CELL
CELL
All living organisms are made of cells and cellproducts. This simple statement, called the cell theory,
was first proposed more than 150 years ago. You may think of a theory as a guess or hypothesis, and
sometimes this is so. A scientific theory, however, is actually the best explanation of all available evidence.All of the evidence science has gathered so far supports the validity of the cell theory.
Cells are the smallest living subunits of a multicellular organism such as a human being. A cell is a complex arrangement of the chemicals discussed in the previous chapter, is living, and carries out specific activities. Microorganisms, such as amoebas and bacteria,are single cells that function independently.Human cells, however, must work together and function interdependently. Homeostasis depends upon the contributions of all of the different kinds of cells.Human cells vary in size, shape, and function. Most human cells are so small they can only be seen with
the aid of a microscope and are measured in units called micrometers (formerly called microns)
Cells are very small—a typical cell is only about 0.1 mm in diameter. As a result, no one could actually examine the structure of a cell until effective microscopes were invented in the 17th century. In 1665, Robert Hooke inspected thin slices of cork and found that they consisted of millions of small, irregular units. In describing his observations, Hooke used the term cell because the many small, bare spaces he saw reminded him of the rooms,or cells, in a prison or monastery. Although Hooke saw only the outlines of the cells, and not the cells themselves, he stimulated
broad interest in the microscopic world and in the nature of cellular life. The research that he began more than 345 years ago has, over time, produced the cell theory in its current form. The basic concepts of this theory can be summarized as follows:
• Cells are the building blocks of all plants and animals.
• All cells come from the division of preexisting cells.
• Cells are the smallest units that perform all vital physiological functions.
• Each cell maintains homeostasis at the cellular level.
Homeostasis at the level of the tissue, organ, organ system,and organism reflects the combined and coordinated actions of many cells.The human body contains trillions of cells, and all our
activities—from running to thinking—result from the combined and coordinated responses of millions or even billions of cells. Many insights into human physiology arose from
studies of the functioning of individual cells. What we have learned over the last 60 years has given us a new understanding of cellular physiology and the mechanisms of homeostatic
control. Today, the study of cellular structure and function, or cytology, is part of the broader discipline of cell biology,which integrates aspects of biology, chemistry, and physics.
The human body contains two general classes of cells: sex cells and somatic cells. Sex cells (also called germ cells or reproductive cells) are either the sperm of males or the oocytes of
females. The fusion of a sperm and an oocyte at fertilization is the first step in the creation of a new individual.
CELL STRUCTURE
Despite their many differences, human cells have several similar structural features: a cell membrane, a nucleus, and cytoplasm and cell organelles. Red blood cells are an exception because they have no nuclei when mature. The cell membrane forms the outer boundary of the cell and surrounds the cytoplasm,organelles, and nucleus.
CELL MEMBRANE
Also called the plasma membrane, the cell membrane is made of phospholipids, cholesterol, and proteins. The phospholipids are diglycerides,and form a bilayer, or double layer, which
makes up most of the membrane. Phospholipids permit lipid-soluble materials to easily enter or leave
the cell by diffusion through the cell membrane. The presence of cholesterol decreases the fluidity of
the membrane, thus making it more stable. The proteins have several functions: Some form channels or pores to permit passage of materials such as water or ions; others are carrier enzymes or transporters that also help substances enter the cell. Still other proteins, with oligosaccharides on their outer surface, are antigens,markers that identify the cells of an individual as“self.” Yet another group of proteins serves as receptor sites for hormones. Many hormones bring about their specific effects by first bonding to a particular receptor on the cell membrane, a receptor with the
proper shape. This bonding, or fit, then triggers chemical reactions within the cell membrane or the
interior of the cell (see Box 10–3 for an illustration involving the hormone insulin).
Many receptors for other molecules are also part of cell membranes. These molecules are part of the
chemical communication networks our cells have. An unavoidable consequence of having so many receptors for chemical communication is that some pathogens have evolved shapes to match certain receptors. For example, HIV, the virus that causes AIDS, just happens to fit a particular surface receptor on our white blood cells. When the virus fits in, the receptor becomes a gateway into the cell, which begins the takeover of the cell by the virus.Most often, however, the cell membrane is a beneficial structure. Although the cell membrane is the outer boundary of the cell, it should already be apparent to you that it is not a static or wall-like boundary,but rather an active, dynamic one. It is much more like a line of tollbooths than a concrete barrier. The cell membrane is selectively permeable; that is, certain substances are permitted to pass through and others
48 Cells
are not. These mechanisms of cellular transport will be covered later in this chapter. The cell membrane is of particular importance for muscle cells and nerve cells because it carries electrical impulses.
