2.1.1(k) - Prokaryotic and eukaryotic cell comparison
This lesson is about comparing two basic kinds of cellular organisation. You are not learning every organelle again; you are learning the precise structural features that let a biologist distinguish a prokaryotic cell from a eukaryotic cell. The specification point also links to microscopy, so the lesson includes one scale calculation using the OCR magnification relationship.
Shared features of cells
Biologists classify cells into prokaryotic cells and eukaryotic cells by how the cell is organised internally. The central idea is not that one cell type is "simple" and the other is "better". The central idea is where the genetic material is, whether the cell has membrane-bound compartments, and which structures are shared.
Prokaryotic cell
A cell that lacks a membrane-bound nucleus and lacks membrane-bound organelles; its DNA is found in the cytoplasm, usually in a region called the nucleoid.
Eukaryotic cell
A cell with DNA enclosed in a membrane-bound nucleus and with membrane-bound organelles such as mitochondria, endoplasmic reticulum and Golgi apparatus.
Structure means the arrangement of visible parts of the cell. Ultrastructure means finer cellular detail, usually the detail revealed by electron microscopy. In this lesson, the key ultrastructure idea is the contrast between a cell without internal membrane-bound compartments and a cell with them.
Typical prokaryotic and eukaryotic cells share a core set of features:
| Feature shared by cells | Why it matters |
|---|---|
| Cell surface membrane | Separates the cell contents from the outside and controls exchange. |
| Cytoplasm | Contains enzymes and is where many metabolic reactions occur. |
| DNA | Carries genetic information. |
| Ribosomes | Are the site of protein synthesis. |
Core Structures
[DIAGRAM: cell_comparison: Lesson 017: Bacterium and animal cell; asset_slug: 017_m02_1_1_prokaryotic_and_eukaryotic_cell_comparison__diagram_01; recommended_method: drawn_biology; description: Comparison of bacterial nucleoid and optional plasmid with a nucleated animal cell; enlarged schematics are not drawn to a common scale.]
The table compares a typical bacterial prokaryotic cell with a typical eukaryotic cell. Optional features such as plasmids and flagella are not universal.
A comparison answer should be built from paired statements:
| Feature | Typical prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | No membrane-bound nucleus. | Has a membrane-bound nucleus. |
| DNA arrangement | DNA is usually circular and lies in the cytoplasm, often in a nucleoid region. | DNA is arranged as linear chromosomes inside the nucleus. |
| Plasmids | Small circular plasmids may be present. | Plasmids are not a normal feature of animal or plant cells at this level. |
| Membrane-bound organelles | Absent. | Present, for example mitochondria, ER and Golgi apparatus. |
| Ribosomes | Smaller ribosomes, commonly described as 70S. | Larger cytoplasmic ribosomes, commonly described as 80S. |
| Cell wall | Typical bacterial cells have a cell wall containing murein, also called peptidoglycan. | Plant and fungal cells may have cell walls, but animal cells do not. Plant walls contain cellulose. |
| Size | Usually smaller, often about 0.1-5 µm. | Usually larger, often about 10-100 µm. |
The phrase "typical bacterial cell" matters. Some bacterial cells have a capsule, pili or a flagellum, but those features are not universal. OCR comparison marks are more likely to reward the stable distinctions: nucleus, membrane-bound organelles, DNA form, ribosome size, cell wall material and relative size.
Compartmentation
The most important difference is compartmentation. A membrane-bound organelle is a cell structure surrounded by its own membrane. Eukaryotic cells use these organelles to separate different jobs into different spaces.
For example, mitochondria carry out aerobic respiration in eukaryotic cells, while the rough endoplasmic reticulum and Golgi apparatus are involved in making and processing proteins. You do not need the full sequence of protein secretion in this lesson, but you do need to recognise that those membrane-bound structures are eukaryotic features.
Prokaryotic cells do not have a nucleus, mitochondria, chloroplasts, ER or Golgi apparatus. Their DNA, ribosomes and many enzymes are in the cytoplasm. Small prokaryotic cells have short internal diffusion distances. The absence of membrane-bound compartments does not mean that they lack metabolism: many reactions occur in their cytoplasm or at their cell-surface membrane.
In a comparison question, do not write only "eukaryotic cells have organelles". Say "eukaryotic cells have membrane-bound organelles, whereas prokaryotic cells do not". Ribosomes are present in both cell types.
Walls, Ribosomes And Scale
Ribosomes are not surrounded by a membrane, so they are found in both cell types. The comparison is their size: prokaryotic ribosomes are smaller, commonly called 70S ribosomes, while eukaryotic cytoplasmic ribosomes are larger, commonly called 80S ribosomes. The "S" is a sedimentation unit, so it should not be treated like an ordinary length or mass unit.
Cell walls are another place where students often lose precision. A typical bacterial prokaryotic cell has a cell wall containing murein, also called peptidoglycan. Some eukaryotic cells have cell walls, but not animal cells. Plant cell walls contain cellulose, so "has a cell wall" on its own is not enough to distinguish a bacterium from every eukaryotic cell.
Scale can support identification in micrographs. Prokaryotic cells are usually much smaller than eukaryotic cells, though there is overlap in real biological examples. Use size as supporting evidence, not as the only proof.
Magnification
Use the same unit for image size and real object size before substituting. A common safe route is to convert millimetres to micrometres using:
Calculating magnification of a bacterial cell image
A bacterial cell has an actual length of 2.0 µm. Its length in a photomicrograph is 18 mm.
Step 1: Convert the image size into micrometres.
Step 2: Substitute into the OCR magnification relationship.
The image is magnified ×9000. This does not mean the real bacterium is 9000 µm long; it means the image is 9000 times larger than the real object.
Identifying cells from evidence
Cell identification needs a combination of independent observations. A cell wall alone could belong to a bacterium, plant or fungus; wall chemistry distinguishes peptidoglycan, cellulose and chitin. A small cell with circular DNA outside a nucleus and no membrane-bound organelles supports a prokaryotic identification. A nucleus and mitochondria support a eukaryotic identification.
Absence is harder to establish than presence: a thin section can miss a nucleus, and small ribosomes may lie below the image resolution. Observe more than one section or field, check the scale, and distinguish “not visible” from “not present”. A specialised mammalian red blood cell has lost its nucleus during development; its origin remains eukaryotic. Such exceptions are why one missing feature alone is weak evidence.