Definition, Nutrition, vs Autotrophs
What are Heterotrophs?
Also refered to as consumers, heterotrophs are
organisms that obtain their energy (nutrition) from organic compounds/materials.
In other words, heterotrophs are organisms that are unable to produce their own
food (unlike autotrophs) and therefore have to consume/ingest organic compounds
as a source of energy.
Compared to autotrophs (which occupy the base of the
food-web triangle), heterotrophs occupy the upper levels of the food web given
that their survival is dependent on the producers (autotrophs). Heterotrophs
consist of all animals ranging from mammals to bacteria.
Some examples of heterotrophs include:
- Human beings
While all heterotrophs survive by
consuming/ingesting food (organic material) this is achieved in different ways
for different types of animals. The following are the three main types of Heterotrophic
- Saprotrophic nutrition
- Holozoic nutrition
This type of nutrition (saprotrophic nutrition)
involves the consumption (through absorption) of food material from decaying
organic matter (dead and decaying animals, plants etc).
Organisms that obtain their
nutrition from dead and decaying organic matter are known as saprotrophs or
saprophytes. These include a variety of fungi and bacteria as well as a number
of other organisms that resemble fungi (such as water molds).
nutritional process involves the breakdown of organic matter in the decaying
matter by saprophytes' enzymes into simpler substances. These are then
absorbed by these organisms as a source of nutritional energy.
While a good
number of saprophytes (e.g. many fungi) may possess plant-like traits, they are
unable to produce their own food and therefore have to ingest organic compounds
as their source of energy. It is for this reason that they are classified as
Saprophytic fungi (Mucor)
Saprophytic fungi like mucor are unable to make their
own food or ingest food as higher animals do. For this reason, they use a
different mechanism to obtain nutrition. At its base, the mucor body is highly divided
into threads that are known as hyphae. As they grow on the decomposing matter,
this network of threads (hyphae) spread through the substance (animal, bread,
leaves etc) from which they absorb nutrients.
The production of digestive
enzymes (cellulases, oxidases, proteases etc) allows for the substance to be
broken down into much simpler molecules that are then absorbed (through
diffusion) by the fungi.
Yeast (Saprophytic yeast)
Yeast is also classified as fungi. Unlike other
fungi like mucor, yeast is single celled organisms that reproduce spores during
reproduction. Given that they are unable to produce hyphae like other fungi,
yeast obtains their nutrients through a process known as extracellular
digestion. Here, the organism produces digestive enzymes that are then released
in to their immediate surrounding environment.
Once the enzymes break down the
material in their surrounding, these simpler nutrients are then taken up by the
organism into the cell.
See Yeast under the Microscope
Some of the conditions required by saprophytes
for nutrition include:
- Water (or moisture)
- Oxygen (some organisms like
yeast do not need oxygen)
- Neutral or mildly acidic pH
- Warm temperature range
* the word sapro refers to decaying material or
Importance of saprophytic fungi
Saprophytic fungi play an important role in
nature. In addition to being natural decomposers, they help break down dead
matter into simpler substances. This is important because it makes it possible
for other plants to be able to easily take up these recycled materials for
These fungi also act as food (mushrooms) that can be
eaten (some can be poisonous). As for single celled fungi like yeast, they play
an important role in fermentation, which makes them important in the bakery and
Saprophytic bacteria can typically be found
growing on dead/decaying matter. As such, they can be found in rotting wood,
decaying plants and animals as well as on decaying organic matter found in
Like other saprophytes, saprophytic bacteria produce a variety
of digestive enzymes for the purposes of breaking down organic matter of
Examples of saprophytic bacteria include:
- Clostridium aceto-butylicum
Saprophytic bacteria are by far the largest
decomposers and thus play an important role of breaking down various waste materials
Some of the other important roles of these bacteria include:
- Organic manure - Given that they break down organic matter, they are often used in farms for the purposes of
breaking down various complex organic material (dung etc) into manure that is
then used to promote the growth of plants. This is largely due to the fact that
plants are well able to take up simpler material (such as ammonium compounds)
broken down from complex organic matter.
- Food spoilage - While saprophytic bacteria are important in breaking down
decaying matter, they also accelerate food spoilage (fruits, bread etc)
- Saprophytic bacteria are
also used for fermentation and for producing vinegar among others.
* the word holozoic is from the Greek words;
"holos" which means whole and "zoic" which means animal
Compared to saprophytic nutrition where the organism
absorbs nutrients, holozoic nutrition involves the ingestion of the food
material by the organism. Therefore, the organism consumes the solid food
substance, which is then broken down further within the organism and
transported into the cells of the organism.
Examples of animals that use
holozoic mode of nutrition include:
- Human beings,
- Other mammals,
- Single celled organisms
Holozoic organisms are divided into:
Herbivores include animals that only consume
plants as their source of energy. Depending on the animal, they eat different
types of plants or given parts of a specific plant (leaves, fruits, bark etc).
Therefore, not all animals are capable of eating all types/parts of plants.
instance, unlike cows, the tongue of a giraffe allows it to eat leaves of
thorny plants (the tongue can sort out leaves from thorns).
There are two main
types of herbivores based on their eating habits:
- Browsers - Browsers are herbivores that typically feed on such plants as
leaves, fruits, shrubs and soft plant shoots. Some examples of browsers include
the wild goats, Alpine ibex, the Nubian ibex as well as Walia ibex among others.
