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B3.2 · Compare prokaryotes, eukaryotes, and viruses

Learn to compare prokaryotes, eukaryotes, and viruses through clear examples and targeted practice.

Ontario Grade 11 Biology

Diversity of Living Things

A Grade 11 guide to three kinds of biological organization

In SNC2D, you learned that cells are the basic units of living things and that cells have structures with different roles. A bacterium, a human cell, and a virus are not organized in the same way. This lesson compares prokaryotes, eukaryotes, and viruses by looking at their structures and how they reproduce. The comparison also shows why a virus is not classified as a cell.

What you will learn

Describe key features of prokaryotic cells, eukaryotic cells, and viruses.
Compare the three groups using cell structures and ability to reproduce.
Use evidence in a description to decide which group it best fits.

Start with the biological question

Imagine that a microscope image shows a tiny object. How could you tell whether it is a prokaryotic cell, a eukaryotic cell, or a virus? Size alone is not enough. Instead, compare features such as whether it is a cell, whether it has a nucleus, and whether it can reproduce without a host cell.

A cell is the basic organized unit of living things. The cell membrane is a boundary around a cell. Cytoplasm is the material inside the membrane. DNA is genetic material that carries information used by cells. These terms help describe both major cell types.

A model is a simplified way to represent something. In this lesson, the comparison table is a model: it focuses on a few useful features, rather than showing every detail of every organism.

Look for several features, not just size.
Prokaryotes and eukaryotes are cell types; viruses are not cells.

Compare the two cell types

Prokaryotes are organisms made of prokaryotic cells. These cells do not have a nucleus. A nucleus is a membrane-bound structure that contains a cell's DNA. Prokaryotic cells have a cell membrane and cytoplasm, and their DNA is in the cell but not enclosed in a nucleus. They do not have membrane-bound organelles. An organelle is a structure inside a cell that carries out a particular role.

Bacteria are prokaryotes. Many are single-celled, meaning one cell makes up the organism. The word prokaryote describes a type of cell organization; it does not mean that these organisms have no internal structure.

Eukaryotes are organisms made of eukaryotic cells. Their cells have a nucleus that contains DNA, as well as membrane-bound organelles. Animals, plants, fungi, and protists are eukaryotes. A protist is a eukaryotic organism that is not classified as an animal, plant, or fungus in this broad grouping. Some eukaryotes are single-celled, while others have many cells.

Both types of cells have a cell membrane, cytoplasm, and DNA. The clearest comparison is whether a nucleus and membrane-bound organelles are present. Cell size can be a useful general clue, since prokaryotic cells are generally smaller than eukaryotic cells, but size alone does not identify an unknown sample.

Prokaryotic cells have no nucleus; eukaryotic cells have a nucleus.
Both cell types have a cell membrane, cytoplasm, and DNA.
Eukaryotes include single-celled and multicellular organisms.

Where viruses fit

A virus is not a cell. It consists of genetic material, which is DNA or RNA, surrounded by a protein coat. RNA is another kind of genetic material. Some viruses also have an outer envelope, a membrane-like covering. Not every virus has an envelope.

Viruses do not have the full cellular organization described for prokaryotes and eukaryotes. In particular, a virus is not a cell with cytoplasm and the cell structures listed in the comparison. A virus can reproduce only inside a host cell. A host cell is a cell that a virus enters and uses to make more viruses. This dependence is an important difference from cells, which can reproduce as cells.

The comparison does not require deciding whether viruses are living. It is more precise here to describe their structures and dependence on host cells. Saying that viruses are not cells is a structural comparison, not a claim that they are simply very small cells.

A virus has genetic material and a protein coat; some also have an envelope.
Viruses are not cells and reproduce only inside host cells.
Do not classify a virus as prokaryotic or eukaryotic.

Use the comparison as evidence

To classify a description, identify what the evidence says about the object. A nucleus points to a eukaryotic cell. DNA in a cell without a nucleus points to a prokaryotic cell. Genetic material inside a protein coat, with reproduction dependent on a host cell, points to a virus.

These are patterns, not a complete inventory of every possible feature. For example, the absence of a nucleus is useful when the object is known to be a cell. It is not enough by itself to call something a prokaryote, because a virus is not a cell either. First decide whether the evidence describes a cell or a virus.

A microscope image or written description gives observations. The comparison model helps explain those observations. If the evidence is incomplete, the best answer may be that more information is needed rather than making a confident identification.

First decide whether the object is described as a cell.
Use a combination of structural clues and reproduction information.
An incomplete description may not support a definite classification.

