All typical cells have four structures in common: a plasma membrane, cytoplasm, DNA and ribosomes. This applies to both prokaryotic and eukaryotic cells and is required at both IB Biology SL and HL.
| Common structure | Function |
|---|---|
| Plasma membrane | Encloses the cell and controls the movement of substances between the cell and its environment. It consists mainly of a phospholipid bilayer with embedded proteins. |
| Cytoplasm | Provides an aqueous environment in which many enzyme-catalysed metabolic reactions occur. It is composed mainly of water and contains dissolved substances. |
| DNA | Stores genetic information in its base sequence and provides instructions for producing RNA and proteins. |
| Ribosomes | Carry out translation, synthesizing polypeptides according to the base sequence of mRNA. |
These structures are necessary for the basic functions of life. The plasma membrane maintains a distinct internal environment, while the cytoplasm provides the medium for metabolism. DNA stores inherited information, and ribosomes use expressed genetic information to produce polypeptides.
The structures differ between cell types. Prokaryotic cells contain naked DNA, usually in a loop, and 70S ribosomes. Eukaryotic cells usually contain linear DNA associated with histones inside a nucleus and have 80S ribosomes in the cytoplasm. Eukaryotic cells also contain membrane-bound organelles, but these are not shared by prokaryotes.
A common misconception is that all cells have a nucleus, mitochondria or a cell wall. These structures are not universal: prokaryotes lack a nucleus and mitochondria, while animal cells lack a cell wall. Some highly specialized cells, such as mature human erythrocytes, also lose structures including the nucleus during differentiation, so IB questions usually refer to the shared organization of typical cells.
Exam technique: For a “state” question, list the four structures without explanation. For an “explain” or “compare” question, connect each structure to its function and distinguish prokaryotic from eukaryotic organization using precise terminology.