Autotrophs are any organisms that are capable of producing their own food. For most, this is achieved by using light energy, water and carbon dioxide. Rather than using energy from the sun, some will use chemical energy to make their own food.
All autotrophs use non-living material (inorganic sources) to make their own food. Because of their ability to make their own food, autotrophs are also commonly refered to as primary producers and thus occupy the base of the food chain. They vary widely from those found on land (soil) to those that live in aquatic environments.
chain - Food chain refers to a linear sequence through which food energy is
transferred when one organism consumes another. This chain is divided into
different trophic/nutritional levels.
Since autotrophs do not depend on organic
matter and are capable of making their own food from inorganic sources, they
occupy the base of the food chain (first trophic/nutritional level) with
herbivores and carnivores (as well as omnivores) occupying the second and third
trophic levels respectively.
While there are a wide variety of organisms that
are classified as autotrophs, there are two main types based on
how they produce their food. These organisms live in different environments and
use different mechanisms (and material) to produce energy.
The two types are:
Basically, phototrophy involves the use of light
energy (from the sun) for photosynthesis. Here, light energy obtained from the
sun is used to produce food material (organic material) from carbon-dioxide and
Most of the organisms that use this method to produce food have
chloroplast (membrane bound) as well as a membrane bound nucleus. As such, they
are eukaryotic organisms.
There are various prokaryotes that are also
capable of photosynthesis. This includes a number of bacteria.
As mentioned, all photoautotrophs have chlorophyll.
While some like cyanobacteria may not have a chloroplast that contains the
chlorophyll, they have chlorophyll in place to capture light energy to be used
In higher plants, photosynthesis takes place in
the mesophyll layer of the leaf where chloroplasts are located. Carbon-dioxide required
for photosynthesis gets into the mesophyll layer and into the chloroplast
through small openings on the leaves known as stomata.
openings are located underside of leaves to minimize water loss during
transpiration. Whereas carbon-dioxide is taken in through the stomata, water is
absorbed through osmosis from the soil (by specialized root hairs). Water is
then transported to the leaves (and other parts of the plant) through the xylem
(one of the plants vascular tissues).
Within the chloroplast, chlorophyll is located
in the innermost membrane known as the thylakoid membrane. This pigment
captures/absorbs the red and blue wavelengths of light (visible spectrums) that
produces the energy required for photosynthesis.
Photosynthesis occurs in two main phases, these
Light-Dependent Phase (Light Dependent Reactions)
This is the first phase of the photosynthesis
and takes place in the thylakoid membrane of the chloroplast.
photosystems known as Photosystem I and Photosystem II (PSI and PSII) have a
variety of pigments including chlorophyll molecule that absorb light energy.
This provides energy required to move electrons from water molecules through
the photosystems to make NADPH (Nicotinamide adenine dinucleotide phosphate)
and ATP (Adenosine triphosphate).
The first phase of photosynthesis is refered
to as light dependent because it only takes place in the presence of sunlight.
The primary purpose of this phase is to convert light energy from the sun into
chemical energy (ATP and NADPH). Using this chemical energy, plants are then
able to synthesis organic material such as sugars.
In plants, the light-independent reactions take
place in the absence of sunlight. Because the first phase (light dependent reactions)
successfully produced energy in the form of ATP and NADPH, sunlight is no
longer required given that these sources of energy provide the required energy
for sugar synthesis. Here, the Calvin cycle is used to describe the light-independent
In the Calvin cycle, carbon-dioxide combines
with ribulose-1, 5-bisphosphate (RuBP) in the presence of RuBP
carboxylase/oxygenase, (RuBisCo) enzyme to produce two
molecules of 3-phosphoglyceric acid (3-PGA) which is a three-carbon compound.
This is the first stage of light-independent reaction and is known as carbon
The second phase is known as reduction and requires ATP and NADPH. In
this stage, the two energy sources provide the energy required to convert
3-phosphoglyceric acid into glyceraldehyde-3-phosphate (G3P) which is a
Lastly, in the third stage known as regeneration, some
molecules of glyceraldehyde-3-phosphate are used to produce sugar molecules
(glucose) while others are recycled in order to regenerate RuBP for more
reactions. This stage is fueled by ATP which acts as the source of energy.
For photoautotrophs, chlorophyll is a very
important pigment. This is because it helps capture sunlight that is then used
during photosynthesis. All organisms that carry out photosynthesis have
There are two main types of chlorophyll including:
Chlorophyll a - Chlorophyll a is the most common chlorophyll
and can be found in the majority of the photoautotrophs including
cyanobacteria, higher plants and algae. Chlorophyll (a) captures blue-violet
and orange-red light (at 675nm) while reflecting green light (thus appearing
green in color). Energy from these wavelengths is then used for photosynthesis.
