Microfilaments and microtubules are key components of the cytoskeleton in eukaryotic cells. A cytoskeleton provides structure to the cell and connects to every part of the cell membrane and every organelle. Microtubules and microfilaments together allow the cell to hold its shape, and move itself and its organelles.
| Double Helix | Helical lattice |
| 7 nm in diameter | 20-25 nm in diameter |
| Predominantly composed of contractile protein called actin. | Composed of subunits of protein tubulin. These subunits are termed as alpha and beta. |
| Flexible and relatively strong. Resist buckling due to compressive forces and filament fracture by tensile forces. | Stiff and resist bending forces. |
| Micro-filaments are smaller and thinner and mostly help cells move | Microtubules are shaped similarly but are larger, and help with cell functions such as mitosis and various cell transport functions. |
Microtubules are composed of globular proteins called tubulin. Tubulin molecules are bead like structures. They form heterodimers of alpha and beta tubulin. A protofilament is a linear row of tubulin dimers. 12-17 protofilaments associate laterally to form a regular helical lattice.
Formation of Microfilaments
Individual subunits of microfilaments are known as globular actin (G-actin). G-actin subunits assemble into long filamentous polymers called F-actin. Two parallel F-actin strands must rotate 166 degrees to layer correctly on top of each other to form the double helix structure of microfilaments. Microfilaments measure approximately 7 nm in diameter with a loop of the helix repeating every 37 nm.
Functions of Microfilaments
- Microfilaments form the dynamic cytoskeleton, which gives structural support to cells and links the interior of the cell with the surroundings to convey information about the external environment.
- Microfilaments provide cell motility. e.g., Filopodia, Lamellipodia.
- During mitosis, intracellular organelles are transported by motor proteins to the daughter cells along actin cables.
- In muscle cells, actin filaments are aligned and myosin proteins generate forces on the filaments to support muscle contraction.
- In non-muscle cells, actin filaments form a track system for cargo transport that is powered by non-conventional myosins such as myosin V and VI. Non-conventional myosins use the energy from ATP hydrolysis to transport cargo (such as vesicles and organelles) at rates much faster than diffusion.
Functions of Microtubules
- Microtubules determine the cell structure.
- Microtubules form the spindle apparatus to divide the chromosome directly during cell division (mitosis).
- Microtubules provide transport mechanism for vesicles containing essential materials to the rest of the cell.
- They form a rigid internal core that is used by microtubule-associated motor proteins (MAPs) such as Kinesin and Dyenin to generate force and movement in motile structures such as cilia and flagella. A core of microtubules in the neural growth cone and axon also imparts stability and drives neural navigation and guidance.
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Does a cell have, or even need, a "skeleton"?
What do you get if you take some tubing, and make the tubes smaller and smaller and smaller? You get very small tubes, or microtubes. Very small tubes, or microtubules, together with microfilaments, form the basis of the "skeleton" inside the cell.
The cytoplasm consists of everything inside the plasma membrane of the cell, excluding the nucleus in a eukaryotic cell. It includes the watery, gel-like material called cytosol, as well as various structures. The water in the cytoplasm makes up about two thirds of the cell’s weight and gives the cell many of its properties.
The cytoplasm has several important functions, including:
- suspending cell organelles.
- pushing against the plasma membrane to help the cell keep its shape.
- providing a site for many of the biochemical reactions of the cell.
The cytoskeleton is a cellular "scaffolding" or "skeleton" that crisscrosses the cytoplasm. All eukaryotic cells have a cytoskeleton, and recent research has shown that prokaryotic cells also have a cytoskeleton. The eukaryotic cytoskeleton is made up of a network of long, thin protein fibers and has many functions. It helps to maintain cell shape. It holds organelles in place, and for some cells, it enables cell movement. The cytoskeleton also plays important roles in both the intracellular movement of substances and in cell division. Certain proteins act like a path that vesicles and organelles move along within the cell. The threadlike proteins that make up the cytoskeleton continually rebuild to adapt to the cell's constantly changing needs. Three main kinds of cytoskeleton fibers are microtubules, intermediate filaments, and microfilaments.
