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Superkingdom Eukaryotae

Cells structure

typical plant cell, amyloplasts, cell parts, ostrich egg, algal cells

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 giraffeís 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 cellís 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 cellís 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.

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.



Article key phrases:

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