8. NADPH is recycled during photosynthesis. It is produced during the _____ and used in the _____.

a. electron transport system of photosystem I, Calvin cycle

b. process of chemiosmosis, Calvin cycle

c. electron transport system of photosystem II, electron transport system of photosystem I

d. light-independent reactions, light-dependent reactions

9. The overall purpose of the Calvin cycle is to

a. generate molecules of ATP.

b. generate NADPH.

c. build sugar molecules.

d. produce oxygen.

10. Many plants cannot carry out the typical C₃ photosynthesis in hot weather, so some plants

a. use the ATP cycle.

b. use C₄ photosynthesis or CAM.

c. shut down photosynthesis completely.

d. All of these are true for different plants.