Cells

CELL.  The smallest unit of living matter that can exist by itself is the cell. Some plants and animals consist of only a single cell. Others are composed of many billions of cells.

   Cells exist in a variety of shapes and sizes. They may, for example, be cube-shaped or flat. Scientists who study cells have determined that a single cell may be as large as a tennis ball or so small that thousands would fit on the period at the end of this sentence. The yolk of a hen's egg is actually a very large cell (see Egg). By contrast, bacteria each one of which is a tiny cell are among the smallest cells (see Bacteria). Regardless of its shape or size, every cell contains the needed to maintain life (see Living Things). While normally cells function with great efficiency, they are subject to various disorders that result in disease. (See also Cancer; Disease, Human; Virus.)

   The size of cells is usually measured in microns. A micron is a millionth of a meter, and about 25,000 microns equal one inch. The smallest bacteria are about 0.2 micron in diameter. The average cell in the human body about ten microns in diameter is a speck barely visible without the aid of a microscope.

   The study of cells is the branch of biology called cytology. The science that deals with cells on the smallest structural and functional level is called molecular biology.

Parts of a Typical Cell

   All cells consist of protoplasm, the jelly.  The protoplasm of a typical cell forms three vital parts the cell membrane, the cytoplasm, and the nucleus. The membrane encloses the other cell structures. Much of the chemical work of the cell is done in the cytoplasm, which surrounds the nucleus. The nucleus, enclosed by its own membrane, is the control center of the cell.

The Cell Membrane

   Cells can survive only in a liquid medium that brings in food and carries away waste. For one-celled organisms this fluid is an external body of water the ocean, a lake, or a stream. For many-celled plants and animals, however, the medium is part of the organism in plants, the sap; in animals, the blood.

   The cell membrane is semi permeable, or differentially permeable some substances can pass through it, but others cannot. This characteristic enables the cell to admit useful substances and to reject harmful substances from the surrounding fluid as well as to force out, or excrete, waste products into the fluid.

   The cell membrane is an extremely thin but tough band of protein and phospholipid molecules (see Protein). Phospholipids are chemicals similar to stored fat (see Fats and Oils). Substances probably pass through the cell membrane in several ways. On the evidence of electron micrographs, biologists believe that it has pores through which certain small molecules pass intact. Large molecules, however, enter the cell in other ways. By a process called diffusion, they may be dissolved by substances in the cell membrane. They can then pass through the membrane without difficulty. Some cells take in large molecules by means of pinocytosis. In this process, the cell membrane forms a pocket around large molecules floating against it. The molecule- and water-filled pocket then breaks loose from the membrane to become a bubble like vacuole, and the vacuole then drifts into the cytoplasm. Finally, the vacuole wall breaks up and the molecules are released into the cytoplasm.

The Cytoplasm

   The cytoplasm is mainly water. Its water content varies from a minimum of about 65 percent to a maximum of about 95 percent. The solids in the cytoplasm include granular proteins, carbohydrates, droplets of fat, and pigments. The cytoplasm is thus a colloidal system (see Colloid).

   The cytoplasm may be either watery or syrupy, depending on the concentration of solids dispersed in the fluid. When the concentration of solids increases, membranes and fibrous structures appear in the cytoplasm. When the solid content decreases, these structures seem to vanish. Changes in concentration also produce an apparent streaming of the cytoplasm from place to place within the cell. This occurs, for example, when food molecules enter the cell.

Cytoplasm Structures and Their Functions

   Most of the cell's constant work of keeping alive is performed in the cytoplasm. Here food molecules are changed into the material needed for energy. Here materials for growth or stiffening of the cell membrane are manufactured. The cytoplasm in the specialized cells of a many-celled plant or animal may produce substances needed by the rest of the organism. For example, plant cells containing chlorophyll manufacture glucose a plant food from carbon dioxide and water (see Plant). Certain cells in the alimentary canal of an animal make digestive juices.

   One of the cytoplasm's key energy transactions occurs in the sausage-shaped mitochondria. Each mitochondrion has an inner and an outer membrane. Like the cell membrane, the membranes of the mitochondrion are semi permeable. Food molecules that pass into the cytoplasm are taken into the mitochondria and oxidized, or burned, for energy. (For a more complete discussion of the energy transactions that take place within a cell, see Biochemistry.)

