4.3: Metabolism
- Page ID
- 131132
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Metabolism is the name given to all of the chemical reactions within cells that are necessary to sustain life. Metabolism involves reactions that provide cells with energy as well as those that transform nutrients into biological molecules such as nucleic acids, proteins, lipids, and more. How redox reactions are involved depends on the type of metabolism.
A wide diversity of metabolic strategies is possible among microorganisms. To help organize our knowledge about metabolisms, we can classify organisms based on how they obtain energy and carbon. Based on energy sources, major categories of organisms are chemotrophs and phototrophs (Table \(4.4\)). Chemotrophs generate energy by catalyzing redox reactions whereas phototrophs generate energy from light. Based on carbon source, major categories of organisms are heterotrophs and autotrophs. Heterotrophs obtain carbon for biomass from organic compounds whereas autotrophs obtain carbon from inorganic compounds (e.g., \(\text{CO}_{2}\), \(\text{HCO}_{3}^{-}\)). Both chemotrophs and phototrophs can be either heterotrophic or autotrophic. In addition to these major categories, mixotrophs are organisms that can use a mix of different sources of energy and carbon rather than a single category (e.g., use of both heterotrophic and autotrophic pathways).
Chemotrophs are also commonly further classified based on the nature of the electron donor (i.e., what is being oxidized) in their redox reaction. If the electron donor is an organic compound (e.g., acetate, lactate, glucose), they can be described as chemoorganotrophs. If the electron donor is inorganic (e.g., \(\text{H}_{2}\), \(\text{H}_{2} \text{S}\), \(\text{Fe}^{2+}\), \(\text{NH}_{4}^{+}\)), they can be described as chemolithotrophs. Note that here, the reference to inorganic and organic carbon does not indicate the type of carbon used to make biomass. Instead, it reflects the energy source used by the microorganism.
Lastly, you may also encounter the terms assimilatory and dissimilatory metabolism. Some inorganic compounds such as nitrate, ferric iron, sulfate, and carbon dioxide can be reduced when they are used as electron acceptors in chemotrophic reactions that generate energy. When that is the case, those compounds are said to be dissimilated. However, they can also be reduced for use as a nutrient source in biosynthesis. When that is the case, they are said to be assimilated.
| Classification | Subgroup | Energy Source | Carbon Source |
|---|---|---|---|
| Chemotroph | Chemoheterotroph | Redox reaction | Organic |
| Chemoautotroph | Redox reaction | Inorganic | |
| Phototroph | Photoheterotroph | Solar radiation | Organic |
| Photoautotroph | Solar radiation | Inorganic |