5.3 Energy flow  (Page 4/3)

All flesh is grass.

All living things require energy in one form or another. Energy is required by most complex metabolic pathways (often in the form of adenosine triphosphate, ATP), especially those responsible for building large molecules from smaller compounds, and life itself is an energy-driven process. Living organisms would not be able to assemble macromolecules (proteins, lipids, nucleic acids, and complex carbohydrates) without a constant energy input. And yes, Isaiah was correct; your flesh and all flesh originated in the green plants of this planet.

It is important to understand how organisms acquire energy and how that energy is passed from one organism to another through food webs and their constituent food chains. Food webs illustrate how energy flows directionally through ecosystems, including how efficiently organisms acquire it, use it, and how much remains for use by other organisms of the food web. The transfer of energy from one trophic level to the next is an important concept when examining the effects of human disturbance on an ecosystem.

How organisms acquire energy in a food web

Energy is acquired by living things in three ways: photosynthesis , chemosynthesis , and the consumption and digestion of other living or previously living organisms. Organisms using the first two methods are called autotrophs (auto=self, these organisms feed themselves). Those that rely on consumption of others are called heterotrophs (hetero=other, these organisms are fed by others). Photosynthetic autotrophs (photoautotrophs) use sunlight as an energy source, whereas chemosynthetic autotrophs (chemoautotrophs) use inorganic molecules as an energy source. Autotrophs are critical for all ecosystems. Without these organisms, energy would not be available to other living organisms and life itself would not be possible.

Photoautotrophs, such as plants, algae, and photosynthetic bacteria, serve as the energy source for a majority of the world’s ecosystems. These ecosystems are often described by grazing food webs. Photoautotrophs harness the solar energy of the sun by converting it to chemical energy in the form of ATP (and NADP). The energy stored in ATP is used to synthesize complex organic molecules, such as glucose.

Chemoautotrophs are primarily bacteria that are found in rare ecosystems where sunlight is not available, such as in those associated with dark caves or hydrothermal vents at the bottom of the ocean ( [link] ). Many chemoautotrophs in hydrothermal vents use hydrogen sulfide (H ₂ S) released from the vents as a source of chemical energy. This allows chemoautotrophs to synthesize complex organic molecules, such as glucose, for their own energy and in turn supplies energy to the rest of the ecosystem, making chemoautotrophs the primary producers of their ecosystem.