   The endoplasmic reticulum, a network of membranous tubes, runs through the cytoplasm. In the opinion of some biologists, this network is a continuous structure that begins at the cell membrane, twists through the cytoplasm, and ends at the membrane surrounding the nucleus. Located along the endoplasmic reticulum as well as elsewhere in the cytoplasm are numerous ribosomes. These tiny granules consist in part of ribonucleic acid (RNA). Proteins are manufactured at the ribosomes. The Golgi complex, or Golgi apparatus, is a membranous structure composed of stacks of thin sacs. Newly made proteins move from the endoplasmic reticulum to the Golgi complex, where they are stored for later secretion.

   Vacuoles drift through the cytoplasm. They usually carry food molecules in solution. Vacuoles also regulate the water content of certain one-celled animals. When an amoeba, for example, absorbs too much water, it forms a contractile vacuole against the membrane. The vacuole fills with water and then contracts to squeeze the liquid out of the cell. This pumping action continues until all excess water has been forced from the amoeba (see Amoeba).

   Lysosomes are structures somewhat similar in appearance to vacuoles but denser. They appear to have a digestive function. Each lysosome is filled with digestive enzymes and encased in a membrane. Lysosomes are believed to break down food substances brought into the cell by pinocytosis. It has been suggested that the Golgi complex plays a part in the formation of lysosomes.

   Plastids in the cytoplasm of plants and one-celled animals synthesize and store food and other materials, such as starch or a working chemical. Plant plastids that hold chlorophyll are called chloroplasts (see Plant, of Plants. Many-celled animals store their food in special cells.

The Nucleus

   Near the center of the cell is a roundish or oval-shaped nucleus. The nucleus controls the growth and division of the cell. It also contains the structures that transmit hereditary traits (see Genetics).

   Enclosed by a two-layered membrane, the nucleus contains a liquid called nucleoplasm as well as strands of deoxyribonucleic acid (DNA) covered with a coating of protein. A strand of DNA consists of a long series of genes, which are the units of heredity of plants and animals. Genes determine the characteristics of a cell. They do this by regulating the production of RNA, which in turn controls the manufacture of specific proteins.

   Human cells, for example, make only proteins unique to human beings. DNA strands are usually too thinly strung out to be seen with an optical microscope. Because the strands are readily stained with dyes, they are called chromatin. When a cell begins to divide, however, the chromatin thickens into the form of chromosomes.

   A nucleus not undergoing division has at least one nucleolus. The nucleolus contains a concentration of RNA. Biologists think RNA is made initially in the nucleolus according to a DNA and stored there until needed for protein manufacture.

   Near the nucleus of animal cells is a spherical structure called the centrosome, from which asters radiate. The centrosome contains a pair of rodded structures, called centrioles, which usually lie at right angles to each other. Although centrioles and centrosomes have not been seen in plant cells, biologists believe that plant cells contain similar structures.

How Cells Divide

   The cells of one- or many-celled organisms grow until they reach their full size. Then structures in the nucleus usually initiate cell division, the process by which a cell divides into two daughter cells.

   Prior to the division of body cells, or mitosis, the chromatin thickens into rodlike chromosomes the bearers of hereditary traits. Each chromosome then makes an exact duplicate of itself. A knot called the centromere holds the original and duplicate chromosomes together.

   As mitosis begins, the paired chromosomes gather in a line at mid-cell. In animal cells, the two centrioles divide to form a second pair, each pair having its associated asters. (In plant cells only the asters appear.) The centriole pairs move to opposite ends of the cell, sending out spindle fibers as they move. The centromeres split, separating each chromosome from its partner. The chromosomes move along the spindle toward their respective set of centrioles. Eventually the cell divides.

   Each cell of a given species has a characteristic number of chromosomes. Human body cells normally contain 46 chromosomes 23 pairs. Mitosis ensures that both daughter cells have the full set of chromosomes characteristic of the species.

   Sex cells produce gametes sperm and eggs by meiosis. This involves two divisions. During the first meiotic division, homologous chromosomes pair and duplicate themselves. The first division results in the formation of two cells, each with a full complement of chromosomes. During the second division, however, the chromosomes in the two cells do not duplicate themselves. Thus, each of the four gametes produced receives only one of each chromosome pair, or only half the number of chromosomes characteristic of the species. A human gamete, for example, has only 23 chromosomes.

   The full complement of chromosomes is restored when a male gamete fertilizes an egg. Half the chromosomes and therefore half the hereditary determinants of the fertilized egg are received from each parent. Consequently, the offspring resembles both parents. (See also Genetics; Reproductive System.)