- Microtubules, shown in Figure below (a), are hollow cylinders and are the thickest of the cytoskeleton structures. They are most commonly made of filaments which are polymers of alpha and beta tubulin, and radiate outwards from an area near the nucleus called the centrosome. Tubulin is the protein that forms microtubules. Two forms of tubulin, alpha and beta, form dimers (pairs) which come together to form the hollow cylinders. The cylinders are twisted around each other to form the microtubules. Microtubules help the cell keep its shape. They hold organelles in place and allow them to move around the cell, and they form the mitotic spindle during cell division. Microtubules also make up parts of cilia and flagella, the organelles that help a cell move.
- Microfilaments, shown in Figure below (b), are made of two thin actin chains that are twisted around one another. Microfilaments are mostly concentrated just beneath the cell membrane, where they support the cell and help the cell keep its shape. Microfilaments form cytoplasmatic extensions, such as pseudopodia and microvilli, which allow certain cells to move. The actin of the microfilaments interacts with the protein myosin to cause contraction in muscle cells. Microfilaments are found in almost every cell, and are numerous in muscle cells and in cells that move by changing shape, such as phagocytes (white blood cells that search the body for bacteria and other invaders).
- Intermediate filaments differ in make-up from one cell type to another. Intermediate filaments organize the inside structure of the cell by holding organelles and providing strength. They are also structural components of the nuclear envelope. Intermediate filaments made of the protein keratin are found in skin, hair, and nails cells.
(a) The eukaryotic cytoskeleton. Microfilaments are shown in red, microtubules in green, and the nuclei are in blue. By linking regions of the cell together, the cytoskeleton helps support the shape of the cell. (b) Microscopy of microfilaments (actin filaments), shown in green, inside cells. The nucleus is shown in blue.
| Fiber Diameter | About 25 nm | 8 to 11 nm | Around 7 nm |
| Protein Composition | Tubulin, with two subunits, alpha and beta tubulin | One of different types of proteins such as lamin, vimentin, and keratin | Actin |
| Shape | Hollow cylinders made of two protein chains twisted around each other | Protein fiber coils twisted into each other | Two actin chains twisted around one another |
| Main Functions | Organelle and vesicle movement; form mitotic spindles during cell reproduction; cell motility (in cilia and flagella) | Organize cell shape; positions organelles in cytoplasm structural support of the nuclear envelope and sarcomeres; involved in cell-to-cell and cell-to-matrix junctions | Keep cellular shape; allows movement of certain cells by forming cytoplasmatic extensions or contraction of actin fibers; involved in some cell-to-cell or cell-to-matrix junctions |
| Representation |
The cytoskeleton is discussed in the following video: //www.youtube.com/watch?v=5rqbmLiSkpk (4:50).
- The cytoplasm consists of everything inside the plasma membrane of the cell.
- The cytoskeleton is a cellular "skeleton" that crisscrosses the cytoplasm. Three main cytoskeleton fibers are microtubules, intermediate filaments, and microfilaments.
- Microtubules are the thickest of the cytoskeleton structures and are most commonly made of filaments which are polymers of alpha and beta tubulin.
- Microfilament are the thinnest of the cytoskeleton structures and are made of two thin actin chains that are twisted around one another.
Use this resource to answer the following questions.
- What is the role of the cytoskeleton?
- What is the subunit of microfilaments and microtubules?
- Describe the main function of microtubules.
- What is the role of microtubules during mitosis?
- How are microtubules associated with locomotion?
- Describe the roles of microfilaments.
- What is the difference between cytoplasm and cytosol?
- List two roles of the cytoplasm.
- Name the three main types of cytoskeleton fibers.
- List two functions of the eukaryotic cytoskeleton.
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