Prokaryotic cells lack a nucleus and membrane-bound organelles, whereas eukaryotic cells contain a nucleus and have a compartmentalized cytoplasm with membrane-bound organelles. This distinction is required at both IB Biology SL and HL in subtopic A2.2, Cell structure.
Key Structural Differences
Feature | Prokaryotic cells | Eukaryotic cells |
|---|---|---|
Nucleus | No nucleus; DNA occupies a nucleoid region | Nucleus enclosed by a double membrane with pores |
DNA | Usually one circular, naked DNA molecule; additional plasmids may occur | Multiple linear DNA molecules associated with histone proteins |
Ribosomes | 70S ribosomes | 80S ribosomes in the cytoplasm |
Membrane-bound organelles | Absent | Present, such as mitochondria, Golgi apparatus and endoplasmic reticulum |
Compartmentalization | Limited internal compartmentalization | Extensive compartmentalization separates different processes |
Typical size | Usually smaller | Usually larger |
Cell wall | Present in the prokaryotic cells studied by IB; bacterial walls contain peptidoglycan | Present in plants and fungi but absent from animal cells; composition differs from bacterial walls |
Cell division | Binary fission | Mitosis or meiosis, followed by cytokinesis |
In prokaryotes, the DNA is not separated from the cytoplasm. Transcription and translation can therefore occur in the same cellular region. In eukaryotes, transcription occurs in the nucleus, while translation occurs at ribosomes in the cytoplasm or on the rough endoplasmic reticulum.
Compartmentalization allows eukaryotic cells to concentrate enzymes and metabolites and to separate incompatible biochemical processes. For example, aerobic cell respiration occurs within mitochondria rather than freely throughout the cytoplasm.
A common misconception is that prokaryotic cells have “no organelles.” The accurate statement is that they have no membrane-bound organelles; they still contain structures including ribosomes and a plasma membrane.
Exam Technique
For a distinguish question, make paired statements about both cell types rather than describing only one. In micrograph questions, identify a prokaryote using several featuressuch as no nucleus, no membrane-bound organelles and relatively small sizerather than relying on size alone.
