Key Eukaryotic Organelles: Structure and Roles
Centrosome: Structure and Functions
The centrosome is a dense material, often referred to as the centrosphere, with few radial microtubules. Each centriole consists of nine groups of microtubule triplets arranged as a cylinder, held together by proteins. These triplets remain united by proteins.
Functions:
- Microtubule organizing centers.
- Involved in the formation of cilia and flagella.
- Forms the mitotic spindle during cell division.
- Contributes to the structure of the cytoskeleton.
Ribosomes: Essential for Protein Synthesis
Ribosomes are small particles where protein synthesis takes place. A ribosome is a complex of ribosomal RNA (rRNA) and associated proteins. It is formed by two subunits that dissociate at the end of the protein synthesis process.
Localization:
- Free in the cytosol.
- Bound to the endoplasmic reticulum (ER).
- Found in mitochondria and chloroplasts.
The ribosome binds to a specific point on messenger RNA (mRNA) and translates triplet codons into amino acids. At the end, the newly synthesized protein is released, and the two subunits dissociate, remaining free in the cytosol. It is called a polysome or polyribosome when several ribosomes simultaneously translate the same mRNA molecule.
Endoplasmic Reticulum: Synthesis and Transport Hub
The endoplasmic reticulum (ER) is a complex, interconnected membrane network that extends throughout the cytoplasm, forming a unique internal space called the lumen. It has different functions, including lipid and protein synthesis, as well as transport and secretion to the exterior. It is categorized as rough ER or smooth ER depending on the presence or absence of ribosomes on its cytosolic surface.
Rough Endoplasmic Reticulum
The rough ER is highly developed in secretory cells and synthesizes proteins for export. These proteins are released into the lumen for secretion. Glycosylation of these proteins also occurs here.
Smooth Endoplasmic Reticulum
The smooth ER lacks ribosomes. Its functions focus on metabolism:
- Lipid synthesis (e.g., cholesterol and steroids).
- Synthesis and storage of fatty acids.
- Release of accumulated calcium.
- Detoxification processes (elimination of harmful substances).
Golgi Apparatus: Processing and Packaging Center
The Golgi apparatus is formed by groups of stacked, flattened cisternae called dictyosomes. It is usually found near the nucleus and the centrosome. Its development depends on the cell type and its physiological state.
Structure: It distinguishes a proximal cis face (input) and a distal trans face (output), from where vesicles exit to their destinations: the plasma membrane, lysosomes, and secretory vesicles. Proteins and lipids, transferred from the ER, enter the dictyosomes and undergo a series of modifications, resulting in their secretion.
Functions:
- Involved in secretion processes (releasing contents outside via exocytosis).
- Recycling of the plasma membrane.
- Glycosylation of lipids and proteins (a process started in the ER).
- Formation of lysosomes and vacuoles.
- Synthesis of pectins and hemicellulose for plant cell walls.
Mitochondria: The Cell’s Powerhouses
Mitochondria are organelles that function as the cell’s power plants, generating cellular energy. Eukaryotic respiration takes place within them, a process where energy is harvested from the oxidation of organic molecules and converted into ATP.
Mitochondrial Outer Membrane
The outer membrane contains numerous transmembrane proteins that form important channels for substance entry.
Mitochondrial Inner Membrane
The inner membrane contains:
- Protein conveyors for metabolites.
- Respiratory chain enzymes essential for oxidative phosphorylation and ATP synthesis.
- The ATP synthase complex that catalyzes ATP formation.
Mitochondrial Matrix
The matrix contains many enzymes, ATP substrates, ions, etc. It also contains mitochondrial DNA, RNA, and its own ribosomes. Within the mitochondrial matrix, the Krebs cycle, fatty acid oxidation, and mitochondrial DNA replication, transcription, and translation occur.
Functions: Cellular respiration, which is the oxidation of molecules by oxygen to obtain energy in the form of ATP.