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Chapter 3. Energetics Respiration Growth. Types of energy metabolism. Chemoheterotrophs Energy source (electron donor):organic carbon Carbon source: organic carbon. An electron donor is an energy source An electron acceptor is a respiratory substrate. Chemoheterotrophs. Electron donor.

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Chapter 3

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Chapter 3




Types of energy metabolism

  • Chemoheterotrophs

    • Energy source (electron donor):organic carbon

    • Carbon source: organic carbon

An electron donor is an energy source

An electron acceptor is a respiratory substrate


Electron donor

Carbon source

Organic C

Carbon dioxide


Electron acceptor

O2aerobic respiration

NO3, Fe(III), Mn(IV), SO4, CO2anaerobic respiration

Types of energy metabolism

  • Chemoautotrophs

    • Energy source (electron donor): reduced form of an inorganic chemical

    • Carbon source: inorganic carbon-CO2


NH4+, Fe(II), Mn(II), HS-

Electron donor

Carbon source

Carbon dioxide

Inorganic C

Organic carbon

Electron acceptor

O2aerobic respiration


NO3, Fe(III), Mn(IV), SO4, CO2anaerobic


N2O, Fe(II), Mn(II), H2S, CH4

Types of energy metabolism

  • Photoautotrophs

    • Energy source: light

    • Electron donor: water

    • Carbon source: inorganic carbon


Energy source

Electron donor

Light (l)


Carbon source, electron acceptor

Carbon dioxide

Inorganic C (CO2)

Organic carbon

Light (l)

Electron donor



Types of energy metabolism

  • Photoheterotrophs

    • Energy source: light

    • Electron donor: water

    • Carbon source: organic carbon


Energy source

Light (l)

Electron donor


Carbon source

Organic C


Organic C

Terminal electron acceptor

Fe-S clusters

in Photo System 1

Energy Yield

In a chemical reaction, only part of the energy is used to do work. Energy available for work is called “free energy” or DG.

The rest of the energy is lost to entrophy.

DG = -RT log Keq where Keq = [C] [D] / [A] [B] from rxn:

A + B C + D

If logKeq is a negative value, this means the reaction can only proceed if energy is added (endothermic rxn).

When logKeq is a negative value, DG is positive.

If logKeq is a positive value, this means the reaction is favored and, in fact, gives off energy (exothermic rxn).

When logKeq is a positive value, DG is negative.

Energy yield from different electron acceptors

  • 6O2 6H2Oaerobic respiration-686

  • 24 NO3 12N2anaerobic respiration -36

  • SO4 H2S anaerobic respiration -40

  • CO2 CH2O photosynthesis+115

Terminal electron acceptor

DG (kcal/mol)


Which ones are exothermic and which ones are endothermic?

Reduction Potential

Redox couples




NO3/ N2





oxidation state (volts)







(CH2O CO2) electron donor


Energy yield relationship between electron acceptor and electron donor

Electron ReductionElectron Reduction Difference

AcceptorPotential (V) Donor Potential (V)(V)

O2 H2O+0.81 CH2O CO2 -0.47 -1.28

NO3 N2 +0.75 CH2O CO2 -0.47 -1.22

SO4 H2S -0.22 CH2O CO2 -0.47 -0.25

The sign and magnitude of the difference represents how much

free energy is available to the cell to do work.

Bacterial Growth

Batch Culture-closed system




Cell Density









N2 -


= #cells produced/unit time (growth rate)

t2 – t1

Natural systems

  • Natural environments that behave as closed systems

    • Not many exist. There is always some energy coming in from outside the system

  • Natural environments that behave as open systems

    • Most inhabitable environments

Continuous Culture

More closely mimics natural systems

  • Chemostat

    • Control flow rate and concentration of growth-limiting nutrient of liquid medium entering and exiting a growth chamber (bioreactor)

    • Control

      • pH

      • Temperature

      • Concentration of terminal electron acceptor (TEA)

      • Concentration of toxic by-products of metabolism

Engineered wastewater treatment system operates like a chemostat

Chemostat:open system

X=cell number

S=limiting nutrient conc.