NUCLEUS
With the exception of mature red blood cells, all human cells have a nucleus. The nucleus is within the cytoplasm and is bounded by a double-layered nuclear membrane with many pores. It contains one or more nucleoli and the chromosomes of the cell .A nucleolus is a small sphere made of DNA, RNA, and protein. The nucleoli form a type of RNA calledribosomal RNA, which becomes part of ribosomes (a cell organelle) and is involved in protein synthesis.The nucleus is the control center of the cell because it contains the chromosomes. Even with amicroscope, the 46 chromosomes of a human cell are usually not visible; they are long threads called chromatin.Before a cell divides, however, the chromatin coils extensively into visible chromosomes. Chromosomes
are made of DNA and protein. Some chromosomalproteins provide the structural framework for
the coiling of the chromatin into chromosomes so that cell division can take place. Other chromosomal proteins help regulate the activity of the DNA.Remember from our earlier discussion that the DNA is the genetic code for the characteristics and activities of the cell. Although the DNA in the nucleus of each cell contains all of the genetic information for all human traits, only a small number of genes (a gene is the genetic code for one protein) are actually active
CYTOPLASM AND CELL ORGANELLES
Cytoplasm is a watery solution of minerals, gases,organic molecules, and cell organelles that is found
between the cell membrane and the nucleus. Cytosol is the water portion of cytoplasm, and many chemical reactions take place within it. Cell organelles are intracellular structures, often bounded by their ownQUESTION: How do cilia differ in structure from microvilli?membranes, that have specific functions in cellular metabolism..The endoplasmic reticulum (ER) is an extensive network of membranous tubules that extend from the nuclear membrane to the cell membrane. RoughER has numerous ribosomes on its surface, whereas smooth ER has no ribosomes. As a network of interconnected tunnels, the ER is a passageway for the transport of the materials necessary for cell function within the cell. These include proteins synthesized by the ribosomes on the rough ER, and lipids synthesized by the smooth ER. Ribosomes are very small structures made of protein
and ribosomal RNA. Some are found on the surface of rough ER, while others float freely within the
cytoplasm. Ribosomes are the site of protein synthesis.The proteins produced may be structural proteins such as collagen in the skin, enzymes, or hormones such as insulin that regulate cellular processes. These proteins may function within the cell or be secreted
from the cell to be used elsewhere in the body.Our protein molecules are subject to damage, and
some cellular proteins, especially regulatory proteins,may be needed just for a very short time. All such proteins must be destroyed, and this is the function of proteasomes. A proteasome is a barrel-shaped organelle made of enzymes that cut protein molecules apart (protease enzymes). Proteins that are to be destroyed,that is, those no longer needed or those that are damaged or misfolded, are tagged by a protein called ubiquitin (sort of a cellular mop or broom) and carried into a proteasome. The protein is snipped into peptides or amino acids, which may be used again for protein synthesis on ribosomes. Proteasomes are particularly important during cell division and during
embryonic development, when great changes are taking place very rapidly as cells become specialized. Many of our cells have secretory functions, that is,they produce specific products to be used elsewhere in tissues. Secretion is one task of the Golgi apparatus,a series of flat, membranous sacs, somewhat like a stack of saucers. Carbohydrates are synthesized within the Golgi apparatus, and are packaged, along with other materials, for secretion from the cell.Proteins from the ribosomes or lipids from the smooth endoplasmic reticulum may also be secreted in this way. To secrete a substance, small sacs of the Golgi membrane break off and fuse with the cell membrane, releasing the substance to the exterior of the cell. This is exocytosis, exo meaning “to go out”
of the cell. Mitochondria are oval or spherical organelles bounded by a double membrane. The inner membrane has folds called cristae. Within the mitochondria,the aerobic (oxygen-requiring) reactions of cell respiration take place.
