Common characteristics

Each cell is at least somewhat self-contained and self-maintaining. The following are characteristics of every cell irrespective of its type:

Cell division is the name for the processes resulting in the physical partitioning and separation of a cell into daughter cells. [GO:0051301].

The growth and division of cells is named cell cycle, which can be defined as:

The progression of biochemical and morphological phases and events that occur in a cell during successive cell replication or nuclear replication events. Canonically, the cell cycle comprises the replication and segregation of genetic material followed by the division of the cell, but in endocycles or syncytial cells nuclear replication or nuclear division may not be followed by cell division [GO:0007049]

All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, separates its interior from its environment, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Inside the membrane, a salty cytoplasm takes up most of the cell volume.

The plasma or cytoplasmic membrane surrounds a cell and separates the cell from its external environment. It consists of a phospholipid bilayer and associated proteins. [GO:0005886]

All of the contents of a cell excluding the plasma membrane and nucleus, but including other subcellular structures. [GO:0005737]

All cells possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery. Also, inside the cells there are other organelles, organized structures of distinctive morphology and function.

Figure 1

Prokaryotic cells

Many prokaryotic cells have a protective cell wall.

The rigid or semi-rigid envelope lying outside the cell membrane of plant, fungal, and most prokaryotic cells, maintaining their shape and protecting them from osmotic lysis. In plants it is made of cellulose and, often, lignin; in fungi it is composed largely of polysaccharides; in bacteria it is composed of peptidoglycan. [GO:0005618]

Also, some prokaryotic cells have a flagellum:

Long whiplike or feathery structures borne either singly or in groups by the motile cells of many bacteria and unicellular eukaryotes and by the motile male gametes of many eukaryotic organisms, which propel the cell through a liquid medium .[ GO:0019861]

Figure 2

Ribosomes are a kind of intracellular organelle, about 200 A in diameter, consisting of RNA and protein. It is the site of protein biosynthesis resulting from translation of messenger RNA (mRNA). It consists of two subunits, one large and one small, each containing only protein and RNA. Both the ribosome and its subunits are characterized by their sedimentation coefficients, expressed in Svedberg units (symbol: S). Hence, the prokaryotic ribosome (70S) comprises a large (50S) subunit and a small (30S) subunit, while the eukaryotic ribosome (80S) comprises a large (60S) subunit and a small (40S) subunit. Two sites on the ribosomal large subunit are involved in translation, namely the aminoacyl site (A site) and peptidyl site (P site). Ribosomes from prokaryotes, eukaryotes, mitochondria, and chloroplasts have characteristically distinct ribosomal proteins.

Eukaryotic cells

The main characteristic feature of eukaryotic cells is that they are organized into complex structures by internal membranes and a cytoskeleton. There are a number of organelles that are present in many eukaryotic cells. Organelles are:

Organized structure of distinctive morphology and function. Includes the nucleus, mitochondria, plastids, vacuoles, vesicles, ribosomes and the cytoskeleton. Excludes the plasma membrane. [GO:0043226]

Among the organelles in eukaryotes, mitochondria are:

Semiautonomous, self replicating organelles that occurs in varying numbers, shapes, and sizes in the cytoplasm of virtually all eukaryotic cells. It is notably the site of tissue respiration.[GO:0005739]

Figure 3