All Free Biology MCQs with Answers

2181. The common waste material in bacteria is____________?

• A. Glycogen
• B. Lactic acid
• C. Ammonia
• D. Urea

Explanation: Many bacteria release organic acids such as lactic acid as metabolic waste products, particularly during fermentation when oxygen is unavailable or insufficient. Students may choose ammonia because it is a common nitrogenous waste, but in this basic bacterial-metabolism context lactic acid is the expected answer.

2182. Example of bacteria requiring low concentration of oxygen is__________?

• A. Campylobactor
• B. spirocheta
• C. E-coli
• D. Pseudomonas

Explanation: Campylobacter is microaerophilic, meaning it grows best at an oxygen concentration below that of normal air. Students may choose Pseudomonas because it is strongly associated with aerobic respiration, but it generally does not require the low-oxygen conditions characteristic of microaerophiles.

2183. Purple non-sulphur bacteria is an example of___________?

• A. Heterotrophic bacteria
• B. Chemosynthetic bacteria
• C. Photosynthetic bacteria
• D. Saprotrophic bacteria

Explanation: Purple non-sulphur bacteria use light energy to make ATP, so they are classified as photosynthetic bacteria, although many obtain organic compounds from their environment. Students may choose heterotrophic bacteria because these organisms often use organic carbon, but the defining feature in this question is their use of light.

2184. Those bacteria which are fully dependent upon their host for nutrition are called__________?

• A. Heterotrophic bacteria
• B. Chemosynthetic bacteria
• C. Saprophytic bacteria
• D. Parasitic bacteria

Explanation: Parasitic bacteria obtain nutrients from a living host and depend on that host for at least part of their nutrition. Students may choose heterotrophic bacteria because parasites are heterotrophs, but heterotrophy is a broad category that also includes saprophytic bacteria.

2185. Nitrifying bacteria are the examples of___________?

• A. Hetrotrophic bacteria
• B. Chemosynthetic bacteria
• C. Saprophytic bacteria
• D. Parasitic bacteria

Explanation: Nitrifying bacteria obtain energy by oxidizing inorganic nitrogen compounds such as ammonia or nitrite, so they are chemosynthetic, specifically chemoautotrophic, bacteria. The likely alternative is heterotrophic bacteria, but heterotrophs obtain organic nutrients rather than energy from inorganic chemical oxidation.

2186. E. coli and example of____________?

• A. Aerobic bacteria
• B. Anaerobic bacteria
• C. Facultative bacteria
• D. Microaerophilic

Explanation: E. coli is a facultative anaerobe because it can respire aerobically when oxygen is available but can also grow without oxygen by anaerobic respiration or fermentation. Students often choose aerobic bacteria because E. coli grows well in oxygen, overlooking its ability to grow in its absence.

2187. Rapid growth at exponential ration occurs in which phase of bacterial growth:

• A. Lag phase
• B. Log phase
• C. Stationary phase
• D. Death / decline phase

Explanation: During the log phase, bacteria divide at a rapid and relatively constant rate, producing exponential population growth. The common alternative is the lag phase, but cells mainly adapt and synthesize enzymes there rather than multiplying rapidly.

2188. Cyanobacteria have ________ cell wall?

• A. Gram +ve.
• B. Gram - ve.
• C. Acid fast
• D. Cellulose rich

Explanation: Cyanobacteria have a Gram-negative-type cell envelope, with a thin peptidoglycan layer and an outer membrane. The likely mistake is choosing Gram-positive because cyanobacteria perform photosynthesis like some familiar Gram-positive-looking organisms, but their envelope structure is Gram-negative.

2189. Cyanobacteria reproduce by____________?

• A. Binary fission
• B. Mitosis
• C. Meiosis
• D. Conjugation

Explanation: Cyanobacteria are prokaryotes and reproduce asexually, commonly by binary fission or fragmentation. Mitosis and meiosis are eukaryotic nuclear division processes, while conjugation is genetic exchange rather than ordinary reproduction.

2190. Approximately _____ species of bacteria are known to cause diseases in humans.

• A. 100
• B. 200
• C. 300
• D. 400

Explanation: Standard introductory microbiology texts commonly state that about 200 bacterial species are known to cause disease in humans. A student may select 100 or 300 because the figure is approximate, but 200 is the conventional estimate used in this classification context.