Identify bacterial cell structures and explain their functions
Distinguish circular DNA and plasmids from a membrane-bound nucleus.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall the distinction between plant, animal and bacterial cells. Read the supplied evidence before drawing a conclusion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is a plasmid in this bacterial model?
What you will learn
- Label the six required bacterial structures.
- Link each structure to support, exchange, reactions, proteins or genetic information.
A cell without an enclosed nucleus
The bacterial model has a cell wall outside a cell membrane, cytoplasm and ribosomes. The wall supports the cell, the membrane controls exchange, cytoplasm contains reaction sites and ribosomes synthesise proteins. Its wall is not a plant cellulose wall.
The main bacterial DNA is circular and lies in the cytoplasm without a membrane-bound nucleus. Smaller circular DNA molecules called plasmids can carry additional genes. Plasmids are not miniature nuclei, and not every bacterium necessarily carries the same plasmids.
Read labels by position and form
In this schematic, the large DNA loop represents the main circular DNA, and the small loop a plasmid. Dots represent ribosomes. These are symbolic drawings, not a scale photograph; the components have been enlarged for identification.
Bacteria do not have mitochondria or chloroplasts in this model. A cell can respire without mitochondria, and a surface photograph cannot establish the absence of internal structures merely because they are not visible. Compare the bacterial diagram with the supplied microscope-image limits.
Sources for this lesson
Pause and explain
Which pair belongs in a bacterial cell?
Worked example
A bacterial sketch labels large circular loop A, small circular loop B and dots C. Identify them and one function of each.
Show the worked solution
- A main circular DNA: genetic information.
- B plasmid: additional genetic information.
- C ribosomes: protein synthesis.
Answer A circular DNA; B plasmid; C ribosomes.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Name the outer supporting boundary and inner exchange-controlling boundary of a bacterium.
Give me a hint
Keep wall and membrane separate.
Compare my reasoning
- Outer wall provides support.
- Inner membrane controls entry and exit.
Cell wall and cell membrane, respectively.
Look for these in your work
- Correct positions.
- Correct roles.
Explain the difference between a ribosome and a plasmid.
Give me a hint
One makes protein; the other is DNA.
Compare my reasoning
- Ribosome is a protein-synthesis site.
- Plasmid carries additional genetic information.
Ribosome: protein synthesis; plasmid: small circular DNA with genes.
Look for these in your work
- Distinguish material and task.
- Do not call a plasmid a nucleus.
An image shows only bacterial surfaces. A learner labels an unseen central nucleus. Explain two errors.
Give me a hint
Check both the cell type and what the image reveals.
Compare my reasoning
- The bacterial model has no membrane-bound nucleus.
- A surface view does not reveal its internal DNA organisation.
Wrong bacterial structure and unsupported internal label from a surface view.
Look for these in your work
- State no nucleus.
- Identify the visibility limit.
Common mistakes
- No membrane-bound nucleus surrounds bacterial DNA.
- Plasmids are DNA molecules, not ribosomes.
What can you explain now?
How does bacterial DNA organisation differ from a typical animal cell?
Compare with the explanation
Bacterial main DNA is not enclosed in a membrane-bound nucleus.
A typical animal cell has a nucleus; the bacterial model does not.
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Make it stick
Draw and label all six bacterial structures, then explain each function.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.