Photosynthesis In Different Climates

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.

CAM plants: photosynthesizing in the desert

Although C₄ plants are well adapted to occasional periods of drought, they cannot cope well with desert conditions. A group of plants including cacti and pineapples have evolved a third type of carbon dioxide fixation which enables them to survive in very dry climates. These plants are called CAM plants. CAM is an abbreviation for crassulacean acid metabolism, a type of metabolism first observed in the family of plants called Crassulaceae (which includes the stonecrops, fleshy-leaved plants that will grow on rocks and walls).

CAM plants conserve water by only opening their stomata at night. During the night, they fix carbon dioxide into oxaloacetate which is converted into malate. This acts as a carbon dioxide storage compound. During the day, malate releases carbon dioxide into the Calvin cycle. This allows photosynthesis to take place on hot, dry, sunny days, even though the stomata are closed.

CAM plants conserve water very well and are able to survive in extremely dry conditions, but CAM plants do not photosynthesise very efficiently. Most are very slow growing. Where there is plenty of water, CAM plants cannot compete well with C₃ and C₄ plants.

CAM plants and C₄ plants have a similar metabolism: carbon dioxide is first fixed into a four-carbon intermediate before it enters the Calvin cycle. However, in CAM plants the initial fixation and the Calvin cycle occur at separate times, whereas in C₄ plants the initial fixation and the Calvin cycle are separated structurally but both occur during the day. C₄ plants live in hot, very sunny, and periodically dry environments but where lack of water is rarely a limiting factor (partly because the plants can reduce water losses due to their C₄ metabolism) and annual rainfall is high (typically, tropical rainforest-type climates); CAM plants are desert plants that live in areas of very low annual rainfall. Note that C₃, C₄, and CAM plants all eventually use the Calvin cycle to make glucose from carbon dioxide.

The Leaf

The leaf is the main site of photosynthesis, the process by which green plants manufacture their own food. The lamina or blade of a leaf is flat and thin. Its shape provides a large surface area for absorption of light and carbon dioxide. The leaf is attached to a stem or branch by a leaf stalk or petiole. The stalk holds the leaf in a position such that its surface is exposed to the maximum amount of light. From the stalk, the main vein leads down the leaf with side veins branching out on either side. These veins connect the leaf to the rest of the plant, bringing the leaf some of the raw materials required for photosynthesis, and carrying products of photosynthesis away from it. This veins also provide mechanical support, maintaining the shape of the leaf. The stem and branches raise the leaves above the ground so they are exposed to the light. On many plants the leaves are arranged on branches in such a way that they do not shade one another.