Structure of the cytoplasm and its matrix
Globally, the cellular volume is formed by several components: the plasma membrane, ground substance, organelles, and inclusions.
Cytoplasmic matrix (also called hyaloplasm or cytosol) is a colorless colloidal solution. It is not just "water," but a complex, dynamic environment saturated with enzymatic systems.
Chemical composition of hyaloplasm:
- Water: accounts for about 90% of the total volume.
- Organic compounds: proteins, free amino acids, nucleotides, and fatty acids.
- Inorganic components: ions of various salts.
The main physical characteristic of the cytosol is its ability to change state. It can exist as a sol (a more liquid form) or transition into a gel (a thick, viscous state). This provides an optimal environment for metabolic processes and enzymatic reactions.
Characteristics of organelles in different organisms
Organelles are permanent specialized structures of the cytoplasm. Their set differs radically depending on the type of cellular organization.
Prokaryotes (pre-nuclear organisms):
- Completely devoid of membrane-bound organelles.
- Their functions are partially taken over by mesosomes — specific invaginations of the plasmalemma that participate in metabolism and cell division.
- Contain a huge number of small ribosomes (type 70S) evenly distributed throughout the cytoplasm. They can account for up to 40% of the total cell mass.
Eukaryotes (nuclear organisms):
- Possess a complex and developed system of membranous structures: the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, mitochondria, and (in plants) plastids.
- A non-membranous cell center (centrosome) is present.
- The main cytoplasmic ribosomes are large (type 80S). They can float freely in the hyaloplasm or attach to the ER membranes. Interestingly, double-membrane organelles (mitochondria and plastids) contain their own small 70S-type ribosomes similar to prokaryotic ones 70S.
Classification of organelles
To systematize knowledge, all cell organelles are divided into groups based on two main criteria: functions (purpose) and structural features.
1. By purpose:
- General-purpose organelles. Found in almost all cells as they ensure basic vital activity. These include mitochondria, the ER, the Golgi apparatus, lysosomes, ribosomes, the cell center, and plastids.
- Special-purpose organelles. Formed only in highly specialized tissues to perform narrow-profile tasks. Examples: myofibrils (muscle cell contraction), neurofibrils (support and transport function in nerve cells), as well as movement organelles — cilia and flagella.
2. By structure:
- Non-membranous (lack a lipid envelope).
- Membranous. In turn, they are divided into single-membrane and double-membrane structures.
Briefly: answers to topical questions
Which organelles are single-membrane?
Single-membrane organelles include structures of the vacuolar system and various microbodies.
- Endoplasmic reticulum (ER) — a system of cisternae and tubules (smooth and rough).
- Golgi apparatus — stacks of flat cisternae with vesicles.
- Endosomes and lysosomes — vesicles containing hydrolytic enzymes.
- Microbodies — peroxisomes, glyoxysomes, and glycosomes.
- Vacuoles — cavities filled with cell sap in plants.
Which cell organelles are double-membrane?
Double-membrane cell organelles include mitochondria and plastids.
- Mitochondria — standalone structures possessing two membranes, with the inner one forming infoldings called cristae.
- Plastids — double-membrane organelles characteristic of plant cells, containing their own 70S-type ribosomes.
Which cell structures are non-membranous organelles?
Non-membranous organelles include granular and fibrillar structures, as well as derivatives of fibrillar structures.
- Ribosomes — ribonucleoprotein complexes of two subunits; consist of rRNA and protein.
- Fibrillar organelles of the cytoskeleton — microfilaments (actin), intermediate filaments, and microtubules (tubulin).
- Contractile structures — myofibrils and myofilaments.
- Centrioles and the cell center — centrioles are hollow cylinders made of microtubules; a pair of centrioles (diplosome) forms the cell center.
- Derivatives of fibrillar structures — the framework of microvilli and the axoneme as the axial structure of cilia and flagella.
Class and exam questions
What is the difference between the cytoplasm and the hyaloplasm?
Cytoplasm is a broad concept that includes the entire internal environment of the cell along with organelles. Hyaloplasm (cytosol) is only the liquid ground substance in which these organelles are suspended.
Do bacteria have a Golgi apparatus and mitochondria?
No, bacteria are prokaryotes, which completely lack membrane-bound organelles. Some of their functions are compensated by mesosomes.
How do eukaryotic and prokaryotic ribosomes differ?
Eukaryotes have large 80S-type ribosomes in their cytoplasm, while prokaryotes have small 70S-type ribosomes. However, eukaryotic mitochondria and plastids also contain 70S ribosomes.
What else to study on this topic
- Structure and functions of the endoplasmic reticulum (ER)
- Morphology of the Golgi apparatus and its connection with lysosomes
- Double-membrane organelles: mitochondria and plastids
- The cell center and its role in division
- Cytoplasmic inclusions: classification and significance
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