Flagella are long filiform formations on the surface of the cell, ensuring its active spatial movement. Despite the diversity of organisms, these structures within each suprasternum (prokaryotes or eukaryotes) are characterized by a general structure scheme.
General characteristics of flagella
In pre-nuclear organisms (bacteria and archaea) flagella are the main mode of transportation. Among eukaryotes, these locomotor structures are mainly present in unicellular organisms, which are protozoa, but are also characteristic of gamete plants and animals. In some multicellular invertebrates, for example, sponges, the flagella perform the function of moving the liquid substrate relative to the immobile cell layer.
Morphologically, the flagellum consists of a base fixed in the thickness of the cell and a long outer thread that rotates along a spiral path. The structure and mechanism of work of these parts in prokaryotes and eukaryotes are very different, in connection with which two corresponding classes of flagella are distinguished.
Features of flagella prokaryotes and eukaryotes
The outer thread of the flagellum is called filament. In prokaryotes, it consists of the flagellin protein and passively moves due to the rotation of the basal motor. The filament of nuclear cells is much more complex and, thanks to the interaction of tubulin and dynein proteins, can bend independently.
The main differences between the classes of flagella | in prokaryotes | in eukaryotes |
| organelle sizes (thickness, nm; length, microns) | 10-30 nm, 6-15 microns | 200 nm, 100 μm |
| filament proteins | flagellin | tubulin and dynein |
| membrane around filament | is absent | is present |
| degree of rotation | 360 ° | 180 ° |
| energy source | transmembrane potential (archaea may have ATP) | ATP |
| thread movement | passive | active |
| substructures | filament, hook, basal body | filament, basal body (kinetosome) |
| filament structure | solid (in archaea) or hollow protein cylinder | microtubule doublets |
| basal structure | a rod fixed in a complex system of rings or membrane-like organelles (in archaea) | microtubule triplets |
Such a number of differences indicates the absence of homology between these organoids, that is, they are not identical in origin and structure, although they perform similar functions.
The kingdom of prokaryotes includes the kingdoms of archaea and bacteria. The locomotor structures of these taxa are also not homologous to each other, but they are very similar in structure. Flagella of archaea have been studied much worse.
Flagella of archaea and bacteria
By the method of movement, mobile bacteria are divided into floating and moving. Flagella are a locomotor organ of floating microorganisms, allowing them to develop a speed of 20 to 200 microns / sec.
The movement can be spontaneous (if the physicochemical characteristics of the medium are the same in all directions) or purposeful when the bacterium tends to fall into the most favorable conditions for it. In adaptive movement, the rotation of the basal motor is controlled by sensor systems.
By the number and location of flagella on the cell, bacteria distinguish four morphological types of microorganisms:
- monotrichi - have a single flagellum;
- lofotrichi - characterized by a flagellum bundle at one of the cell poles;
- amphitrichi - have one or more flagella at both ends of the cell;
- peritrichi - covered with many flagella on all sides.
The type of flagellation can be either a species trait or the result of a change in cultivation conditions or stage of the bacterial life cycle.
The archaea flagellum is in many respects similar to the bacterial one, however, it has a number of differences in ultrastructure and movement mechanism. So, the archaea filament is thinner, built from another type of flagellin, there is no hollow tubule in the thread. The length of the hook is variable, the basal body has a completely different structure and functions, most likely, based on the energy of ATP. Archaea move much slower than bacteria.
The structure and functioning of the bacterial flagellum
The bacterial flagellum is formed by three substructures: an external thread (filament), a flexible joint (hook), and a basal body anchored in the cell membrane. The synthesis and assembly of these elements encode about 50 fla genes. Mot-genes are responsible for the operation of the motor, and che-genes are responsible for adaptive reactions.
The flagellum filament is a relatively rigid protein helix twisted counterclockwise to form a central hollow channel with a diameter of up to 3 nm. This design contributes to the formation of a spiral trajectory of the thread. Flagellin molecules (FliC) are transported through the filament channel.
The hook connects the thread to the basal body of the flagellum and consists of two types of protein: FlgE and FlgKl. The joint length is constant and is about 50 nm. Due to the curved shape of the hook during rotation of the motor, the base of the fibril describes a circle, so that the flagellum can spiral in motion.
The basal body is fixed in the cell wall and cytoplasmic membrane of bacteria. This substructure performs not only a fixing function, but also is a flagellum motor. The structure and localization of the basal body depends on the type of cell wall of the microorganism. In gram-negative bacteria, it consists of two inner (M and S) and two outer (P and L) rings strung on a rod connected to the hook. The basal body also includes an export system transporting protein elements for the assembly of the flagellum.
The structure of the basal motor
The MS-complex includes moving structures called the rotor, and a rotation direction switch, which in more detailed diagrams of the structure is designated as a C-ring. Around the rotor are concentrated ion channels formed by MotAB proteins - stators. The motor works due to the energy of the proton (H + ) or sodium (Na + ) gradient.
The location of the ring subunits in the cell membrane corresponds to the following scheme:
- "M" - cytoplasmic membrane;
- "S" - periplasmic space or cell wall in G + bacteria;
- "P" is the peptidoglycan layer;
- "L" is the outer membrane.
The outer rings P and L are stationary and perform a supporting function. Gram-positive bacteria are absent.
Nuclear flagellum
The eukaryotic flagellum is a cytoplasmic outgrowth of a cell, consisting of the outer part surrounded by a membrane (undulipodia) and the basal body immersed in the cytoplasm (kinetosomes).
The structural basis of undulipodia is the axoneme, which consists of a system of protein cylinders connected to each other - microtubules. Their location corresponds to the formula 9 × 2 + 2, that is, nine peripheral doublets and two single tubes in the center (singlets).
Doublets are formed by paired cylinders A and B, constructed from tubulin protein subunits. From each A-tube to the side of the adjacent pair, dynein arms extend, which convert the ATP energy into mechanical motion. Doublets are connected to singlets by radial knitting needles, and to each other by nexin bundles. The space between the structural elements of undulipodia is filled with cytoplasm.
The structure of the kinetosome is represented by nine triplets of microtubules (formula 9 + 0), which anchor the flagellum in a eukaryotic cell. There are no singlets in the basal body.