from the light reactions
Figure 10.10 The Calvin cycle (light-independent reactions) of photosynthesis. The cycle takes place in the stroma of chloroplasts where each step is mediated by an enzyme. Carbon dioxide molecules from the air enter the cycle one at a time, making six turns of the cycle necessary to produce one molecule of a 6-carbon sugar such as glucose (C6H12O6).
5. This leaves a net gain of two GA3P molecules, which can contribute either to an increase in the carbohydrate content of the plant (glucose, starch, cellulose, or related substances) or can be used in pathways that lead to the net gain of lipids and amino acids.
Since rubisco catalyzes formation of the 3-carbon compound 3PGA as the first isolated product in these light-independent reactions, plants demonstrating this process are called C3 plants. However, as indicated by its name, the enzyme RuBP carboxylase/oxygenase has the potential to fix both CO2 through its carboxylase activity described by the Calvin cycle and O2 through its oxygenase activity. The oxygenase activity of rubisco makes C3 plants vulnerable to a process called photorespiration.
Photorespiration When the weather is hot and dry, stomata are generally closed. The closed stomata prevent carbon dioxide from entering the leaf, and the concentration of carbon dioxide within drops steadily while the relative concentration of oxygen increases. When the carbon dioxide concentration drops below roughly 50 parts per million, rubisco starts fixing oxygen instead of carbon dioxide, initiating photorespiration. Like photosynthesis, the process of photorespiration requires light because regeneration of RuBP requires ATP and NADPH, presumably from the light-dependent reactions. When photorespiration occurs, RuBP reacts with oxygen, releases carbon dioxide, and if the Calvin cycle is functioning, is in competition with it. Accordingly, photorespiration is generally thought of as an inefficient pathway associated with C3 plants under hot, dry conditions. Although photorespiration often increases under drought stress, the oxygen-binding properties of rubisco are not directly affected by the reduced availability of water. Rather, water stress induces stomatal closure, which then decreases the CO2:O2 ratio in the leaf, favoring respiration. While the stomata are closed, the concentration of carbon dioxide within the leaf drops steadily while the relative concentration of oxygen increases.
The products of photorespiration are the 2-carbon phos-phoglycolic acid, which in mitochondria and peroxisomes ultimately releases carbon dioxide in mitochondria and perox-isomes, and the 3-carbon phosphoglyceric acid that can reenter the Calvin cycle. No ATP is produced by photorespiration.
The 4-Carbon Pathway Sugar cane, corn, sorghum, and at least 1,000 other species of tropical grasses or arid region plants subject to high temperature stresses have a distinctive leaf anatomy called Kranz anatomy (Fig. 10.11). Kranz anatomy leaves have two forms of chloroplasts. In the bundle sheath cells surrounding the veins, there are large chloroplasts, often with few to no grana. The large chloro-plasts have numerous starch grains, and the grana, when present, are poorly developed. The chloroplasts of the mesophyll cells are much smaller, usually lack starch grains, and have well-developed grana.
Plants with Kranz anatomy produce a 4-carbon compound, oxaloacetic acid, instead of the 3-carbon 3-phospho-glyceric acid during the initial steps of the light-independent reactions. Oxaloacetic acid is produced when a 3-carbon compound, phosphoenolpyruvate (PEP), and carbon dioxide are combined in mesophyll cells with the aid of a different carbon-fixing enzyme, PEP carboxylase. Depending on the species, the oxaloacetic acid may then be converted to
bundle sheath stoma epidermal xylem bundle sheath stoma epidermal xylem
aspartic, malic, or other acids. PEP carboxylase is not sensitive to oxygen and hence has a greater enzyme affinity for carbon dioxide, so there is no photorespiratory loss of carbon dioxide captured in the organic acids. The carbon dioxide is transported to the bundle sheath cells where it is released and enters the normal Calvin cycle just as in C3 plants. The carbon dioxide concentration can be kept high in relation to the oxygen concentration in the bundle sheath cells, thereby keeping the reaction of rubisco with oxygen very low (Fig. 10.12).
Because the PEP system produces 4-carbon compounds, plants having this system are called 4-carbon, or C4, plants, to distinguish them from plants that have only the 3-carbon, or C3, system. Besides Kranz anatomy, C4 plants have other characteristic features:
1. High concentrations of PEP carboxylase are found in the mesophyll cells. This is significant because PEP car-boxylase allows the conversion of carbon dioxide to carbohydrate at much lower concentrations than does rubisco (found only in the bundle sheath cells), the corresponding enzyme in the Calvin cycle.
2. The optimum temperatures for C4 photosynthesis are much higher than those for C3 photosynthesis, allowing C4 plants to thrive under conditions that would adversely affect C3 plants.
Obviously, the C4 pathway furnishes carbon dioxide to the Calvin cycle in a more roundabout way than does the C3 pathway, but the advantage of this extra pathway is a major reduction of photorespiration in C4 plants. Also, in C4 plants, the C4 pathway in the mesophyll cells results in carbon dioxide being picked up even at low concentrations (via the enzyme PEP carboxylase) and in carbon dioxide being concentrated in the bundle sheath, where the Calvin cycle takes place almost exclusively. The Calvin cycle enzyme, rubisco, will catalyze the reaction in which RuBP will react with carbon dioxide rather than oxygen. Consequently, C4 plants, which typically photosynthesize at higher temperatures, have photosynthetic rates that are two to three times higher than those of C3 plants. However, at lower temperatures, C3 photosynthesis is more efficient than that of C4 plants because the cost of photorespiration at those temperatures is usually less than the two extra ATPs required for the C4 pathway.
CAM Photosynthesis Crassulacean acid metabolism (CAM) photosynthesis is found in plants of about 30 families, including cacti, stonecrops, orchids, bromeliads, and other succulents that are often stressed by limited availability of water. A few succulents do not have CAM photosynthesis, however, and several nonsucculent plants do. Many CAM plants are facultative C3 plants that can switch to C3 photosynthesis during the day after a good rain or when night temperatures are high. Plants with CAM photosynthesis typically do not have a well-defined palisade mesophyll in the leaves, and, in contrast to the chloroplasts of the bundle sheath cells of C4 plants, those of CAM photosynthesis plants resemble the mesophyll cell chloroplasts of C3 plants.
CAM photosynthesis is similar to C4 photosynthesis in that 4-carbon compounds are produced during the light-independent reactions. In these plants, however, the organic acids
Plant Metabolism vascular bundle (vein)
air space beneath stoma mesophyll cells bundle sheath cell portion of a cross section of a leaf with C4 photosynthesis mesophyll cell
, CO2 from air oxaloacetic acid PEP \ ^—NADPH
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.