Energy Sources

All living things need energy to fuel the biochemical pathways that enable them to grow, reproduce and move.

The Law of Conservation of Energy states that energy can neither be created nor destroyed. It can only be transformed or transferred from one form to another.

In other words, living things cannot make their own energy, but must acquire it somehow. They must continually input energy into their systems, where it can then drive those systems.

For some organisms, their energy source comes directly from the sun, via light photons.
Other organisms must break the chemical bonds in molecules to release — and use — the energy stored within.

This energy, whether released from photons or bonds, is then stored as potential energy in adenosine triphosphate (ATP), carbohydrates, lipids (a more accurate word than ‘fats’), and proteins until needed for living, growth and reproduction.

An organism will die without an energy source.

Carbon Sources

All organisms are made of carbohydrates, lipids and proteins, which are needed for growth, reproduction and movement. These molecules all contain carbon, making them organic molecules.

The Law of Conservation of Mass states that the quantity of mass in a closed system must remain constant over time. This means that mass can neither be created nor destroyed. It can only be rearranged/converted into new forms, with no net loss or gain.

In other words, living things cannot create from nothing the carbohydrates, lipids and proteins they need to live, grow and reproduce — they must continually obtain a source of carbon (and other elements) from which to synthesise these. These carbon (and other) atoms must enter the organism in a form the organism can make use of, where, through biochemical pathways they ultimately end up in other carbon molecules the organism then uses for life or expels as waste.

Some organisms use the carbon in an inorganic carbon source (carbon dioxide, or CO₂) to make their carbohydrates, lipids and proteins.
Other organisms use the carbon in organic carbon sources (carbohydrates, lipids and proteins) to make their own particular carbohydrates, lipids and proteins.

An organism will eventually die if a usable carbon source is not available.

‘-troph’

The suffix ‘-troph’ is from the Ancient Greek word τροφή (trophḗ, or ‘nourishment’).

The words this suffix is appended to tells us the energy and carbon sources a particular group of organisms uses, or its metabolism.

Classification by Energy Source

The prefixes ‘photo-’ and ‘chemo-’ define the energy source:

Phototroph

‘Nourishment from light’.
From the Ancient Greek combining form φωτω- (phōtō-, from φῶς, phôs, ‘light’) + τροφή (trophḗ, or ‘nourishment’).

Phototrophs convert photon energy into the chemical potential energy of adenosine triphosphate (ATP), carbohydrates, lipids and proteins.

Chemotroph

‘Nourishment from chemicals’.
From the combining word chemo- (‘chemical’) + τροφή (trophḗ, or ‘nourishment’).

Chemotrophs release energy by breaking the bonds of chemical compounds, which is converted into the chemical potential energy of adenosine triphosphate (ATP), carbohydrates, lipids and proteins.

Classification by Carbon Source

The prefixes ‘auto-’ and ‘hetero-’ define the carbon source:

Autotroph

‘Self-nourishment’.
From the Ancient Greek combining form αὐτο- (auto-, from αὐτός, autós, ‘self’) + τροφή (trophḗ, or ‘nourishment’).

An autotroph is an organism that uses inorganic carbon (carbon dioxide) as its carbon source. Think plants.

The take-home message is that it does not need a (once-)living (organic) carbon source (such as carbohydrates, lipids and proteins) to make its own carbohydrates, fats and proteins.

Autotrophs are also known as primary producers, as they produce the food source (eg grass, leaves) at the beginning of all food chains and webs.

Heterotroph

‘Nourishment from other (source)’.
From the Ancient Greek ἕτερος (héteros, ‘other, another, different’) + τροφή (trophḗ, or ‘nourishment’).

A heterotroph is an organism that uses organic compounds as its carbon source. Think animals.

The take-home message is that it does need a (once-)living (organic) carbon source (such as carbohydrates, fats and proteins) to make its own carbohydrates, fats and proteins.

Heterotrophs are also known as primary, secondary, tertiary and quarternary consumers as they are consumers of autotrophs (primary consumers) and/or other heterotrophs (secondary, tertiary and quarternary consumers).

Combinations

The flowchart below shows how the energy source and carbon source classifications combine:

We can be even more precise when considering a third aspect of metabolism:

Classification by Electron Donor Source

The subject of electron donors is a very technical subject at the heart of all biochemistry, as electron donors are the means by which energy is released from the energy source. Without electron donors there can be no biochemistry.

This is beyond the scope of this post, but it’s still worth mentioning ever so briefly here.

The prefixes ‘organo-’ and ‘litho-’ define the source of electrons needed to release energy from the energy source:

Organotroph

‘Nourishment from organic (materials)’.
From the combining word organo- (‘organic’) + τροφή (trophḗ, or ‘nourishment’).

Organotrophs use organic compounds as electron donors.

Lithotroph

‘Nourishment from rock (inorganic materials)’.
From the Ancient Greek λίθος (líthos, ‘stone’) + τροφή (trophḗ, or ‘nourishment’).

Lithotrophs use inorganic compounds as electron donors.

Putting it All Together: In Order, Classification by Energy Source, Electron Donor Source, and Carbon Source

Combining all three defines a particular metabolism precisely. Writing this in the order by which energy enters and moves through an organism, from energy source, to electron donor, to carbon source, we have all these possible metabolisms:

• Photoorganoautotroph
• Photoorganoheterotroph
• Photolithoautotroph
• Photolithoheterotroph

• Chemoorganoautotroph
• Chemoorganoheterotroph
• Chemolithoautotroph
• Chemolithoheterotroph

Examples

Plants are photolithoautotrophs, as they obtain energy from photons (photo), use water as the electron donor (litho), and obtain carbon from carbon dioxide (auto). The simpler words autotroph and phototroph can also be used: in fact autotroph is the word usually taught in high school ecology classes for ‘plants’.

(Plants are actually phototrophic during the day when sunlight is present and they are photosynthesising, but switch to heterotrophy at night to utilise the carbon compounds they made during the day.)

Animals are chemoorganoheterotrophs, as they obtain energy from organic compounds (chemo), use organic compounds as electron donors (organo), and obtain carbon from organic compounds (hetero). The simpler words chemotroph and heterotroph can also be used, and heterotroph is the word learnt in high school for ‘animals’.

Bacteria and/or archaea (similar to bacteria but with enough significant differences to justify their own domain) are found in all of the above groups. Some of these groups play important roles in soil formation (pedogenesis) and making essential nutrients available to plants.

For example, the photolithotrophic cyanobacteria were the very organisms to oxygenate the planet two billion years ago and make multicellular life possible. To this day some have roles in nitrogen fixation and soil stabilisation through the production of polysaccharides that bind sand particles together and absorb water.

Exploring the metabolisms of soil bacteria reveals a lot of useful information about pedogenesis and nutrient availability, and these are topics well worth exploring in future posts.