Cell. Prokaryotic and eukaryotic cells

The cell is the basic structural and functional unit of all known living organisms, and the smallest unit of life.

There are two kinds. A prokaryote has no nucleus. Its DNA is circular and sits free in a region called the nucleoid, it has a cell wall, it is always unicellular, and it measures 0.1 to 5 μm. Bacteria and archaea are the examples.

A eukaryote has a membrane-enclosed nucleus, linear double-helix DNA, a full set of membranous and non-membranous organelles and a cytoskeleton, and measures 10 to 100 μm. Animals, humans and plants are eukaryotes.

The single most useful contrast: a prokaryote has no membranous cytoplasmic structures at all. Every organelle a eukaryote has that is made of membrane is simply absent.

Cells are at the centre of the organisation of the human body:

• They are the basic structural and functional unit of all known living organisms.
• They are the smallest unit of life, the building block of life.

The levels of organisation run:

cell → tissue → organ → organ system → organism

Prokaryotes are single-celled organisms, so they are always unicellular.

• They have no nucleus. Instead they have a nucleoid, which holds circular DNA.
• They have no membranous cytoplasmic structures.
• They have a cell wall.
• The examples are bacteria and archaea.
• Their size is 0.1 to 5 μm.

Eukaryotes have a cell membrane and a membrane-enclosed nucleus surrounded by cytoplasm, which is the fluid containing a system of membranous and non-membranous organelles, and a cytoskeleton.

• Their DNA is linear, a double helix.
• They are usually multicellular, sometimes unicellular.
• Animals, humans and plants are eukaryotes.
• Their size is 10 to 100 μm.

This is the table to be able to reproduce.

Three things are shared: the cytoplasm, the ribosome and the plasma membrane. Two belong to the prokaryote alone: the flagellum and the pili. Everything else on the list is a eukaryotic organelle the prokaryote does without.