Cells are categorized as being either prokaryotic or eukaryotic, which is determined by the organization of cell structures. A prokaryote is generally a unicellular organism with no internal membrane-bound structures. The basic structures of prokaryotes are simple compared to eukaryotes. They have a nucleoid, the area inside a prokaryotic cell where genetic material (DNA) is found. A eukaryote is an organism characterized by membrane-bound structures, such as the Golgi apparatus. They also have other structures that perform particular tasks. Each of these is called an organelle. Eukaryotes also have a nucleus (plural, nuclei), a membrane-bound organelle that contains most of the genetic material (DNA); this structure directs a cell's growth, division, and death. Eukaryotic cells are found in plants, animals, fungi, and protists.
Prokaryotic and eukaryotic cells have several structures and functions in common. Common structures between both cell types include a cell membrane, cytoplasm, ribosomes, and chromosomes or genetic material.
The cell membrane is the structure that encloses a cell. It is made up of two layers (a bilayer) of biological molecules called lipids, as well as proteins. The specific lipids in cell membranes are called phospholipids. Phospholipids have heads that are water-loving (hydrophilic) and two tails that are water-hating (hydrophobic). The phospholipids form the bilayer, with the heads on the outside of both layers and the tails facing each other on the inside of the bilayer. Some substances, such as oxygen, cross the cell membrane easily, while others, such as large carbohydrates or proteins, are prevented from freely crossing. The protein component of the membrane performs many additional specialized functions, like the passage of food and waste molecules.
The inside of the cell contains cytoplasm, a combination of cytosol, a jellylike fluid primarily made up of water and dissolved substances that fill the spaces around the internal cell structures, and organelles (excluding the nucleus). A ribosome is a structure composed of RNA and protein that constructs proteins based on the instructions provided by DNA. Each ribosome consists of two subunits composed of ribosomal RNA and protein. "Free" ribosomes are suspended in the cytoplasm and make proteins that remain in the cell. The genetic material for organisms to live and reproduce is a large molecule of deoxyribonucleic acid (DNA) (an organic molecule containing coded instructions for the life processes of an organism, which consists of nucleotides bonded together in the form of a double helix), which is wound into a chromosome. A chromosome is a structure that contains DNA, the genetic material that is passed from one generation to the next. DNA in the nucleoid of a prokaryotic cell is a single circular chromosome. The nucleus of a eukaryotic cell contains multiple chromosomes; the number varies by species. DNA is also found in mitochondria and chloroplasts in eukaryotic cells.
Prokaryotic versus Eukaryotic Cells
|Characteristic||Prokaryotic Cell||Eukaryotic Cell|
|Location of DNA||Nucleoid||Nucleus|
|Cell diameter||0.1–5.0 µm||10–100 µm|
|Membrane-bound organelles||Absent in most||Many|
µm = 1 micron, or 1 millionth of a meter