Bacteria are unstructured bags of enzymes with no cytoskeleton or organized interior.

For much of the twentieth century, the contrast between complex cells and bacteria seemed obvious. Animal and plant cells contained visible internal structures and a cytoskeleton—a framework of proteins that helps organize the cell. Bacteria looked like tiny, almost featureless bags of enzymes.

Their size was part of the problem. Many bacterial structures are too small to resolve with ordinary light microscopes, so an organized interior could easily look empty.

Genetics and better imaging changed that picture. The bacterial protein FtsZ, a relative of the tubulin used in the cytoskeleton of animal and plant cells, forms a ring where a bacterium is about to divide. Another protein, MreB, is related to actin and helps organize cell shape.

Other bacterial systems position chromosomes, enzymes, protein complexes, and specialized compartments inside the cell. Bacteria generally do not have the same large membrane-bound organelles found in animal and plant cells, but their interiors are not unstructured. They are organized on a smaller scale and in different ways.