Eukaryotic Cells: Structures and Functions

Posted on Dec 9, 2024 in Biology

Mitochondria

Mitochondria are organelles surrounded by two membranes, present in all eukaryotic cells. They are considered the power plants of the cell since they are the site of the oxidation of fatty acids and the aerobic phase of cellular respiration.

  • Outer membrane: Smooth and permeable.
  • External chamber: Located between the two membranes, with a composition very similar to the cytosol because of the permeability of the outer membrane.
  • Internal membrane: Presents a series of folds directed toward the inside, called mitochondrial cristae. This membrane regulates the exchange between the inside and outside of the mitochondria due to its permeability. Particles F1 appear at the crests (complex for ATP synthase, function: coupling between redox reactions), which consist of a head facing the matrix, a pendulum, and a small base embedded in the membrane. The respiratory chain is located in the membrane, whose function is the transport of electrons from atoms or H2 to O2.
  • Internal chamber: Occupied by a material rich in proteins, the matrix, where small circular DNA strands and ribosomes similar to those of bacteria are observed.

Cytoskeleton

The cytoskeleton consists of a network of protein filaments that extend throughout the cytoplasm, serving as internal support. This determines the shape of the cell, its internal organization, and movement. It is constituted by:

  • Microfilaments: Thin filaments composed of actin, present as globular actin G. They are involved in different processes:
    • Muscle contraction: Microfilaments associated with another protein, myosin, are responsible for the contraction of muscle cells.
    • Emission of pseudopods: These are cytoplasmic extensions that enable amoeboid movement and phagocytosis.
    • Formation of a contractile ring: This ring, formed by actin and myosin at the end of cell division, causes the bottleneck that leads to the separation of the two daughter cells.
  • Intermediate filaments: Intermediate in diameter between microtubules and microfilaments. Their composition and functions vary according to the structure and cell type. They perform structural roles, such as keratin filaments and neurofilaments.
  • Microtubules: Hollow cylinders whose wall consists of protein subunits of tubulin. They are found scattered throughout the cytoplasm, constituting part of the cytoskeleton. Their functions include:
    • Mechanical: They form a sort of frame responsible for maintaining the shape of the cell.
    • Transport of intracellular organelles: They constitute a kind of lane along which organelles and vesicles can move.
    • Movement of chromosomes: They form the mitotic spindle, which directs the movement of chromosomes.
    • Organization of the cytoskeleton: They are involved in the distribution of the remaining filaments.

Lysosomes

Lysosomes are vesicles surrounded by a membrane, whose interior contains a large number of hydrolytic enzymes or hydrolases capable of digesting most biological macromolecules. They are classified as:

  • Primary: Not yet involved in any digestion process. Their formation involves the rough endoplasmic reticulum (RER) and the Golgi apparatus (GA).
  • Secondary: Originate from the primary lysosomes when they are involved in an intracellular digestion process.

Functions

  • Extracellular digestion: The primary lysosome membrane fuses with the plasma membrane, and through exocytosis, enzymes are released, so they can digest extracellular material.
  • Intracellular digestion of exogenous materials: Solid particles and macromolecules can be incorporated into the cell through a process of endocytosis. This assists in digestion, nutrition, and defense against pathogenic microorganisms.
  • Intracellular digestion of endogenous materials: Through the process of autophagy, which aims to restore cellular components. Autophagy ensures the replacement of cellular components, as well as the nutrition of the cell under fasting conditions.

Golgi Apparatus

The Golgi apparatus is a membranous complex, comprising a set of units called dictyosomes. It consists of:

  • Cisternae: Flattened vesicles grouped in a stack.
  • Transition vesicles: Small vesicles closely associated with cisternae, serving as transport vehicles.
  • Secretory vesicles: Larger and placed at the edges of the dictyosomes.

Functions

Formation of primary lysosomes and secretion.

Process

From the RER, proteins are encapsulated in transition vesicles that move to the Golgi apparatus, joining the part of the dictyosomes next to the RER or cis cisterna, specializing in obtaining substances. Then, other transition vesicles transport proteins through the medial cisternae. As they go through the dictyosomes, proteins are modified according to their destination. In the final compartment (trans cisterna), proteins are sorted and packaged into vesicles for their final destination: some are primary lysosomes, others join the plasma membrane and are incorporated into it, and others form secretion granules, responsible for expelling substances outside the cell.