Eukaryote - biology. I INTRODUCTION Eukaryote, organism whose cells contain a nucleus, a saclike structure that encloses the cell's hereditary materials. The presence of a nucleus distinguishes eukaryotes from prokaryotes, those simple, one-celled organisms in which the hereditary material floats free within the cell. Unlike prokaryotes, eukaryotes display a tremendous diversity of form, from complex, single-celled amoebas, diatoms, and dinoflagellates to multicellular plants, animals, and fungi. Only the eukaryotic cell is capable of a high degree of specialization, and specialization is what makes multicellular organisms possible. Just as banks, post offices, and other specialized workplaces are intrinsic to a city, cells tailored for certain jobs are intrinsic to more-complex organisms. Working in concert, specialized cells can create a higher level of organization known as tissues, such as the growing shoot of a plant or the spiny skin of a sea star. Coordinated tissues form organs and, in animals, these organs combine to form complex organ systems, such as the circulatory, digestive, and respiratory systems. The orchestration of these organ systems makes up the organism. II SIZE AND STRUCTURE The complexity of eukaryotic cells is reflected in their size. In general, the diameter of eukaryotic cells, which range in size from 0.01 mm to 1 mm (0.000394 in to 0.0394 in), is 10 to 100 times that of typical prokaryotic cells. An average-sized animal cell measures about 0.020 mm (0.0008 in), about one-fifth the thickness of the page of a book, while a typical plant cell is slightly larger, about 0.035 mm (about 0.0014 in). The eukaryotic cell with the greatest diameter is the ostrich egg, which measures about 120 mm (4.72 in). The longest eukaryote cells on record are the nerve cells that extend 3 m (10 ft) down a giraffe's neck. Eukaryotes house an assortment of structures, called organelles, within the cytoplasm, a gel-like substance that fills the cell. Like the unique toolbox of a carpenter or electrician, the different organelles house different molecular tools, including the specialized proteins called enzymes needed to accomplish the cell's work. With all the enzymes needed for a particular job clustered in one organelle, the eukaryotic cell can work efficiently. The largest and most conspicuous organelle is the nucleus. The nucleus encloses and protects the cell's genetic material, deoxyribonucleic acid (DNA), so that it is not damaged by biochemical reactions in the cell. Within the eukaryotic nucleus, DNA is wrapped around specialized proteins called histones, like a thread wound around a series of spools. Each DNA strand and its histones fold back and forth several times to form a compact, stick-shaped structure called a chromosome. Depending on the organism, the nucleus contains from one to over a thousand chromosomes. Surrounding the nucleus is the nuclear envelope, a membrane with numerous pores. The pores, ringed by special protein, regulate the flow of substances into and out of the nucleus. The most extensive organelle in the cell is the cytoskeleton, a web of protein filaments that branches extensively throughout the cytoplasm and gives the cell its shape. The cytoskeleton proteins, as well as other proteins in the cell, are made by tiny spherical organelles known as ribosomes. Several other important organelles are found in the cells. Among them are the lysosomes, membranous sacs storing enzymes that digest and recycle worn out cell parts; and the mitochondria, sacs where the cell's energy is generated. The endoplasmic reticulum, another organelle, is an extensive network of membrane folds and tubes that serves in part as the cell's factory floor where large molecules, such as lipids, are manufactured. These large molecules are sent to another organelle, the Golgi apparatus, which consists of layers of membranes where the molecules are modified, sorted, and packaged for transport. Plants, seaweeds, and microscopic algae are specialized eukaryotes that carry out photosynthesis, the process by which light is used to convert carbon dioxide and water into sugar. In addition to the organelles described above, photosynthetic eukaryotes contain chloroplasts. Chloroplasts house the green pigment chlorophyll that is used in photosynthesis. Photosynthetic eukaryotes also may house chromoplasts, sacs of yellow, orange, or red pigments responsible for color in flowers and fruits. Photosynthetic eukaryotes may also have organelles called amyloplasts, where energy-rich starch is stockpiled. Passage of materials into and out of the cell is regulated by the plasma membrane. Plant cells and many algal cells are further surrounded by a tough cell wall composed of cellulose, a carbohydrate. Cells of fungi, on the other hand, are surrounded by a cell wall made of chitin, another type of carbohydrate. In addition, on the outside of the plasma membrane, many eukaryotic cells bear either cilia or flagella, slender filaments that propel cells through liquid. These slender filaments enable aquatic organisms, for example, to navigate through the water. Cilia and flagella may also move liquid past a stationary cell. For example, cilia on the cells that line the lungs and trachea push mucus and dirt particles up out of the lung. III CELL DIVISION Eukaryotes carry out cell division to make the new cells needed for growth, to repair damaged cells, and to replace worn out, dying cells. Most eukaryotic cells divide by mitosis, a process that produces two cells