D=dilution (flow) rate

Energy source (electron donor)

Inoculate vessel

(Terminal electron acceptor)

  • By controlling flow rate or dilution rate, one can control growth rate (m) of the bacteria

  • dX/dt = mX –DX, where X is cell biomass in mass/volume, m is specific growth rate (1/t), and D is the dilution rate (1/t)

  • At steady state, when biomass in reactor remains constant, m = D

  • A chemostat reactor allows the maintenance of steady state conditions for extended periods of time

Processes occurring in a chemostat

Stationary phase


m = D


Limiting nutrient

  • In previous slide, the substrate that was plotted was the nutrient that limited the rate of growth

    • This could be any nutrient

      • Carbon source

      • Nitrogen source

      • Phosphorus source

      • Electron donor or electron acceptor

    • The chemistry of the limiting nutrient will influence how much cell biomass (cell yield) is produced at mmax

Cell Yield (Y)

  • Not all of the carbon added as the carbon source is converted to cell biomass

  • A fraction is respired as CO2 during the transformation of the carbon to energy (ATP)

  • Cell yield coefficient is defined as the amount of biomass produced per unit substrate consumed

Yield coefficient

Carbon source



Pentachlorophenol (PCP)




Cell Yield

Biochemical basis of cell yield

  • In case of PCP, it is a new chemical that microbes have only encountered since its initial production in 1936

  • Consequently, microbes have not had time to evolve efficient enzyme reactions and metabolic pathways to convert it to biomass

  • A lot of energy is required to break the C-Cl bonds-energy not available for biomass production

Biochemical basis of cell yield

  • In case of octadecane, it is a component of crude oil that microbes have encountered for millions of years

  • Consequently, microbes have had time to evolve efficient enzyme reactions and metabolic pathways to convert it to biomass.

  • Octadecane is a highly reduced form of carbon (contains only C-H bonds) and can thus store more energy than compounds that are less reduced or have more oxygen atoms such as carbohydrates (CH2O)

Study calculations

  • Go over Example Calculation on page 50 of text

  • cells are growing on glucose

    • yield coefficient = 0.4 (0.4 g cell mass produced from 1 g of glucose consumed)

  • What percentage of glucose carbon is converted to cell mass and what percentage to CO2?

1 mol of glucose is equivalent to 180g

cell mass produced from 1 mol glucose = 180g x 0.4 = 72g

cell mass is expressed as C5H7NO2 (mol. wt. = 113g/mol)

moles of cell mass produced from 1 mol of glucose

72g cell mass/113 g/mol cell mass = 0.64 mol cell mass

In terms of carbon:

for cell mass,

(0.64 mol cell mass)(5 mol C/mol cell mass)(12 g/mol C) = 38.4g C

for substrate (glucose),

(1 mol substrate)(6 mol C/mol glucose)(12 g/mol C) = 72 g C

% of glucose carbon found in cell mass is (38.4 g C/72 g/C)x100 =53%

by difference 47% glucose carbon is released as CO2





% substrate consumed

that ends up in cell mass

How do metabolic pathways evolve in bacteria

  • Lateral gene transfer

    • plasmids

    • bacteriophage

  • Naked DNA in the environment (tranformation)

Developing genetically-engineered microbes and plants to carry out specific remediation activities when indigenous organisms can’t do the job

  • Naphthalene-degrading plasmid (NAH)

  • Xylene-degrading plasmid (XYL)

  • Conjugation occurred between 2 cells that had the different plasmids containing genes that encode both degradative pathways. One recombinant cell can now degrade both compounds

  • Same genetic manipulations were carried out with octane-degrading plasmid (OCT) and camphor-degrading plasmid (CAM).

  • The plasmids fused into one plasmid when transferred into a bacterial strain to form a genetically-engineered bacterial strain that could degrade both octane and camphor.

  • The strain carrying the NAH & XYL plasmids were mated with the strain that carried the OCT/CAM plasmid to produce a new construct that did a good job growing on and degrading crude oil.


  • Microbes as a group are metabolically diverse

  • Amount of energy a cell can extract from a chemical is determined by intrinsic chemistry of the chemical

  • Abundance of the limiting nutrient in the environment controls the growth rate of the cell

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