Glaucoma A disease of the eye characterized by loss of vision due to an increase in the pressure of fluid within the eye (intraocular pressure) that leads to damage to the optic nerve and loss of vision in both eyes. Can eventually lead to blindness. The disease affects about 6 million people worldwide. In the United States, about 3 million people are affected.
Glial cell Nonimpulse-conducting cells that make up half the weight of the brain; in the central nervous system they are 10 times as numerous as neurons and act as support cells by forming insulation around the neurons to protect them. Their support functions provide myelin for axons, and they act as housekeepers after cell damage or death by cleaning up. Glial cells also play an important role in the early and continuing development of the brain. A small hormonelike protein, called the glial growth factor, induces the growth of glial cells. A type of brain tumor that forms in the glial tissue is called glioblastoma multiform.
There are several types of glial cells in the central nervous system, including oligodendrocytes, astrocytes, and microglia. It is the oligodendrocytes that produce the fatty protein myelin that insulates the axons by wrapping them in layers of myelin. Star-shaped astro-cytes lay down scar tissue on damaged neurons and hold the neurons in place as well as supply potassium and calcium and regulate neurotransmitter levels. Along with microglia cells, astrocytes remove dead cells and other matter from the central nervous system.
Similar functionary types of cells in the peripheral nervous system are the Schwann (provide insulation via myelin) and satellite cells (support cells).
A debilitating disease caused by the demyelinating of neurons is multiple sclerosis, and stem-cell research is being conducted into developing new treatments.
Glomerulus A structure, a tiny ball, between the afferent arterioles and efferent arterioles within the proximal part of the nephron of the kidney; located within the Bowman’s capsule. It is composed of a cluster of capillary blood vessels and is involved in the filtration of blood. The glomerulus is a semipermeable structure that allows water and soluble wastes to pass through and then discharges them out of the Bowman’s capsule as urine waste at a rate of about 160 liters or 42.7 gallons per 24-hour period. Most of this is reab-sorbed back into the blood. The filtered blood then leaves the glomerulus by way of the efferent arteriole to the interlobular vein. Each kidney contains about 1 million glomeruli. Changes in the glomerulus may be responsible for diabetic kidney disease.
Also a nest of nerves in invertebrates found in invertebrate olfactory processing centers; a discrete, globular mesh of densely packed dendrites and axons found in the vertebrate olfactory bulb.
Glucagon A protein hormone released by the pancreas via alpha cells in the islets of Langerhans with the purpose of breaking down glycogen, in the liver, which releases glucose and increases blood levels of glucose. Glucagon works with insulin to maintain normal blood sugar levels.
Glucocorticoid A class of stress-related steroids (hormones) produced by the adrenal glands (cortex) that respond to the stimulation by adrenocorticotropic hormone (ACTH) that comes from the pituitary gland. They are involved in carbohydrate, lipid, and protein metabolism by promoting gluconeogenesis and the formation of glycogen, as well as effects on muscle tone, circulation, blood pressure, and more. They possess anti-inflammatory and immunosuppressive properties. Cortisol (hydrocortisone) is the major natural glucocorticoid.
Synthetically produced ones—mostly derived from cortisol, such as cortinsone, prednisone, prednisolone, methylprednisolone, betamethasone, and dexametha-sone—regulate metabolism of lipids, carbohydrates, and protein and work without the use of ACTH.
Glucose A form of six carbon sugar (C6H12O6) that is the most common energy source and is the usual form in which carbohydrates are assimilated by animals. It is carried through the bloodstream and is made not only from carbohydrates but from fats and protein as well. Glucose is known as a dextrorotatory sugar (a chiral molecule that rotates plane-polarized light to the right), which is sweet, colorless, and soluble.
Colored scanning electron micrograph (SEM) of a healthy kidney glomerulus. The glomerulus is a tight ball of capillaries where blood passing through the kidneys is filtered. Fluid passes out of the capillaries into the cavity of the surrounding Bowman’s capsule (not seen) and drains into a long tube, from which essential substances and some water are reabsorbed. The remaining unwanted fluid, containing toxins from the blood, drains to the bladder as urine. Magnification unknown.
Glycocalyx A thick, (7.5-200 nm) extracellular, sticky coating of oligosaccharides linked to plasma membrane glycoprotein and glycolipids; found around the outside of eukaryote cells, and used to adhere to surfaces. Also called the cell coat.
Glycogen A large polysaccharide; stored energy found in the muscles and liver. It consists of many monosaccharide glucose molecules linked together and is used as a fuel during exercise, broken down as needed; glycogen is the primary storage form of glucose in animals. Also known as stored sugar or animal starch.
Glycolysis The anaerobic pathway or enzymatic conversion (using 11 different enzymes) in the cell’s cytoplasm of glucose to simpler compounds. Glucose, a six-carbon sugar, is converted into two molecules of pyruvic acid of three carbons each, with two molecules of NADH and two ATPs as by-products. It is the most universal and basic energy harvesting system; it transforms glucose into lactic acid in muscles and other tissues for energy production when there is not enough oxygen available.
