The Cell: Structure and Function of Organelles

Posted on Oct 31, 2024 in Biology

The Cytoskeleton

The cytoskeleton is the set of protein filaments located in the cytosol that contribute to the morphology, internal organization, and movement of the cell.

Microfilaments

Microfilaments are composed of actin in eukaryotic cells and are 7 nm in diameter. They come in two forms:

G-actin: A globular protein attached to another protein, profilin, which prevents polymerization.
F-actin: A polymer consisting of two double strands of actin in a helix. Polymerized actin is called F-actin.

These microfilaments contain actin-associated proteins (ABPs) that modify their properties. For example, some ABPs are involved in the structural union of actin filaments with the plasma membrane (e.g., α-actinin) and others are myosin regulatory proteins that are involved with muscle contraction (e.g., tropomyosin). There are also non-motor regulatory proteins like thymosin.

Functions of Microfilaments:

Muscle Contraction: Actin is associated with myosin, allowing actin microfilaments to shorten and slide over one another, enabling contraction.
Maintaining Cell Shape: They form a dense network structure under the plasma membrane, called the cortex, which maintains cell shape.
Amoeboid Movement: Amoebas are able to move through the formation of pseudopods (strand extensions).
Formation of Microvilli: They form a rigid beam of actin filaments in microvilli (e.g., in the bowel).
Cytokinesis: After telophase, the cell forms a contractile ring of actin and myosin in the equatorial zone, whose contraction causes the separation of the cells.
Cyclosis: Circular motion within the cell, between the vacuole membrane and the rest of the space.

Intermediate Filaments

Intermediate filaments are 10 nm in diameter and are structures formed by fibrous proteins found in all eukaryotic cells, but specific for each cell type. Associated proteins are called IFAPs (e.g., filaggrin). They form intermediate filament networks surrounding the nucleus and extending to the cell periphery.

Types of Intermediate Filaments:

Neurofilaments: Located in the axon and dendrites of neurons.
Keratin Filaments (Tonofilaments): Found in epithelial cells, providing great mechanical resistance.
Vimentin Filaments: Found in various cells, including fibroblasts.
Desmin Filaments: Found in muscle cells.

Functions of Intermediate Filaments:

Provide structural support.
Maintain cell shape.
Found in cells subjected to mechanical forces (epithelial and muscle).

Microtubules

Microtubules are cylindrical formations found in the cytoplasm or forming part of cilia, flagella, and centrioles. They are dynamic structures formed according to the needs of the cell. They have variable length and are 24 nm in diameter. They are formed by 13 protofilaments, leaving a central cavity. They are composed of α-tubulin and β-tubulin, which join together to form dimers, which then form protofilaments, which in turn form the microtubule.

Functions of Microtubules:

Formation of the Spindle: The structure responsible for the movement of chromosomes during cell division.
Intracellular Transport: Vesicles move through the cell cytoplasm along microtubules.
Formation of Cilia and Flagella: They constitute the structural basis of cilia and flagella.

Ribosomes

Ribosomes are particles without a membrane that are formed by rRNA and proteins in equal parts. They are ribonucleoproteins found in all cells (except in sperm cells and red blood cells). They can be found:

Free in the cytoplasm, either isolated or united into polyribosomes.
Attached to the outer surface of the rough endoplasmic reticulum (RER) or the cytoplasmic face of the outer nuclear membrane.
Free in the matrix of mitochondria and chloroplasts.

Ribosomes are formed by two uneven subunits, one large and one small, separated by a cleft. Each subunit has a different sedimentation coefficient. In prokaryotes, the sedimentation coefficient is 70S, while in eukaryotes it is 80S. The two subunits are formed in the nucleolus, where they join with rRNA and ribosomal proteins. Subunits exit the nucleus through the nuclear pores into the cytoplasm, where they unite to form a ribosome.

Functions of Ribosomes:

Involved in the synthesis of proteins.
Proteins synthesized in the cytoplasm by free ribosomes remain in the cytosol.
Proteins synthesized by ribosomes attached to the RER pass to other organelles of the cell or are secreted.

Cilia and Flagella

Cilia and flagella are derived from centrioles. Cilia are short and numerous, while flagella are long and there is usually only one.

Structure of Cilia and Flagella:

Stem or Axoneme: Has nine pairs of microtubules forming a circle and two in the center. This is called the 9+2 structure. The two central microtubules have 13 protofilaments, while the external microtubules share protofilaments, with the inner microtubule being complete and the outer one incomplete. The microtubules are linked by a protein called tektin. The inner microtubules have dynein arms that extend clockwise. Each pair of microtubules is connected to the next by nexin.
Transition Zone: The base of the cilium or flagellum where the central microtubules disappear and the basal plate begins.
Basal Corpuscle (Basal Body): Has a 9+0 structure because it consists of 9 triplets of microtubules and no central ones. In the triplets, the microtubules share protofilaments, so the inner one will have 13, while the other two will be incomplete and have 10 protofilaments each.
Ciliary Roots: Microfilaments emerging from the basal corpuscle. Their function is to coordinate the movement of the cilia.

Functions of Cilia and Flagella:

Related to motion. They allow the cell to move in a liquid medium. Cilia move all at once or in waves, and flagella have a whip-like motion.

Cell Wall

The cell wall is a thick, rigid, external layer that acts as an exoskeleton. It is found in the cells of plants, algae, bacteria, and fungi. It is composed of polysaccharides. In fungi, it is made of chitin, and in most algae and plants, it is made of cellulose, a linear polymer composed of glucose monomers. Thousands of glucose monomers form long chains that join to form microfibrils. These are not the only cell wall polysaccharides; there are also hemicellulose and pectin. The cell wall of newly formed plant cells has a primary wall and a middle lamella. Growth ends when the secondary wall is formed.

Components of the Cell Wall:

Middle Lamella: Located between the primary walls of adjacent cells, except in plasmodesmata, which are cellular bridges for intercommunication. It is composed of pectin.
Primary Wall: Characteristic of growing cells. It is thin and flexible, which allows the cell to expand and grow. It is composed of cellulose, hemicellulose, and pectin.
Secondary Wall: A thicker and more rigid layer composed of cellulose and lignin that forms when growth stops. It may also contain lignin. It is found in cells involved in support and transport (e.g., xylem).

Functions of the Cell Wall:

Acts as an exoskeleton that protects the cell, gives it shape, and confers resistance.
Prevents the cell from rupturing.
Involved in the maintenance of osmotic pressure.

Centrosome

The centrosome is a structure without a membrane that is found in all animal cells. It consists of two centrioles surrounded by pericentriolar material called the microtubule organizing center (MTOC).

Structure of the Centrosome:

Centrioles: The walls of centrioles are composed of 9 triplets of microtubules, forming the 9+0 structure. These triplets are joined to each other through nexin. The internal microtubule has 13 protofilaments, but the other two only have 10 each. This structure is also known as the cartwheel.
Pericentriolar Material (PCM): Surrounds the centrioles and is responsible for nucleating and organizing microtubules.

Functions of the Centrosome:

Forms structures such as cilia and flagella, which are formed by microtubules.
Plays a crucial role in cell division, organizing the mitotic spindle.