One-Celled and Many-Celled Plants and Animals

   Bacteria are microscopic specks of cytoplasm surrounded by a tough cell wall. Spherical or dumbbell-shaped nuclei containing DNA have been found in the cytoplasm of some bacteria. The cytoplasm of bacteria also contains grains of stored food, vacuoles, and ribosomes. Many types of bacteria live as parasites on higher organisms.

   Bacteria are plantlike, but most lack the chlorophyll to make their food. However, some types of one-celled plants called algae do bear chlorophyll. Some species of algae clump in great numbers to form a green scum on quiet ponds (see Algae).

   The simplest animals are one-celled organisms called protozoans (see Protozoan). Although there are about 20,000 species of protozoans, the amoeba and the paramecium are perhaps the most commonly known. Many species of protozoans can hunt and catch food.

   Large plants and animals are made up of millions or billions of cells. Any one of these cells may be no larger in size than a microscopic one-celled organism. Even the largest species of plants and animals are therefore assemblages of tiny cells. Most many-celled organisms have various kinds of cells. These form different structures and perform different functions. They work together to sustain the life of the entire organism. Certain types of animal cells, for example, form muscles, eyes, or teeth. Specialized plant cells form flowers, fruits, and seeds.

   A full-grown human has thousands of billions of cells in his body. A drop of blood has about 300 million red cells. The most specialized human cells are the nerve cells, which extend delicate fibers to receive and carry impulses. Some of these fibers are several feet long (see Nervous System).

Differences Between Plant and Animal Cells

   The greatest difference between plant and animal cells occurs at the cell membrane. The cell membrane of a typical plant cell unlike that of an animal cell is covered with a protective wall of cellulose (see Carbohydrates). A plant's combined cell walls give it stiffness and help it grow tall. Cellulose is secreted by the plant cell's cytoplasm. The cell wall may also contain lignin, which like cellulose is an important component of wood (see Wood).

   Vacuoles in some types of plant cells serve to build stalks and stems. Cambium cells develop large central vacuoles. If a cambium cell is to become bark or wood, its membrane grows into the vacuole and lays out additional amounts of cell wall, thus gaining stiffness. If the cambium cell is to become part of a vascular bundle for transmitting sap, it becomes cylindrical and develops openings at each end that pass sap from cell to cell.

   The membranes of animal cells contain a flexible protein material that provides protection yet permits the cells to change in size and shape. This flexibility makes it possible for animals to move about. Most many-celled animals need structures such as bones to support their bodies and shells or tough skins to protect them. Special cells secrete the substances needed to form supportive or protective tissue. (See also Bone; Shell.)

History of the Cell Theory

   Cells were first described by the English scientist Robert Hooke, who in 1665 published a book about his findings. Hooke had sliced off thin sections of cork. With a microscope of his own design he was able to see the minute, boxlike units of which the cork was made up. Hooke called these structures because he thought the Boxeslooked like monastery cells. The first description of living cells was provided by the Dutch scientist Anthony van Leeuwenhoek in 1683 (see Leeuwenhoek).

   More detailed investigations became possible with the development of improved compound microscopes. The Scottish botanist Robert Brown discovered the cell nucleus in 1831. In the 1830s two German scientists, Matthias J. Schleiden and Theodor Schwann, concluded independently that cells were the basis of all life, a view called the cell theory. Rudolf Virchow, another German scientist, stated in 1858 that all cells develop from previously existing cells. During the late 19th century, techniques of fixing and staining tissues to preserve cells in as lifelike a state as possible opened the way for intensive cell research.

   In most laboratory light microscopes, the background is brightly lighted, the objects studied are dark, and the power of magnification is about 1,000. In some instruments the background is dark, and the objects examined are bright. Ultraviolet microscopes achieve magnifications two to three times greater than those obtained with these light microscopes. Phase-contrast instruments use special equipment to reveal the refraction, or bending, of light passing through objects, enabling the viewer to see cell details not visible with other microscopes. The electron microscope uses waves of electrons (negatively charged particles), rather than light, and magnetic fields, rather than lenses, to get an image. With the aid of electron microscopes 200 times more powerful than the best light instruments, molecular biologists have learned a great deal more about the tiny structures within cells. (See also Biochemistry; Biology; Microscope.)

 [1]