The cytoplasm is composed of a viscous and very fluid part, the cytosol, made up of water (which represents 75-85% of the total weight of the cell), of inorganic substances dissociated in ionic form (especially ions K +, Na +, Ca ++ and Mg ++) and from various organic molecules (including proteins with enzymatic or structural function).
Numerous cytoplasmic structures of various types are included in the cytoplasm of eukaryotic cells, which perform very specific functions (in prokaryotes cellular activities exist, but are not carried out by distinct structures). Some of these structures (also called organelles) are delimited by a structured membrane such as the plasma membrane, but with modifications in the type and number of phospholipids and proteins that allow them to perform particular functions.
- endoplasmic reticulum
- Golgi apparatus
- Eyelashes and flagella
The endoplasmic reticulum is a complex network of communicating tubules, bags, and channels, which open up at the level of the nuclear membrane. The endoplasmic reticulum is of two types; the rough type is a continuation of the nuclear membrane and carries millions of tiny granules, called ribosomes. On the external surface, they are linked to the synthesis of proteins; the smooth type is free of ribosomes and is responsible for lipid synthesis. Finally, the space between the folded membranes of both types of endoplasmic reticulum is used to store and transfer molecules from one point to another in the cell.
Ribosomes are the site of protein synthesis, i.e., the assembly of amino acids to form proteins. They consist of two subunits of unequal size, made up of ribonucleic acid (RNA) and proteins. They can be associated with the rough endoplasmic reticulum (in this case, they synthesize proteins destined to be secreted outside the cell) or free in the cytoplasm (they synthesize proteins that the cell retains inside).
The Golgi apparatus consists of tubules flattened in the center and swollen at the ends, stacked on each other and ending at the bottom of the blind, where various materials (hormones, proteins, lipids) are modified and accumulated, which will be transported to other parts of the cell or expelled. For this purpose, small vesicles detach from the ends of the flattened sacs and migrate towards the plasma membrane and merge with it; the contents of the vesicles are thus poured out. Lysosomes are vesicles that derive from the Golgi apparatus and contain hydrolytic enzymes (capable of demolishing proteins and lipids). The cell uses lysosomes to recycle worn parts of organelles or to "digest" an entire cell (for example, a bacterium).
Mitochondria are roundish or ovoid organelles, delimited by a double membrane; the internal one is folded on itself so as to form septa (mitochondrial crests), which increase the internal surface of the organelle. The space enclosed by the internal membrane is the matrix, while the space between the two membranes is the inter-membrane space. Cellular respiration occurs in the mitochondria, through which energy is extracted from nutrients (sugars and fats) following their oxidation and subsequent demolition up to carbon dioxide and water. The energy obtained from this process is stored in the form of ATP molecules; at the appropriate time, the hydrolysis of ATP makes available the energy necessary for the cell to carry out its activities.
For this reason, cells with a high metabolism (for example, muscle cells) have a greater number of mitochondria; instead, the red blood cells are free. A peculiarity of mitochondria is the ability to self-replicate, allowed by the presence of a mitochondrial DNA, ribosomes, and all the molecules necessary for the duplication of the genetic information. Mitochondria, in fact, live only a few days and must, therefore, be continuously produced (this occurs by detaching vesicles with a double membrane containing mitochondrial DNA from the primitive mitochondria).
The organelles are wrapped and supported by protein-like fibers that form a network, the cytoskeleton. The cytoskeleton also provides support for cells without a rigid wall and has an active part in cell division and in the movements of the organelles and the entire cell. The cytoskeleton is not a rigid and permanent structure since the fibers that constitute it are continuously assembled and disassembled. These are divided into three groups based on their dimensions, microfilaments (5-7 nm in diameter), intermediate filaments (8-10 nm in diameter) and microtubules, hollow tubules with a diameter of about 25 nm, which also constitute the centrioles, eyelashes, and flagella.
The centrioles are hollow cylindrical structures, consisting of 9 triplets fused with microtubules, which are found in all animal cells (two per cell) and in a few plant cells. The centrioles intervene during cell division to correctly distribute the chromosomes in the two daughter cells.
The eyelashes and flagella are filiform and mobile appendages with identical structure, 9 pairs of microtubules welded to form a ring around two central microtubules. They are distinguished by the length and the number in which they are present on the plasma membrane. The eyelashes are numerous and short (10-25 µm), the flagella are few and long (50-75 µm). Their coordinated movements move the cell into the surrounding environment or create currents in the extracellular liquid that cause a constant flow of the suspended particles.
The centrioles are organelles in the form of a hollow cylinder that performs the function of organizing centers of the internal structure of eyelashes and flagella, and in this case, they are called basal bodies. In addition, all cells have a structure called the centrosome, consisting of a pair of centrioles, which performs an important function when a cell reproduces itself. Eyelashes and flagella are very thin mobile extensions present on the surface of many types of cells; they consist of specifically organized microtubules. Eyelashes are hair-like appendages that have the function of moving extracellular fluids, but can also provide some cells with movement. Each eyelash individually performs a whiplash-like movement, and as a whole, the lashes move in sync, creating a wave movement on the cell surface. Protozoa use their eyelashes both to move and to obtain food particles, making the particles that are found in the liquids outside the cell advance with their beat.