← Back to 1. The Foundations of Biochemistry

📝 Lithotrophs vs Organotrophs electron sources (15 MCQs)

📖 From Principles of Biochemistry • 1. The Foundations of Biochemistry • 15 questions available

←Previous Topic:Autotrophs vs Heterotrophs carbon sources📝 Lithotrophs vs Organotrophs electron sourcesEscherichia coli as a model prokaryote:Next Topic→

What is Lithotrophs vs Organotrophs electron sources?

Definition:
Lithotrophs and organotrophs are classifications based on the source of electrons used in metabolic redox reactions, where lithotrophs obtain electrons from inorganic compounds such as hydrogen sulfide, ammonia, or ferrous iron, while organotrophs acquire electrons from organic compounds like glucose, and this distinction determines their energy-yielding pathways and ecological niches.

Working:
Lithotrophs work by oxidizing inorganic molecules to release electrons that enter the electron transport chain, generating a proton motive force for ATP synthesis, with a general reaction like $2H_2S + O_2 \rightarrow 2S + 2H_2O + Energy$ , whereas organotrophs oxidize organic substrates, such as glucose, through glycolysis and oxidative phosphorylation, where the electron donor is the organic molecule itself.

Example:
A simple example is Nitrosomonas, a lithotroph that oxidizes ammonia ( $NH_3$ ) to nitrite ( $NO_2^-$ ) to obtain energy, while humans are organotrophs that oxidize glucose via the equation $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + Energy$ , using organic matter as the electron source.

Reason:
Understanding lithotrophs vs organotrophs is important because lithotrophs play critical roles in biogeochemical cycles, such as nitrogen and sulfur cycles, and are key in environmental bioremediation, while organotrophs represent most heterotrophic organisms including humans, influencing our understanding of metabolism, nutrition, and microbial ecology.

Easy

Medium

Hard

📝 All Lithotrophs vs Organotrophs electron sources MCQs

Q1. Which statement best distinguishes lithotrophic and organotrophic energy strategies?

A.Lithotrophs obtain electrons from inorganic substances, whereas organotrophs obtain electrons from organic substances ✅

B.Lithotrophs always use light, whereas organotrophs always use oxygen

C.Lithotrophs consume only carbon dioxide, whereas organotrophs consume only minerals

D.Lithotrophs are always autotrophs, whereas organotrophs are always heterotrophs

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: Lithotrophy and organotrophy classify organisms according to the nature of their electron donors. Lithotrophs use reduced inorganic substances such as hydrogen or sulfide, whereas organotrophs obtain electrons from reduced organic compounds. These categories do not automatically determine carbon source or energy source.

Q2. An organism oxidizes $H_2S$ to obtain electrons but obtains its cellular carbon primarily from $CO_2$ . How should its metabolism be classified with respect to electron and carbon sources?

A.Organolithotrophic heterotroph

B.Lithoautotrophic ✅

C.Organoheterotrophic

D.Photoheterotrophic

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The organism uses an inorganic electron donor, $H_2S$ , so it is lithotrophic. Because it obtains carbon from $CO_2$ , it is autotrophic. Combining the two classifications gives lithoautotrophic metabolism rather than organotrophic or heterotrophic metabolism.

Q3. A microbe grows using an organic compound as its electron donor but obtains carbon from $CO_2$ . Which conclusion is most defensible?

A.It is necessarily a lithotroph because

CO_2

is inorganic

B.It is an organoautotroph because its electron donor is organic and its carbon source is

CO_2

✅

C.It must be an organoheterotroph because every organotroph uses organic carbon

D.It cannot grow because organisms cannot combine organic electron donors with inorganic carbon

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Electron source and carbon source are independent metabolic classifications. An organic electron donor makes the organism organotrophic, while $CO_2$ as the principal carbon source makes it autotrophic. Therefore, organoautotroph is the appropriate combined description.

Q4. A researcher supplies two cultures with identical $CO_2$ concentrations. Culture X grows when hydrogen is supplied, while Culture Y grows when glucose is supplied. What is the strongest inference?

