M I C R O  -  O R G A N I C S





Mitochondria contains the molecular machinery for the release of energy from the phosphate bonds of adenosine triphosphate (ATP), which becomes available to power cellular functions. Mitochondria consists mainly of protein, but also contains lipid, DNA and RNA. Under the electron microscope, thesespherical organelles have an outer membrane surrounding an inner membrane that folds as cristae into a scaffolding for oxidative phosphorylation and electron transport enzymes. The mitochondrial matrix contains the enzymes of the citric acid cycle and fatty acid oxidation.

Mitochondrial DNA is double-stranded and circular. Mitochndrial RNA are classified as ribosomal, messenger and transfer. The metabolic activity of the cell is proportional to the number of mitochondria and the number of cristae. Cells with high metabolic activity have large amounts of well-developed mitochondria. 

Ribosomes are small organelles composed of ribosomal RNA (rRNA) and 80 different proteins.  Ribosomal subunits are assembled in nucleolus from rRNA and imported proteins from the cytoplasm. Following assembly, the ribosomal subunits pass to the cytoplasm where they participate in protein synthesis. Some ribosomes are free in the cytoplasm and can integrate with a polyribosomal structure; other ribosomes are attached to the endoplasmic reticulum. Synthesised proteins may be designated for secretion, storage, or membrane-integration.

Within the cytoplasm of cells is a configuration of connecting and branching channels constituted by a continuous membrane known as the endoplasmic reticulum (ER). ER are classified as rough ER when ribosomes are attached to the cytosolic side of the membrane, and classified as smooth ER when ribosomes are absent. The main function of rough ER is the segregation of proteins destined for export from the cell or for intracellular use. Proteins are modified within the ER by the addition of carbohydrate, removal of a signal sequenc, and other post-translational modifications.

Smooth ER appears more tubular than rough ER. Smooth ER allows the performance of specialized functions: steroid sysnthesis, metabolism and detoxification of substances in the liver, phospholipid synthesis, and excitation-contraction in skeletal muscle.

The Golgi, a curved membrane stack, finishes the post-transitional modifications, concentrates and packages proteins for export or storage. Proteins made within the rough ER bud off in vesicles and are transported to the Golgi appartus where the vesicles fuse with the membrane and the components are further modified, concentrated and packaged, before budding off as vesicles on the opposite side. 

Lysosomes are vesicles containing more than 40 hydrolytic enzymes that can digest most macromolecules. These organelles are the sites of intracellular digestion. Common lysosomal enzymes include acid phosphatase, ribonuclease, deoxyribonuclease, proteases, sulfatases, and lipases. The limiting membrane and the inactivity of the enzymes to function at the cytosol pH prevents the cell from digesting itself. 

Primary lysosomes are small concentrated sacs of enzymes. Primary lysosomes fuse with a phagocytic vacuole to become secondary lysosomes where digestion begins. As the substances are digested the nutrients diffuse through the lysosomal membrane to the cytosoplasm. Residual bodies are formed when indigestable elements are retained. Lysosomes also participate in the turnover of cellular organelles.

These small organelles contain oxidative enzymes. Peroxisomes contain amino oxidases, hydroxyacid oxidase and catalase. Catalase protects the cell from hydrogen peroxide damage. Enzymes involved in lipid metabolism are also found in peroxisomes.

Secretory granules are found in cells that store and release hormones, neurotransmitters or digestive enzymes. These vesicles contain a concentrated form of the secretory product.

The cytoskeleton is a complex network of filaments, anchor proteins, and protein motors that constitute a support and transport scaffold. This network confers cellular shape and facilitates cellular movement.

These variable length tubules have a dense wall and central hollow core. Microtubules are composed of repeating heterodimers of alpha and beta tubulin. 
Microtubules provide the necessary intracellular transportation system for the movement of organelles and vesicles from one place to another. Molecular motors such as dynien and kinesin transport packages via the microtubules in an energy-requiring process.

Actin and myosin are microfilament proteins responsible for contraction in muscle cells. Myosin in motile nonmuscle cells polymerizes to effect cell movement. Microfilaments form thin sheaths beneath cell membranes associated with endocytosis, exocytosis and cell movement. 

Intermediate filaments are made of several proteins. These filaments include keratins, vinmentin, desmin, glial fibrillary acidic protein, neurofilaments, and nuclear laminins.

These cylindric organelles participate in cell division as microtubule organizing centers. Centrioles are composed of tubulin in a characteristic nine microtubular triplets arrangement. A single pair of centrioles is sited near the Golgi complex in non-dividing cells. During cell division, a pair of centrioles moves to opposite poles of the cell to become organizing centers for the mitotic spindle.

Cilia and flagella have a microtubular core. Both cilia and flagella have a core 9 + 2 arrangement of microtubules covered by a cell membrane. This structure, axoneme, consists of 9 microtubular doublets surrounding an inner core of two sheathed microtubules. Adjacent doublets are linked to one another by protein bridges called nexins, and are connected to the central pair by radial spokes. The tubules of each peripheral pair are called subfibers A and B. At the base of each cilia or flagella is a centriole-like structure called the basal body. Cilia and flagella are motile structures. Movement is effected by the sliding of adjacent doublets over one another by an energy-requiring process.

The nucleus is a spherical, sometimes elongated structure that contains the genetic material, the DNA, of the cell. The highly organized chromatin is enveloped by a nuclear membrane that contains numerous pores for the transport of molecules to the cytoplasm. The nuclear envelope consists of two parallel unit membranes with an intervening perinuclear space. 

Chromatin consists of coiled DNA bound to basic proteins called histones and non-histone proteins. The degree of coiling of the chromatin varies during cell activity. There are two types of chromatin: dark, dense heterochromatin and light euchromatin. The basic structure of the chromatin is the nucleosome, which consists of a core of histones with 166 DNA base pairs wrapped around the core. Nucleosomes have an intervening 48 base-pair linking segment.

This spherical structure within the nucleus is composed of DNA, RNA and protein. The DNA present codes for rRNA, and are known as nucleolar organizers. Proteins synthesized in the cytoplasm pass through the nuclear pores and become associated with the newly made rRNA in the nucleolus. Thereafter, the ribosomal subunits migrate to the cytoplasm.

The nuclear matrix, consisting of proteins, metabolites and ions, is a fibrillar nucleo-skeleton that forms a scaffold for the folded DNA. A fibrous lamina  lies just under the nuclear envelope which dissociates and reforms during cell division.