Book : Cell Biology
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Microtubules

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Microtubules are a component of the cytoskeleton, found throughout the cytoplasm. These tubular polymers of tubulin can grow as long as 50 micrometres and are highly dynamic. The outer diameter of a microtubule is about 24 nm while the inner diameter is about 12 nm. They are found in eukaryotic cells, as well as some bacteria, and are formed by the polymerization of a dimer of two globular proteins, alpha and beta tubulin. Microtubules are very important in a number of cellular processes. They are involved in maintaining the structure of the cell and, together with microfilaments and intermediate filaments, they form the cytoskeleton. They also make up the internal structure of cilia and flagella.They provide platforms for intracellular transport and are involved in a variety of cellular processes, including the movement of secretory vesicles, organelles, and intracellular macromolecular assemblies (see entries for dynein and kinesin). They are also involved in chromosome separation (mitosis and meiosis), and are the major constituents of mitotic spindles, which are used to pull apart eukaryotic chromosomes

These straight, hollow cylinders are found throughout the cytoplasm of all eukaryotic cells (prokaryotes don't have them) and carry out a variety of functions, ranging from transport to structural support. Microtubules, which are about 25 nanometers in diameter, form part of the cytoskeleton that gives structure and shape to a cell, and also serve as conveyor belts moving other organelles throughout the cytoplasm. In addition, microtubules are the major components of cilia and flagella, and participate in the formation of spindle fibers during cell division (mitosis). The length of microtubules in the cell varies between 200 nanometers and 25 micrometers, depending upon the task of a particular microtubule and the state of the cell's life cycle.

Microtubules are biopolymers that are composed of subunits made from an abundant globular cytoplasmic protein known as tubulin, as illustrated in Figure 1. Each subunit of the microtubule is made of two slightly different but closely related simpler units called alpha-tubulin and beta-tubulin that are bound very tightly together to form heterodimers. In a microtubule, the subunits are organized in such a way that they all point the same direction to form 13 parallel protofilaments. This organization gives the structure polarity, with only the alpha-tubulin proteins exposed at one end and only beta-tubulin proteins at the other.

By adding or removing globular tubulin proteins, the length of polymeric microtubules can be increased or decreased. Because the two ends of a microtubule are not the same, however, the rate at which growth or depolymerization occurs at each pole is different. The end of a polarized filament that grows and shrinks the fastest is known as the plus end and the opposing end is called the minus end. For all microtubules, the minus end is the one with exposed alpha-tubulins. In an animal cell, it is this end that is located at the centriole-containing centrosome found near the nucleus, while the plus end, comprised of exposed beta-units, is projected out toward the cell's surface. Microtubules are continuously being assembled and disassembled so that tubulin monomers can be transported elsewhere to build microtubules when needed.



                                Figure 1: Central Vacuole

A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain cases they may contain solids which have been engulfed. Vacuoles are formed by the fusion of multiple membrane vesicles and are effectively just larger forms of these. The organelle has no basic shape or size; its structure varies according to the needs of the cell.
Vacuoles are membrane-bound sacs within the cytoplasm of a cell that function in several different ways. In mature plant cells, vacuoles tend to be very large and are extremely important in providing structural support, as well as serving functions such as storage, waste disposal, protection, and growth. Many plant cells have a large, single central vacuole that typically takes up most of the room in the cell (80 percent or more). Vacuoles in animal cells, however, tend to be much smaller, and are more commonly used to temporarily store materials or to transport substances. 

The central vacuole in plant cells (see Figure 1) is enclosed by a membrane termed the tonoplast, an important and highly integrated component of the plant internal membrane network (endomembrane) system. This large vacuole slowly develops as the cell matures by fusion of smaller vacuoles derived from the endoplasmic reticulum and Golgi apparatus. Because the central vacuole is highly selective in transporting materials through its membrane, the chemical palette of the vacuole solution (termed the cell sap) differs markedly from that of the surrounding cytoplasm. For instance, some vacuoles contain pigments that give certain flowers their characteristic colors. The central vacuole also contains plant wastes that taste bitter to insects and animals, while developing seed cells use the central vacuole as a repository for protein storage.

Among its roles in plant cell function, the central vacuole stores salts, minerals, nutrients, proteins, pigments, helps in plant growth, and plays an important structural role for the plant. Under optimal conditions, the vacuoles are filled with water to the point that they exert a significant pressure against the cell wall. This helps maintain the structural integrity of the plant, along with the support from the cell wall, and enables the plant cell to grow much larger without having to synthesize new cytoplasm. In most cases, the plant cytoplasm is confined to a thin layer positioned between the plasma membrane and the tonoplast, yielding a large ratio of membrane surface to cytoplasm.

The structural importance of the plant vacuole is related to its ability to control turgor pressure. Turgor pressure dictates the rigidity of the cell and is associated with the difference between the osmotic pressure inside and outside of the cell. Osmotic pressure is the pressure required to prevent fluid diffusing through a semipermeable membrane separating two solutions containing different concentrations of solute molecules. The response of plant cells to water is a prime example of the significance of turgor pressure. When a plant receives adequate amounts of water, the central vacuoles of its cells swell as the liquid collects within them, creating a high level of turgor pressure, which helps maintain the structural integrity of the plant, along with the support from the cell wall. In the absence of enough water, however, central vacuoles shrink and turgor pressure is reduced, compromising the plant's rigidity so that wilting takes place.

Plant vacuoles are also important for their role in molecular degradation and storage. Sometimes these functions are carried out by different vacuoles in the same cell, one serving as a compartment for breaking down materials (similar to the lysosomes found in animal cells), and another storing nutrients, waste products, or other substances. Several of the materials commonly stored in plant vacuoles have been found to be useful for humans, such as opium, rubber, and garlic flavoring, and are frequently harvested. Vacuoles also often store the pigments that give certain flowers their colors, which aid them in the attraction of bees and other pollinators, but also can release molecules that are poisonous, odoriferous, or unpalatable to various insects and animals, thus discouraging them from consuming the plant.


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