A.X is likely lithotrophic and Y is likely organotrophic ✅

B.X is necessarily heterotrophic and Y is necessarily autotrophic

C.Both must be organotrophs because

CO_2

is present

D.Both must be lithotrophs because their carbon source is

CO_2

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Hydrogen is an inorganic electron donor, supporting classification of Culture X as lithotrophic. Glucose is an organic electron donor, supporting classification of Culture Y as organotrophic. The shared $CO_2$ supply does not determine their electron-donor categories.

Q5. A wastewater treatment system contains abundant reduced inorganic compounds but very little organic matter. A microbial population increases rapidly. Which experimental result would most strongly support a lithotrophic explanation?

A.Growth decreases when inorganic electron donors are removed, even though

CO_2

remains available ✅

B.Growth increases only after glucose is added

C.Growth stops when

CO_2

is removed but is unaffected by inorganic electron donors

D.Growth occurs only under intense illumination regardless of chemical composition

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: If growth depends strongly on reduced inorganic electron donors while $CO_2$ remains available, that pattern directly supports lithotrophic metabolism. The other outcomes emphasize organic substrates, carbon availability, or light rather than the defining electron-donor characteristic.

Q6. An industrial culture initially receives an inorganic electron donor and $CO_2$ . Growth is slow. After glucose is added, growth increases sharply. Which interpretation best fits the observation?

A.The organism may be capable of organotrophic metabolism in addition to using inorganic electron donors ✅

B.The organism must have permanently changed from an autotroph into a lithotroph

C.Glucose proves that the organism cannot use inorganic electron donors

D.The increased growth proves that glucose is being converted directly into inorganic electron donors

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: A rapid response to glucose suggests that the organism can exploit an organic electron donor, indicating organotrophic capability. This does not prove that its previous metabolism was impossible or that its classification permanently changed. Microbial metabolism can be flexible.

Q7. A student argues: 'Any organism that uses $CO_2$ must be a lithotroph because $CO_2$ is inorganic.' What is the central error?

A.They confused carbon source with electron donor ✅

B.They confused oxygen use with carbon fixation

C.They assumed all inorganic compounds provide electrons

D.They assumed organotrophs cannot use

CO_2

under any condition

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Lithotrophy refers to the use of inorganic electron donors, not simply the use of inorganic carbon. An organism may use $CO_2$ as its carbon source while obtaining electrons from organic compounds, so carbon-source and electron-donor classifications must be separated.

Q8. A student claims, 'If an organism is organotrophic, it must be heterotrophic.' Which example most directly disproves this claim?

A.An organism using glucose as an electron donor while fixing

CO_2

✅

B.An organism using sulfide as an electron donor while consuming glucose

C.An organism using light as an energy source and organic carbon

D.An organism using

CO_2

as both its electron donor and carbon source

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Organotrophy describes the nature of the electron donor, whereas heterotrophy describes the carbon source. An organism can therefore use an organic electron donor such as glucose while obtaining cellular carbon from $CO_2$ , making the student's conclusion invalid.

Q9. A culture is tested under four conditions. Growth relative to the control is: no added electron donor $=5\%$ , hydrogen $=80\%$ , glucose $=12\%$ , sulfide $=75\%$ . Which conclusion is best supported?

A.The culture preferentially uses inorganic electron donors ✅

B.The culture is strictly dependent on glucose

C.The culture is incapable of using inorganic electron donors

D.The culture must obtain all cellular carbon from hydrogen

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: Hydrogen and sulfide are inorganic electron donors and produce much stronger growth than glucose or the no-donor control. The pattern therefore supports preferential lithotrophic electron acquisition, although the experiment alone does not establish the organism's carbon source.

Q10. A graph plots growth yield against increasing concentrations of an electron donor. Curve A rises steeply with increasing sulfide concentration, while Curve B rises steeply with increasing glucose concentration. Which interpretation is most appropriate?

A.A shows stronger evidence of lithotrophic metabolism, while B shows stronger evidence of organotrophic metabolism ✅

B.Both curves demonstrate identical electron-donor strategies

C.A demonstrates heterotrophy, while B demonstrates autotrophy

D.The graph alone proves that A and B use identical carbon sources

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Sulfide is an inorganic electron donor, so growth responding strongly to sulfide supports lithotrophic metabolism. Glucose is an organic electron donor, so the response of Curve B supports organotrophic metabolism. Carbon-source classification still requires separate evidence.

