The cell is the fundamental structural unit of all living organisms. Some cells are complete organisms, such as the unicellular bacteria and protozoa; others, such as nerve, liver, and muscle cells, are specialized components of multi-cellular organisms. Cells range in size from the smallest bacteria-like mycoplasmas, which are 0. 1 micrometer in diameter, to the egg yolks of ostriches, which are about 8 cm (about 3 in) in diameter.
Although they may differ widely in appearance and function, all cells have a surrounding membrane and an internal, water-rich substance called the cytoplasm, the composition of which differs significantly from the external environment of the cell. Within the cell is genetic material, deoxyribonucleic acid (DNA), containing coded instructions for the behavior and reproduction of the cell and also the chemical machinery for the translation of these instructions into the manufacture of proteins.
Viruses are not considered cells because they lack this translation machinery; they must parasitize cells in order to translate their own genetic code and reproduce themselves. Cells are of two distinctly different types, prokaryotes and eukaryotes; thus, the living world is divided into two broad categories. The DNA of prokaryotes is a single molecule in direct contact with the cell cytoplasm, whereas the DNA of eukaryotes is much greater in amount and diversity and is contained within a nucleus separated from the cell cytoplasm by a membranous nuclear envelope.
Many eukaryotic cells are further divided into compartments by internal membranes in addition to the nuclear envelope, whereas prokaryotic cells never contain completely internal membranes. The prokaryotes include the mycoplasmas, bacteria, and blue-green algae. The eukaryotes comprise all plant and animal cells. In general, plant cells differ from animal cells in that they have a rigid cell wall exterior to the plasma membrane; a large vacuole, or fluid-filled pouch; and chloroplasts that convert light energy to chemical energy for the synthesis of glucose.
Structure and Function Cells are composed primarily of oxygen, hydrogen, carbon, and nitrogen, the elements that make up the majority of organic compounds. The most important organic compounds in a cell are proteins, nucleic acids, lipids, and polysaccharides (carbohydrates). The \”solid\” structures of the cell are complex combinations of these large molecules. Water makes up 60 to 65 percent of the cell, because water is a favorable environment for biochemical reactions.
All cells are dynamic at some stage of their life cycle, in the sense that they use energy to perform a variety of cell functions: movement, growth, maintenance and repair of cell structure, reproduction of the cell, and manufacture of specialized cell products such as enzymes and hormones. These functions are also the result of interactions of organic molecules. Plasma Membrane The plasma membrane, a continuous double layer of phospholipid molecules 75 to 100 angstroms thick, constitutes the boundary between the cell and its external environment.
In addition to lipids, the plasma membrane has protein components (polypeptides) that are associated with either the outer or inner surfaces of its layers or are buried within them. The structure as a whole is selectively permeable, or semipermeable; that is, it permits the exchange of water and selected atoms and molecules between the cell exterior and interior. This is vital to the cell because while the plasma membrane helps maintain high local concentrations of organic molecules within the cell, it also allows interaction between the cell and its external environment.
The plasma membrane mediates such interactions in various ways. The exchange of mineral ions and small nutrient molecules is controlled by plasma membrane proteins that act as pumps, carriers, and channels. The plasma membrane also participates in the exchange of larger molecules through phagocytosis, the engulfing of large food particles; endocytosis, the intake of fluids and membrane components; and exocytosis, the expulsion of cell products or cell waste. In addition, the plasma membrane contains receptors that selectively receive nerve and hormone signals and transmit them to the interior of the cell.
Finally, direct cell-to-cell interactions can occur through specialized regions of the plasma membrane known as junctions. Organs such as the skin and the small intestine consist of cells held together by tight junctions and local thickenings, or desmosomes, which constitute another type of junction. Cells can communicate electrically through a third type of junction, called a gap junction, that consists of tiny protein \”tunnels\” between two cells, through which tiny \”message\” molecules and ions may be passed.
When the plasma membranes of two cells are continuous, an actual bridge of cytoplasm forms between them; in plants these bridges are called plasmodesmata. Cell Walls Exterior to the plasma membrane of most plant cells and bacteria is a cell wall, a cell product made largely of complex polysaccharides. In higher plants the polysaccharide is cellulose. The presence of a cell wall makes these cells rigid and sturdy, but it poses special problems for the transport of substances into and out of the cell.
