The cytoskeleton is a complex network of fibers that supports the interior of a cell. Cross-linked by molecular connectors into systems that support cellular membranes, it holds internal structures, such as the nucleus, in place and controls various kinds of cell movement.
Virtually all eukaryotic cells, including plant cells, have a cytoskeleton. Cytoskeletal systems extend internally from the membrane covering the cell surface to the surface of the membrane system surrounding the cell’s nucleus. There are indications that a cytoskeletal support system reinforces the interior of the nucleus as well.
The fibers of the cytoskeleton also anchor cells to external structures through linkages that extend through the surface membrane. The cytoskeletal material, rather than being fixed and unchanging, varies in makeup and structure as cells develop, move, grow, and divide.
The cytoskeleton, depending on the cell type, is assembled from one or more of three major structural fibers: microtubules, microfilaments, and intermediate filaments.
Microtubules are fine, unbranched hollow tubes with walls built from subunits consisting of the protein tubulin. Microtubules are about 25 nanometers in diameter, have walls about 4 to 5 nanometers thick, and range in length from a few to many micrometers.
These structural elements, which may be arranged singly or in networks or parallel bundles, probably provide tensile strength and rigidity to cellular regions containing them. A tubular form combines lightness with strength and elasticity.
Microfilaments, also called actin filaments, are linear, unbranched fibers built up from the protein actin. Microfilaments are solid fibers that are much smaller than microtubules—about 5 to 7 nanometers in diameter, not much thicker than the wall of a microtubule. Microfilaments occur singly, in networks, and in parallel bundles in the cytoskeleton.
The consistency of the cytoplasm (the living matter of a cell, exclusive of the nucleus), which can vary from highly liquid to solid and gel-like, is regulated by the degree to which microfilaments are cross-linked into networks.
Microfilaments are also arranged in parallel bundles that give tensile strength and elasticity to cell regions and extensions. Many cell types contain numerous finger like extensions that are reinforced internally by internal parallel bundles of microfilaments.
Both microtubules and microfilaments form the basis for almost all cellular movements. In these motile systems, microtubules and microfilaments are acted upon by motile proteins that are able to convert chemical energy into the mechanical energy of movement.
The motile proteins cause the microtubules or microfilaments to slide forcefully, or move cell structures and molecules over the surfaces of the two elements.
Microtubules andmicrofilaments occur as structural supports of the cytoskeleton of all plant, animal, fungal, and protozoan cells. The third structural element, the intermediate filament, is more abundant in animal cells than in plant cells.
This type of fiber, called “intermediate” because its dimensions fall between those of microtubules and microfilaments, is about 10 nanometers in diameter.
Unlike microtubules and microfilaments, which are each highly uniform in structure and made from a single type of protein, intermediate filaments occur in six different types, each made up of a different protein or group of proteins.
Although the proteins making up the various intermediate filaments are different, they are related in both their three-dimensional structures and amino acid sequences.
Intermediate filaments occur in networks and bundles in the cytoplasm. They appear to be much more flexible than either microtubules or microfilaments, so it is considered likely that they form elastic ties holding cell structures in place, much like cellular rubber bands. However, the actual roles of these elements in the cytoskeleton remain uncertain in plant cells.
Both microtubules and microfilaments can be readily converted between assembled and disassembled forms. In the conversion, the protein subunits of microtubules and microfilaments are exchanged rapidly between the fully assembled element and large pools of disassembled subunits in solution in the cytoplasm. Cells can control the balance between assembly and disassembly with high precision.
As a result, the protein subunits can be recycled, and cytoskeletal structures containing microtubules and microfilaments can be set up or taken apart as the cell changes its function. As cell division occurs, for example, microtubules and microfilaments forming cytoskeletal structures typical of growing cells are rapidly disassembled and then reassembled into structures that take part in cell division.
The assembly-disassembly reactions of microtubules and microfilaments proceed so readily that it is relatively easy to carry the mout in a test tube. Microtubules and microfilaments, in fact,were among the first cell structures to be taken apart and put back together experimentally.
Cytoplasmic Streaming and Cell Division
Among the cell activities with which microfilaments are associated is cytoplasmic streaming, orcyclosis. The primary function of cytoplasmic streaming, which occurs within all live cells, is unknown.
However, moving currents of cytoplasm are thought to facilitate the transport of nutrients, enzymes, and other substances between the cell and its surroundings, and within the cell itself.
A typical plant cell consists of a cell wall and its contents, called the protoplast. The protoplast consists of the cytoplasm and a nucleus. Within the cytoplasm are organelles, membranes, and other structures. Suspended in the cytoplasmic fluid is one or more liquid-filledvacuoles, and a vacuole is bounded by a membrane called the tonoplast.
In cytoplasmic streaming, the organelles and other substances travel within moving currents in between the microfilaments and the tonoplast. The organelles in the streaming cytoplasm are thought to be indirectly attached to the microfilaments, and this attachment creates a pulling or towing motion, responsible for the movement of cytoplasmic particles.
Themicrofilaments, in their constantly changing arrays, also facilitate specific activities within the cell, including cell cleavage during mitosis. Microfilaments mediate the movement of the cell nucleus before and following cell division.
The microtubules, which are longer, move the split chromosomes to the newly forming cells in mitosis, and they play a role in cell plate formation in dividing cells.
In organizing other components of the cell, the cytoskeleton is thus intimately involved in the processes of cell division, growth, and differentiation. The cytoskeleton maintains the cell’s overall shape and is responsible for the movement of various organelles within it. In single-celled organisms such as the amoeba, the cytoskeleton is responsible for the locomotion of the cell itself.