The cytoplasm/endoplasmic reticulum
The clear fluid portion of the cytoplasm in which many particles are dispersed is called cytosol; this contains mainly dissolved proteins, electrolytes, and glucose and minute quantities of lipid compounds. Dispersed in the cytoplasm are neutral fat globules, glycogen granules, ribosomes, secretory granules and five especially important organelles; the endoplasmic reticulum, the Golgi apparatus , the mitochondria , the lysosomes and the peroxisomes.
THE ENDOPLASMIC RETICULUM
Figure illustrates in the cytoplasm a network of tubular and flat vesicular structures called the endoplasmic reticulum. The tubules and vesicles all interconnect with each other. Their walls are constructed of lipid bilayer membranes containing large amounts of proteins similar to the cell membrane. The total surface area of this structure in some cells ---the liver cells, for instance can be as much as 30 to 40 times as great as the cell membrane area. The detailed structure of a small portion of endoplasmic reticulum is illustrated in the figure
The space inside the tubules and vesicles is filled with endoplasmic matrix, a fluid medium that is different from fluid outside the endoplasmic reticulum. Substances formed in some parts of the cell enter the space of the endoplasmic reticulum and are then conducted to other parts of the cell. Also the vast surface of the reticulum plus multiple enzyme systems attached to its membranes provides the machinery for a major share of the metabolic functions of the cell.
RIBOSOME'S/THE GRANULAR ENDOPLASMIC RETICULUM
Attached to the outer surface of many parts of the endoplasmic reticulum are large numbers of small granules particles called ribosomes. Where these are present the reticulum is called the granular endoplasmic reticulum. The ribosomes are composed of a mixture of ribonucleic acid (RNA) protein in the cells as discussed later.
THE AGRANULAR ENDOPLASMIC RETICULUM
Part of the endoplasmic reticulum has no attached ribosomes. This part is called the agranular or smooth endoplasmic reticulum. The agranular reticulum functions in the synthesis of lipid substances and in many other enzymatic process of the cell.
GOLGI APPARATUS
The Golgi apparatus illustrated in the figure is closely related to the endoplasmic reticulum. It has membranes similar to those of the agranular endoplasmic reticulum. It is usually composed of four or more stacked layers of thin, flat enclosed vesicles lying adjacent to one pole of the nucleus. This apparatus plays a major role in synthesizing substances that are to be secreted or to be used elsewhere in the cell. Therefore it is very prominent in secretory cells. The Golgi apparatus functions in association with the endoplasmic reticulum. As illustrated in figure above small "transport vesicles" also called endoplasmic reticulum vesicles or simply ER vesicles, continually pinch off from the endoplasmic reticulum and shortly thereafter fuse with the Golgi apparatus. In this way substances are transported from the endoplasmic reticulum to the Golgi apparatus. The transported substances are then processed in the Golgi apparatus to from lysosomes secretory vesicles or other cytoplasmic components.
THE LYSOSOMES
Lysosomes are vesicular organelles formed by the Golgi apparatus that then become dispersed throughout the cytoplasm. The lysosomes provide an intracellular digestive system that allows the cell to digest intracellular substances, including especially ingested food particles, damaged cellular structures or bacteria that have entered the cell.
The lysosomes are quite different from one cell to another, but it is usually 250 to 750 nanometers in diameter. It is surrounded by a typical lipid bilayer membranes and is filled with large numbers of small granules, which are protein aggregates of hydrolytic (digestive) enzymes. A hydrolytic enzyme is capable of splitting an organic compound into two or more parts by combining hydrogen from a water molecule with part of the compound and combining the hydroxyl portion of the water molecule with the other part of the molecule. For instance protein is hydrolyzed to form amino acids and glycogen is hydrolyzed to form glucose. About 40 different acid hydrolase enzymes have been found in lysosomes and the principal substances that they digest are proteins, carbohydrates, lipids and derivatives of these such as nucleic acids and mucopolysaccharides.
Ordinarily the membrane surroundings the lysosome prevents the enclosed hydrolytic enzymes from coming in contact with other substances in the cell and therefore impedes their digestive actions. However many different conditions of the cell will break the membranes of some of the lysosomes allowing release of the enzymes. Theses enzymes then split the organic substances with which they come in contact into small, highly diffusible substances, such as amino acids and glucose. Some of the more specific functions of lysosomes are discussed later on.
