Autotrophs vs. Heterotrophs Advanced
Douglas Wilkin, Ph.D.
Barbara Akre
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Printed: October 6, 2015
AUTHORS
Douglas Wilkin, Ph.D.
Barbara Akre
www.ck12.org
C HAPTER
Chapter 1. Autotrophs vs. Heterotrophs - Advanced
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Autotrophs vs.
Heterotrophs - Advanced
• Analyze the way in which autotrophs obtain energy and evaluate the importance of autotrophs to energy for
all life.
• Explain the relationship between autotrophs and heterotrophs.
Name one major difference between a plant and an animal.
There are many differences, but in terms of energy, it all starts with sunlight. Plants absorb the energy from the sun
and turn it into food. You can sit in the sun for hours and hours. You will feel warm, but you’re not going to absorb
any energy. You have to eat to obtain your energy. You, of course, can go to the kitchen and cook something to eat.
Other animals can also eat, but a plant cannot.
How Do Organisms Get Energy? Autotrophs vs. Heterotrophs
Living organisms obtain chemical energy in one of two ways.
Autotrophs, shown in the Figure 1.1, store chemical energy in carbohydrate food molecules they produce themselves. Food is chemical energy stored in organic molecules. Food provides both the energy to do work and the
carbon to build the organic structures from cells to organisms. Because most autotrophs transform sunlight to make
or synthesize food, we call the process they use photosynthesis. The food produced via this process is glucose. Only
three groups of organisms - plants, algae, and some bacteria - are capable of this life-giving energy transformation.
Autotrophs make food for their own use, but they make enough to support other life as well. Almost all other
organisms depend absolutely on these three groups for the food they produce. The producers, as autotrophs are also
known, begin food chains which feed all life. Food chains will be discussed in the Ecology concepts.
Heterotrophs cannot make their own food, so they must eat or absorb it. For this reason, heterotrophs are also
known as consumers. Consumers include all animals and fungi and many protists and bacteria. They consume
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FIGURE 1.1
Photosynthetic autotrophs, which make
food for more than 99% of the organisms
on earth, include only three groups of organisms: plants such as the redwood tree
(a), algae such as kelp (b), and certain
bacteria like this Anabaena (c).
either autotrophs or other heterotrophs. Heterotrophs show great diversity and may appear far more fascinating than
producers. But heterotrophs are limited by their utter dependence on those autotrophs which originally made the
food. If plants, algae, and autotrophic bacteria vanished from Earth, animals, fungi, and other heterotrophs would
soon disappear as well. All life requires a constant input of energy. Only autotrophs can transform that ultimate,
solar source into the chemical energy in food which powers life, as shown in Figure 1.2.
FIGURE 1.2
Food chains carry energy from producers (autotrophs) to consumers (heterotrophs). 99% of energy for life comes
from the sun via photosynthesis. Note
that only nutrients recycle. Energy must
continue to flow into the system. Though
this food chains "ends" with decomposers, do decomposers, in fact, digest
matter from each level of the food chain?
(See the Energy Transfer: Decomposers
(Advanced) concept).
Photosynthesis provides over 99% of the energy supply for life on Earth. A much smaller group of autotrophs mostly bacteria in dark or low-oxygen environments - produce food using the chemical energy stored in inorganic
molecules such as hydrogen sulfide, ammonia, or methane. While photosynthesis transforms light energy to chemical energy, this alternate method of making food transfers chemical energy from inorganic to organic molecules. It
is therefore called chemosynthesis, and is characteristic of the tubeworms shown in Figure 1.3. Some of the most
recently discovered chemosynthetic bacteria inhabit deep ocean hot water vents or “black smokers.” There, they use
the energy in gases from the Earth’s interior to produce food for a variety of unique heterotrophs: giant tube worms,
blind shrimp, giant white crabs, and armored snails. Some scientists think that chemosynthesis may support life
below the surface of Mars, Jupiter’s moon, Europa, and other planets as well. Ecosystems based on chemosynthesis
may seem rare and exotic, but they too illustrate the absolute dependence of heterotrophs on autotrophs for food.
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Chapter 1. Autotrophs vs. Heterotrophs - Advanced
FIGURE 1.3
Tubeworms deep in the Gulf of Mexico get
their energy from chemosynthetic bacteria living within their tissues. No digestive
systems needed!
