Cellular Organelles: Structure and Function in Eukaryotic Cells
Centrosome
The centrosome is a cellular organelle found in animal cells. It is not surrounded by a membrane and consists of two paired centrioles embedded in a set of protein aggregates that surround it, called the pericentriolar material. The walls of centrioles are composed of nine groups of three microtubules (9 + 0 structure). Its primary function is the nucleation and anchoring of microtubules (MTs), so that generically these structures (in conjunction with polar bodies spindle in yeast) are called MT organizing centers. Around the centrosomes, a set of microtubules forms radially, creating an aster. The centrosomes play a key role in establishing the network of interphase MTs and the spindle apparatus. During the interphase of the cell cycle, MTs determine the cell shape, polarity, and motility, whereas during mitosis, they form the spindle apparatus required for the segregation of chromosomes between two daughter cells. Its functions are related to cell motility and cytoskeletal organization. During cell division, the centrosomes are directed to opposite poles of the cell, organizing the spindle (or mitotic) apparatus. In the period between anaphase, microtubules in the aster stretch and contribute to the separation of chromosomes to chromatids and the division of the cytoplasm. Mature neurons do not have a centrosome and therefore do not multiply.
Cilia and Flagella
Cilia and flagella are centriolar derivatives located on the free surface of some cells. The cilia are short and very numerous, while the flagella are long and few. They respond to the same structural pattern, consisting of:
- Axoneme: Contains nine pairs of peripheral microtubules and a pair of central microtubules, all parallel to the axis of the cilium or flagellum, forming the 9 + 2 structure. All microtubules are composed of protofilaments of tubulin dimers.
- Transition Area: The base of the cilium or flagellum. In this area, the central tubules disappear, and the basal plate appears, connecting the base of the cilium or flagellum with the plasma membrane.
- Basal Body: Presents a structure composed of nine peripheral triplets of microtubules. Closest to the nucleus, it has radial fibrils that traverse the microtubule to the center.
- Ciliary Rootlet: Microfilament stretched, leaving the bottom of the basal body, whose function is related to the coordination of the cilia and the stimuli.
Function: It is related to movement. They allow a cell to actively move through a liquid medium, but can also cause the liquid to move.
Ribosomes
Ribosomes, discovered in 1953, are compact particles formed in equal parts of rRNA and protein. They exist in all cells, although they are rare in red blood cells and not present in sperm. They can be found free in the cytoplasm forming polyribosomes, attached to the outer membrane of the rough endoplasmic reticulum (RER) or the cytoplasmic face of the outer nuclear membrane, or free in the mitochondrial matrix and chloroplasts. In prokaryotes, they have a sedimentation coefficient of 70S, and in eukaryotes, 80S. Ribosomes are the organelles responsible for protein synthesis, a process known as translation. The information required for this synthesis is in the messenger RNA (mRNA), whose sequence of nucleotides determines the amino acid sequence of the protein. In turn, the mRNA sequence is derived from the transcription of a gene on DNA. Transfer RNA carries amino acids to the ribosomes, where they are incorporated into the growing polypeptide.
Organelles
Mitochondria
Mitochondria are cytoplasmic organelles bearing a double membrane found in most eukaryotic cells. Their size varies between 0.5-10 microns in diameter. Mitochondria are sometimes described as “power generators” of cells because they produce most of the supply of adenosine triphosphate (ATP), which is used as an energy source. Besides providing energy to the cell, mitochondria are involved in other processes such as cell signaling, cell differentiation, cell cycle control, and cell growth. Some features are unique to mitochondria. Their number varies widely depending on the type of organism or tissue. Some cells have no mitochondria or only one, while others may contain several thousand. This organelle is composed of compartments that carry out specialized functions. These include the outer mitochondrial membrane, intermembrane space, inner mitochondrial membrane, the cristae, and the mitochondrial matrix. Mitochondrial proteins vary depending on the tissue and species: in humans, 615 different types of proteins have been identified in mitochondria of cardiac muscle. Although most of the cell’s DNA is in the nucleus, mitochondria have their own genome, which shows many similarities with bacterial genomes.
Functions: Krebs cycle in the mitochondrial matrix, oxidative phosphorylation in the inner mitochondrial membrane (cristae), protein synthesis, decarboxylation of pyruvic acid, and concentration of substances in the inner chamber.
Lysosomes
Lysosomes are relatively large, acidic organelles formed by the rough endoplasmic reticulum (RER) and then packaged by the Golgi complex. They contain hydrolytic and proteolytic enzymes used to digest materials of external or internal origin that come to them. That is, they are responsible for cellular digestion. Those formed from the Golgi are called primary, and secondary lysosomes are formed from the fusion of primary lysosomes, hydrolases, and their substrates.
Peroxisome
Peroxisomes are very common cytoplasmic organelles in the form of vesicles containing oxidases and catalases. These enzymes act as cellular detoxification agents. Like most organelles, peroxisomes are found only in eukaryotic cells. They are equipped with a semipermeable membrane, which contains several enzymes that produce or use hydrogen peroxide. More than 50 enzymes have been identified in peroxisomes of different tissues. Peroxisomes are formed by budding away from the smooth endoplasmic reticulum (SER), but they may also enlarge a certain portion of their membrane to produce new peroxisomes without spilling their contents into the cytoplasm. This membrane protects the cell from the harmful effects inside the peroxisome. Particles inside are usually crystallized. Peroxisomes have a central role in the shortening of very long-chain fatty acids for complete oxidation in mitochondria and the oxidation of the side chain of cholesterol necessary for the synthesis of bile acids. They are also involved in the synthesis of glycerolipids, plasmalogens, and isoprenoids. They also contain enzymes that oxidize amino acids, uric acid, and other substrates using molecular oxygen with the formation of hydrogen peroxide.
Vacuole
Vacuoles are cellular organelles, non-exclusive membranous cisterns of plant cells. They consist of a tonoplast membrane that separates the cytoplasm. Inside is the vacuolar juice, mostly composed of water. The main functions are to maintain cell turgor and osmotic pressure (which is very high), aid in digestion, and store cellular substances.