MITOCHONDRIA
The mitochondria called the "powerhouse" of the cell. With out them the cells would be unable to extract signification amounts of energy from the nutrients and oxygen, and as a consequence essentially all cellular functions would cease. Mitochondria are present in essentially all portions of the cytoplasm but the total number per cell varies from less than a hundred up to several thousand, depending on the amount of energy required by each cell.
The basic structure of the mitochondria it to be composed mainly of two lipid bilayer –protein membranes; an outer membrane and an inner membrane. Many infoldings of the inner membrane form shelves onto which oxidative enzymes are attached. In addition the inner cavity of the mitochondrion is filled with a matrix containing large quantities of dissolved enzymes that are necessary for extracting energy from nutrients. These enzymes operate in association with the oxidation enzymes on the shelves to cause oxidation of the nutrients, thereby forming carbon dioxide and water. The liberated energy is used to synthesize a high-energy substance called adenosine triphosphate (ATP). ATP is then transported out of the mitochondrion and it diffuses throughout the cell to release its energy whenever it is needed for performing cellular functions.
Mitochondria are self-replicative which means that one mitochondrion can form a second one, a third one, and so on whenever there is need in the cell for increased amounts of ATP.
OTHER CYTOPLASM STRUCTURES AND ORGANELLES
Throughout this text we learn that there are literally hundreds of different types of cells and each of these has special unique structures. For instance some of the cells are rigid. This is usually achieved by the presence in the cytoplasmic compartment of large numbers of filamentous or tubular structures composed of Fibrillar proteins. Also some of the tubular structures called microtubules can actually transport substance from one part of a cell to an other thus providing an intracellular circulatory system. It is structures composed of microtubules that from the rigid structures of (a) cilia that protrude from certain cell surface and move fluids along the surfaces (b) the tail of the sperm that beats rhythmically and propels the sperm through the genital tract of the female and (c) the mitotic apparatus that plays an essential role in cell division.
Fibrillar proteins also form the contractile apparatus of muscle cells. Finally one of the important functions of many cells is to secrete special substances such as digestive enzymes, mucus and so forth. Almost all of these substances are formed by the endoplasmic reticulum----Golgi apparatus system and are released from the Golgi apparatus into the cytoplasm inside storage vesicles called secretory vesicles or secretory granules. Then after a period of storage in the cell these are expelled through the cell membrane to be used elsewhere in the body as we shall discuss later.( endoplasmic reticulum)
THE ENDOPLASMIC RETICULUM
Figure illustrates in the cytoplasm a network of tubular and flat vesicular structures called the endoplasmic reticulum. The tubules and vesicles all interconnect with each other. Their walls are constructed of lipid bilayer membranes containing large amounts of proteins similar to the cell membrane. The total surface area of this structure in some cells ---the liver cells, for instance can be as much as 30 to 40 times as great as the cell membrane area. The detailed structure of a small portion of endoplasmic reticulum is illustrated in the figure
The space inside the tubules and vesicles is filled with endoplasmic matrix, a fluid medium that is different from fluid outside the endoplasmic reticulum. Substances formed in some parts of the cell enter the space of the endoplasmic reticulum and are then conducted to other parts of the cell. Also the vast surface of the reticulum plus multiple enzyme systems attached to its membranes provides the machinery for a major share of the metabolic functions of the cell.
RIBOSOME'S/THE GRANULAR ENDOPLASMIC RETICULUM
Attached to the outer surface of many parts of the endoplasmic reticulum are large numbers of small granules particles called ribosomes. Where these are present the reticulum is called the granular endoplasmic reticulum. The ribosomes are composed of a mixture of ribonucleic acid (RNA) protein in the cells as discussed later.
THE AGRANULAR ENDOPLASMIC RETICULUM
Part of the endoplasmic reticulum has no attached ribosomes. This part is called the agranular or smooth endoplasmic reticulum. The agranular reticulum functions in the synthesis of lipid substances and in many other enzymatic process of the cell.