with the same genetic information as the original cell. Single-celled eukaryotes, such as amoebas and diatoms, commonly reproduce by mitosis. In an adult human, mitosis spawns an estimated three million new cells every second, replacing the battered cells that line the digestive tract; dead, sloughed off skin cells; worn out red blood cells; and other cells exposed to constant abrasion or intense use. Many eukaryotes also undergo a second type of cell division, called meiosis, which is designed for sexual reproduction, the union of male and female sex cells. In meiosis, two cell divisions occur in which the genetic material is rearranged, resulting in four genetically unique cells, each of which contains only half the number of chromosomes as the parent cell. When two cells with half the number of chromosomes unite, the new cell contains the full complement of chromosomes needed to produce the new organism. IV OBTAINING NUTRIENTS To function, eukaryotes need organic molecules: carbohydrates such as sugar and starch; proteins; lipids, which include fats and oils; and nucleic acids such as DNA. Sugars such as glucose are particularly important because eukaryotes use energy from sugar to build proteins, lipids, and other organic molecules. Photosynthetic eukaryotes are known as autotrophs, a group that includes plants, seaweeds, and microscopic algae, all of which can make their own sugar. Those that must take in sugar from outside sources are called heterotrophs. Among the heterotrophs are many single-celled eukaryotes, and all fungi and animals. Heterotrophic eukaryotes typically absorb the nutrients in food through the plasma membrane. To accomplish this task, they must first break down, or digest, the food. Fungi secrete digestive enzymes onto the surface of their food--often decaying leaves or branches--and then absorb the enzyme-released nutrients across the cell wall and plasma membrane. In contrast, animals first ingest their food into some sort of digestive structure such as the stomach. There, digestive enzymes break down the food, and the nutrients are then absorbed into the cells. Some single-celled eukaryotes, such as amoebas, use a process called endocytosis. In endocytosis, these organisms extrude part of the plasma membrane, scoop up a food particle, and drag it into the cell, where they digest it using enzymes within the cell. In these eukaryotes, large waste molecules typically are expelled from the cell by a reverse process called exocytosis. The waste is bundled into a sac called a vesicle and transported to the plasma membrane, where it fuses with the membrane. The waste is then expelled through a hole in the fused membrane. In complex animals, cells generate wastes such as urea when nutrients are broken down within cells. These wastes are transported by blood to the kidneys. The kidneys process the waste and produce urine, which is removed from the body through the bladder. Undigested food travels through the tubelike intestines and is eliminated through the digestive system. V EVOLUTIONARY ORIGINS Eukaryotes evolved much later than prokaryotes, whose origins date to about 3.5 billion to 3.8 billion years before present. Alga-like fossils from ancient rocks suggest that eukaryotes may have evolved about 2.1 billion years before present. Other fossil remains indicate that eukaryotes were well established 1.6 billion years before present. These fossils, called acritarchs, are hollow spheres that appear to be spores or cysts of eukaryotic algae. Eukaryotic cells are thought to have evolved from primitive prokaryotes. Evidence for this view is found in the archaea, prokaryotes that resemble both bacteria and eukaryotes. Like bacteria, the archaea lack a nucleus and most other organelles. Like eukaryotes, they display flexible cell membranes and histone proteins, and have certain segments of DNA in common. This evidence, along with other molecular studies, leads many scientists to conclude that archaea, bacteria, and eukaryotes arose from a common ancestral prokaryote similar to the archaea. However, according to a theory developed by American microbiologist Carl Woese, the archaea, bacteria, and eukaryotes may have arisen, not from a single common ancestor, but from a group of genetically diverse, primitive prokaryotes. The nucleus and endoplasmic reticulum of eukaryotes probably evolved from internal folds of the plasma membrane of prokaryote ancestors. Scientists once thought that mitochondria and chloroplasts arose in the same way. But a theory proposed by American microbiologist Lynn Margulis holds that these organelles arose from certain prokaryotes, which established mutually beneficial relationships with the earliest eukaryotes. In the distant past, these free-living prokaryotes were consumed by early eukaryotes, but managed to survive within the cytoplasm. The autotrophic prokaryotes proved useful to their hosts because they produced glucose through photosynthesis. The heterotrophic prokaryotes also were useful because they could generate the energy source adenosine triphosphate (ATP) for the host. The host in turn provided protection for the engulfed prokaryotes. Over time, the autotrophic prokaryotes developed into chloroplasts and the heterotrophic prokaryotes developed into mitochondria. These organelles still contain their own DNA, with bacteria-like genes, and their own ribosomes, relics handed down from their distant bacterial ancestors. Contributed By: Ben Waggoner Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
