# 46.2A: Strategies for Acquiring Energy

• Boundless
• Boundless
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##### Learning Objectives
• Distinguish between photoautotrophs and chemoautotrophs and the ways in which they acquire energy

## How Organisms Acquire Energy in a Food Web

All living things require energy in one form or another since energy is required by most, complex, metabolic pathways (often in the form of ATP ); life itself is an energy-driven process. Living organisms would not be able to assemble macromolecules (proteins, lipids, nucleic acids, and complex carbohydrates) from their monomeric subunits without a constant energy input.

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. Energy is acquired by living things in three ways: photosynthesis, chemosynthesis, and the consumption and digestion of other living or previously-living organisms by heterotrophs.

Photosynthetic and chemosynthetic organisms are grouped into a category known as autotrophs: organisms capable of synthesizing their own food (more specifically, capable of using inorganic carbon as a carbon source ). Photosynthetic autotrophs (photoautotrophs) use sunlight as an energy source, whereas chemosynthetic autotrophs (chemoautotrophs) use inorganic molecules as an energy source. Autotrophs act as producers and 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. Many chemoautotrophs in hydrothermal vents use hydrogen sulfide (H2S), which is 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.

Heterotrophs function as consumers in the food chain; they obtain energy in the form of organic carbon by eating autotrophs or other heterotrophs. They break down complex organic compounds produced by autotrophs into simpler compounds, releasing energy by oxidizing carbon and hydrogen atoms into carbon dioxide and water, respectively. Unlike autotrophs, heterotrophs are unable to synthesize their own food. If they cannot eat other organisms, they will die.

## Key Points

• Food webs illustrate how energy flows through ecosystems, including how efficiently organisms acquire and use it.
• Autotrophs, producers in food webs, can be photosynthetic or chemosynthetic.
• Photoautotrophs use light energy to synthesize their own food, while chemoautotrophs use inorganic molecules.
• Chemoautotrophs are usually bacteria that live in ecosystems where sunlight is unavailable.
• Heterotrophs cannot synthesize their own energy, but must obtain it from autotrophs or other heterotrophs; they act as consumers in food webs.

## Key Terms

• photoautotroph: an organism that can synthesize its own food by using light as a source of energy
• chemoautotroph: a simple organism, such as a protozoan, that derives its energy from chemical processes rather than photosynthesis
• heterotroph: an organism that requires an external supply of energy in the form of food, as it cannot synthesize its own

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