In aerobic respiration, the two pyruvic acids are further used in the krebs cycle.
The anaerobic pathway or enzymatic conversion (using 11 different enzymes) in the cell’s cytoplasm of glucose to simpler compounds.
Glycoprotein (conjugated protein) Glycoproteins are complexes in which carbohydrates are attached cova-lently to asparagine (N-glycans) or serine/threonine (O-glycans) residues of peptides. A protein coated with a sugar is termed glycosylated and is described or named with the initials "gp" along with its molecular weight, e.g., gp160. Several gps are associated with HIV infection, since they are the outer-coat proteins of HIV: gp41 plays a key role in HIV’s infection of CD4+ T cells by facilitating the fusion of the viral and cell membranes. The protein gp120 is one of the proteins that forms the envelope of HIV; it projects from the surface of HIV and binds to the CD4 molecule on helper T cells. GPs are found in mucus and mucins, y-globulins, a1-globulins, a2-globulins, and transferrin, an ion-transporting protein. They act as receptors for molecular signals originating outside the cell. Attachment of oligosaccharides to peptides increases solubility, covers the antigenic domains, and protects the peptide backbone against proteases.
Gold drugs Gold coordination compounds used in the treatment of rheumatoid arthritis, examples being auranofin, (tetraacetylthioglucosato-S)(triethyl-phosphane)gold(I), and myocrisin, disodium thioma-lonatogold(I).
Golgi, Camillo (1843-1926) Italian Medical Teacher Camillo Golgi was born in Corteno, near Brescia, Italy, on July 7, 1843, the son of a physician. He studied medicine at the University of Pavia, and after graduating in 1865, he continued working in Pavia at the Hospital of St. Matteo. Golgi was influenced by the scientific methods of Giulio Bizzozero, who introduced general pathology in the programs of the medical school at the University of Pavia. In 1872 Golgi accepted the post of chief medical officer at the Hospital for the Chronically Sick at Abbiategrasso and began his investigations into the nervous system.
Golgi returned to the University of Pavia as extraordinary professor of histology, left the university, and returned as the chair for general pathology in 1881, succeeding his teacher Bizzozero. He also married Donna Lina, a niece of Bizzozero.
Golgi developed an interest in the causes of malaria and determined the three forms of the parasite and their associated fevers. He developed a photographic technique to document the most characteristic phases of malaria in 1890.
While Golgi never practiced medicine, he was a famous and popular teacher as director of the Department of General Pathology at St. Matteo Hospital. He also founded and directed the Instituto Sieroterapico-Vaccinogeno of the Province of Pavia. Golgi also became rector of Pavia University and was made a senator of the Kingdom of Italy.
During World War I, he assumed the responsibility for a military hospital in Pavia and created a neu-ropathological and mechanotherapeutical center for the study and treatment of peripheral nervous lesions and for the rehabilitation of the wounded.
His greatest contribution seems to be his revolutionary method of staining individual nerve and cell structures, known then as the "black reaction" and now called Golgi staining. It allowed a clear visualization of a nerve cell body with all its processes.
Golgi shared the Nobel Prize for 1906 with Santiago Ramon y Cajal for their work on the structure of the nervous system. He retired in 1918 but remained as professor emeritus at the University of Pavia. The Historical Museum at the University of Pavia dedicated a hall to Golgi, where more than 80 certificates of honorary degrees, diplomas, and awards are exhibited.
Golgi’s discovery of the black reaction and further research provided a major contribution to the advancement of the knowledge on the structural organization of nerve tissues. He also described the morphological features of glial cells and the relationships between their processes and blood vessels; described two fundamental types of nerve cells today called Golgi type I and Golgi type II; discovered the Golgi tendon organs; explained the cycle of plasmodium (malaria) and the cell organelle, today called the Golgi apparatus.
Golgi died at Pavia on January 21, 1926. In 1994 the Italian Ufficio Principale Filatelico issued a stamp to celebrate his work.
Golgi apparatus or complex Part of a cell, a cup- or disclike organelle in cells, usually near the nucleus and composed of a number of flattened or folded sacs, called cisternae, with vacuoles and vesicles. They act as an assembly line in sorting, modifying, and packaging proteins and lipids produced on the endoplasmic reticulum,located in the cytoplasm of the cell, for various parts of the cell. Named after the Nobel Prize recipient, Italian neurologist and histologist Camillo Golgi (1843-1926). They are the formation site of the carbohydrate side chains of glycoproteins and mucopolysaccharides. The vacuoles release these by migrating through the cell membrane. Some of the vesicles send molecules to the cellular membrane, where they are excreted, and some are used for selective excretion.
