Which of the following statements is a correct distinction between autotrophs and heterotrophs?

a. Autotrophs, but not heterotrophs, can nourish themselves beginning with CO2 and other nutrients that are inorganic. b. Only heterotrophs require chemical compounds from the environment. c. Cellular respiration is unique to heterotrophs. d. Only heterotrophs have mitochondria. e. Only autotrophs can live on nutrients that are entirely inorganic.

The correct answer is a. Autotrophs, but not heterotrophs, can nourish themselves beginning with CO2 and other nutrients that are inorganic.

Photo of a fern , a photoautotroph
Photo of a fern, a photoautotroph (Louise Wolff –darina 00:22, 7 May 2005 (UTC) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)])

There are different ways in which living organisms can obtain nutrients. Some of the main modes of nutrition include being an autotroph, heterotroph, saprotroph or parasite.

Autotrophs can either use chemicals (chemoautotroph) or sunlight (photoautotroph) to form sugar. Photoautotrophs use sunlight and carbon dioxide to make glucose in a series of reactions.

Plants and many types of bacteria and Protista are autotrophic and form the base of food chains where they are known as primary producers.

Heterotrophs include bacterial, protistan and animal species that take in solid particles as food items. Animals that feed on plants are known as herbivores and are also called the primary consumers of a food chain.

Secondary consumers are carnivores that then feed on these herbivores, and are thus meat eaters. There are however many animals that are omnivores which feed on both meat and plant material.

Protist heterotrophs also exist and a few species such as Euglena, are able to switch between heterotrophy and autotrophy.

Saprotrophs are ogranisms such as fungi and bacteria which choose to consume dead and decaying organic matter. They are thus important in recycling nutrients and minerals through the environment.

Parasitism is a mode that is common in some animals and protists, and even a few plants. Well-known protist parasites of humans include the malaria parasite and various amoebas.

Animal parasites include various roundworms and tapeworms that rely on host animals to obtain nutrients.

Types of nutrition

Living organisms have different ways of obtaining nutrients. The main division is between those organisms that can make their own food using sunlight or chemicals and those that have to take in nutrients from another organism.

Autotrophic organisms either use sunlight (photoautotroph) or chemicals (chemoautotrophs) to make sugars. These organisms include single-celled protists, plants, and various bacterial species.

Heterotrophs include bacteria, protists, and animals. These organisms cannot make their own food and thus have to take in nutrients from another organism.

There are several different kinds of heterotrophs present that occur in the categories of Protista, animals, and bacteria. There are also organisms such as fungi, that are known as saprotrophs that feed on dead and decaying organic material.


There are species found in the domain Bacteria which can make food using chemicals. These are known as chemoautotrophs. Other species are photoautotrophs that use sunlight and carbon dioxide to form glucose.

There are many species of protists that are photoautotrophic. Many of these organisms are found in the oceans where they make up much of the phytoplankton found in the surface layers of water.

Diatoms, dinoflagellates and various species of green algae are examples of photoautotrophic phytoplankton.

The main photosynthetic pigments that these protists use are chlorophyll a and c. There are exceptions though and some protists can actually use other pigments such as phycoerythrins, fucoxanthins, xanthophylls, and carotenes for photosynthetic reactions.


The plants use mainly chlorophyll a and b to capture light energy for photosynthesis. Being multicellular organisms they have modifications of the plant body to maximize photosynthesis.

The presence of leaves containing cells packed with chloroplasts helps in the process as does the presence of stomata in the epidermis to enable carbon dioxide to enter cells for photosynthesis.

Plants and other autotrophic organisms form the base of many food chains and are thus known as primary producers.


Heterotrophic life forms are not able to photosynthesize and thus rely on taking in nutrients from other organisms as food.

The heterotrophic animals that feed on plant material directly are called herbivores. These animals are called primary consumers and they include many types of animal species including insects and various mammals such as cows and deer.

The next level in a food chain is the secondary consumer which is a heterotrophic organism that feeds on a primary consumer. For example, a bird may feed on a caterpillar which had been eating the leaves of a plant.

Animals that feed on other animals are meat eaters known as carnivores. There are however animals that feed more opportunistically and will feed on both vegetation and meat. These animals are known as omnivores and include many species of birds and primates.

Even grain-eating birds tend to also consume insects, especially when they have chicks in the nest that need protein for growth.

There are also Protista which are heterotrophic such as Paramecium and Amoeba. Some protists such as Euglena can even switch between autotrophy and heterotrophy modes of nutrition.


Organisms that have a saprotrophic mode of nutrition will feed on decaying and dead organic matter. Many species of bacteria and fungi are saprotrophs, which are important in recycling nutrients and minerals through their digestive action.

Saprotrophic life forms are often overlooked in ecosystems but are crucial in nutrient cycles and ensure that dead organisms can be broken down and nutrients returned to the water and soil.


Parasitism is a way in which some organisms are able to feed and reproduce. There are many examples of parasites in the Protista and Animals. There are even a few examples of plants that are also parasitic, for instance, the Branched broomrape plant.

Examples of protistan parasites include various important human parasites such as Plasmodium which is found in the blood and causes debilitating disease and many deaths every year.

Various species of Entamoeba and Giardia are also parasites causing illness in many animals including humans.

Animal parasites that obtain nutrition from our intestines include a range of tapeworms such as Taenia species and roundworms such as Ascaris species. These animals are morphologically adapted to live inside a host animal and obtain nourishment directly from the host.


  1. Editors of Encyclopedia Britannica (2019). Heterotroph. Retrieved from Encyclopedia Britannica.
  2. AW Wilson (2019). Saprotroph. Retrieved from Encyclopedia Britannica.
  3. Editors of Encyclopedia Britannica (2019). Phytoplankton. Retrieved from Encyclopedia Britannica.
  4. JH Westwood (2019). Parasitic plants. Retrieved from Encyclopedia Britannica.
  5. SW Jeffrey (1968). Photosynthetic Pigments of the Phytoplankton of Some Coral Reef Waters. Limnology and Oceanography.


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