Therefore, mitochondria are the site of ATP (and hence energy) production. Cells that require large amounts of ATP, such as muscle cells, have many mitochondria to meet their need for energy. Mitochondria contain their own genes in a single DNA molecule and duplicate themselves when a cell divides. An individual’s mitochondrial DNA (mDNA) is of maternal origin, that is, from the mitochondria that were present in the ovum, or egg cell,
which was then fertilized by a sperm cell. The mitochondria of the sperm cell usually do not enter the
ovum during fertilization, because they are not found in the head of the sperm with the chromosomes
Lysosomes are single-membrane structures that contain digestive enzymes. When certain white blood
cells engulf bacteria, the bacteria are digested and destroyed by these lysosomal enzymes. Worn-out cell parts and dead cells are also digested by these enzymes.This is a beneficial process, and is necessary before tissue repair can begin. But it does have a disadvantage in that lysosomal digestion contributes to inflammation in damaged tissues. An excess of inflammation can start a vicious cycle, actually a positive feedback mechanism,that results in extensive tissue damage.Many of our cells are capable of dividing, or reproducing,themselves. Centrioles are a pair of rodshaped
structures perpendicular to one another,located just outside the nucleus. Their function is to
organize the spindle fibers during cell division. The spindle fibers are contracting proteins that pull the two sets of chromosomes apart, toward the ends of the original cell as it divides into two new cells. Each new cell then has a full set of chromosomes.Cilia and flagella are mobile thread-like projections through the cell membrane; each is anchored by a basal body just within the membrane. Cilia are usually shorter than flagella, and an individual cell has many of them on its free surface. The cilia of a cell beat in unison and sweep materials across the cell surface.Cells lining the fallopian tubes, for example, have cilia to sweep the egg cell toward the uterus. The only human cell with a flagellum is the sperm cell. The flagellum provides motility, or movement, for the sperm
cell.Microvilli are folds of the cell membrane on the free surface of a cell. These folds greatly increase the surface area of the membrane, and are part of the cells lining organs that absorb materials. The small intestine,for example, requires a large surface area for the absorption of nutrients, and many of its lining cells have microvilli. Some cells of the kidney tubules also
have microvilli that provide for the efficient reabsorption of useful materials back to the blood.
REFERENCES
All living organisms are made of cells and cellproducts. This simple statement, called the cell theory,
was first proposed more than 150 years ago. You may think of a theory as a guess or hypothesis, and
sometimes this is so. A scientific theory, however, is actually the best explanation of all available evidence.All of the evidence science has gathered so far supports the validity of the cell theory.
Cells are the smallest living subunits of a multicellular organism such as a human being. A cell is a complex arrangement of the chemicals discussed in the previous chapter, is living, and carries out specific activities. Microorganisms, such as amoebas and bacteria,are single cells that function independently.Human cells, however, must work together and function interdependently. Homeostasis depends upon the contributions of all of the different kinds of cells.Human cells vary in size, shape, and function. Most human cells are so small they can only be seen with
the aid of a microscope and are measured in units called micrometers (formerly called microns)
Cells are very small—a typical cell is only about 0.1 mm in diameter. As a result, no one could actually examine the structure of a cell until effective microscopes were invented in the 17th century. In 1665, Robert Hooke inspected thin slices of cork and found that they consisted of millions of small, irregular units. In describing his observations, Hooke used the term cell because the many small, bare spaces he saw reminded him of the rooms,or cells, in a prison or monastery. Although Hooke saw only the outlines of the cells, and not the cells themselves, he stimulated
broad interest in the microscopic world and in the nature of cellular life. The research that he began more than 345 years ago has, over time, produced the cell theory in its current form. The basic concepts of this theory can be summarized as follows:
• Cells are the building blocks of all plants and animals.
• All cells come from the division of preexisting cells.
• Cells are the smallest units that perform all vital physiological functions.
• Each cell maintains homeostasis at the cellular level.
Homeostasis at the level of the tissue, organ, organ system,and organism reflects the combined and coordinated actions of many cells.The human body contains trillions of cells, and all our
activities—from running to thinking—result from the combined and coordinated responses of millions or even billions of cells. Many insights into human physiology arose from
studies of the functioning of individual cells. What we have learned over the last 60 years has given us a new understanding of cellular physiology and the mechanisms of homeostatic
control. Today, the study of cellular structure and function, or cytology, is part of the broader discipline of cell biology,which integrates aspects of biology, chemistry, and physics.