Gonadotropins A group of hormones that are produced in the pituitary gland and regulate the development and function of the testis and ovary. The group includes the follicle-stimulating hormone (FSH), which promotes male and female gamete formation, and luteinizing hormone (LH), which stimulates the secretion of the male and female testosterone and estrogen. Gonadotropin itself is controlled by the gonadotropin releasing hormone (GnRH), a hormone that controls the production and release of gonadotropins and is secreted by the hypothalamus every 90 minutes or so, which enables the pituitary to secrete LH and FSH.
Gonads The male and female sex organs. In the male, they are glands located inside the scrotum, behind and below the penis, which produce sperm and are the primary source of testosterone. Also called the testes. In the female, they are ovaries, two almond-sized glands located on either side of the uterus. They produce and store the oocytes and the female sex hormones estrogen and progesterone.
Gondwanaland (Gondwana) The large southern protocontinent, derived from the supercontinent Pan-gaea, that, when fragmented, formed Africa, South America, Antarctica, Australia, and India during the Paleozoic era more than 200 million years ago as an event of plate tectonics (continental drift).
G protein A class of heterotrimeric proteins important in signaling pathways in the plasma membrane in mammalian cells. Regulated by the guanine nucleotides GDP (guanosine diphosphate) and GTP (guanosine triphosphate), they participate in cell signal pathways by usually binding a hormone or signal ligand to a seven-pass trans-membrane receptor protein; activates intracellular messenger systems when the signaling molecule (typically a hormone) binds to the transmembrane receptor. The proteins are located on the inner surface of the plasma membrane and transmit signals from outside the membrane, via transmembrane receptors, to adenylate cyclase, which then catalyzes the formation of the second messenger, cyclic adenosine monophosphate (AMP), inside the cell.
Graded potential A nerve impulse that is initially proportional to the intensity of the stimulus that produces it, then declines in intensity; membrane potentials that vary in magnitude.
Gradualism An Earth model of evolution that assumes a slow, steady rate of change, with slow steps instead of quick leaps, and with new variation arising by mutation and recombination. A view held by Charles Darwin.
Grafting The process of attaching two different plant parts, usually shoots, to each other to create a single new plant. Used in propagating trees and shrubs.
Gram staining An important laboratory technique to distinguish between two major bacterial groups, based on stain retention by their cell walls. Bacteria smears are fixed by flaming, then stained with crystal violet followed by iodine solution, and then rinsed with alcohol or acetone, decolorized, and coun-terstained with safranin. Gram-positive bacteria are stained bright purple or purple-black, while gram-negative bacteria are pink. This staining technique is useful in bacterial taxonomy and identification and in indicating fundamental differences in cell-wall structure. Gram-negative bacteria lack peptidoglycan in the cell wall, while gram-positive bacteria have about 90 percent of their cell wall composed of peptidoglycan.
Granum A series of disk- or saclike structures called thaylakoid disks—specialized membrane structures located in the inner membrane of chloroplasts—where photosynthesis takes place. They appear as green granules under a microscope and contain the light-reactant chemicals chlorophyll and carotenoid pigments. ATP is generated during photosynthesis by chemiosmosis.
Gravid Used in relation to pregnant insects and meaning heavy with fully developed eggs or denoting an advanced stage of pregnancy.
Gravitropism The ability of an organism or specific cells to respond, e.g., bend, to the gravitational pull; a growth curvature induced by gravity. See also geotropism.
Greenhouse effect The warming of an atmosphere by its absorbing and reemitting infrared radiation while allowing shortwave radiation to pass on through.
Certain gaseous components of the atmosphere, called greenhouse gases, transmit the visible portion of solar radiation but absorb specific spectral bands of thermal radiation emitted by the Earth. The theory is that terrain absorbs radiation, heats up, and emits longer wavelength thermal radiation that is prevented from escaping into space by the blanket of carbon dioxide and other greenhouse gases in the atmosphere. As a result, the climate warms. Because atmospheric and oceanic circulations play a central role in the climate of the Earth, improving our knowledge about their interaction is essential. See also carbon dioxide.
Gross primary productivity (GPP) The total energy fixed by plants in a community through photosynthesis (such as repackaging inorganic energy to organic energy) per unit area per unit time; total carbon assimilation by plants; total mass or weight of organic matter created by photosynthesis over a defined time line.
Ground meristem Meristem is embryonic tissue. Ground meristem is one of the primary meristem tissues that are differentiated from the apical meristem. The primary meristem tissues include three different tissues: protoderm, ground meristem, and procambi-um. The protoderm differentiates into the epidermis; the procambium differentiates into the vascular tissue; and the ground meristem differentiates into three regions: the cortex, which is several layers of parenchyma cells under the epidermis of the stem and root; pith ray, the parenchyma cells between the vascular bundles in the primary dicot stem; and pith, which are parenchyma cells in the center of the primary dicot stem. Parenchyma are isodiametric cells, i.e., approximately equal in length, width, and height, that are thin walled and not extremely specialized. Ground meris-tem gives rise to cells of the ground tissue system. The ground tissue of a leaf, located between the upper and lower epidermis and specialized for photosynthesis, is called the mesophyll.
