The light energy breaks down the water and creates chemical energy. This in turn is used to split apart the carbon dioxide into carbon and oxygen. The oxygen is released back into the environment while carbon and hydrogen are combined to make something new: glucose! It can be converted into a tough, durable molecule called cellulose, which forms the cell walls of the plant and allows it to continue to grow.
The baking sun and lack of water alone would be enough to kill most plants. Soft, broad leaves might be good at photosynthesis, but they lose a lot of water due to evaporation. These new leaves were good at two things: avoiding water loss and protecting the plant On this topic, I wrote a post on why cacti adapted to have spines instead of leaves.
They were useless when it came to photosynthesis, though, so instead, the stems of the cacti were put to work. Unlike many large plants, the stems of cacti are usually green with chlorophyll, indicating that this is where photosynthesis is taking place. The biggest reason for water loss is transpiration, or evaporation from the leaves or surface of the plant.
But too much can cause the plant to shrivel and die. These pores are also where the plant releases the oxygen it creates during photosynthesis, and takes up the carbon dioxide in needs to continue building sugars. Leaving the stomata open during the heat of the day drastically speeds up the loss of water, so instead, cacti hold their breath until evening. With no light energy to drive the process, cacti have to store their carbon dioxide until dawn, at which point they can begin to photosynthesize as usual.
The carbon dioxide molecules are stored in the form of malic acid, which is part of the reason this unusual adaptation is called Crassulacean Acid Metabolism , or CAM. CAM is used by lots of different types of plants, especially those that live in resource-poor environments. Most desert-dwellers use some form of CAM, whether all the time or just when water is particularly tight. Other types of plants, including some aquatic ones, use CAM to conserve carbon dioxide instead of water. In environments that have little carbon dioxide, it makes sense to gather and store as much as possible for later use.
CAM to the rescue! C4 and CAM photosynthesis are both adaptations to arid conditions, because they are more efficient in the conservation of water. CAM plants take in carbon dioxide during the night hours, fixing it within the plant as an organic acid with the help of an enzyme.
The stomata pores can be open during the evening when the temperature is lower and humidity relatively higher. During the day, the stomata can remain closed, using the internally released carbon dioxide and thus sealing the plant off from the outside environment. This is probably a six to 10 times more efficient way to prevent water loss compared to normal plant respiration. This modified effect seems to work best when there is a considerable difference between daytime and nighttime temperatures.
Photorespiration basically occurs when the enzyme rubisco that grabs carbon dioxide for photosynthesis grabs oxygen instead, causing respiration that blocks photosynthesis and thus causes a slowing of the production of sugars.
The majority of plants fall into the C3 category and are best adapted to rather cool, moist temperatures and normal light conditions. Their stomata are usually open during the day. When conditions are extremely arid, CAM plants can just leave their stoma closed night and day, and the organic cycle is fed by internal recycling of the nocturnally fixed respiratory carbon dioxide.
Of course, this is somewhat like a perpetual motion machine, and because there are costs in running this machinery, the plant cannot CAM-idle for very long. To find more examples of succulents, go to the Succulent Gallery. Physiological adaptations to drought conditions. Plants that grow in dry environments face a major problem. In order to grow, they need to absorb carbon dioxide from the atmosphere and convert it into sugars photosynthesis by using energy obtained from light.
Plants gain their carbon dioxide by opening small pores, called stomata on the leaf or stem surface. But opening of the stomata during the hot, daytime hours leads to loss of water from the tissues.
The cacti and many other succulent plants have overcome this dilemma by using a special biochemical process called crassulacean acid metabolism CAM because it was first discovered in plants of the Crassulaceae family. Details of this can be found on another page, but basically these plants open their stomata by night, when the temperature is cool, and absorb carbon dioxide which they store by chemically combining it with an organic compound containing 3 carbon atoms, producing a 4-carbon organic acid.
During the day, when the stomata are closed, the carbon dioxide is released from this organic acid and used to synthesize sugars, using the energy of light. These plants do exactly the opposite of normal plants, which open their stomata by day when light is available for photosynthesis, and close them at night. Some other plants of hot environments have developed a further physiological adaptation, called C 4 metabolism, which contrasts with the C 3 metabolism of most plants. This is found especially in many tropical and subtropical grasses, including those of the warmer deserts of North America.
Details can be found on a separate page. Cacti as integral components of natural habitats. Cacti may be fascinating organisms, but the question we will ask here is: how important are they as members of the desert community? How do they fit into the whole scene? The answer is that some of the larger cacti can be major components of the desert vegetation, and provide food and shelter for other organisms.
The large cacti support a range of animals such as the specialised cactus bees which use cactus pollen to feed their larvae , javelinas which each large amounts of prickly pear pads, and bats which depend on the nectar of cardon, saguaro and organ pipe cactus during their annual northern migration from southern Mexico into northern Mexico and Arizona.
Fruits of the larger cacti provide food for many birds, while the plants themselves provide shelter and nest sites for woodpeckers and flickers, cactus wrens, curve-billed thrashers, owls and hawks, etc.
Without these large cacti, there would be significantly less animal diversity. But the largest cacti are confined to the smi-tropical environment of the Sonoran Desert of North America and, apart from the chollas and prickly pears, the other deserts have much smaller cacti that are minor components of the total habitat.
In fact, few if any cacti can be considered to be the primary determinants of desert communities, because almost all cacti depend on the shade and shelter of other plants in order for their seedlings to become established and to survive the early years of growth. The evidence of this is seen in the frequent occurrence of cacti under nurse plants, including the major nitrogen-fixing trees and sub-trees of the Sonoran Desert. This helps us to see the role of cacti in perspective, and to recognise the vital roles that other, less spectacular, plants play in the community structure of deserts.
Cactus Gallery? North American Deserts Gallery? Succulent Gallery? Ribbed stem of the cardon cactus Ferocious spines of a barrel cactus Sun-protective spines of a cholla cactus Spineless living rock cactus. A cactus left and two Euphorbia species from Africa centre and right Close-up of flowers on the cactus-like Euphorbia. Remains of a flower are seen in the plant at the left Flowers of living stone plants.
0コメント