Prokaryotic Growth, Metabolism, and Adaptation (2B) - Bio & Biochem

1. Which oxygen-handling category grows best with low $O_2$ levels (below atmospheric concentration)?

   Microaerophile. They require oxygen for respiration but are damaged by atmospheric levels of $O_2$.

2. Which bacterial growth phase has net zero growth because division rate equals death rate?

   Stationary phase. Nutrient exhaustion and toxin buildup cause the rate of cell death to match the rate of division.

3. Which growth phase is defined by rapid, constant-rate cell division and maximal metabolism?

   Log (exponential) phase. Abundant nutrients and space allow cells to divide at their maximum rate with high metabolic activity.

4. Which bacterial growth phase is characterized by a net decrease in viable cells over time?

   Death (decline) phase. Continued resource depletion results in cell death exceeding division, leading to population decline.

5. What is the definition of a facultative anaerobe regarding metabolism with or without $O_2$?

   Uses $O_2$ when present; can ferment/anaerobically respire without it. They flexibly switch between aerobic respiration and anaerobic pathways for energy production.

6. What is the definition of bacterial transduction as a mechanism of genetic exchange?

   DNA transfer mediated by bacteriophages. Phages accidentally package host DNA during infection, transferring it to new bacterial cells.

7. Which method measures viable bacteria by counting colonies after dilution and plating?

   Colony-forming unit (CFU) count. Each colony originates from a single viable cell, providing a count of culturable bacteria.

8. What is the definition of the bacterial generation time during exponential growth?

   Time required for a population to double in number. This interval reflects the duration of one complete cycle of binary fission under optimal conditions during the log phase.

9. Identify $n$ in terms of time $t$ and generation time $g$ for binary fission growth.

   $n = \frac{t}{g}$. The number of generations equals the total elapsed time divided by the time per generation.

10. State the formula for bacterial population after $n$ generations starting from $N_0$ cells.

    $N = N_0 \times 2^n$. Binary fission doubles the cell count each generation, yielding exponential growth.

11. Which metabolic category uses light as an energy source and $CO_2$ as the carbon source?

    Photoautotroph. These organisms fix inorganic carbon using energy from photosynthesis, like cyanobacteria.

12. Calculate $n$ if a culture increases from $N_0 = 10^3$ to $N = 8 \times 10^3$ cells.

    $n = 3$ generations. The population increases eightfold, corresponding to $2^3$ doublings via binary fission.

13. Which bacterial growth phase is characterized by little net growth while cells adjust to a new medium?

    Lag phase. Cells synthesize enzymes and repair damage to prepare for division without significant population increase.

14. What is the definition of bacterial transformation as a mechanism of genetic exchange?

    Uptake of naked DNA from the environment. Competent cells incorporate exogenous DNA, enabling horizontal gene transfer and adaptation.

15. Calculate $N$ if $N_0 = 500$ cells and $n = 3$ generations have occurred.

    $N = 4000$ cells. Each generation doubles the population, so three generations multiply the initial count by $2^3 = 8$.

16. What is the primary purpose of bacterial endospore formation under harsh conditions?

    Long-term survival via a highly resistant dormant state. Endospores withstand extreme conditions like heat and desiccation, allowing revival when favorable.

17. What is the definition of an obligate aerobe in terms of oxygen requirement?

    Requires $O_2$ for growth; uses aerobic respiration. Oxygen serves as the essential terminal electron acceptor in their electron transport chain.

18. Which method measures total cell number by light scattering but cannot distinguish live from dead cells?

    Optical density (turbidity) measurement. Light absorbance correlates with cell density but includes both viable and non-viable cells.

19. Which enzyme detoxifies hydrogen peroxide by converting $2H_2O_2$ to $2H_2O + O_2$?

    Catalase. It breaks down toxic hydrogen peroxide, a byproduct of superoxide dismutase activity, into harmless products.

20. What is the definition of an obligate anaerobe in terms of oxygen tolerance?

    Cannot grow in $O_2$; oxygen is toxic. They lack protective enzymes against reactive oxygen species, making aerobic environments lethal.

21. Identify the term for the maximum population size an environment can sustain for a species.

    Carrying capacity. Resources like nutrients and space limit indefinite population growth in a given environment.

22. What is the key difference between batch culture and continuous culture (chemostat)?

    Chemostat adds nutrients and removes waste to maintain steady state. This open system sustains exponential growth by controlling flow rates, unlike closed batch cultures.

23. Which metabolic category uses chemical compounds for energy and organic molecules for carbon?

    Chemoheterotroph. They derive energy from oxidizing organic compounds and use them as carbon sources, common in pathogens.

24. What is the definition of bacterial conjugation including the key structure required?

    Direct DNA transfer via cell-to-cell contact using a sex pilus. The pilus forms a bridge for plasmid DNA transfer from donor to recipient, promoting genetic diversity.

25. Which enzyme converts superoxide radicals $O_2^-$ into $H_2O_2$ and $O_2$?

    Superoxide dismutase. It neutralizes harmful superoxide ions generated during aerobic metabolism to prevent oxidative damage.

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