An overview of the cell

Cells whose genetic material is enclosed by a nuclear envelope are eukaryotic; cells without a membrane-bound nucleus are prokaryotic.

Both kinds are filled with cytoplasm, a semi-fluid matrix in which most of the cell's chemical reactions take place.

Eukaryotic cells also carry distinct membrane-bound organelles such as the endoplasmic reticulum, Golgi complex, lysosomes, mitochondria, microbodies and vacuoles; prokaryotes lack all of these.

Ribosomes are membrane-less and are found in every cell, prokaryotic or eukaryotic. Inside a eukaryotic cell they occur in the cytoplasm, in chloroplasts (in plants), in mitochondria and on the rough ER.

The centrosome is a non-membrane-bound organelle present in animal cells and helps in cell division.

Cells vary widely in size: Mycoplasmas are the smallest cells, bacteria may be 3–5 μm, a human red blood cell is about 7.0 μm across, and the largest isolated single cell is the ostrich egg.

Nerve cells are among the longest cells, and cell shape usually reflects function: disc-like, polygonal, columnar, cuboidal, thread-like or irregular.