College practice · Worked answers
Microbiology exam 1 practice test
Use this short college microbiology test to find the topics you need to revisit. It covers cell types, microscopy, staining, and growth. “Exam 1” means different things in different courses, so compare the topic list with your syllabus before using it as a study plan.
Build a study route from your misses
| Topic | Be able to explain | Study next |
|---|---|---|
| Microbial groups | Cells versus acellular agents; prokaryotes versus eukaryotes | Chapter 1 concepts |
| Microscopy and stains | Magnification versus resolution; what a Gram stain reports | Chapter 2 concepts |
| Cell structures | Nucleoid, ribosome, membrane, and wall | Chapter 3 concepts |
| Growth | Binary fission, doubling, and batch-culture phases | Chapter 9 concepts, if assigned |
The chapter numbers follow OpenStax, not every instructor’s course order. Use your own learning objectives to decide what belongs on your exam.
Three contrasts worth explaining aloud
Prokaryotic does not mean empty: bacteria and archaea have membranes, genetic material, and ribosomes, but lack a membrane-bound nucleus. A yeast cell is eukaryotic. A virus is acellular and relies on a host cell for replication.
Bigger does not always mean clearer: magnification enlarges an image; resolution is the ability to distinguish nearby details. Increasing magnification alone cannot recover detail the instrument did not resolve.
More cells does not mean bigger cells: microbial population growth usually refers to an increase in cell number. In a simple binary-fission model, one division doubles the population. Do not apply that model through an entire closed-culture growth curve.
Use the test in two passes
- Answer the ten questions without notes. Write one sentence explaining each choice.
- Check the feedback. For a miss, identify the confused pair of ideas, not just the letter of the answer.
- Read the matching textbook section, then retry without looking at the solution.
- Explain a new example. If you can only recall this question’s wording, the concept still needs work.
Work through two examples
Classifying an unfamiliar microbe
- A description says the organism is a single cell with a membrane-bound nucleus.
- The nucleus supports a eukaryotic classification. Cell count alone does not decide this: both prokaryotes and eukaryotes can be unicellular.
- Do not classify it as a bacterium just because it is small. Yeasts and many protists are microbial eukaryotes.
Counting doublings
- A hypothetical culture starts with 75 cells and completes three binary-fission generations with no cell loss.
- Use a factor of 2 for each generation: 75 × 2 × 2 × 2.
- The model predicts 600 cells. Adding 3 or multiplying by 3 would confuse generations with the growth factor.
Try it yourself
Choose an answer, then check your reasoning. You can retry any question or open its solution.
Show solution 1
DNA in a nucleoid, with no membrane-bound nucleus. A prokaryote has DNA and ribosomes but lacks a membrane-bound nucleus. A nucleus is a eukaryotic feature.
Why the other choices are wrong:
- No DNA or ribosomes: Prokaryotic cells have DNA and ribosomes; they lack a membrane-bound nucleus.
- A membrane-bound nucleus: A membrane-bound nucleus is the defining feature of eukaryotic cells.
- Membrane-bound organelles such as mitochondria: Mitochondria and other membrane-bound organelles are eukaryotic features. Prokaryotes don’t have them.
Show solution 2
Magnification only. The image got larger, so magnification increased. Distinguishing adjacent details would demonstrate better resolution.
Why the other choices are wrong:
- Resolution only: Better resolution would let you tell nearby cells apart. The question says you still can’t.
- Both necessarily: A bigger image isn’t automatically a sharper one. Magnification can go up while resolution stays the same.
- Neither, since the image is still blurry: The image did get bigger, so magnification went up. The blur tells you resolution didn’t improve.
Show solution 3
A virus. Viruses are not cells. Yeasts are eukaryotic cells; bacteria and archaea are prokaryotic cells.
Why the other choices are wrong:
- A yeast: Yeasts are fungi, which are eukaryotic cells, even though they are small.
- A bacterium: Bacteria are prokaryotic cells. Being tiny is not the same as being acellular.
- An archaeon: Archaea are prokaryotic cells, like bacteria. They are not acellular.
Show solution 4
480. Two generations give a factor of 2² = 4. The count is 120 × 4 = 480 cells.
Why the other choices are wrong:
- 240: 240 is one doubling. Two generations means doubling twice: 120 × 2 × 2.
- 122: That adds the number of generations to the count. Each generation multiplies the population by 2.
- 360: That adds 120 cells each generation, which is straight-line growth. Binary fission doubles the whole population, so the second generation adds 240.
Show solution 5
Gram-positive bacilli. Purple rods are reported as Gram-positive bacilli when controls pass. Stain appearance does not establish a species.
Why the other choices are wrong:
- Gram-negative cocci: Purple means Gram-positive when the stain worked, and rods are bacilli, not cocci.
