C₃ versus C₄ Plants

In your reading in chapter 10 you learned about the process of photorespiration, in which the enzyme rubisco catalyzes the addition of oxygen (rather than CO₂) to the compound RuBP, the starting molecule for the Calvin Cycle. When O₂ is added instead of CO₂, RuBP cannot enter the Calvin cycle and is instead oxidized to CO₂, without production of ATP. In other words, O₂ and CO₂ are competitors for the same active site on the rubisco enzyme. In addition, rubisco is one of the slowest enzymes known, which means that the concentration of rubisco proteins in the chloroplast is much higher than that of most enzymes. When the concentration of CO₂ is high and that of O₂ is low, the addition of CO₂ is favored and carbohydrate synthesis by the Calvin cycle proceeds. But when the reverse conditions prevail—when the concentration of CO₂ is low and that of O₂ is high—O₂ is added and RuBP is broken down to CO₂. Photorespiration is a rather wasteful process, short-circuiting the Calvin cycle while generating very little energy for use in the cell.

Recall that in the Calvin cycle, CO₂ is initially “fixed” (incorporated into) RuBP, which immediately splits into two three-carbon molecules (PGA, or 3-phosphoglycerate). Accordingly, plants that fix CO₂ in this way are referred to as C₃ plants. An alternative method of fixing CO₂ is found in plants with a distinct leaf structure known as Kranz anatomy, from the German word Kranz, for “wreath,” referring to the ringlike arrangement of photosynthetic cells around the leaf veins of these plants (Fig. B). In Kranz plants, unlike C₃ plants, the bundle-sheath cells surrounding the vein have numerous chloroplasts, and the mesophyll cells that correspond to the palisade layer are clustered in a ring arrangement around the bundle-sheath cells. The chloroplasts in the bundle sheath cells and mesophylls usually differ in a number of ways. In the bundle–sheath cells the chloroplasts are bigger, they accumulate large amounts of starch in the presence of light, and the grana are few and poorly developed; in the mesophyll cells the chloroplasts are smaller, they usually do not accumulate much starch, and they have numerous large grana (see figure). (Recall that the light-dependent reactions of photosynthesis occur in the grana whereas the Calvin cycle takes place in the stroma.

Why are Kranz plants called C₄ plants? Under conditions of high temperature and intense light, the stomates of all plants tend to close and most plants (C₃ plants) will carry on photorespiration, but Kranz plants do not, due to their special way of fixing CO₂ initially. In Kranz plants—also known as C₄ plants—CO₂ is combined with a three-carbon compound in the mesophyll cells, forming a four-carbon compound (C₄) that passes into the bundle-sheath cells. In the bundle-sheath cells the C₄ compound is then broken down to CO₂ and another C₃ compound. The C₃ compound moves back into the mesophyll cells, where it is converted into another compound and starts the C₄ cycle over again. The CO₂, however, remains in the bundle-sheath cells, where it can enter the Calvin cycle and be incorporated into carbohydrate. In effect, the mesophyll cells act as CO₂ pumps, transferring enough CO₂ (via the C₄ intermediate) into the bundle-sheath cells to maintain an artificially high CO₂ concentration in which the Calvin cycle is able to function. Study the slides of Ligustrum and Zea in the next section to learn the anatomy of the two types of leaves. Also study Fig. 10.18 (p. 192) of your text to learn the reactions involved in the C₄ pathway.

The combination of Kranz anatomy and C₄ photosynthesis has evolved independently in a variety of unrelated plants, including a number of major crops such as corn, sugarcane, and sorghum. It is therefore not only an impressive illustration of the intimate relationship between structure and function in living systems, but also provides an important source of nutrients for human consumption.

Figure A. Anatomy of C₃ and C₄ (Kranz) leaves: In a C₃ leaf the palisade mesophyll cells typically form a layer in the upper part of the leaf; the corresponding mesophyll cells in a C₄ leaf are usually arranged in a ring around the bundle sheath. While the bundle-sheath cells of C₄ plants have chloroplasts (dark green), those of C₃ leaves usually lack them.

Two Kinds of Chloroplasts in a C₄ Leaf

(click on image for larger version)

Figure B. In this electron micrograph a segment of a corn leaf is part of a bundle-sheath cell; its chloroplasts have a small grana and contain starch grains (light areas). At right and bottom are parts of two mesophyll cells; their chloroplasts are smaller and contain numerous grana but no starch grains.

Comparison of Photosynthetic Efficiency in C₃ and C₄ Plants

Figure C. (click image at left to enlarge)

(A) Corn can fix carbon at CO₂ concentrations as low as one part per million, and it carries out photosynthesis at a very high rate at concentrations of 200-300 ppm (a normal concentration of CO₂ in the atmosphere is about 330 ppm). By contrast, beans perform no net carbon fixation at CO₂ concentrations below about 50 ppm, and their rate of photosynthesis at concentrations of 200-300 ppm is not very high.

(B) Photosynthesis in corn shows no inhibition at all at O₂ concentrations below 65 percent, whereas the photosynthetic rate of beans falls steadily as the O₂ concentration rises (a normal concentration of O₂ in the atmosphere is about 21 percent). Both A and B are for a temperature of 20^oC and a light intensity of 2,000 foot-candles. Obviously, C₄ photosynthesis is superior under these conditions. However, when the temperature or illumination varies, while the O₂ and CO₂ concentrations remain at normal levels, a very different picture emerges.

For example, as the temperature drops (C), C₃ plants clearly perform more efficiently, so they have the advantage in cooler (that is, more temperate) climates.