Every living organism on Earth is built up of one or more cells. Based on their cell structure, organisms can be grouped as prokaryotes and eukaryotes.
Prokaryotes are unicellular organisms that lack a cell nucleus, while the cells of Eukaryotes have a membrane-bound nucleus.
Prokaryotic and eukaryotic cells differ in several respects. Such differences are their size and structure: prokaryotic cells are about ten times smaller and much simpler in structure than eukaryotic cells.
cell wall - Based on the type of their cell walls, bacteria are either Gram-negative or Gram-positive. The basis of differentiation is that Gram staining produces different results in the cell walls of different types of bacteria because of their different structure. Gram-positive bacteria retain the dye, while Gram-negative ones do not.
cell membrane - A double lipid layer.
DNA - The DNA of bacteria forms a closed loop and is not surrounded by a nuclear envelope (or membrane), bacteria are therefore prokaryotic cells ('before kernel' in Greek). In the cells of eukaryotic organisms, however, DNA does not form a closed loop and is surrounded by a nuclear envelope. Humans are made up of eukaryotic cells.
plasmid - A small DNA molecule within cells that is physically separated from the chromosomal DNA and can replicate independently. It is most commonly found in bacteria as circular DNA molecules.
cilium - A thin, tubular protein structure. Some cells have a few hundred cilia. Their primary function is to provide adhesion to solid surfaces or other cells. Some types of cilia also serve locomotion.
flagellum - A long protein structure that serves the locomotion of the bacterium.
pilus - An appendage on certain Gram-negative bacteria, longer than a cilium. They serve bacterial conjugation, that is, to connect two bacteria for the transfer of plasmids.
The genetic material of prokaryotic cells consists of a single circular chromosome which is not separated from other parts of the cell by a nuclear membrane.
These simple cells do not have any membrane-bound organelles or a cytoskeleton either. The cytoplasm is surrounded by a cell membrane, which, in turn, is covered by a polysaccharide and peptide cell wall that provides external protection for the cells. Prokaryotic cells may have one or numerous appendages extending from their surface. These can be cilia, flagella or pili.
Bacteria (formerly: Eubacteria) and Archea are the two main groups of prokaryotes.
nucleus - It contains chromatin, a complex of DNA and proteins. The cells of animals, plants and fungi are eukaryotic, that is, they contain a nucleus enclosed by a membrane. Prokaryotic cells (bacteria) do not have a membrane-bound nucleus, their DNA is found in the cytoplasm.
mitochondrion - The cell's power station: it produces ATP by breaking down organic molecules. ATP is the primary energy-transfer molecule in the cell.
cell membrane - A lipid membrane that encloses the cell.
endoplasmic reticulum - A complex, interconnected network of membranes inside the cell. It plays an important role in protein synthesis, protein processing, lipid synthesis and the breaking down of certain substances.
Golgi apparatus - It plays an important role in protein processing.
vesicle - Substances within the cell are transported wrapped in membrane bubbles or vesicles. One type of vesicle is the lysosome, in which certain substances are digested and waste is broken down.
cytoskeleton - It plays important roles in the positioning and movement of vesicles and organelles, and provides animal cells - which do not have cell walls - with structure and shape.
Eukaryotic cells have real nuclei, that is, the genetic material is enclosed by a membrane. The nucleus contains several linear chromosomes.
These cells also contain membrane-bound cell organelles besides the nucleus. The most important of these are the mitochondria and the chloroplasts, the latter characteristic of plant cells only.
The endoplasmic reticulum, connected to the cell nucleus, also plays an important role in the cell: this is the site of protein synthesis.
The cytoplasm of all eukaryotic cells is surrounded by a cell membrane, but only plant and fungal cells have cell walls.
Eukaryotes are usually grouped in four kingdoms: unicellular eukaryotes (protozoa), fungi, plants and animals.