Cellular Structures: Microtubules, Filaments, and Extracellular Components
Microtubules
Definition and Structure:
Hollow rods found in all eukaryotic cells.
Constructed from globular proteins called tubulins.
Each tubulin protein is a dimer, made of two slightly different polypeptides: -tubulin and -tubulin.
Growth and Dynamics:
Grow in length by adding tubulin dimers.
Can be disassembled and their tubulins reused to build microtubules elsewhere in the cell.
Have a distinct polarity due to tubulin dimer orientation: one end (the "plus end") can accumulate or release tubulin dimers at a much higher rate, causing significant growth and shrinkage during cellular activities. The "plus end" designation indicates higher "on" and "off" rates, not exclusively addition.
Functions:
Shape and support the cell.
Serve as tracks for organelles equipped with motor proteins.
Guide vesicles from the Endoplasmic Reticulum (ER) to the Golgi apparatus and from the Golgi to the plasma membrane.
Involved in the separation of chromosomes during cell division.
Centrosomes and Centrioles
Centrosomes:
The region in animal cells from which microtubules grow, typically located near the nucleus.
Microtubules originating from centrosomes act as compression-resisting girders of the cytoskeleton.
Centrioles:
Located within the centrosome.
Consists of a pair of centrioles, each approximately () in diameter.
Each centriole is composed of nine sets of triplet microtubules arranged in a ring.
Although they aid in organizing microtubule assembly in animal cells, many other eukaryotic cells lack centrosomes with centrioles and organize microtubules by alternative means.
The two centrioles are typically at right angles to each other.
Cilia and Flagella
General Characteristics:
Cellular extensions containing microtubules, found in some eukaryotic cells.
Responsible for the beating of these structures.
Bacterial flagella have a completely different structure.
Locomotor Appendages:
Many unicellular protists use cilia or flagella for propulsion through water.
Sperm of animals, algae, and some plants possess flagella.
Fluid Movement:
In tissue layers, cilia extending from tightly attached cells can move fluid over the tissue surface.
Example: Ciliated lining of the trachea sweeps mucus containing debris out of the lungs.
Example: Cilia lining the oviducts in women help move an egg toward the uterus.
Differences between Motile Cilia and Flagella:
Motile Cilia: Usually occur in large numbers per cell, shorter than flagella, have an alternating power and recovery stroke (like oars of a boat), beating at a rate of about to strokes per second.
Flagella: Usually limited to one or a few per cell, longer than cilia, have an undulating motion (like a fish tail), driving the cell in the same direction as the flagellum's axis.
Non-motile (Primary) Cilia
Function:
Can act as a signal-receiving "antenna" for the cell.
Generally non-motile, with only one per cell.
Almost all vertebrate animal cells possess such a primary cilium.
Membrane proteins on these cilia transmit molecular signals from the cell's environment to its interior, triggering signaling pathways that can alter cell activities.
Cilium-based signaling is crucial for brain function and embryonic development.
Common Structure of Motile Cilia and Flagella
Sheathed Microtubules:
Each motile cilium or flagellum has a group of microtubules sheathed in an extension of the plasma membrane.
"9+2" Arrangement:
Nine doublets of microtubules are arranged in a ring with two single microtubules in the center.
This pattern is characteristic of nearly all eukaryotic flagella and motile cilia.
Non-motile primary cilia have a "9+0" pattern, lacking the central pair of microtubules.
Basal Body:
The microtubule assembly is anchored in the cell by a basal body.
Structurally very similar to a centriole, with microtubule triplets in a "9+0" pattern.
In many animals (including humans), the basal body of the fertilizing sperm's flagellum enters the egg and becomes a centriole.
Mechanism of Bending:
Bending movements are produced by large motor proteins called dyneins, attached along each outer microtubule doublet.
Dynein proteins have two "feet" that "walk" along the microtubule of the adjacent doublet, using ATP for energy.
One foot maintains contact while the other releases and reattaches.
Flexible cross-linking proteins hold the outer doublets and central microtubules together.
The coordinated walking movement on one side of the circle causes the microtubules and the entire organelle to bend, rather than sliding past each other.
Microfilaments (Actin Filaments)
Definition and Structure:
Thin solid rods, also called actin filaments.