The human body contains two general classes of cells: sex cells and somatic cells. Sex cells (also called germ cells or reproductive cells) are either the sperm of males or the oocytes of
females. The fusion of a sperm and an oocyte at fertilization is the first step in the creation of a new individual.
CELL STRUCTURE
Despite their many differences, human cells have several similar structural features: a cell membrane, a nucleus, and cytoplasm and cell organelles. Red blood cells are an exception because they have no nuclei when mature. The cell membrane forms the outer boundary of the cell and surrounds the cytoplasm,organelles, and nucleus.
CELL MEMBRANE
Also called the plasma membrane, the cell membrane is made of phospholipids, cholesterol, and proteins. The phospholipids are diglycerides,and form a bilayer, or double layer, which
makes up most of the membrane. Phospholipids permit lipid-soluble materials to easily enter or leave
the cell by diffusion through the cell membrane. The presence of cholesterol decreases the fluidity of
the membrane, thus making it more stable. The proteins have several functions: Some form channels or pores to permit passage of materials such as water or ions; others are carrier enzymes or transporters that also help substances enter the cell. Still other proteins, with oligosaccharides on their outer surface, are antigens,markers that identify the cells of an individual as“self.” Yet another group of proteins serves as receptor sites for hormones. Many hormones bring about their specific effects by first bonding to a particular receptor on the cell membrane, a receptor with the
proper shape. This bonding, or fit, then triggers chemical reactions within the cell membrane or the
interior of the cell (see Box 10–3 for an illustration involving the hormone insulin).
Many receptors for other molecules are also part of cell membranes. These molecules are part of the
chemical communication networks our cells have. An unavoidable consequence of having so many receptors for chemical communication is that some pathogens have evolved shapes to match certain receptors. For example, HIV, the virus that causes AIDS, just happens to fit a particular surface receptor on our white blood cells. When the virus fits in, the receptor becomes a gateway into the cell, which begins the takeover of the cell by the virus.Most often, however, the cell membrane is a beneficial structure. Although the cell membrane is the outer boundary of the cell, it should already be apparent to you that it is not a static or wall-like boundary,but rather an active, dynamic one. It is much more like a line of tollbooths than a concrete barrier. The cell membrane is selectively permeable; that is, certain substances are permitted to pass through and others
48 Cells
are not. These mechanisms of cellular transport will be covered later in this chapter. The cell membrane is of particular importance for muscle cells and nerve cells because it carries electrical impulses.
NUCLEUS
With the exception of mature red blood cells, all human cells have a nucleus. The nucleus is within the cytoplasm and is bounded by a double-layered nuclear membrane with many pores. It contains one or more nucleoli and the chromosomes of the cell .A nucleolus is a small sphere made of DNA, RNA, and protein. The nucleoli form a type of RNA calledribosomal RNA, which becomes part of ribosomes (a cell organelle) and is involved in protein synthesis.The nucleus is the control center of the cell because it contains the chromosomes. Even with amicroscope, the 46 chromosomes of a human cell are usually not visible; they are long threads called chromatin.Before a cell divides, however, the chromatin coils extensively into visible chromosomes. Chromosomes
are made of DNA and protein. Some chromosomalproteins provide the structural framework for
the coiling of the chromatin into chromosomes so that cell division can take place. Other chromosomal proteins help regulate the activity of the DNA.Remember from our earlier discussion that the DNA is the genetic code for the characteristics and activities of the cell. Although the DNA in the nucleus of each cell contains all of the genetic information for all human traits, only a small number of genes (a gene is the genetic code for one protein) are actually active
CYTOPLASM AND CELL ORGANELLES
Cytoplasm is a watery solution of minerals, gases,organic molecules, and cell organelles that is found
between the cell membrane and the nucleus. Cytosol is the water portion of cytoplasm, and many chemical reactions take place within it. Cell organelles are intracellular structures, often bounded by their ownQUESTION: How do cilia differ in structure from microvilli?membranes, that have specific functions in cellular metabolism..The endoplasmic reticulum (ER) is an extensive network of membranous tubules that extend from the nuclear membrane to the cell membrane. RoughER has numerous ribosomes on its surface, whereas smooth ER has no ribosomes. As a network of interconnected tunnels, the ER is a passageway for the transport of the materials necessary for cell function within the cell. These include proteins synthesized by the ribosomes on the rough ER, and lipids synthesized by the smooth ER. Ribosomes are very small structures made of protein
