Microtubules are hollow, tubular polymers made from repeating units of the protein tubulin. They are found universally in all eukaryotic cells, including those of plants and animals. As a major part of the cytoskeleton, they provide cells with mechanical support and organization. Microtubules are dynamic protein filaments that constantly assemble and disassemble, allowing cells to rapidly remodel their internal structure for tasks like maintaining shape or executing cell division.
Shared Functions of Microtubules
Microtubules in both plant and animal cells maintain cellular form and enable movement within the cytoplasm. They function as rigid components of the cytoskeleton, helping to establish and maintain the overall shape of the cell. This structural role is particularly important in animal cells, which lack the rigid support of a cell wall.
These filaments also act as tracks for intracellular transport throughout the cell’s interior. Motor proteins, such as kinesin and dynein, bind to cellular cargo like organelles and vesicles, using the microtubules as a substrate to walk along. Kinesin generally moves cargo toward the plus end, while dynein moves toward the minus end, facilitating organized and directed movement.
This coordinated transport delivers newly synthesized proteins, membrane components, and signaling molecules to their correct destinations. The dynamic nature of microtubules, which allows them to constantly grow and shrink, also contributes to cytoplasmic organization. By continually adjusting their length and position, these filaments help localize organelles and establish the internal polarity of cells.
Structural Organization Differences
The primary difference between plant and animal microtubules lies in how their assembly is organized. In animal cells, the primary Microtubule Organizing Center (MTOC) is the centrosome, typically located near the nucleus. The centrosome is composed of two perpendicular centrioles surrounded by pericentriolar material. Microtubules radiate outward from this centralized MTOC, which acts as the main initiation site for their growth, dictating a radial arrangement.
Plant cells completely lack centrosomes and centrioles, making their organization acentriolar. They still possess MTOCs, but these sites are diffuse and not defined by a single, permanent organelle. Microtubules are organized from various locations, such as the nuclear envelope during the cell cycle or the plasma membrane in non-dividing cells.
In non-dividing plant cells, a prominent array of cortical microtubules forms just beneath the plasma membrane. This array guides the deposition of cellulose microfibrils, which determines the structure and strength of the cell wall. Plant microtubule networks are often described as self-organizing systems that emerge from dispersed microtubules, rather than being centrally nucleated.
Specialized Roles in Cell Division
Both cell types rely on microtubules to form the mitotic spindle, the apparatus responsible for separating duplicated chromosomes. The mitotic spindle is a bipolar array of microtubules that aligns chromosomes at the cell’s center and pulls the sister chromatids toward opposite poles. This mechanism of chromosome segregation is conserved in both plant and animal cells, ensuring each daughter cell receives the correct genetic material.
The process of cell separation, or cytokinesis, differs vastly due to the presence of a rigid cell wall in plants. Animal cells achieve cytokinesis through the formation of a cleavage furrow, where a contractile ring of actin and myosin filaments pinches the cell membrane inward. Microtubules are involved in this process, but the final separation is driven by the actin cytoskeleton.
Plant cells cannot use a contractile ring, so they employ a unique, microtubule-based structure called the phragmoplast. The phragmoplast is a dense array of microtubules that forms between the two separating nuclei after mitosis. This structure acts as a scaffold to guide Golgi-derived vesicles carrying cell wall material and membrane components to the cell’s midline.
The vesicles fuse along the midline of the phragmoplast, forming the cell plate, which grows outward toward the existing parental cell wall. This cell plate matures into the new cell wall and plasma membranes that separate the two daughter cells. The orientation of the future cell plate is often predicted by the preprophase band, a plant-specific ring of cortical microtubules that briefly appears before mitosis.