Text 7.3. Photosynthesis In Different Climates

■ Essential targets:

By the end of this text you should be able to:

· distinguish between C₃ and C₄ plants;

· explain the advantages and disadvantages of crassulacean acid metabolism (CAM);

· give examples of C_3, C_4, and CAM plants.

Pre-reading

■ With a partner, consider the following questions and try to answer them. Then quickly scan the text to check your answers.

1. What is necessary for plants to survive in different climates?

2. Do you know the ways of fixing carbon dioxide?

■ Read the given text and make your essential assignments:

Green plants thrive in environments ranging from hot and dry equatorial regions to freezing-cold polar regions. Their success depends on their adaptability. To survive and breed, each plant has had to evolve specific adaptations to cope with the demands of its particular environment. These adaptations include ways of fixing carbon dioxide.

C₃ plants: fixing directly into the Calvin cycle

C₃ plantsfix carbon dioxide directly into the Calvin cycle as the three-carbon compound glycerate 3-phosphate (GP). Common and widely distributed, they include some of our most important crop plants such as wheat, soya beans, and rice. C₃ plants function efficiently in temperature conditions. However, they suffer two major disadvantages in hot, dry environments.

First, to obtain sufficient carbon dioxide, C₃ plants must open their stomata (small pores in their leaves). Unfortunately, when stomata are open, they not only allow carbon dioxide to enter the plant, but also allow water to escape. So in hot dry conditions C₃ plants have to either cease photosynthesising or run the risk of wilting and dying.

The second disadvantage relates to the ability of ribulose biphosphate carboxylase (ribosco) to combine with oxygen. Ribosco is the enzyme that catalyses carbon dioxide fixation. On a hot, sunny day carbon dioxide concentrations around photosynthesising cells decrease, because a large proportion of the carbon dioxide is being used up on photosynthesis. In these conditions, ribosco combines with oxygen rather than carbon dioxide in a process called photorespiration. The process results in the loss of fixed carbon dioxide from the plant, reducing photosynthetic efficiently and plant growth. Unlike photosynthesis, photorespiration does not produce sugar molecules; and unlike respiration, it yields no ATP. As much as half of the carbon dioxide fixed in the Calvin cycle may be released by photorespiration. Therefore, in hot, arid conditions, or in conditions where carbon dioxide levels are low, C₃ plants do not grow well.

C₄ plants: the Hatch-Slack pathway

C₄ plantshave evolved a special metabolic adaptation which reduces photorespiration. They do not use ribulose biphosphate (RuBP) to fix carbon dioxide directly into the Calvin cycle. Instead, they use phosphoenolpyruvate (PEP) to fix carbon dioxide as a four-carbon compound, oxaloacetate. The reaction is catalysed by phosphoenolpyruvate carboxylase (PEP, carboxylase). This enzyme cannot combine with oxygen. Consequently C₄ plants can continue to fix carbon dioxide even when its concentration is very low.

The leaves of C₄ plants are specially adapted to carry out this initial fixation. A ring of large closely packed cells called the bundle sheath surrounds the leaf veins. Surrounding the bundle sheath is a smaller ring of mesophyll cells. The distinctive arrangement is called Kranz anatomy and can be used to identify C₄ plants (“Kranz” means crown or halo and refers to the two distinctive rings). The initial fixation of carbon dioxide into oxaloacetate takes place in the small ring of mesophyll cells. Then the oxaloacetate is converted to malate, another four-carbon compound. Malate is transported into the bundle sheath cells where it releases carbon dioxide. Once released, the carbon dioxide is reassimilated by RuBP and enters the Calvin cycle in the same way as described for C₃ plants. The metabolic pathway that transports carbon dioxide into the bundle sheath cells is called the Hatch-Slack pathway. As a result of this pathway, the concentration of carbon dioxide in the bundle sheath cells is 20 to 120 times higher than normal.

C₄ plants have two main advantages in hot, dry environments. First, because PEP carboxylase has a high affinity for carbon dioxide and does not combine with oxygen, C₄ plants can continue to photosynthesise even when their stomata are closed for long periods. This reduces water loss and photorespiration. C₄ plants need only about half as much water as C₃ plants for photosynthesis. Secondly, because high carbon dioxide concentrations can be maintained in the bundle sheath cells, C₄ plants can increase their photosynthetic efficiency.

These adaptations enable C₄ plants to outcome C₃ plants in hot and very sunny conditions, but not in temperate conditions. Fewer than 0.5 per cent of plant species are C₄ plants, yet they include economically important crops such as maize, sugar cane, and millet.

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