Cytoplasm The cytoplasm is the water-rich matrix within a cell that contains and surrounds the other cellular contents. It is more like a viscous gel than a watery solution, but it liquefies when shaken or stirred. Such gel-to-sol transitions are thought by some cell biologists to play a role in the movement of a cell’s components from place to place within the cell. Rapid movement of cell components is called either streaming or cyclosis, depending on whether it occurs linearly or circularly.
Through an electron microscope the cytoplasmic gel appears as a three-dimensional lattice of slender, protein-rich strands in a continuous water-rich phase. Because the latticework is reminiscent of the internal structure of spongy bone, which is composed of many struts, or trabeculae, it is called the microtrabecular lattice (MTL). The MTL appears to interconnect and support the other \”solid\” structures of the cell. The composition and function of the MTL are as yet still unknown, but it is thought to control the spatial arrangement of cell components within the cytoplasm.
Cytoskeleton The so-called cytoskeleton influences the shape of the cell in much the same way tent poles determine the shape of a tent. Without the cytoskeleton a cell tends to become spherical. The cytoskeleton probably gives direction to the movement of components within the cytoplasm as well and participates in movement of the cell itself. The cytoskeleton is composed of three main filament types: the microtubules, microfilaments, and intermediate-sized filaments that are supported and distributed within the MTL. Microtubules are long rigid cylinders that act as the bones of the cell.
They also may act as tracks along which intracellular components are transported. The walls of the cylinders are composed of two proteins, alpha- and beta-tubulin. Microfilaments are composed of actin, a major protein of muscle. They often occur in long bundles called stress fibers and may act as the muscles of the cell. The intermediate-sized filaments are a heterogeneous class of proteins whose function is largely unknown. Nucleus The membrane-bounded structures contained within the cytoplasm of eukaryotes are referred to as organelles.
The nucleus is the most easily recognizable of these. DNA, combined with protein, is organized inside the nucleus into structural units called chromosomes, which usually occur in identical pairs. The DNA in each chromosome is a single, very long, highly coiled molecule subdivided into functional subunits called genes. Genes contain the coded instructions for the assembly of polypeptides and larger proteins. Together the chromosomes contain all the information needed to build an identical functioning copy of the cell.
The nucleus is surrounded by an envelope of two concentric membranes. Interaction between the nuclear contents and the surrounding cytoplasm is permitted through holes, called nuclear pores, in this envelope. The nucleus also contains a specialized region, the nucleolus, where nucleoprotein particles are assembled. These particles migrate through the nuclear pores into the cytoplasm, where they are modified to become ribosomes. Ribosomes Ribosomes are the \”factories\” where the instructions encoded in the DNA of the nucleus are translated to make proteins.
The instructions are carried from the DNA to the ribosomes by long nucleic-acid molecules called messenger ribonucleic acids (RNAs). Endoplasmic Reticulum (Er) Among the other membranous structures within the eukaryotic cell are extensive membrane systems that make up the smooth and the rough endoplasmic reticulum. The smooth endoplasmic reticulum often takes the form of branching tubes The rough endoplasmic reticulum is made up of sheet-like flattened sacs, which often are stacked one on top of the other; the term rough refers to the numerous ribosomes that dot the cytoplasmic surfaces of the sacs.
The rough endoplasmic reticulum is one of the sites of protein synthesis in the cytoplasm. Proteins are synthesized on the cytoplasmic surface and pass through the membrane to become sequestered within the sacs. These packaged proteins are destined for secretion to the outside of the cell. Other proteins, synthesized on ribosomes that are not attached to membranes, are not secreted and remain as structural proteins or metabolic enzymes. Golgi Apparatus Similar in appearance to and perhaps continuous with the ER is a region of smooth, stacked membranous sacs known as the Golgi apparatus.
