General Classification of Organelles
All cellular organelles are traditionally divided into two major categories based on the presence or absence of a lipid bilayer. This division is fundamental to understanding the structural organization of the cytoplasm:
- Membranous organelles — structures separated from the hyaloplasm (cytosol) by one or two membranes.
- Non-membranous organelles — structures lacking a membrane envelope, representing complex protein or ribonucleoprotein complexes.
Membranous Organelles
This group can be subdivided into a unified vacuolar system, where elements are functionally interconnected, and distinct double-membraned structures.
The vacuolar system (single-membrane organelles) includes:
- Endoplasmic reticulum (ER): a complex spatial network of cisternae, small vacuoles, and tubules.
- Golgi apparatus: stacks of flattened cisternae from the edges of which transport vesicles continuously bud off.
- Endosomes and lysosomes: specialized vesicles containing a set of hydrolytic enzymes, such as proteases, nucleases, and lipases.
- Peroxisomes: vesicles containing oxidases and enzymes required for hydrogen peroxide degradation.
Standing apart in this classification are mitochondria. Unlike the structures of the vacuolar system, they are bounded by two membranes. The inner membrane forms characteristic folds (invaginations) projecting into the matrix, known as cristae.
Non-membranous Organelles
Non-membranous cytoplasmic structures are subdivided into two main subgroups: granular and fibrillar.
Granular organelles are represented by ribosomes. These are complex ribonucleoprotein complexes, each consisting of exactly two subunits (large and small).
Fibrillar organelles have a thread-like structure and are divided into two groups:
- Group I (Contractile structures): includes myofibrils and myofilaments, which provide motility.
- Group II (Cytoskeletal elements): form the internal framework of the cell.
| Cytoskeletal element | Diameter | Principal protein | Arrangement |
|---|---|---|---|
| Microfilaments (MF) | 5–7 nm | Actin | Tangential (along the long axis) |
| Intermediate filaments | 10 nm | Tissue-specific proteins | Depends on tissue type |
| Microtubules (MT) | 24 nm | Tubulin | Radial (hollow inside) |
Derivatives of Fibrillar Structures
Certain intracellular and extracellular structures are built upon the previously discussed cytoskeletal elements:
- Microfilament derivatives: actin filaments form the rigid internal core of microvilli.
- Microtubule derivatives: tubulin cylinders form more complex systems. First, centrioles are hollow cylinders made of microtubules; two such centrioles (a pair termed a diplosome) together form the centrosome. Second, the axoneme is a complex axial structure that forms the core of cilia and flagella.