Objectives
Compare and contrast how autotrophs and heterotrophs obtain
food.
Explain how cellular respiration harvests the energy in food.
Key Terms
autotroph
photosynthesis
producer
heterotroph
consumer
cellular respiration
Look up on a clear day, and you will see the sun burning brightly
almost 150 million kilometers away. A tiny fraction of the light energy
radiated from the sun reaches Earth's surface. And a tiny fraction of that
sunlight powers most of the planet's life. This is true because certain
organisms can convert the energy of sunlight to the chemical energy in
food—sugars and other organic molecules.
Obtaining Food
All organisms need food for energy and building materials. Biologists
classify organisms according to how they obtain food.
Autotrophs An organism such as a plant that makes its own food is
called an autotroph, which means "self-feeder" in Greek. Starting with
inorganic molecules, autotrophs make organic molecules. For example,
plants use the sun's energy to convert water and carbon dioxide into
sugars. This process is called photosynthesis (from the Greek photomeaning "light," and synthesis meaning "making something"). You will
read more about photosynthesis in Chapter 8.
Autotrophs are also called producers because they produce the organic
molecules that serve as food for the organisms in their ecosystem. On
land, plants are the major producers. In oceans, lakes, and streams,
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producers include algae and certain photosynthetic bacteria.
Heterotrophs Organisms that cannot make their own food, such as
humans, are called heterotrophs, meaning "other eaters." Heterotrophs,
also called consumers, must obtain food by eating producers or other
consumers. Heterotrophs depend on producers to supply energy and
materials for life and growth. Since most producers depend on sunlight
as their energy source, you could say that life on Earth is solarpowered.
Harvesting the Energy in Food
As you have read, plants and certain other producers use light energy to
make organic molecules. These organic molecules are a source of
energy and building materials for organisms.
Many organisms, including both producers and consumers harvest the
energy stored in foods through cellular respiration. Cellular respiration
is a chemical process that uses oxygen to convert the chemical energy
stored in organic molecules into another form of chemical energy—a
molecule called adenosine triphosphate (ATP). Cells in plants and
animals then use ATP as their main energy supply. You will read about
cellular respiration and ATP in more detail later in this chapter.
The processes of photosynthesis and cellular respiration recycle a
common set of chemicals: water, carbon dioxide, oxygen, and organic
compounds such as glucose. The diagram in Figure 7-3 visually
summarizes this chemical recycling.
Figure 7-3
The products of photosynthesis are the
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chemical ingredients for cellular respiration,
while the products of cellular respiration
are the chemical ingredients for
photosynthesis.
Water and carbon dioxide are the raw ingredients for photosynthesis.
Plants use energy from sunlight to rearrange the atoms of water and
carbon dioxide, producing glucose and oxygen. Oxygen is used by both
plant and animal cells during cellular respiration to release the energy
stored in glucose. The released energy enables cells to produce ATP.
Cellular respiration also produces carbon dioxide and water. The result
is a continual cycling of these chemical ingredients.
Concept Check 7.1
1. Define autotroph and heterotroph, and give an example of each.
2. Explain the role of food (glucose) in both photosynthesis and
cellular respiration.
3. Explain how life on Earth depends on the sun.
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Wednesday, October 19, 2011 9:18:19 AM CT