TOEFL Academic Passage Practice 007Photosynthesis in Extreme Environments

Photosynthesis is a biological process in which plants, algae, and some bacteria convert light energy into chemical energy stored in organic compounds. Because these compounds can serve as food for the organisms that produce them and for organisms that consume them, photosynthesis supports most ecosystems. The process usually depends on three environmental resources: light, water, and carbon dioxide. The amount and availability of these resources vary greatly from place to place. In deserts, water is scarce; in polar regions, low temperatures and seasonal darkness limit the amount of usable light; and in the deep ocean, sunlight is almost completely absent. Nevertheless, organisms in each of these settings have evolved ways to obtain or use energy under difficult conditions.

Desert plants face a particularly serious problem because photosynthesis requires carbon dioxide to enter through openings called stomata, but opening those structures can allow water to escape. During hot, dry daylight hours, this loss can be dangerous. Many desert plants therefore use a form of photosynthesis called CAM, or Crassulacean Acid Metabolism. CAM plants open their stomata mainly at night, when temperatures are lower and the air is often less dry. They take in carbon dioxide during this period and temporarily store it in chemical compounds. During the following day, the stored carbon dioxide can be used for photosynthesis while the stomata remain closed or less open. This schedule reduces water loss compared with taking in carbon dioxide during the hottest part of the day. Cacti are familiar examples of plants that use this adaptation, although not every desert plant relies on CAM photosynthesis.

Polar environments create a different combination of difficulties. Low temperatures can slow biological processes, while snow and ice may block or scatter incoming light. In addition, winter can bring extended periods of darkness, leaving photosynthetic organisms with little time in which to gather energy. Some algae and mosses, however, can continue photosynthesizing under very low light. Light that penetrates through thin layers of ice or snow may be sufficient for these organisms, especially during periods when daylight returns. Some polar species also possess specialized pigments. These pigments absorb available wavelengths of light efficiently, helping the organisms capture more energy during the short daylight period of the polar summer. Their ability to use weak light does not eliminate the challenges of winter, but it helps them remain active when ordinary photosynthetic conditions are not present.

The deepest parts of the ocean present an even more fundamental obstacle: sunlight does not reach most hydrothermal vents. As a result, organisms living near these vents cannot depend on ordinary light-driven photosynthesis as the primary source of energy. Instead, certain bacteria use chemosynthesis. These bacteria obtain energy by carrying out chemical reactions involving hydrogen sulfide released from the Earth’s crust. They use that energy to produce organic compounds, which can then support other organisms. In this way, chemosynthetic bacteria form the base of food webs near the vents. Animals and other organisms in these communities can depend indirectly on chemical energy even though the surrounding water is dark. The bacteria therefore demonstrate that an ecosystem can be productive without sunlight, although this process is chemically different from photosynthesis.

The examples from deserts, polar regions, and hydrothermal vents illustrate the flexibility of life in extreme environments. Desert plants adjust the timing of gas exchange to conserve water. Polar algae and mosses use pigments and other traits that help them make use of weak or limited light. Bacteria near deep-sea vents draw energy from chemicals rather than from sunlight. These strategies do not make the environments easy to inhabit; instead, they allow particular organisms to use resources that remain available. By modifying how they capture light, obtain carbon dioxide, or acquire energy, these organisms can survive and support communities in places that would otherwise be inhospitable to most forms of life.