- Grazers - Unlike browsers, grazers often eat grass. These include such
animals as cows and Kangaroos.
* Some herbivores like sheep possess an anatomy
(narrow muzzle and larger rumen) that allows them to be intermediate feeders
(they are both grazers and browsers).
Herbivores are the primary consumers given that
they only eat plants (producers). As such, they are the second to the producers
in given that they are source of food for animals higher in the
Apart from being important source of food for animals higher in the
food web (carnivores) herbivores also play such important roles as seed
dispersion as well as pollination (bees etc).
* The anatomy of herbivores allows them to eat
and digest plants. The lower teeth and hard upper gum of goats
allow them to pull the bark off some trees and shrubs while their intestine
allows them to successfully break down the plants further.
Unlike herbivores, carnivores are adapted to
hunting, killing and eating other animals. As such, they only eat meat, which
is their source of energy. Being higher in the food web, some carnivores can
not only eat herbivores, but also omnivorous animals as well as other
carnivores. For instance, carnivores like hyenas can hunt and eat lion cubs.
Carnivores require more energy so as to be able to hunt and kill prey.
Herbivores, omnivores as well as other carnivores are a good source of meat
(proteins) that is required to provide such energy. Because they have to hunt
and kill other animals, carnivores are well adapted to run faster (or fly) with
longer canines that allow them to kill prey and tear off flesh (at least for
most land carnivores). However, it is worth noting that different types of
carnivores are adapted in different ways for their survival.
Whereas a good number of snakes have poisonous fangs that they use to kill
prey, other carnivores like eagles have sharp claws and a string, curved beak
that they used to catch, kill and eat their prey.
* the word carnivore was derived from the words
"carnis" - flesh and "vorate" meaning eat
Omnivores eat both plants and animals. This is
of great benefit for most omnivore given that this characteristic allows them
to live in many environments where they can get meat and plant foods. Some
examples of omnivores include human beings, brown bears, raccoons and the
woodpecker among others.
The availability of these different sources of food
(meat and plants) provides different types of nutritional need that these animals
* Omnivores are well adapted to eat both plants
and meat. For instance, human beings have longer canine teeth (compared to the
other teeth) that allow them to tear food while the molar allow for chewing or
both meat and plant foods.
A parasitic model of nutrition refers to a type
of nutrition where one organism depends on another organism (host) for
nutrition. In this type of nutrition, the organism that depends on the other is
known as the parasite while the animal that is being depended on for nutrition
is the host.
While the parasite benefits from the nutrition it gets from the
host, the host does not benefit from this relationship. In most cases, the
parasitic mode of nutrition ends up causing harm to the host.
Common examples of
- Mosquitoes and malaria
- Bed bugs
* Both plants and animals can be parasites
This mode of nutrition (parasitic nutrition) can
be divided into:
Total parasites (endo-parasites) - A total parasite refers to a parasite that is
wholly dependent on the host for water, food as well as shelter. Examples of
total animal parasites include tape worm and malaria parasites.
live within the host and are unable to survive on their own (outside the host).
Among plants, Cuscuta is a good example of parasite that depends on such plant
hosts as Zizyphus for survival. Because Cuscuta lacks proper roots, their
sucking roots grow in to the stem of the host where they obtain nutrients from
the xylem and phloem of the host plant.
Partial parasites - Compared to total parasites, partial parasites largely depend
on the host for food. A mosquito is a good example of a partial parasite. While
they depend on the host for food, they do not need to live in or on the host
for survival. In plants, partial parasites are capable of synthesizing their own
food through photosynthesis, but depends on other plants for water and minerals.
Viscum is a good example of a plant partial parasite that tends to grow on the
branches of other plants like Oak trees.
* Ecto parasite refers to parasites that live
outside the body of the host (tick, lice) while endo parasites live inside the
body of the host (such as tapeworms that live in the animal's intestine)
See also Parasites under the Microscope
Heterotrophs Vs Autotrophs
Whereas heterotrophs are unable to
make/synthesize their own food, autotrophs have the ability to make their own
food using carbon dioxide in the presence of sunlight (where sunlight is the
source of energy).
These autotrophs are commonly refered to as photoautotroph/phototrophs
(e.g. cyanobacteria). While photoautotroph need light energy to make food, chemoautotrophs
(e.g Hydrogenovibrio crunogenus) are a type of autotrophs that achieve this by
* Photoautotrophs are able to produce food
through photosynthesis by using light energy from the sun. Chemoautotrophs on
the other hand produce their own food by using chemicals through a process known
One of the biggest differences between
autotrophs and heterotrophs is that autotrophs have Chlorophyll. This is an
important pigment present in all autotrophs (from plants to bacteria). Using
chlorophyll, plants (as well as algae and various bacteria etc) are able to
trap light energy and use it to produce food.
This ability to produce their own
food has made autotrophs the most important part of the food chain. Because
they alone are able to make their own food, autotrophs are the primary
producers and thus the foundation of the ecosystem.
Return from Heterotrophs to MicroscopeMaster Home
DK Rao and JJ Kaur. Living Science Biology 10.
Publisher: Ratna Sagar P. Ltd.; CCE edition (1 September 2010)
Veer Bala Rastoji. Modern Biology. Originally