Microtubules are exactly how they sound: microscopic hollow tubes found inside eukaryotic cells and some prokaryotic bacteria cells that provide structure and motor functions for the cell. Biology students learn during their studies that there are only two types of cells: prokaryotic and eukaryotic.
Prokaryotic cells make up the single-celled organisms found in the Archaea and Bacteria domains under the Linnaean taxonomy system, a biological classification system of all life, while eukaryotic cells fall under the Eukarya domain, which oversees the protist, plant, animal and fungi kingdoms. The Monera kingdom refers to bacteria. Microtubules contribute to multiple functions within the cell, all of which are important to cellular life.
TL;DR (Too Long; Didn't Read)
Microtubules are tiny, hollow, bead-like tubular structures that help cells maintain their shape. Along with microfilaments and intermediate filaments, they form the cytoskeleton of the cell, as well as participate in a variety of motor functions for the cell.
Main Functions of Microtubules Within the Cell
As part of the cytoskeleton of the cell, microtubules contribute to:
- Giving shape to cells and cellular membranes.
- Cell movement, which includes contraction in muscle cells and more.
- Transportation of specific organelles within the cell via microtubule "roadways" or "conveyor belts."
- Mitosis and meiosis: movement of chromosomes during cell division and creation of the mitotic spindle.
What They Are: Microtubule Components and Construction
Microtubules are small, hollow, bead-like pipes or tubes with walls constructed in a circle of 13 protofilaments that consist of polymers of tubulin and globular protein. Microtubules resemble miniaturized versions of beaded Chinese finger traps. Microtubules can grow 1,000 times as long as their widths. Fabricated by the assembly of dimers – a single molecule, or two identical molecules joined together of alpha and beta tubulin – microtubules exist in both plant and animal cells.
In plant cells, microtubules form at many sites within the cell, but in animal cells, microtubules begin at the centrosome, an organelle near the nucleus of the cell that also participates in cell division. The minus end represents the attached end of the microtubule while its opposite is the plus end. The microtubule grows at the plus end through polymerization of tubulin dimers, and the microtubules shrink with their release.
Microtubules give structure to the cell to help it resist compression and to provide a highway in which vesicles (sac-like structures that transport proteins and other cargo) move across the cell. Microtubules also separate replicated chromosomes to opposite ends of a cell during division. These structures can work alone or in conjunction with other elements of the cell to form more complicated structures like centrioles, cilia or flagella.
With diameters of only 25 nanometers, microtubules often disband and reform as quickly as the cell needs them to. The half-life of tubulin is only about a day, but a microtubule may exist for only 10 minutes as they are in a constant state of instability. This type of instability is called dynamic instability, and microtubules can assemble and disassemble in response to the cell's needs.
Microtubules and the Cell's Cytoskeleton
The components that make up the cytoskeleton include elements made from three different types of proteins – microfilaments, intermediate filaments and microtubules. The narrowest of these protein structures include microfilaments, often associated with myosin, a thread-like protein formation that, when combined with the protein actin (long, thin fibers that are also called "thin" filaments), helps to contract muscle cells and provide stiffness and shape to the cell.
Microfilaments, small rod-like structures with an average diameter of between 4 to 7 nm, also contribute to cellular movement in addition to the work they perform in the cytoskeleton. The intermediate filaments, an average of 10 nm in diameter, act like tie-downs by securing cell organelles and the nucleus. They also help the cell withstand tension.
Microtubules and Dynamic Instability
Microtubules may appear completely stable, but they are in constant flux. At any one moment, groups of microtubules may be in the process of dissolving, while others may be in the process of growing. As the microtubule grows, heterodimers (a protein consisting of two polypeptide chains) provide caps to the end of the microtubule, which come off when it shrinks for use again. The dynamic instability of the microtubules is considered to be a steady state as opposed to a true equilibrium because they have intrinsic instability – moving in and out of form.
Microtubules, Cell Division and the Mitotic Spindle
Cell division is not only important to reproduce life, but to make new cells out of old. Microtubules play an important role in cell division by contributing to the formation of the mitotic spindle, which plays a part in the migration of duplicated chromosomes during anaphase. As a "macromolecular machine," the mitotic spindle separates replicated chromosomes to opposite sides when creating two daughter cells.
The polarity of microtubules, with the attached end being a minus and the floating end being a positive, makes it a critical and dynamic element for bipolar spindle grouping and purpose. The two poles of the spindle, made from microtubule structures, help to segregate and separate duplicated chromosomes reliably.
Microtubules Give Structure to Cilia and Flagellum
Microtubules also contribute to the parts of the cell that help it move and are structural elements of cilia, centrioles and flagella. The male sperm cell for example, has a long tail that helps it reach its desired destination, the female ovum. Called a flagellum (the plural is flagella), that long, thread-like tail extends from the exterior of the plasma membrane to power the cell's movement. Most cells – in cells that have them – generally have one to two flagella. When cilia exist on the cell, many of them spread along the full surface of the cell's outer plasma membrane.
