Bacterial Respiration Investigation Worksheet
Biology Lab Investigation
Cellular Respiration in Bacterial Cultures
Name:
Date:
Period:
Investigation Prompt: Students perform a scientific investigation using several types of bacteria grown in liquid cultures. After incubating the bacterial cultures in test tubes for several days, they observe some of the bacteria growing close to the surface of the liquid and other types of bacteria growing at the bottom of the test tubes.
Experimental Results: Test Tube Culture Observations
Condition
Incubation Period
72 Hours
Temp: 37°C • Static
Picture A: Tube 1 (Surface) Clear Broth Bacterial Growth Surface layer O₂ GRADIENT High O₂ Decreasing Zero O₂ Air Diffusion Picture B: Tube 2 (Bottom) No Growth Bacterial Growth Bottom pellet
1
Which cellular process does the bacteria that grow close to the surface of the liquid culture (Picture A) most likely use to produce energy?
A DNA replication
B DNA transcription
C aerobic respiration
D anaerobic respiration
2
Which statement best explains why the bacteria in Picture B grow exclusively at the bottom of the test tube?
A They require high oxygen levels to run the electron transport chain.
B They are obligate anaerobes that produce ATP without oxygen, which is toxic to them.
C They perform photosynthesis using ambient light focused by the curved base of the tube.
D They sink due to their high density and lack motility structures to swim to the surface.
3
Why does a dissolved oxygen gradient form between the surface and the bottom of the liquid culture?
A Atmospheric oxygen diffuses in at the surface, creating decreasing concentrations deeper down.
B Gravity pushes gaseous oxygen molecules upward so they cannot dissolve into the lower liquid.
C Nutrients at the bottom of the tube actively react with oxygen and convert it into carbon dioxide.
D Water molecules at the top surface naturally decompose into oxygen gas during warm incubation.
4
In terms of ATP production, what is the primary biochemical advantage for the bacteria growing in Picture A over Picture B?
A Aerobic pathways yield approximately 36 to 38 ATP per glucose, compared to only 2 ATP in anaerobic pathways.
B Surface bacteria bypass glycolysis entirely and directly synthesize ATP from atmospheric nitrogen gas.
C Lactic acid fermentation produces four times as much ATP as oxidative phosphorylation.
D Anaerobic pathways require double the ATP activation energy to break down each pyruvate molecule.
Standard: NGSS HS-LS1-7 Cellular Respiration & Anaerobic Energy Production Choose the single best answer for each question
Bacterial Respiration Answer Key
Teacher Resource • Answer Key
Cellular Respiration in Bacterial Cultures: Answer Key
Course: Biology / Life Science
Standard: NGSS HS-LS1-7
Quick Reference Key:
Q1 C aerobic respiration
Q2 B obligate anaerobes
Q3 A O₂ diffusion gradient
Q4 A 36–38 vs. 2 ATP
1 Surface Bacteria Energy Pathway
Correct Answer: C
Scientific Rationale: Bacteria growing near the liquid surface are obligate aerobes. They cluster at the top meniscus because they require atmospheric oxygen as the final electron acceptor in the electron transport chain to perform aerobic respiration and generate ATP.
Distractor Breakdown: A & B are genetic processes that synthesize nucleic acids rather than producing cellular energy; D occurs in oxygen-deprived environments and would not cause bacteria to cluster at the oxygen-rich surface.
2 Bottom-Dwelling Bacteria (Tube 2 / Picture B)
Correct Answer: B
Scientific Rationale: Bacteria growing strictly at the bottom of the tube are obligate anaerobes. They generate ATP via anaerobic respiration or fermentation. Because they lack protective enzymes (such as catalase and superoxide dismutase), oxygen gas is lethal to them, restricting their survival to the anoxic zone.
Distractor Breakdown: A contradicts their location away from oxygen; C is invalid because standard culture media are heterotrophic and test tubes are not light-dependent; D is a common physical misconception—bacteria actively grow where biochemical conditions permit.
3 Dissolved Oxygen Gradient Formation
Correct Answer: A
Scientific Rationale: Liquid media (such as fluid thioglycollate) allow gaseous oxygen to dissolve and diffuse into the liquid surface from the airspace. Diffusion through static liquid is slow and limited by depth, naturally establishing a steep vertical concentration gradient from saturated O₂ at the top to zero O₂ at the bottom.
Distractor Breakdown: B is scientifically false—gases dissolve under pressure and do not "rise" out of liquid purely due to gravity; C incorrectly attributes gas removal to nutrient destruction; D confuses physical gas diffusion with the chemical splitting of water.
4 Comparative ATP Energy Yields
Correct Answer: A
Scientific Rationale: Complete aerobic cellular respiration utilizes glycolysis, the citric acid cycle, and oxidative phosphorylation with oxygen as the final electron acceptor, yielding approximately 36 to 38 ATP per glucose. Anaerobic fermentation yields only 2 net ATP from glycolysis alone.
Distractor Breakdown: B is biologically impossible (atmospheric nitrogen cannot replace glucose in ATP synthesis); C misrepresents fermentation yields; D glycolysis activation energy (2 ATP) is identical in both aerobic and anaerobic initial stages.
Instructional Extension & Class Discussion Prompts
Inquiry Prompt: Ask students what they would expect to see if a facultative anaerobe (like E. coli) were inoculated into Tube 3. (Expected: growth throughout the entire tube, with a denser concentration near the surface where aerobic ATP yield is highest.)
Key Misconception: Students often assume all bacteria require oxygen or that anaerobic bacteria "choose" the bottom. Reinforce that oxygen is an active cellular poison to obligate anaerobes because they lack superoxide dismutase.
Curriculum Key: HS-LS1 From Molecules to Organisms: Structures and Processes Print / Projection Ready Teacher Reference
