Prokaryotic Cell Structure
Prokaryotic cells, such as bacteria, are simple cellular structures defined by the absence of a true nucleus. Understanding their key components is crucial for distinguishing them from more complex eukaryotic cells.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- The genetic material is a single, circular loop of chromosomal DNA that is located in the cytoplasm, in a region called the nucleoid, not enclosed within a nucleus.
- In addition to the main chromosome, many bacteria contain plasmids: small, independent rings of DNA carrying non-essential genes, such as those for antibiotic resistance.
- The cytoplasm is a gel-like substance filling the cell, where all metabolic reactions occur. It contains ribosomes for protein synthesis but lacks membrane-bound organelles like mitochondria.
- Prokaryotic cells have a cell membrane that controls the passage of substances in and out of the cell.
- Surrounding the cell membrane is a rigid cell wall which provides structural support and protection against osmotic pressure.
Definitions
- Prokaryotic Cell
- A type of cell that lacks a membrane-bound nucleus and other complex organelles. Bacteria are prokaryotes.
- Plasmid
- A small, circular strand of DNA found in the cytoplasm of bacteria, separate from the main chromosome. Plasmids often carry advantageous genes.
- Nucleoid
- The specific region within the cytoplasm of a prokaryotic cell where the main chromosomal DNA is concentrated.
Worked example
A scientist identifies a single-celled organism. Its genetic material consists of a main circular chromosome and three smaller rings of DNA. All genetic material is found within the cytoplasm. Which of the following components would you also expect to find in this organism?
- 1
Identify the cell type from the description.
The presence of a circular chromosome and smaller rings (plasmids) in the cytoplasm, with no mention of a nucleus, indicates a prokaryotic cell (a bacterium).
- 2
Recall the fundamental structures of a prokaryotic cell from the specification:
cell membrane, cytoplasm, cell wall, chromosomal DNA, and plasmid DNA.
- 3
Eliminate structures that are exclusive to eukaryotic cells.
For example, a true nucleus, mitochondria, or chloroplasts would not be present.
- 4
Conclude that a cell wall, providing structural support, is a standard component of prokaryotic cells and would be expected to be present.
Answer: A cell wall
Common mistakes
- ×Applying eukaryotic features to prokaryotes. A common mistake is to state that bacterial DNA is in a nucleus or that bacteria have mitochondria. Remember: prokaryotes lack a nucleus and all other membrane-bound organelles.
- ×Confusing the roles of the chromosomal DNA and plasmid DNA. The main chromosome carries all the essential genetic information for survival and reproduction, while plasmids carry non-essential, 'bonus' genes.
- ×Assuming the cell wall has the same function as the cell membrane. The cell wall provides rigid structural protection, while the cell membrane is selectively permeable and controls what enters and leaves the cell.
No-calculator tips
- ✓Use the mnemonic 'Pro means No'. Prokaryotes have NO nucleus and NO complex organelles.
- ✓For multiple-choice questions about prokaryotes, immediately eliminate any answers that mention a nucleus, mitochondria, or chloroplasts. This can often narrow the options down to the correct one.
- ✓Visualise the structure as a simple nested set of layers: Cytoplasm → Cell Membrane → Cell Wall. The DNA is free within the cytoplasm.