GOLGI APPARATUS
The Golgi apparatus illustrated in the figure is closely related to the endoplasmic reticulum. It has membranes similar to those of the agranular endoplasmic reticulum. It is usually composed of four or more stacked layers of thin, flat enclosed vesicles lying adjacent to one pole of the nucleus. This apparatus plays a major role in synthesizing substances that are to be secreted or to be used elsewhere in the cell. Therefore it is very prominent in secretory cells. The Golgi apparatus functions in association with the endoplasmic reticulum. As illustrated in figure above small "transport vesicles" also called endoplasmic reticulum vesicles or simply ER vesicles, continually pinch off from the endoplasmic reticulum and shortly thereafter fuse with the Golgi apparatus. In this way substances are transported from the endoplasmic reticulum to the Golgi apparatus. The transported substances are then processed in the Golgi apparatus to from lysosomes secretory vesicles or other cytoplasmic components.
THE LYSOSOMES
Lysosomes are vesicular organelles formed by the Golgi apparatus that then become dispersed throughout the cytoplasm. The lysosomes provide an intracellular digestive system that allows the cell to digest intracellular substances, including especially ingested food particles, damaged cellular structures or bacteria that have entered the cell.
The lysosomes are quite different from one cell to another, but it is usually 250 to 750 nanometers in diameter. It is surrounded by a typical lipid bilayer membranes and is filled with large numbers of small granules, which are protein aggregates of hydrolytic (digestive) enzymes. A hydrolytic enzyme is capable of splitting an organic compound into two or more parts by combining hydrogen from a water molecule with part of the compound and combining the hydroxyl portion of the water molecule with the other part of the molecule. For instance protein is hydrolyzed to form amino acids and glycogen is hydrolyzed to form glucose. About 40 different acid hydrolase enzymes have been found in lysosomes and the principal substances that they digest are proteins, carbohydrates, lipids and derivatives of these such as nucleic acids and mucopolysaccharides.
Ordinarily the membrane surroundings the lysosome prevents the enclosed hydrolytic enzymes from coming in contact with other substances in the cell and therefore impedes their digestive actions. However many different conditions of the cell will break the membranes of some of the lysosomes allowing release of the enzymes. Theses enzymes then split the organic substances with which they come in contact into small, highly diffusible substances, such as amino acids and glucose. Some of the more specific functions of lysosomes are discussed later on.
MITOCHONDRIA
The mitochondria called the "powerhouse" of the cell. With out them the cells would be unable to extract signification amounts of energy from the nutrients and oxygen, and as a consequence essentially all cellular functions would cease. Mitochondria are present in essentially all portions of the cytoplasm but the total number per cell varies from less than a hundred up to several thousand, depending on the amount of energy required by each cell.
The basic structure of the mitochondria it to be composed mainly of two lipid bilayer –protein membranes; an outer membrane and an inner membrane. Many infoldings of the inner membrane form shelves onto which oxidative enzymes are attached. In addition the inner cavity of the mitochondrion is filled with a matrix containing large quantities of dissolved enzymes that are necessary for extracting energy from nutrients. These enzymes operate in association with the oxidation enzymes on the shelves to cause oxidation of the nutrients, thereby forming carbon dioxide and water. The liberated energy is used to synthesize a high-energy substance called adenosine triphosphate (ATP). ATP is then transported out of the mitochondrion and it diffuses throughout the cell to release its energy whenever it is needed for performing cellular functions.
Mitochondria are self-replicative which means that one mitochondrion can form a second one, a third one, and so on whenever there is need in the cell for increased amounts of ATP.
OTHER CYTOPLASM STRUCTURES AND ORGANELLES
Throughout this text we learn that there are literally hundreds of different types of cells and each of these has special unique structures. For instance some of the cells are rigid. This is usually achieved by the presence in the cytoplasmic compartment of large numbers of filamentous or tubular structures composed of Fibrillar proteins. Also some of the tubular structures called microtubules can actually transport substance from one part of a cell to an other thus providing an intracellular circulatory system. It is structures composed of microtubules that from the rigid structures of (a) cilia that protrude from certain cell surface and move fluids along the surfaces (b) the tail of the sperm that beats rhythmically and propels the sperm through the genital tract of the female and (c) the mitotic apparatus that plays an essential role in cell division.
Fibrillar proteins also form the contractile apparatus of muscle cells. Finally one of the important functions of many cells is to secrete special substances such as digestive enzymes, mucus and so forth. Almost all of these substances are formed by the endoplasmic reticulum----Golgi apparatus system and are released from the Golgi apparatus into the cytoplasm inside storage vesicles called secretory vesicles or secretory granules. Then after a period of storage in the cell these are expelled through the cell membrane to be used elsewhere in the body as we shall discuss later.( endoplasmic reticulum)
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