Cell biologists think that the apparatus modifies proteins, after they are synthesized and packaged on the rough endoplasmic reticulum, by linking them with sugars or other molecules. Lysosomes Lysosomes are membrane-bounded bags, or vesicles, containing digestive enzymes. Their normal function is digestion of complex nutrients and broken-down organelles. In disease fighting, the lysosomes of white blood cells aid in the digestion of engulfed bacteria and other foreign or toxic materials. Mitochondria and Chloroplasts (Plastids)
Mitochondria are the powerhouses of the animal cell, where the products of the enzymatic breakdown, or metabolism, of nutrients such as glucose are converted into energy in the form of the molecule adenosine triphosphate (ATP). This process uses up oxygen and is called aerobic respiration. Plants possess, in addition to mitochondria, similar organelles called chloroplasts. Each chloroplast contains the green pigment chlorophyll, which is used to convert light energy from the sun into ATP. This process is called photosynthesis.
Cilia and Flagella Some cells have flexible, whip-like external appendages called cilia and flagella, which are used for locomotion and for capturing food. Cilia are 3 to 10 micrometers long and are found on protozoa as well as in human oviducts and respiratory tracts. In the respiratory tract they sweep large particles up the trachea and prevent them from passing into the lungs. Flagella, which may be ten times as long, are found on some protozoa and unicellular plants, and they are used for locomotion by the sperm of higher organisms.
Eukaryotic cilia and flagella are composed of microtubules covered by a membrane sheath. Prokaryotic flagella are more slender and are composed of the protein flagellin. They propel the cell by rotating like the propeller of a ship rather than by a whipping motion. Centrioles and Basal Bodies All animal and some plant cells contain a pair of centrioles, which are cylindrical structures composed of short microtubules. They are surrounded by a cloud of fuzzy material, the exact function of which is unknown. Centrioles control the arrangement of microtubules in the cell cytoskeleton.
Basal bodies, which are similar, are structures that anchor cilia and flagella within the cytoplasm, just inside the plasma membrane. Centrioles and basal bodies both contain DNA and apparently can duplicate themselves independently of duplication of the entire cell. Division, Reproduction, and Differentiation All cells are the products of the division of preexisting cells. Simple cell division, or asexual reproduction, normally results in the production of two identical daughter cells, each containing a set of chromosomes identical with those of the parent cell.
Before the onset of division, a cell grows to roughly twice its original size. In doing so it duplicates its DNA, so that each chromosome is doubled. During division the duplicate sets are physically separated, following longitudinal splitting of each double chromosome, and are transported into opposite sides of the cell. The cell then constricts around its equator and pinches in two. In cells that contain chromosomes, the separation of chromosomes during division requires an oblong scaffold of parallel microtubules, along which the chromosomes are moved.
This scaffold, called the spindle, forms at the beginning of mitosis under the direction of the centrioles. Sexual Reproduction Sexual reproduction is the mingling of the DNA of two different organisms of the same species to produce a cell, or cells, with a new combination of genes. When this occurs between single-celled organisms, it is called conjugation. In multi-cellular organisms, sexual reproduction requires the production of male and female germ cells by a process called meiosis. During this process a cell divides twice; but its chromosomes are duplicated only once.
Thus, four germ cells are produced, each containing half the normal number of chromosomes. In the male organism the germ cells develop into sperm; in the female they develop into eggs. A sperm and an egg then unite to form a new cell, called a zygote, that has a complete set of chromosomes and has received half its genetic information from each parent, thus making it a new individual. Differentiation Differentiation is the process by which a cell daughter becomes different from its parent in appearance or function, or both, even though both parent and daughter cell contain identical genetic information.
The appearance and function of identical daughter cells are initially specified by two kinds of information inherited by each in equal measure from the parent: cytoplasmic and nuclear information. Alterations in either kind of information will result in daughter cells being unlike their parents. Cytoplasmic information consists chiefly of cell organelles and messenger RNAs ready for translation into proteins, whereas nuclear information is contained in the genetic code.
Changes in cytoplasmic information generally are the result of unequal divisions that produce an asymmetrical distribution of cytoplasmic organelles and messenger RNAs between daughter cells. Changes in nuclear information involve restriction of the use of some portion of the cell’s genes, because genes can be turned on or off by the cell in response to cellular environmental signals. The behavior of a cell at any given time in differentiation largely depends on which subset of genes is turned on.