Built from molecules of actin, a globular protein.
A microfilament is a twisted double chain of actin subunits.
Can form structural networks when proteins bind along their side, allowing new filaments to branch.
Prevalence: Present in all eukaryotic cells.
Structural Role (Bearing Tension):
Bear tension (pulling forces) in the cytoskeleton, contrasting with the compression-resisting role of microtubules.
Cortical microfilaments: A network just inside the plasma membrane that supports cell shape, giving the outer cytoplasmic layer (cortex) a semisolid, gel-like consistency.
Microvilli: Bundles of microfilaments form the core of microvilli, delicate projections in cells like nutrient-absorbing intestinal cells, which increase surface area.
Role in Cell Motility:
Involved in muscle cell contraction: thousands of actin filaments interact with thicker myosin filaments, with myosin projections ("heads") walking along actin to shorten the muscle cell.
Amoeboid movement: Localized contractions by actin and myosin enable cells like Amoeba and white blood cells to crawl by extending pseudopodia ("false feet").
Cytoplasmic streaming: Actin-protein interactions contribute to the circular flow of cytoplasm within plant cells, speeding organelle movement and material distribution.
Intermediate Filaments
Naming and Diameter:
Named because their diameter is larger than microfilaments but smaller than microtubules.
Prevalence: Found only in the cells of some animals, including vertebrates.
Structural Role (Bearing Tension):
Specialized for bearing tension, similar to microfilaments.
A diverse class of cytoskeletal elements, with each type constructed from a specific molecular subunit (e.g., keratins).
Unlike microtubules and microfilaments, which have consistent diameter and composition across eukaryotic cells, intermediate filaments are more varied.
Permanence:
More permanent fixtures of cells compared to microfilaments and microtubules, which are frequently disassembled and reassembled.
Their networks often persist even after cells die (e.g., keratin filaments in the outer layer of skin).
Functions:
Are especially sturdy and play an important role in reinforcing cell shape.
Fix the position of certain organelles; for instance, the nucleus is typically held within a cage of intermediate filaments.
Form the nuclear lamina, which lines the interior of the nuclear envelope.
Generally function as the permanent framework of the entire cell.
Extracellular Components and Connections Between Cells
Cells synthesize and secrete materials extracellularly (outside the cell).
These extracellular materials and structures are vital for many essential cellular functions.
Cell Walls of Plants
Distinguishing Feature: An extracellular structure that differentiates plant cells from animal cells.
Functions:
Protects the plant cell.
Maintains cell shape.
Prevents excessive uptake of water.
Provides structural support to the whole plant against gravity.
Occurrence: Also found in prokaryotes, some protists, and fungi.
Dimensions and Composition:
Much thicker than the plasma membrane, ranging from to several micrometers.
Chemical composition varies by species and cell type, but basic design is consistent.
Cellulose microfibrils (a polysaccharide) are synthesized by cellulose synthase and secreted to the extracellular space.
Microfibrils are embedded in a matrix of other polysaccharides and proteins, forming a strong fiber-in-ground-substance architectural design (similar to steel-reinforced concrete or fiberglass).
Layers:
Primary Cell Wall: A relatively thin and flexible wall secreted first by young plant cells.
Middle Lamella: A thin, sticky layer rich in pectins (polysaccharides) located between the primary walls of adjacent cells, serving to glue them together.
Secondary Cell Wall: Added by some plant cells between the plasma membrane and the primary wall after maturation and growth cessation.
Often deposited in several laminated layers.
Has a strong and durable matrix, providing significant protection and support (e.g., wood consists mainly of secondary walls).
Extracellular Matrix (ECM) of Animal Cells
Lack of Cell Walls: Animal cells do not have cell walls like plant cells.
Composition:
An elaborate matrix outside the cells.
Main ingredients are glycoproteins (proteins with covalently bonded carbohydrates) and other carbohydrate-containing molecules secreted by cells.
Key Components:
Collagen: The most abundant glycoprotein in the ECM of most animal cells, forming strong fibers outside the cells.
Accounts for approximately of the total protein in the human body.
Proteoglycans: Collagen fibers are embedded in a network woven out of proteoglycans, which are also secreted by the cells.