Phototrophs are organisms that capture light energy and convert it to chemical energy inside their cell. Most
phototrophs are the autotrophs that perform photosynthesis, which are also known as photoautotrophs. These organisms have the ability to fix carbon from carbon dioxide into organic compounds, such as glucose. Chemotrophs,
on the other hand, do not get their energy from carbon. These are organisms that break down either organic or
inorganic molecules to supply energy for the cell through chemosynthesis. Chemotrophs can be either autotrophic
(chemoautotrophs) or heterotrophic (chemoheterotrophs). Chemoautotrophs derive their energy from chemical
reactions, and synthesize all necessary organic compounds from carbon dioxide. Chemoheterotrophs are unable to
fix carbon to form their own organic compounds. The various types of metabolisms are discussed in the Prokaryotes:
Nutrition and Metabolism (Advanced) concept.
Vocabulary
• autotroph: Organism that produces organic compounds from energy and simple inorganic molecules; also
known as a producer.
• chemosynthesis: The process by which carbon dioxide molecules are converted to carbohydrates; uses energy
from the oxidation of inorganic compounds.
• chemotroph: An organism that breaks down either organic or inorganic molecules to supply energy for the
cell.
• consumer: A heterotrophic organism; must eat or absorb organic food molecules.
• food: Organic (carbon-containing) molecules which store energy in the chemical bonds between their atoms.
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FIGURE 1.4
This flowchart helps to determine if a
species is an autotroph or a heterotroph,
a phototroph or a chemotroph. For example, “Carbon obtained from elsewhere?”
asks if the source of carbon is another
organism.
If the answer is “yes”, the
organism is heterotrophic. If the answer
is “no,” the organisms is autotrophic.
• food chain: A pathway which traces energy flow through an ecosystem.
• heterotroph: Organisms which must consume organic molecules; also known as a consumer.
• inorganic molecule: Molecule which does not contain carbon (with a few exceptions such as carbon dioxide);
a molecule not necessarily made by living organisms.
• organic molecule: A molecule which contains carbon, made by living organisms; examples include carbohydrates, lipids, proteins and nucleic acids.
• photosynthesis: The process by which carbon dioxide and water are converted to glucose and oxygen, using
sunlight for energy.
• phototroph: An organism that captures light energy from the sun and converts it into chemical energy inside
their cell.
• producer: Organism that produces organic compounds from energy and simple inorganic molecules; an
autotroph.
Summary
• Food is chemical energy stored in organic molecules.
• Food provides both the energy to do life’s work and the carbon to build life’s bodies.
• Autotrophs make their own carbohydrate foods, transforming sunlight in photosynthesis or transferring chemical energy from inorganic molecules in chemosynthesis.
• Heterotrophs consume organic molecules originally made by autotrophs.
• All life depends absolutely upon autotrophs to make food molecules.
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Chapter 1. Autotrophs vs. Heterotrophs - Advanced
Review
1. Water and carbon dioxide molecules are reactants in the process of photosynthesis. Does this mean they are
“food” for plants, algae, and blue-green bacteria? Use the definition of “food” to answer this question.
2. Compare autotrophs to heterotrophs, and describe the relationship between these two groups of organisms.
3. Name and describe the two types of food making found among autotrophs, and give an example of each.
Which is quantitatively more important to life on earth?
4. Define chemosynthesis.
5. Trace the flow of energy through a typical food chain (describing "what eats what"), including the original
source of that energy and its ultimate form after use. Underline each form of energy or energy-storing
molecule, and boldface each process which transfers or transforms energy.
References
1. (a) Courtesy of National Park Service; (b) Courtesy of Shane Anderson, National Oceanic and Atmospheric
Administration; (c) Courtesy of US Environmental Protection Agency. (a) http://commons.wikimedia.org/wik
i/File:Riesenmammutbaum.jpg; (b) http://commons.wikimedia.org/wiki/File:Kelp_300.jpg; (c) http://commons
.wikimedia.org/wiki/File:Anabaenaflosaquae_EPA.jpg . Public Domain
2. Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. CK-12 Foundation . CC BY-NC 3.0
3. Charles Fisher. http://commons.wikimedia.org/wiki/File:Lamellibrachia_luymesi1.png . CC BY 2.5
4. Laura Guerin. CK-12 Foundation . CC BY-NC 3.0
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