Differentiation primarily occurs through activation and deactivation of genes in a programmed succession to produce orchestrated changes in cell characteristics. During differentiation certain genes often are irreversibly turned off, and the change becomes permanent. This limits the variety of ways in which a cell can respond to an environmental signal, as well as the variety of signals to which it can respond, and the cell is channeled toward its ultimate differentiated fate. This process is called determination. Thus, a human nerve cell cannot transform into a human muscle cell even though they each contain identical genetic information.
Aging of cells is sometimes viewed as a continuation of their differentiation, with death seen as the final determination. Origin of Life and Evolution of Cells Scientists have formulated many theories about the origin of life and how it evolved into the various forms known today. These ideas are deduced from the evidence of the fossil record, from laboratory simulations of conditions on the primeval earth, and from consideration of the structure and function of cells. The earth was created more than 3 billion years ago, although more than 2 billion years probably passed before life as it is now known developed.
Scientists believe that the atmosphere of the young earth was mostly water vapor, methane, and ammonia, with very little gaseous oxygen. Laboratory simulations have shown that all major classes of organic molecules could have been generated from this atmosphere by the energy of the sun or by lightning and that the lack of oxygen would prevent newly formed organic molecules from being broken down by oxidation. Rain would have carried these molecules into lakes and oceans to form a primordial soup. When the concentration of organic molecules in this soup became high enough, molecules would have begun to form stable aggregates.
For example, lipids might coalesce into droplets the way cooking oil does in water, thus generating simple membranes and trapping other organic molecules in the interior of the droplet. Randomly formed aggregations that could harness energy to grow and reproduce themselves would eventually far outnumber other combinations. DNA may have been an essential component of the self-reproducing aggregates; it and RNA are the only organic molecules able to duplicate themselves. These supramolecular aggregations would have been extremely lifelike and with some refinements would have resembled primitive prokaryotes.
This concept of the origin of life, however, does not explain the development of the genetic code and the precise interdependence between the code and protein synthesis. The relative absence of oxygen from the atmosphere of the young earth meant that no ozone layer existed to screen out ultraviolet radiation and no oxygen was available for aerobic respiration. Therefore, the first cells were probably photosynthetic and used ultraviolet light. Because photosynthesis generates oxygen, the oxygen content of the atmosphere gradually increased.
As a result, cells that could use this oxygen to generate energy, and photosynthetic cells that could use light other than ultraviolet, eventually became predominant. Eukaryotes may have evolved from prokaryotes. This idea comes from speculation about the origin of mitochondria and chloroplasts. These organelles may be the degenerate descendants of aerobic and photosynthetic prokaryotes that were engulfed by larger prokaryotes but remained alive within them. Over the years the host cell became dependent on the endosymbionts for energy (ATP), while they in turn became dependent on the host for most other cell functions.
The fact that mitochondria and chloroplasts are surrounded by two membranes, as if they had originally entered the cell by phagocytosis, supports this theory. In addition, these organelles contain their own DNA and ribosomes, which resemble the DNA and ribosomes of bacteria more than those of eukaryotes. It is possible that other eukaryotic organelles originated similarly. History Cells were first described in 1665 by the English scientist Robert Hooke, who studied the dead cells of cork with a crude microscope.
Living cells were first described in detail in the 1670s by the Dutch scientist Anton van Leeuwenhoek. These early descriptions were not improved on until the early 19th century, when better-quality microscope lenses were developed. In 1839 the German botanist Matthias Schleiden and the German zoologist Theodor Schwann formulated the basic cell theory of today. Struck by the underlying similarity between plant and animal cells, they stated that all living organisms consist of cells and cell products. Thus, a whole organism could be understood through the study of its cellular parts.
In 1858 the German pathologist Rudolf Virchow’s theory, that all cells come from preexisting cells, led to the development of ideas about cell division and cell differentiation. The development in the late 19th century of techniques for staining cell parts enabled scientists to detect tiny cell structures that were not actually seen in detail until the advent of the electron microscope in the 1940s. The development of various advanced optical techniques in the 20th century also increased the detection power of the light microscope for observations of living cells.
The study of cells is not limited to describing structures. A central concept in modern cytology is that each structure has a function that may be understood as a series of biochemical reactions. The understanding of these functions has been greatly aided by the development of cell fractionation techniques, using an ultracentrifuge to separate specific intracellular structures from the rest of the cell. Another technique is tissue culture, by which specific kinds of cells can be isolated and grown for study.