- A proven species identification: A stain shows reaction, shape, and arrangement. Naming a species needs more tests.
- Gram-negative bacilli: The shape is right, but purple means Gram-positive when the controls pass.
Show solution 6
Ribosomes. Both prokaryotes and eukaryotes use ribosomes to make proteins. Prokaryotic ribosomes (70S) are smaller than eukaryotic cytoplasmic ribosomes (80S), but both cell types have them.
Why the other choices are wrong:
- A nucleus: Only eukaryotes have a membrane-bound nucleus. Prokaryotic DNA sits in a nucleoid.
- Mitochondria: Mitochondria are membrane-bound organelles, which are found only in eukaryotes.
- Endoplasmic reticulum: The endoplasmic reticulum is a membrane-bound organelle, so it is eukaryotic only.
Show solution 7
800. Four generations give a factor of 2⁴ = 16. The count is 50 × 16 = 800 cells.
Why the other choices are wrong:
- 200: That multiplies by 4, the number of generations. Four doublings is a factor of 2⁴ = 16.
- 54: That adds 4 cells. Each generation doubles the count; it doesn’t add one cell per generation.
- 400: That is only three doublings. Count all four: 50 → 100 → 200 → 400 → 800.
Show solution 8
Log (exponential). Log, or exponential, phase is when cells divide at a steady rate and the population keeps doubling.
Why the other choices are wrong:
- Lag: In lag phase cells are adjusting, and there is little increase in number even though metabolism can be active.
- Stationary: In stationary phase the viable count levels off because new cells and dying cells roughly balance.
- Death: In death phase the viable count is falling, not doubling.
Show solution 9
Most viruses are smaller than a light microscope can resolve. Conventional light microscopes cannot resolve objects much smaller than about 0.2 µm, and most viruses are smaller than that. Electron beams have much shorter wavelengths, so electron microscopes resolve far finer detail.
Why the other choices are wrong:
- A light microscope just needs more magnification: Extra magnification without better resolution only gives a bigger blur. The limit comes from the wavelength of light.
- Viruses don’t take up any stains: Staining affects contrast, not the resolution limit. Most virus particles are too small for a conventional light microscope to resolve in detail, even when stained.
- Darkfield light microscopes already show virus details: Darkfield adds contrast but still uses visible light, so it has the same resolution limit.
Show solution 10
Protecting the cell from bursting when water rushes in. The cell wall gives the cell its shape and protects it from bursting from osmotic pressure in dilute surroundings.
Why the other choices are wrong:
- Controlling which molecules enter and leave: That is the plasma membrane’s job. The cell wall is relatively porous.
- Making proteins: Ribosomes make proteins, not the cell wall.
- Holding the cell’s DNA: DNA sits in the nucleoid inside the cell, not in the wall.
Answer key
Open all answers
- DNA in a nucleoid, with no membrane-bound nucleus. A prokaryote has DNA and ribosomes but lacks a membrane-bound nucleus. A nucleus is a eukaryotic feature.
- Magnification only. The image got larger, so magnification increased. Distinguishing adjacent details would demonstrate better resolution.
- A virus. Viruses are not cells. Yeasts are eukaryotic cells; bacteria and archaea are prokaryotic cells.
- 480. Two generations give a factor of 2² = 4. The count is 120 × 4 = 480 cells.
- Gram-positive bacilli. Purple rods are reported as Gram-positive bacilli when controls pass. Stain appearance does not establish a species.
- Ribosomes. Both prokaryotes and eukaryotes use ribosomes to make proteins. Prokaryotic ribosomes (70S) are smaller than eukaryotic cytoplasmic ribosomes (80S), but both cell types have them.
- 800. Four generations give a factor of 2⁴ = 16. The count is 50 × 16 = 800 cells.
- Log (exponential). Log, or exponential, phase is when cells divide at a steady rate and the population keeps doubling.
- Most viruses are smaller than a light microscope can resolve. Conventional light microscopes cannot resolve objects much smaller than about 0.2 µm, and most viruses are smaller than that. Electron beams have much shorter wavelengths, so electron microscopes resolve far finer detail.
- Protecting the cell from bursting when water rushes in. The cell wall gives the cell its shape and protects it from bursting from osmotic pressure in dilute surroundings.
Common questions
Is this a complete exam 1 review?
No. It is a starting point for common introductory topics. Your syllabus may include chemistry, metabolism, or other material that this short test does not cover.
How should I use my score?
Use missed topics to choose what to review. Ten questions are too few to predict an exam grade.
Where are the answers?
Each question has a solution, and the answer-key section collects all ten explanations for printing.
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Sources and fact checks
Definitions and methods checked against these textbook references. Practice scenarios and worked numbers were written for this guide.