THE CELL, ITS STRUCTURE AND FUNCTIONS
1. What separates the cell cytoplasm from interstitial fluid?
2. What are organelles?
3. Define the nuclear membrane.
4. What does chromatin become in dividing cells?
5.
Differentiate between granular and agranular endoplasmic reticula and their associated functions.6. Are the vesicular structures of the endoplasmic reticulum separate or interconnected?
7. What is the function of the Golgi apparatus?
8. What organelle is the site of the citric acid cycle?
9. What is the principal substance of lysosomes?
10. What cellular function are the centrioles associated with? What.is their location within the cell known as?
The number of cells in an animal is in the trillions and for the human has been estimated to be about 100 trillion. Each of these cells had their start beginning with fertilization of an oocyte. The appearance of cells varies with the organ of which they are a part and will be shown and described when encountered. Cells are highly organized chemical systems and share many features that are shown schematically in Figure 1-1. The basic components of a cell are the plasma membrane (cell membrane) that bounds the cell and gives it limits; the cytoplasm, which is the homogenous ground substance that forms the background in which the formed elements are suspended; and the nucleus. The nucleus is separated from the cytoplasm by a nuclear membrane and the cytoplasm is separated from the surrounding fluids (interstitial fluid) by a cell membrane. The cell membrane is usually pliable and is composed of phospholipids and proteins. The phospholipid molecules occur in two layers. The protein molecules may be associated with either the outer or inner layer and may penetrate completely or incompletely (see Chapter 2).
■ FIGURE 1-1 Schematic drawing of the general organization of a cell.
(From Eurell JA, Frappier BL. Dellmann’s Textbook of Veterinary Histology. 6th edn. Ames, IA: Blackwell Publishing, 2006.) Because of cell specialization, no cell can be called typical. The cytoplasm is the location of diverse metabolic activities and is filled with both minute and large dispersed particles and organelles.The Organelles
The organelles are highly organized physical structures represented in Figure 1-1 and, in addition to the cell membrane, consist of the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes, and centrioles. These structures assist the cytoplasm with its metabolic activities by receiving materials into the cell, synthesizing new substances, generating energy, packaging materials for transport to other parts of the cell or to the circulation, excretion of waste products, and reproduction.
Nucleus
The nucleus is the control center of the cell, controlling its chemical reactions and reproduction. It contains large quantities of DNA. Nuclear components consist of a nuclear membrane, one or more nucleoli, and chromatin, all bathed in nuclear sap (nucleoplasm). The nuclear membrane (also called nuclear envelope) consists of two membranes wherein the outer membrane is continuous with the endoplasmic reticulum, and the space between the two nuclear membranes, the lumen, is also continuous with the lumen of the endoplasmic reticulum. Numerous nuclear pores penetrate both layers. These pores permit exchange between the nucleoplasm of the nucleus and the cytoplasm of the cell, including movement of RNA synthesized in the nucleus, out into the cytoplasm. The nucleolus does not have a limiting membrane and is a structure that contains large amounts of RNA and proteins that are found in ribosomes. Chromatin appears as dark-staining particles throughout the nucleoplasm in the nondividing cell. In the dividing cell, the chromatin organizes into chromosomes.
Endoplasmic Reticulum
The endoplasmic reticulum (ER) is a network of tubular and flat vesicular (small thin-walled cavity) structures in the cytoplasm that all interconnect with one another.
The fluid within the lumen of the ER is continuous with the fluid in the nuclear envelope and is different from the fluid in the cytoplasm. A large number of small granular particles called ribosomes are attached to the outer surfaces of many parts of the ER. Where these are present they are called the granular or rough ER, and where they are not present they are called the agranular or smooth ER. Ribosomes are composed of a mixture of RNA and proteins and function in the synthesis of proteins. The agranular ER functions in the synthesis of lipid substances and other enzymatic processes of the cell.Golgi Apparatus
The Golgi apparatus is closely related to the ER. It is prominent in secretory cells, being well developed in cells secreting enzymes and hormones. It packages materials made in the cell and transforms them into units that are then distributed outside the cell. The packaging begins when vesicles pinch off from the ER and then fuse with the Golgi apparatus. The vesicular substances are then processed in the Golgi apparatus to form lysosomes or other secretory vesicles, which become surrounded by a membrane. They are then released from the Golgi apparatus for storage or use in the cell or are transported to the cell membrane, where they are released into the extracellular fluid as a secretion.
Mitochondria
Mitochondria are the “powerhouses” of the cell because they are the principal sites for energy production. The number in a cell depends on the amount of energy required, and mitochondria increase in number when cellular needs increase. A mitochondrion is composed of an outer and an inner membrane. The inner membrane has infoldings that provide shelves for the attachment of oxidative phosphorylation enzymes (enzymes for production of energy). The inner cavity consists of a matrix (supporting substance) that contains enzymes and coenzymes (cofactors) required for extracting energy from nutrients. The matrix is the site of the citric acid cycle (also known as the tricarboxylic acid cycle and Krebs cycle).
Lysosomes
The vesicular organelles called lysosomes are formed by the Golgi apparatus and then become dispersed throughout the cytoplasm. Because lysosomes contain digestive enzymes, their presence in the cytoplasm provides an intracellular digestive system allowing digestion of damaged cellular structures, food particles ingested by the cell, and bacterial cells.
Centrosome
The centrosome is located in the cytoplasm, close to the nucleus, and contains two centrioles. The centrioles are usually oriented at right angles to each other and each consists of nine groups of three microtubules arranged in a circle. During cell division the centrosome serves as a spindle pole and helps to organize the microtubules.
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