Q11. A graph shows growth rate on the y-axis and substrate concentration on the x-axis. Culture P reaches a high growth rate with increasing $H_2$ , whereas Culture Q reaches a high growth rate only with increasing acetate. What is the best comparison?

A.P shows evidence of lithotrophy, whereas Q shows evidence of organotrophy ✅

B.P and Q are both necessarily autotrophs

C.P is necessarily heterotrophic, whereas Q is necessarily autotrophic

D.Neither culture can be classified because growth rate contains no metabolic information

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Hydrogen is an inorganic electron donor, so Culture P's response supports lithotrophic metabolism. Acetate is an organic compound that can serve as an electron donor, supporting organotrophic metabolism in Culture Q. Their carbon sources would require additional measurements.

Q12. A researcher observes that adding glucose increases growth, but removing $CO_2$ also sharply decreases growth. Which metabolic model best explains both observations?

A.The organism may use glucose as an electron donor while relying substantially on

CO_2

for carbon ✅

B.The organism must be strictly organoheterotrophic

C.The organism must be strictly lithoautotrophic

D.The observations prove that glucose is the organism's only carbon source

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: Glucose stimulation suggests that an organic compound can support electron acquisition, while sensitivity to $CO_2$ removal suggests an important role for inorganic carbon fixation. Together, these findings are consistent with organoautotrophic metabolism or metabolic flexibility.

Q13. An experiment compares two microbes. Microbe A oxidizes $NH_3$ and fixes $CO_2$ . Microbe B oxidizes lactate and also fixes $CO_2$ . Which statement correctly compares them?

A.A is lithoautotrophic, whereas B is organoautotrophic ✅

B.A is organoheterotrophic, whereas B is lithoheterotrophic

C.Both are lithoautotrophic because both fix

CO_2

D.Both are organoheterotrophic because both obtain energy from chemical oxidation

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: Microbe A uses an inorganic electron donor, $NH_3$ , and inorganic carbon, $CO_2$ , making it lithoautotrophic. Microbe B uses an organic electron donor, lactate, while fixing $CO_2$ , making it organoautotrophic.

Q14. Which experimental design would most reliably distinguish whether a newly isolated organism is lithotrophic or organotrophic?

A.Compare growth and electron-transfer activity using chemically comparable inorganic and organic electron donors while controlling carbon and energy sources ✅

B.Change the carbon source and electron donor simultaneously

C.Measure only the organism's final biomass after growth on a complex medium

D.Expose the organism to different light intensities without controlling chemical substrates

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: A valid comparison should isolate the variable defining lithotrophy versus organotrophy: the electron donor. Keeping carbon and other relevant conditions controlled while comparing inorganic and organic electron donors allows researchers to attribute differences in growth or electron transfer more confidently.

Q15. An organism can use $H_2$ with $CO_2$ and can also use an organic compound with $CO_2$ . Under condition 1, hydrogen provides electrons; under condition 2, the organic compound provides electrons. What does this reveal?

A.The organism is metabolically flexible and can switch between lithotrophic and organotrophic electron acquisition ✅

B.The organism must always be classified only as a lithotroph

C.The organism must always be classified only as an organotroph

D.The organism cannot be autotrophic under either condition

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: The organism demonstrates two distinct electron-donor strategies: inorganic hydrogen supports lithotrophic metabolism, while an organic compound supports organotrophic metabolism. This illustrates why metabolic classifications describe particular physiological strategies rather than necessarily assigning an organism to one rigid pathway.

🔗 Related Topics (MCQs)

📝 Cellular Foundations MCQs 📝 Cell structure and functional units MCQs 📝 Cell dimensions and oxygen diffusion MCQs 📝 Archaea (archaebacteria) MCQs 📝 Bacteria (eubacteria) MCQs

←Previous Topic:Autotrophs vs Heterotrophs carbon sources📝 Lithotrophs vs Organotrophs electron sourcesEscherichia coli as a model prokaryote:Next Topic→