Cytoskeleton, Cell Inclusions, and Cell Wall: Structure and Function
The Cytoskeleton
The cytoskeleton is the set of protein filaments located in the cytosol that contribute to the morphology of the cell, its internal organization and the organelles, and movement. It is formed by microfilaments of actin, intermediate filaments, and microtubules.
Microfilaments of Actin
In cells, they are essential for eukaryotic cell movements. They are structures with different polarities, and extremes can be polymerized and depolymerized easily.
- Actin G: It is a globular protein associated with another protein, profilin, which prevents its polymerization.
- Actin F: It is a polymer composed of two strands of double helix coiled dextrorotatory actin G.
Besides actin, other proteins are called associated proteins, altering their properties:
- Structural proteins: Involved in the union of the actin filaments with the plasma membrane (vinculin).
- Regulatory proteins: (Myosin) acts in muscle contraction.
Intermediate Filaments
They are formed by fibrous proteins present in all eukaryotic cells, specific for each type. They form networks around the nucleus and extend outward. There are three classes:
- Keratin filaments: Referred to as tonofilaments and have great mechanical resistance.
- Neurofilaments: Are in the axon and dendrites of neurons.
- Vimentin filaments of desmin: Abundant in muscle cells, especially smooth muscle.
They are the most resistant of the three types of the cytoskeleton. They form structural features, avoid breakage of the membranes of cells subjected to mechanical stress, and contribute to the cell shape.
Microtubules
Microtubules are uniform cylindrical formations in the cytoplasm or part of cilia and flagella. They have a variable length, and if they are sectioned transversely, they show 13 subunits (protofilaments composed of tubulin) leaving a hollow center. They participate in:
- Mitotic spindle formation, which is responsible for organizing the chromosomes in mitosis.
- Cellular transport of vesicles, some organelles like mitochondria that move through the cytosol.
- Cell movement by forming pseudopods, forming the frame of cilia and flagella.
Inclusions
In the cytoplasm, hydrophobic, inert substances are found in all eukaryotic cells, both plant and animal. Common inclusions are starch and glycogen.
Crystalline Inclusions
They are seen as crystals, and they are protein deposits.
- Plant cells: These inclusions come from crystallized salts forming crystals called drusen and raphides.
- Animal cells: The cells of the seminiferous tubules of mammals have inclusions called Charcot-Bottcher crystals.
Hydrophobic Inclusions
These are products synthesized by the cell.
- Plant cells: They are found in large vacuoles or the cytoplasm. Examples: grains of starch, fat droplets, essential oils, latex.
- Animal cells: Glycogen is in abundance in liver cells, and animal cells use it in muscles. The glycogen accumulated in the liver is the main source of energy. It is also consumed in muscle contraction.
- Lipids: Triglycerides accumulate in fat cells and other cells in the form of droplets.
- Pigments: Dark melanin has a protective function. Lipofuscin is yellow and found in nerve cells and aged cardiac cells. Hemosiderin is formed as degradation products of hemoglobin in the cells of the liver, spleen, and bone marrow.
Cell Wall
The cell wall is an outer cover that acts as an exoskeleton. It is thick and rigid, possessed by plant cells, algae, and fungi on the cell membrane. It is composed mostly of polysaccharides. In fungi, the polysaccharide is chitin, and in algae and higher plants, it is cellulose.
- Middle layer: Located between the primary plates of neighboring cells, except where there are plasmodesmata. It is formed by pectin.
- Primary wall: Found in growing cells, thin and flexible, allowing the cell to continue growing. It is made up of cellulose and pectin.
- Secondary wall: When the cell stops growing, this layer is formed, thicker and more rigid than the primary wall. It is composed of pectin and a small amount of cellulose. It may also contain lignin.
They form an exoskeleton that protects the cell, gives it shape, and provides resistance to plants. It is responsible for plants standing upright and prevents cell rupture.