and ribosomal RNA. Some are found on the surface of rough ER, while others float freely within the
cytoplasm. Ribosomes are the site of protein synthesis.The proteins produced may be structural proteins such as collagen in the skin, enzymes, or hormones such as insulin that regulate cellular processes. These proteins may function within the cell or be secreted
from the cell to be used elsewhere in the body.Our protein molecules are subject to damage, and
some cellular proteins, especially regulatory proteins,may be needed just for a very short time. All such proteins must be destroyed, and this is the function of proteasomes. A proteasome is a barrel-shaped organelle made of enzymes that cut protein molecules apart (protease enzymes). Proteins that are to be destroyed,that is, those no longer needed or those that are damaged or misfolded, are tagged by a protein called ubiquitin (sort of a cellular mop or broom) and carried into a proteasome. The protein is snipped into peptides or amino acids, which may be used again for protein synthesis on ribosomes. Proteasomes are particularly important during cell division and during
embryonic development, when great changes are taking place very rapidly as cells become specialized. Many of our cells have secretory functions, that is,they produce specific products to be used elsewhere in tissues. Secretion is one task of the Golgi apparatus,a series of flat, membranous sacs, somewhat like a stack of saucers. Carbohydrates are synthesized within the Golgi apparatus, and are packaged, along with other materials, for secretion from the cell.Proteins from the ribosomes or lipids from the smooth endoplasmic reticulum may also be secreted in this way. To secrete a substance, small sacs of the Golgi membrane break off and fuse with the cell membrane, releasing the substance to the exterior of the cell. This is exocytosis, exo meaning “to go out”
of the cell. Mitochondria are oval or spherical organelles bounded by a double membrane. The inner membrane has folds called cristae. Within the mitochondria,the aerobic (oxygen-requiring) reactions of cell respiration take place.
Therefore, mitochondria are the site of ATP (and hence energy) production. Cells that require large amounts of ATP, such as muscle cells, have many mitochondria to meet their need for energy. Mitochondria contain their own genes in a single DNA molecule and duplicate themselves when a cell divides. An individual’s mitochondrial DNA (mDNA) is of maternal origin, that is, from the mitochondria that were present in the ovum, or egg cell,
which was then fertilized by a sperm cell. The mitochondria of the sperm cell usually do not enter the
ovum during fertilization, because they are not found in the head of the sperm with the chromosomes
Lysosomes are single-membrane structures that contain digestive enzymes. When certain white blood
cells engulf bacteria, the bacteria are digested and destroyed by these lysosomal enzymes. Worn-out cell parts and dead cells are also digested by these enzymes.This is a beneficial process, and is necessary before tissue repair can begin. But it does have a disadvantage in that lysosomal digestion contributes to inflammation in damaged tissues. An excess of inflammation can start a vicious cycle, actually a positive feedback mechanism,that results in extensive tissue damage.Many of our cells are capable of dividing, or reproducing,themselves. Centrioles are a pair of rodshaped
structures perpendicular to one another,located just outside the nucleus. Their function is to
organize the spindle fibers during cell division. The spindle fibers are contracting proteins that pull the two sets of chromosomes apart, toward the ends of the original cell as it divides into two new cells. Each new cell then has a full set of chromosomes.Cilia and flagella are mobile thread-like projections through the cell membrane; each is anchored by a basal body just within the membrane. Cilia are usually shorter than flagella, and an individual cell has many of them on its free surface. The cilia of a cell beat in unison and sweep materials across the cell surface.Cells lining the fallopian tubes, for example, have cilia to sweep the egg cell toward the uterus. The only human cell with a flagellum is the sperm cell. The flagellum provides motility, or movement, for the sperm
cell.Microvilli are folds of the cell membrane on the free surface of a cell. These folds greatly increase the surface area of the membrane, and are part of the cells lining organs that absorb materials. The small intestine,for example, requires a large surface area for the absorption of nutrients, and many of its lining cells have microvilli. Some cells of the kidney tubules also
have microvilli that provide for the efficient reabsorption of useful materials back to the blood.
REFERENCES
· Raven, Peter H.,
Ray F. Evert, & Susan E. Eichhorn, 2005. Biology of Plants, 7th edition. (New York: W. H. Freeman and Company).
.Becker, Kenneth M. (February 1973). "A Comparison of Angiosperm
Classification Systems
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