A feature-based comparison

Feature	Prokaryotes	Eukaryotes	Viruses
Cell?	Yes	Yes	No
Nucleus?	No	Yes	No
Cell membrane and cytoplasm?	Yes	Yes	Not organized as a cell with these structures
Genetic material	DNA in the cell, not enclosed in a nucleus	DNA in the nucleus	DNA or RNA inside a protein coat
Reproduction	As cells	As cells	Only inside a host cell
Examples	Bacteria	Animals, plants, fungi, and protists	Viruses

Worked example

Classifying a cell with no nucleus

A description says an organism consists of one cell. The cell has a membrane and cytoplasm, and its DNA is not enclosed in a nucleus. Which group best fits?

Check whether it is a cell
The description identifies a cell with a membrane and cytoplasm. That supports classifying it as a cell rather than a virus.
Check for a nucleus
The DNA is not enclosed in a nucleus. Prokaryotic cells lack a nucleus, while eukaryotic cells have one.
Choose the best fit
The evidence supports a prokaryotic cell. It does not identify a particular kind of prokaryote.

Answer: It is a prokaryotic cell.

Check: The cell clues matter as well as the absence of a nucleus. Without knowing it is a cell, that one feature would not be enough to distinguish it from a virus.

Worked example

Distinguishing a single-celled eukaryote

An organism is one cell. The cell has a nucleus and membrane-bound organelles. A learner says it cannot be a eukaryote because it is not multicellular. Is the learner correct?

Use the defining cell features
A nucleus and membrane-bound organelles are features of eukaryotic cells. These clues directly support the eukaryotic classification.
Check the cell number claim
Eukaryotes can be single-celled or multicellular. Being one cell does not rule out eukaryotic organization.
Revise the conclusion
The learner's claim is incorrect. The described organism is a single-celled eukaryote.

Answer: No. It is a single-celled eukaryote.

Check: The presence of a nucleus is a stronger classification clue than the number of cells in the organism.

Worked example

Recognizing a virus from several clues

A tiny infectious object is described as genetic material inside a protein coat. It can make more copies only inside a host cell. The description does not mention a nucleus or cytoplasm. Which group best fits, and what can you conclude about an envelope?

Combine the structural clues
Genetic material inside a protein coat is a virus structure, rather than the description of a prokaryotic or eukaryotic cell.
Use the reproduction clue
The object reproduces only inside a host cell. That dependence also fits viruses.
Avoid adding unsupported detail
Some viruses have an envelope and some do not. Since the description does not state whether one is present, its presence cannot be concluded.

Answer: It is a virus. The description does not establish whether it has an envelope.

Check: The answer uses the features given and does not treat an unspecified feature as known.

Common mistakes and how to avoid them

Calling a virus a prokaryote because it has no nucleus.

Correction: A prokaryote is a cell type. A virus is not a cell, so it is neither prokaryotic nor eukaryotic.

Assuming every eukaryote is multicellular.

Correction: Some eukaryotes consist of one cell. A nucleus is a more useful clue for this comparison.

Assuming every virus has an envelope.

Correction: Some viruses have an envelope, but others do not.

Using size as the only way to classify an unknown object.

Correction: Compare cell organization and other available evidence. Size alone is not enough.

Lesson summary

Prokaryotes and eukaryotes are organisms made of cells; viruses are not cells.
Prokaryotic cells lack a nucleus. Eukaryotic cells have a nucleus and membrane-bound organelles.
Both prokaryotic and eukaryotic cells have a cell membrane, cytoplasm, and DNA.
Viruses have DNA or RNA inside a protein coat; some also have an envelope.
Viruses reproduce only inside a host cell. Use several clues and do not infer details that are not given.

Check your understanding

Question 1

Which feature most clearly supports classifying a cell as eukaryotic?

It has a nucleus.
It is small.
It has genetic material.
It is described as infectious.

Show answer and explanation

It has a nucleus.

A nucleus is a defining difference between the eukaryotic and prokaryotic cell types in this comparison.

Question 2

A description says an object has genetic material inside a protein coat and reproduces only inside a host cell. What is the best classification?

A prokaryote
A eukaryote
A virus
A multicellular organism

Show answer and explanation

A virus

The protein coat and dependence on a host cell fit the virus description.

Question 3

Which statement about prokaryotes and eukaryotes is accurate?

Only eukaryotic cells have DNA.
Both have a cell membrane, cytoplasm, and DNA.
All eukaryotes are multicellular.
Prokaryotic cells have a nucleus.

Show answer and explanation

Both have a cell membrane, cytoplasm, and DNA.

Both cell types have a cell membrane, cytoplasm, and DNA. The other options incorrectly describe their shared or differing features.

Key terms

Cell: The basic organized unit of living things.
Cell membrane: The boundary around a cell.
Cytoplasm: The material inside a cell membrane.
DNA: Genetic material found in cells and in some viruses.
Eukaryote: An organism whose cells have a nucleus and membrane-bound organelles.
Host cell: A cell that a virus enters and uses to make more viruses.
Nucleus: A membrane-bound structure in a eukaryotic cell that contains DNA.
Organelle: A structure inside a cell that carries out a particular role.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Biology (SBI3U), expectation B3.2. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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