The cilia on cells that line a female organism's Fallopian tubes, for example, help to move the ovum to its fateful meetup with the sperm cell on its journey to the uterus. The flagella and cilia of eukaryotic cells are not the same structurally as those found in prokaryotic cells. Built with the same with microtubules, biologists call the microtubule arrangement a "9 + 2 array" because a flagellum or cilium consists of nine microtubule pairs in a ring that encloses a microtubule duo in the center.
Microtubule functions require tubulin proteins, anchoring locations and coordinating centers for enzyme and other chemical activities within the cell. In cilia and flagella, tubulin contributes to the central structure of the microtubule, which includes contributions from other structures like dynein arms, nexin links and radial spokes. These elements allow communication between microtubules, holding them together in a way that's similar to how actin and myosin filaments move during muscle contraction.
Cilia and Flagellum Movement
Even though both cilia and flagellum consist of microtubule structures, the ways in which they move are distinctively different. A single flagellum propels the cell much in the same manner that a fish's tail moves a fish forward, in a side-to-side whip-like motion. A pair of flagella may synchronize their movements to propel the cell forward, like how a swimmer's arms function when she's swimming the breast stroke.
Cilia, much shorter than flagellum, cover the outer membrane of the cell. The cytoplasm signals cilia to move in a coordinated fashion to propel the cell in the direction it needs to go. Like a marching band, their harmonized movements all step in time to the same drummer. Individually, a cilium or flagellum's movement works like that of a single oar, passing through the medium in a powerful stroke to propel the cell in the direction it needs to go.
This activity may occur at dozens of strokes per second, and one stroke may involve the coordination of thousands of cilia. Under a microscope, you can see how fast ciliates respond to obstacles in their environment by changing directions quickly. Biologists still study how they respond so quickly and have yet to discover the communication mechanism by which the inner parts of the cell tell the cilia and flagella how, when and where to go.
The Cell's Transportation System
Microtubules serve as the transportation system within the cell to move mitochondria, organelles and vesicles through the cell. Some researchers refer to way in which this process works by likening microtubules akin to conveyor belts, while other researchers refer to them as a track system by which mitochondria, organelles and vesicles move through the cell.
As energy factories in the cell, mitochondria are structures or little organs in which respiration and energy production occur – both biochemical processes. Organelles consists of multiple small, but specialized structures within the cell, each with their own functions. Vesicles are small sac-like structures that may contain fluids or other substances like air. Vesicles form from the plasma membrane, pinching off to create a sphere-like sac enclosed by a lipid bilayer.
Two Major Groups of Microtubule Motors
The bead-like construction of microtubules serves as a conveyor belt, track or highway to transport vesicles, organelles and other elements within the cell to the places they need to go. Microtubule motors in eukaryotic cells include kinesins, which move to the plus end of the microtubule – the end that grows – and dyneins that move to the opposite or minus end where the microtubule attaches to the plasma membrane.
As "motor" proteins, kinesins move organelles, mitochondria and vesicles along the microtubule filaments through the power of hydrolysis of the energy currency of the cell, adenosine triphosphate or ATP. The other motor protein, dynein, walks these structures in the opposite direction along microtubule filaments toward the minus end of the cell by converting the chemical energy stored in ATP. Both kinesins and dyneins are the protein motors used during cell division.
Recent studies show that when dynein proteins walk to the end of the minus side of the microtubule, they congregate there instead of falling off. They hop across the span to connect to another microtubule to form what some scientists call "asters," thought by scientists to be an important process in the formation of the mitotic spindle by morphing the multiple microtubules into a single configuration.
The mitotic spindle is a "football-shaped" molecular structure that drags chromosomes to opposite ends right before the cell splits to form two daughter cells.
Studies Still Going On
The study of cellular life has been going on since the invention of the first microscope in the latter part of the 16th century, but it's only been in the last few decades that advances have occurred in cellular biology. For example, researchers only discovered the motor protein kinesin-1 in 1985 with the use of a video-enhanced light microscope.
Up until that point, motor proteins existed as a class of mysterious molecules unknown to researchers. As technology developments advance, and studies continue, researchers hope to delve deep into the cell to find out everything they can possibly learn on how the inner workings of the cell operate so seamlessly.
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- Journal of Cell Science: Kinesins at a Glance
- University of California San Francisco: Researchers Invent Reversible 'Off-Switch' for Cellular Proteins
- University of California Davis: How the Cell’s Roadways Are Remodeled for Cell Division
- University of Maryland: Who Invented the First Microscope?
- CK-12: Modern Classification Systems
- Nature Milestones: Milestone 15: Brand New Motor