Number, Size, Shape, and Properties of Cells. part3

Golgi Apparatus
The Golgi apparatus is composed of several Golgi bodies and also represents a system of internal channels taking part in the ingestion and excretion of substances in the form of membrane-bounded secretory vesicles. Lysosomes are also formed by this mechanism. The Golgi bodies have one side for uptake and one for discharge. Precursors of protein secretions migrate from the granular endoplasmic reticulum to the intake side of the Golgi body, where they are loaded into transport vesicles and flushed out of the cell through the discharge side. During this process, the membrane of the vesicle fuses with the cell membrane. Hence the renewal of the cell membrane is an important task of the Golgi apparatus.

Lysosomes
The more or less spherical lysosomes are the digestive organs of the cell. They contain large quantities of enzymes, especially acid hydrolases and phosphatases, with the aid of which they can degrade ingested foreign material or the cell’s own decaying organelles and return them in the form of metabolites for cellular metabolism (recycling). The lysosome’s membrane protects intact cells from uncontrolled activity of the lysosomal enzymes. In damaged cells, the liberated enzymes can contribute to tissue autolysis (e. g., in purulent abscesses).

Centrioles
Centrioles are hollow, open-ended cylinders. Their walls are composed of microtubules, which are rigid, filamentous proteins. Centrioles play a major role in cell division, when they build threadlike spindle structures that are connected with the movement of the chromosomes. Evidently this process determines the polarity of the cell for the direction of a cell division.

Mitochondria
Mitochondria are small filiform structures, 2−6 μm long that are present in varying numbers (a few to more than a thousand) in all cells with the exception of red blood corpuscles. Their walls consist of an inner and an outer elementary membrane. The inner has multiple folds, and so possesses a large surface area. Mitochondria are the “power plants” of the cell, as they provide the energy necessary for all metabolic processes in the form of a universal biological fuel, adenosine triphosphate (ATP). The manufacture of ATP from the three basic materials—proteins, fats, and carbohydrates—takes place almost exclusively in the mitochondria, where the energy liberated as part of a process of oxidative combustion (mitochondrial respiratory sequence) is not dissipated as heat but is stored in the form of high-energy compounds (ATP).

ATP consists of three chemical substances linked to each other by high-energy bonds: a nitrogen-containing adenine, the sugar ribose, and three phosphate molecules (adenosine triphosphate). When one phosphate molecule is split off, energy is liberated and ATP becomes ADP (adenosine diphosphate), which, with added energy, can revert to adenosine triphosphate in the mitochondria.
From the mitochondria, ATP reaches the sites in the cell where energy is utilized. It is needed among other uses for the transport of materials through the cell membrane, for the synthesis of proteins and other cell components, and for muscle movement (contraction).