General classification of organelles
All cellular organelles are traditionally divided into two major categories depending on the presence or absence of a lipid bilayer membrane. This division is fundamental to understanding the structural organization of the cytoplasm:
- Membranous organelles are structures delimited from the hyaloplasm by one or two membranes.
- Non-membranous organelles are structures that lack a membrane envelope and represent complex protein or ribonucleoprotein complexes.
Membranous organelles
This group can be divided into a unified vacuolar system, where the elements are functionally interconnected, and isolated double-membrane structures.
The vacuolar system (single-membrane organelles) includes:
- Endoplasmic reticulum (ER): represents a complex spatial network of cisternae, small vacuoles, and tubules.
- Golgi apparatus: appears as stacks of flattened cisternae, from the edges of which vesicles constantly 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 necessary for the utilization of hydrogen peroxide.
Standing apart in the classification are mitochondria. Unlike the structures of the vacuolar system, they are bounded by two membranes. Furthermore, the inner membrane forms characteristic folds (invaginations) into the organelle lumen, which are called 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 unit of which consists of exactly two subunits (large and small).
Fibrillar organelles have a thread-like structure and are divided into two groups:
- Group I (Contractile structures): these include myofibrils and myofilaments, which provide motor activity.
- Group II (Cytoskeleton elements): form the internal framework of the cell.
| Cytoskeleton element | Diameter | Main protein | Arrangement features |
|---|---|---|---|
| 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
Some intracellular and extracellular formations are built on the basis of the previously discussed cytoskeleton elements:
- Microfilament derivatives: actin filaments form the rigid internal framework of microvilli.
- Microtubule derivatives: tubulin cylinders form more complex systems. First, these are centrioles — hollow cylinders made of microtubules. Two such centrioles (the pair is called a diplosome) together form the cell center (centrosome). Second, this is the axoneme — a complex axial structure that forms the basis of cilia and flagella.
Briefly: answers to topical questions
What proteins are part of the intermediate filaments of the cytoskeleton?
The protein composition of intermediate filaments is tissue-specific and depends on the specific cell type. They include the following proteins:
- Keratin — characteristic of epithelium, where the structures are often called tonofilaments involved in keratinization.
- Vimentin — forms filaments in connective tissue, endothelium, and vascular smooth myocytes.
- Desmin — present in muscle tissue (except vascular), where it connects adjacent telophragmata (Z-lines) and anchors myofibrils to membrane structures.
- Specific neurofilament proteins — form intermediate filaments in nerve cells.
What is the microscopic structure of the axoneme of cilia and flagella?
The axoneme is the axial structure of cilia and flagella, running along the axis of the protrusion. Its ultrastructure is organized according to the (9 × 2) + 2 pattern. The following components are distinguished in the axoneme structure:
- Peripherally located 9 doublets of microtubules.
- In the center, there is 1 pair of microtubules enclosed in a central sheath.
- Dynein arms — elements made of the protein dynein extending from each peripheral doublet to the adjacent one.
- Radial spokes extend from the peripheral doublets to the central sheath.
The surface of the axoneme is covered by the plasmalemma, and the space between the microtubules is filled with hyaloplasm.
What functions are performed by microvilli at the apical pole of the cell?
The main function of microvilli at the apical pole is a dramatic increase in the cell surface area to facilitate absorption and digestive processes. They perform the following functions:
- Absorption — uptake of digestion products in the intestine and intensive reabsorption of substances in the renal tubules.
- Parietal (membrane) digestion — final breakdown of substances mediated by enzymes (aminopeptidases, phosphatases, glycosidases) fixed in the glycocalyx of microvilli.
Microvilli form the specific brush border of epithelial cells, inside of which lies a supporting skeleton of microfilaments.
Single-membrane organelles of the vacuolar system are functionally interconnected. Imagine a factory: the ER is the workshop (tubules and cisternae), the Golgi apparatus is the packaging warehouse (stacks of flat cisternae), and lysosomes and peroxisomes are the couriers (vesicles containing enzymes).
Class and exam questions
What is the main difference between mitochondria and elements of the vacuolar system?
Mitochondria stand apart because they have two membranes, the inner of which forms infoldings called cristae. Organelles of the vacuolar system have only one membrane.
What is the cell center made of?
The cell center consists of a pair of centrioles. This pair is called a diplosome, and the centrioles themselves are hollow cylinders formed by microtubules.
How do microfilaments differ from microtubules in structure?
Microfilaments consist of the protein actin, have a diameter of 5–7 nm, and are arranged tangentially. Microtubules are hollow, consist of tubulin, have a diameter of 24 nm, and are arranged radially.
What else to study on this topic
- Functional relationship between the elements of the vacuolar system
- Tissue specificity of the protein composition of intermediate filaments
- Mechanisms of action of peroxisomal and lysosomal enzymes
- Spatial organization of the axoneme in cilia and flagella
Master the entire topic in the "Histology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Cell Organelles" and 3,485 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 137 video lessons on the subject
Other topics in the "Cytoplasm" section
Free video lessons: histologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.