Chlorophyll b - Chlorophyll b is common in algae and plants
and captures green light (at 640 nm). In the organisms in which it is found,
chlorophyll b passes energy from the light to chlorophyll a thus acting to
complement chlorophyll a. It is particularly useful when there is little light
given that absorbs a broader spectrum than chlorophyll a. As a result, it is
produced in plenty during cases where sunlight is limited.
* Depending on the amount of light available,
chlorophyll may be oxidized to produce chlorophyll b
This reaction is common among many higher
plants, algae as well as cyanobacteria. While cyanobacteria are capable of
producing oxygen and sugar as the final product, other bacteria are not capable
of producing oxygen. As a result, cyanobacteria are the only bacteria that have
been shown to be capable of producing oxygen during photosynthesis.
that do not produce oxygen during photosynthesis are known classified as
obligate anaerobes while they produce through a process refered to as
Some of the organisms that use this mechanism to
While these organisms use light energy to
produce their own energy, they do not use water as the source of protons.
Rather, such gases as hydrogen sulfide are used for reduction. For such
organisms as green sulfur bacteria, such pigments as bacteriochlorophyll (a)
and (b) absorb light energy that is then used or photosynthesis reaction.
Whereas photoautotrophs obtain their energy from
the sun, chemotrophs do not need the sun and thus obtain their energy from
various molecules available in their environment.
Chemotrophs are divided into
two groups including chemoorganotrophs (use organic molecules as a source of
energy) and chemolithotrophs that use inorganic molecules. Here, we shall focus
on chemolithotrophs given that they do not use organic molecules to produce
These organisms are also known as lithotrophs
and include various bacteria including the nitrifying bacteria and bacteria
found in tube worms in deep sea levels. While these organisms live in
environments where there is no sunlight, there is sufficient inorganic material
Essentially, biosynthesis involves the oxidation
of the inorganic material. Here, chemolithotrophs (cells) take in the electron
donor (iron, elemental sulfur and hydrogen sulfide etc) which are then oxidized
to produce energy.
For instance, the oxidation of hydrogen sulfide produces
electrons that are transported through the electron transport chain for
oxidative phospholyration that produces ATP energy. The chemical energy in form
of ATP is then used in biosynthesis to fix carbon in order to produce organic
* This process is different from photosynthesis
where autotrophs are able to produce their own energy by using energy from the
sun (sunlight). Because chemolithotrophs do not have access to sunlight, they
have to rely on inorganic material in their environment.
As mentioned, autotrophs are primary producers
and therefore occupy the base of the food chain at the first trophic level.
This makes them very important in nature given that every other organism
that is not a primary producer relies on them for their survival. For instance,
herbivores rely on plants for their energy and eat various plants (grass, corn,
leaves etc) as their source of food.
and omnivores are dependent on plants and meat as their source of food and
energy. Without autotrophs, which are the primary producers, all these other
organisms at the higher trophic levels would not survive because the food
chain as a whole is dependent on the primary producers.
Apart from simply being the source of food and
energy, they are also important in other ways. The
Thioautotrophic bacteria that live in the giant tube worm (Riftia pachyptila)
uses hydrogen sulfide (oxidation) to produce NADPH and ATP that is then used to
synthesis organic material. This is used as the source of energy by the worm.
This is a symbiotic relationship that allows the two organisms to live and
benefit each other. Here, therefore, this type of autotrophy benefits organisms
that live in tough environments such as the deep sea.
Difference between Heterotrophs and Autotrophs
There are a number of differences between
heterotrophs and autotrophs, these include:
Autotrophs (for the most part) use inorganic
material to produce organic compounds while heterotrophs cannot - Whereas
they use such material as carbon-dioxide and water to produce such
organic compounds as glucose, heterotrophs are simply consumers that require
organic material (organic compounds) as their source of energy.
Autotrophs (phototrophs) have chloroplast or
chlorophyll or the equivalent of chlorophyll pigments while heterotrophs do not
- They need these pigments for the purposes of absorbing light energy for
Because heterotrophs cannot carry out this process,
they do not have nor require these pigments. Autotrophs that do not use light
energy do not have these pigments, but can use inorganic material to make their
own food as a source of energy
Carbon dioxide – a majority of autotrophs need
carbon-dioxide to synthesis their own food as a source of energy. That is,
carbon-dioxide is for the most part the source of carbon that is required to produce
carbon based molecules (organic molecules like glucose).
does not serve the same purpose in heterotrophs like human beings, cows or pigs
etc (in such heterotrophs, carbon-dioxide helps with such functions as vasodilation
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