Kids Research Express

Phloem, in higher plants, vascular tissue that conducts sugars and other synthesized food materials from the regions of manufacture in the plant to those of consumption and storage. Phloem is found in the vascular bundles, the longitudinal strands of conductive tissue, in association with the water-conducting tissue, or xylem . The vascular bundles constitute major structural units in herbaceous stems and are the veins in leaves. In the vascular cylinder traversing the center of the buttercup root, for example, the xylem forms a star-shaped central core, and bands of phloem are present in the grooves of this core. Typically, the xylem is on the side of the vascular bundle closest to the pith, although other arrangements are not uncommon. In the older portions of a plant the soft cells of the phloem are crushed as new phloem is formed in the growing process and pushed outward. This new phloem is formed by the action of the cambium, or growing zone, a layer of cells that separates the xy

Xylem, woody tissue, found in higher plants, that conducts water and inorganic salts throughout the plant and provides it with mechanical support. In leaves, flowers, and young stems, xylem is present in conjunction with phloem in the form of conducting strands called vascular bundles. In roots there is a central core of xylem (see Root ). Xylem that derives from the shoot and root-growing points is called primary xylem. In addition, new xylem, called secondary xylem, may be added by the activity of the cambium, which actively divides cells situated between the xylem and phloem. This division gives rise to new xylem cells toward the center in roots and toward the outside in most stems. Some plants have little or no secondary xylem, in contrast to woody plants in which indefinite amounts of secondary xylem is the botanical term equivalent to wood. See Stem . Xylem may contain three types of elongated cells: tracheids, vessel elements, and fibers. At maturity, when functioning in conduc

Nucleus of a Cell Nucleus of a Cell The nucleus, present in eukaryotic cells, is a discrete structure containing chromosomes, which hold the genetic information for the cell. Separated from the cytoplasm of the cell by a double-layered membrane called the nuclear envelope, the nucleus contains a cellular material called nucleoplasm. Nuclear pores, present around the circumference of the nuclear membrane, allow the exchange of cellular materials between the nucleoplasm and the cytoplasm. Nucleus (biology), membrane-bound structure of a cell that plays two crucial roles. The nucleus carries the cell’s genetic information that determines if the organism will develop, for instance, into a tree or a human; and it directs most cell activities including growth, metabolism, and reproduction by regulating protein synthesis (the manufacture of long chains of amino acids ). The presence of a nucleus distinguishes the more complex eukaryotic cells of plants and animals from the simpler prokaryot

Prokaryote, relatively simple unicellular organism, such as a bacterium, characterized by the absence of a nucleus and other specialized cell structures. Scientists distinguish prokaryotes from eukaryotes, which are more complex organisms with cells that contain a nucleus, such as plants and animals. Scientists classify prokaryotes in different ways, depending on the classification system used. In 1938 American biologist Herbert Copeland proposed that unicellular organisms lacking nuclei be classified in their own kingdom called Kingdom Monera, now called Kingdom Prokaryotae. All bacteria were categorized in this new kingdom. This scheme was the first to establish separate kingdoms for prokaryotes (organisms without nuclei) and eukaryotes (organisms with nuclei). In the 1970s scientists determined that cyanobacteria, formerly known as blue-green algae, have physical features that make them more closely related to bacteria than to algae. Although the exact classification of cyanobacteri

Endoplasmic Reticulum Endoplasmic Reticulum (ER), an extensive network of tubes that manufacture, process, and transport materials within nucleated cells. The ER consists of a continuous membrane in the form of branching tubules and flattened sacs that extend throughout the cytoplasm (the cell’s contents outside of the nucleus) and connect to the double membrane that surrounds the nucleus . There are two types of ER: rough and smooth. The outer surface of rough ER is covered with tiny structures called ribosomes , where protein synthesis occurs. Proteins are created as long polypeptide chains, some of which require modification. These proteins are transported into the rough ER, where enzymes fold and link them into the three dimensional shape that completes their structure. The rough ER also transports proteins either to regions of the cell where they are needed or to the Golgi apparatus , from which they may be exported from the cell. Rough ER is particularly dense in cells that manu

Golgi Apparatus Golgi Apparatus, also Golgi body or Golgi complex, network of stacked sacs found within nucleated cells that store, package, and distribute the proteins and lipids made in the endoplasmic reticulum . The Golgi apparatus was first described by Italian anatomist Camillo Golgi in the late 19th century. Located near the nucleus, each apparatus consists of a stack of six or seven flattened, membrane-bound sacs, or cisternae, each separated by a narrow space. The Golgi apparatus is cup-shaped with the convex end, or cis cisterna, facing the cell nucleus and the concave end, or trans cisterna, facing the cell surface. The number of Golgi apparatus in each cell varies but averages between 10 and 20 in animal cells and up to several hundred in plant cells. Proteins and lipids manufactured in the endoplasmic reticulum bud off in tiny, hollow structures, or vesicles, and fuse with the cis cisterna of the Golgi apparatus. The proteins and lipids move progressively through the stac

Ribosomes Ribosome, cell structure that uses genetic instructions transported in ribonucleic acid (RNA) to link a specific sequence of amino acids into chains to form proteins . Ribosomes, which measure about 0.00025 mm (0.00001 in), are dispersed in the cytoplasm (the cell contents outside the nucleus) of all prokaryotic cells— archaebacteria and bacteria. They are also found in the cytoplasm of all eukaryotic cells —cells of protists, fungi, plants, and animals—where they either float free in the cytoplasm or are bound to networks of membrane-enclosed tubules in the cytoplasm, called the endoplasmic reticulum . In eukaryotic cells, two types of cell structures called mitochondria and chloroplasts also contain ribosomes.