In this article we will discuss about the patterns of carbon and energy metabolism in organisms.
Carbon and energy metabolism are intimately associated in all living organisms. On the basis of the ability of organisms to obtain carbon and energy from usable sources, they can be divided into several categories.
Considering the energy source first, organisms can be divided into two groups:
1. The phototrophs and
2. The chemotrophs.
The phototrophic organisms are capable of utilizing radiant energy i.e. light as energy source by virtue of their ability of carrying out photosynthesis. The chemotrophic organisms, on the other hand, obtain energy from oxidation of chemical compounds.
Both types convert the usable energy source into a readily utilizable source, adenosine triphosphate (ATP). As described earlier, this compound acts as the “energy-currency” in most biological systems. Generation of ATP in photosynthesis is known as photophosphorylation and that of chemotrophs as substrate-level phosphorylation and oxidative phosphorylation.
On the basis of carbon source utilizing ability, organism can also be divided into two broad groups the lithotrophs and the organotrophs. Lithotrophic organisms are able to use inorganic carbon source, mainly carbon dioxide (rarely carbon monoxide), while organotrophic ones depend mainly on organic compounds for supply of carbon. The lithotrophs are commonly known as autotrophs, and the organotrophs as heterotrophs. Both autotrophs and heterotrophs can be phototrophic or chemotrophic.
Thus, combining the abilities of energy and carbon source utilization, living organisms can be divided into four major types:
i. Photo-lithotrophic,
ii. Photo-organotrophic,
iii. Chemolithotrophic, and
iv. Chemoorganotrophic.
A closer look at these four categories of organisms reveals some important differences.
Though all photo-lithotrophs can reduce CO2 to organic compounds with the help of ATP produced by photophosphorylation, they differ among themselves regarding the nature of the compound used as reductant for reducing CO2. In case of majority of photo-lithotrophs, water is used as the reductant or hydrogen donor accompanied by formation of oxygen as a by-product (oxygenic photosynthesis).
In the rest of the photo-lithotrophic organism, some other inorganic reductants are used, like H2S, H2 or other reduced sulfur compounds and no oxygen is produced in their photosynthesis (anoxygenic photosynthesis).
The chemo-organotrophs depend on organic compounds for supply of both carbon and energy. While they oxidize organic compounds, the hydrogen and electrons released from these compounds (substrates) may be finally transferred to oxygen-producing water, or they may be transferred to some other organic acceptor causing their reduction.
In the first case, the organisms carry out an aerobic respiration and the type of metabolism is respiratory. In the second case, the organisms carry out fermentation and the type of metabolism is fermentative. All these process, viz. photosynthesis, respiration and fermentation proceed in living organisms through numerous steps catalysed by specific enzymes.
Classification of organisms based on metabolism is shown in Fig. 8.41:
The phototrophic organisms are the primary producers. Majority of them carry out oxygenic photosynthesis and provide atmospheric oxygen and organic compounds which sustain the life of all chemotrophs. Thus, the phototrophs harness the solar energy for conversion of inorganic carbon to organic matter and the chemotrophs are consumers of the photosynthetic products.
They use these products for building up their cellular constituents as well as for extraction of energy through respiration or fermentation.
In this context, photosynthesis and respiration can be considered as two opposite processes as schematically represented in Fig. 8.42:
