The cells in the tubules are called flame cells or protonephridia because they have a cluster of cilia that looks like a flickering flame when viewed under the microscope. Flame cells function like a kidney, removing waste materials through filtration.
The cilia propel waste matter down the tubules and out of the body through excretory pores that open on the body surface; cilia also draw water from the interstitial fluid, allowing for filtration. After excretion, any useful metabolites are reabsorbed by the cell. Flame cells are found in freshwater invertebrates, such as flatworms, including parasitic tapeworms and free-living planaria.
Earthworms annelids and some other invertebrates, such as arthropods and mollusks, have slightly-more-evolved excretory structures called nephridia. A pair of nephridia is present on each segment of the earthworm. Their excretory system consists of two tubules connected to a highly branched duct system. The cells in the tubules are called flame cells or protonephridia because they have a cluster of cilia that looks like a flickering flame when viewed under the microscope, as illustrated in [link] a.
The cilia propel waste matter down the tubules and out of the body through excretory pores that open on the body surface; cilia also draw water from the interstitial fluid, allowing for filtration.
Any valuable metabolites are recovered by reabsorption. Flame cells are found in flatworms, including parasitic tapeworms and free-living planaria.
Earthworms annelids have slightly more evolved excretory structures called nephridia , illustrated in [link] b. A pair of nephridia is present on each segment of the earthworm. They are similar to flame cells in that they have a tubule with cilia.
Excretion occurs through a pore called the nephridiopore. They are more evolved than the flame cells in that they have a system for tubular reabsorption by a capillary network before excretion. Malpighian tubules are found lining the gut of some species of arthropods, such as the bee illustrated in [link]. They are usually found in pairs and the number of tubules varies with the species of insect.
Malpighian tubules are convoluted, which increases their surface area, and they are lined with microvilli for reabsorption and maintenance of osmotic balance. Malpighian tubules work cooperatively with specialized glands in the wall of the rectum. Body fluids are not filtered as in the case of nephridia; urine is produced by tubular secretion mechanisms by the cells lining the Malpighian tubules that are bathed in hemolymph a mixture of blood and interstitial fluid that is found in insects and other arthropods as well as most mollusks.
Metabolic wastes like uric acid freely diffuse into the tubules. The secretion of ions alters the osmotic pressure which draws water, electrolytes, and nitrogenous waste uric acid into the tubules. Water and electrolytes are reabsorbed when these organisms are faced with low-water environments, and uric acid is excreted as a thick paste or powder. Not dissolving wastes in water helps these organisms to conserve water; this is especially important for life in dry environments.
Link to Learning. See a dissected cockroach, including a close-up look at its Malpighian tubules. Many systems have evolved for excreting wastes that are simpler than the kidney and urinary systems of vertebrate animals. The simplest system is that of contractile vacuoles present in microorganisms. Single- celled organisms use cell division as their method of reproduction. While there are a few cells in the body that do not undergo cell division, most somatic cells divide regularly.
A somatic cell is a general term for a body cell : all human cells , except for the cells that produce eggs and sperm which are referred to as germ cells , are somatic cells. The cell cycle is an ordered series of events involving cell growth and cell division that produces two new daughter cells.
Natural Killer Cells After a pathogen enters the body, infected cells are identified and destroyed by natural killer NK cells , which are a type of lymphocyte that can kill cells infected with viruses or tumor cells abnormal cells that uncontrollably divide and invade other tissue. As such, NK cells offer a complementary check for unhealthy cells , relative to T cells. NK cells are always active; an interaction with normal, intact MHC I molecules on a healthy cell disables the killing sequence, causing the NK cell to move on.
After the NK cell detects an infected or tumor cell , its cytoplasm secretes granules comprised of perforin: a destructive protein that creates a pore in the target cell. Phagocytic cells then digest the cell debris left behind. Regulation of the Cell Cycle by External Events External factors can influence the cell cycle by inhibiting or initiating cell division.
Each cell is produced as part of its parent cell. Some cells do live longer than others, but eventually all cells die when their vital functions cease. Crowding of cells can also inhibit cell division. Another factor that can initiate cell division is the size of the cell ; as a cell grows, it becomes inefficient due to its decreasing surface-to-volume ratio.
Mechanics of Cellular Differentation How does a complex organism such as a human develop from a single cell —a fertilized egg—into the vast array of cell types such as nerve cells , muscle cells , and epithelial cells that characterize the adult?
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