8876 / 2027

Lesson 5 of 7 / Cells and stem cells

A bacterium is a cell without a nucleus

What does a bacterium need to make proteins?

I can describe the defining structures of a typical bacterial cell.

  • I can describe a typical bacterial cell.Syllabus

    Syllabus 8876, Core 1(d). Describe a typical bacterial cell.

About 4 min

Make a guess. You are not marked.

Bacteria have no membrane-bound organelles. Do they have ribosomes?

  1. They take ribosomes from host cells.
  2. They have 70S ribosomes, not membrane-bound.
  3. Bacteria have no organelles of any kind.
Show the answer

They have 70S ribosomes, not membrane-bound.

Ribosomes are not membrane-bound, so bacteria have them. They are 70S, smaller than eukaryotic 80S ones.

The key idea

A typical bacterium is small and unicellular, with circular DNA, 70S ribosomes and a peptidoglycan wall, but no membrane-bound nucleus or other membrane-bound organelles.

Explore the idea

A cell without a nuclear envelope

70S ribosomes

Bacteria synthesise proteins on 70S ribosomes. These are not membrane-bound organelles.

Original schematic. Structures, cell numbers and distances are illustrative, not to scale.

Explanation

Bacteria are cellular organisms. A typical bacterial cell has cytoplasm, a cell surface membrane and a wall made of peptidoglycan. Its wall is chemically different from a plant cellulose wall. It is generally smaller than a typical eukaryotic cell.

The main bacterial chromosome is circular DNA located in a region of the cytoplasm, without a nuclear envelope. Smaller circular plasmids may also occur, but not every bacterium has the same plasmids. The cell has 70S ribosomes for protein synthesis.

Lack of membrane-bound organelles does not mean lack of metabolism. Bacterial enzymes and membranes support essential reactions without mitochondria, chloroplasts, ER or Golgi bodies. Do not label a bacterium as a virus simply because it lacks a nucleus.

Step by step
  1. 1

    Find shared cell features

    Both bacterial and eukaryotic cells possess membranes, cytoplasm, DNA and ribosomes.

  2. 2

    Find the distinguishing organisation

    Look for DNA without a nuclear envelope and absence of membrane-bound organelles.

Worked example

An unknown single cell

A small cell has a peptidoglycan wall, circular DNA and 70S ribosomes. Is the absence of a nucleus evidence that it cannot synthesise proteins?

One way to explain it

No. These features fit a typical bacterial cell. Its DNA provides genetic information and its 70S ribosomes synthesise polypeptides; a membrane-bound nucleus is not required.

Why this answer works
  • Use the full set of features.
  • Distinguish a missing compartment from a missing process.

Watch out for this

A student says: "Bacteria contain no organelles, so they have no ribosomes." What is wrong with this?

Show the answer

They lack membrane-bound organelles but possess non-membrane-bound 70S ribosomes.

Check your understanding

Which pair is characteristic of a typical bacterium?

  1. 70S ribosomes and a peptidoglycan wall.
  2. A nuclear envelope and Golgi body.
  3. A cellulose wall and no ribosomes.
Show the answer

70S ribosomes and a peptidoglycan wall.

Both fit the required bacterial model.

Quick recall

Card 1 of 3. Answer in your head, then check.

What does a bacterium need to make proteins?

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