A bacterium living near a deep-sea hydrothermal vent oxidizes hydrogen sulfide (H₂S) as its energy source and uses CO₂ as its sole carbon source. How is it correctly classified?
APhotoautotroph
BPhotoheterotroph
CChemoheterotroph
DChemoautotroph
Energy from an inorganic chemical reaction (H₂S oxidation) = chemotroph. Carbon from CO₂ fixation = autotroph. Combining these gives chemoautotroph. This is distinct from photoautotrophs like plants and cyanobacteria, which use light energy rather than chemical energy. The deep-sea example is important because it demonstrates that autotrophy does not require sunlight.
Question 2 True / False
Fermentation is a type of anaerobic respiration because both processes occur without oxygen.
TTrue
FFalse
Answer: False
Both fermentation and anaerobic respiration occur without O₂, but they are mechanistically distinct. Anaerobic respiration uses an electron transport chain with a non-oxygen terminal electron acceptor (e.g., nitrate, sulfate). Fermentation has no electron transport chain at all — it regenerates NAD+ by transferring electrons to an organic molecule (like pyruvate), yielding only 2 ATP per glucose. Sharing the absence of O₂ does not make them the same process.
Question 3 Short Answer
Why do facultative anaerobes preferentially perform aerobic respiration when oxygen is available, even though they are capable of surviving without it?
Think about your answer, then reveal below.
Model answer: Aerobic respiration via the electron transport chain yields approximately 30-32 ATP per glucose, compared to only 2 ATP from fermentation, giving cells a strong energetic advantage and faster growth rate when O₂ is present.
The vast difference in ATP yield comes from the electron transport chain and oxidative phosphorylation, which are only possible when a high-energy terminal electron acceptor (O₂) is available. Fermentation short-circuits the process, recovering energy only from substrate-level phosphorylation. Natural selection favors using the more efficient pathway when conditions allow it.