Osmoregulation Definition and Explanation

Osmoregulation Definition and Explanation Osmoregulation is the active regulation of osmotic pressure to maintain the balance of water and electrolytes in an organism. Control of osmotic pressure is  needed to perform biochemical reactions and preserve homeostasis. How Osmoregulation Works Osmosis is the movement of solvent molecules through a semipermeable membrane into an area that has a higher solute concentration. Osmotic pressure is the external pressure needed to prevent the solvent from crossing the membrane. Osmotic pressure depends on the concentration of solute particles. In an organism, the solvent is water and the solute particles are mainly dissolved salts and other ions, since larger molecules (proteins and polysaccharides) and nonpolar or hydrophobic molecules (dissolved gases, lipids) dont cross a semipermeable membrane. To maintain the water and electrolyte balance, organisms excrete excess water, solute molecules, and wastes. Osmoconformers and Osmoregulators There are two strategies used for osmoregulation- conforming and regulating. Osmoconformers use active or passive processes to match their internal osmolarity to that of the environment. This is commonly seen in marine invertebrates, which have the same internal osmotic pressure inside their cells as the outside water, even though the chemical composition of the solutes may be different. Osmoregulators control internal osmotic pressure so that conditions are maintained within a tightly-regulated range. Many animals are osmoregulators, including vertebrates (like humans). Osmoregulation Strategies of Different Organisms Bacteria - When osmolarity increases around bacteria, they may use transport mechanisms to absorb electrolytes or small organic molecules. The osmotic stress activates genes in certain bacteria that lead to the synthesis of osmoprotectant molecules. Protozoa - Protists use contractile vacuoles to transport ammonia and other excretory wastes from the cytoplasm to the cell membrane, where the vacuole opens to the environment. Osmotic pressure forces water into the cytoplasm, while diffusion and active transport control the flow of water and electrolytes. Plants - Higher plants use the stomata on the underside of leaves to control water loss. Plant cells rely on vacuoles to regulate  cytoplasm osmolarity. Plants that live in hydrated soil (mesophytes) easily compensate for water lost from transpiration by absorbing more water. The leaves and stem of the plants may be protected from excessive water loss by a waxy outer coating called the cuticle. Plants that live in dry habitats (xerophytes) store water in vacuoles, have thick cuticles, and may have structural modifications (i.e., needle-shaped leaves, protected stomata) to protect against water loss. Plants that live in salty environments (halophytes) have to regulate not only water intake/loss but also the effect on osmotic pressure by salt. Some species store salts in their roots so the low water potential will draw the solvent in via osmosis. Salt may be excreted onto leaves to trap water molecules for absorption by leaf cells. Plants that live in water or damp environments (hydr ophytes) can absorb water across their entire surface. Animals - Animals utilize an excretory system to control the amount of water that is lost to the environment and maintain osmotic pressure. Protein metabolism also generates waste molecules which could disrupt osmotic pressure. The organs that are responsible for osmoregulation depend on the species. Osmoregulation in Humans In humans, the primary organ that regulates water is the kidney. Water, glucose, and amino acids may be reabsorbed from the glomerular filtrate in the kidneys or it may continue through the ureters to the bladder for excretion in urine. In this way, the kidneys maintain the electrolyte balance of the blood and also regulate blood pressure. Absorption is controlled by the hormones aldosterone, antidiuretic hormone (ADH), and angiotensin II. Humans also lose water and electrolytes via perspiration. Osmoreceptors in the hypothalamus of the brain monitor changes in water potential, controlling thirst and secreting ADH. ADH is stored in the pituitary gland. When it is released, it targets the endothelial cells in the nephrons of the kidneys. These cells are unique because they have aquaporins. Water can pass through aquaporins directly rather than having to navigate through the lipid bilayer of the cell membrane. ADH opens the water channels of the aquaporins, allowing water to flow. The kidneys continue to absorb water, returning it to the bloodstream, until the pituitary gland stops releasing ADH.

International Community Essay Example | Topics and Well Written Essays - 9000 words

International Community - Essay Example From the appalling crackdown on the visionary expectation of the ardent student protesters some16 years ago to the subsequent repression of all political dissidence, Communist leaders have time and again tended to reassert a blind obeisance to a dogged system of harsh and mechanical one-party rule. Equally alarming is the ostentatious display of military power in response to Taiwanese aspirations for independence over the last two decades, along with the troubling detection of furtive deliveries of military know-how to unstable areas in the Middle East, the Persian Gulf, and South Asia. These measures scarcely serve to improve international confidence in the Chinese bid for a responsible position on the world stage of the new century. Admittedly China has recently begun to evince a new, less menacing and forward-looking stance toward proving itself as a responsible collaborator in global interaction as the nation begins to assume a substantial position in the world economy. By assisting in negotiations on the Korean Peninsula, China, in effect, was able to take advantage of her influence in the region in the service of broader international concerns. Domestically, the People's Republic of China has taken steps to free a token number of leading political dissidents in response to Western appeals, and, in diverse rural villages, elections for local leaders have been authorized. Chinese rulers also seem to be dealing moderately with the lately re-annexed regions of Hong Kong and Macau, and so far seem to be restricting intimidation tactics to verbal posturing and a display of military muscle in response to Taiwanese desires for independence.1 It is possible that the menace that some perceive in China as an up-and-co ming superpower with overall goals that threaten American wellbeing and commerce, might, in truth, be an emerging new Chinese generation: a forthright forward-looking dynamic keenly aware that the future depends on a critical modification of policies from the worn-out revolutionary era in the interest of asserting China's rightful place in the broader scheme of a world growing ever more and more interdependent. At least one Chinese insider seems to believe so. From his years spent in the People's Republic as a Red Guard during the infamous Cultural Revolution, Professor Chen Jian brings a quite singular experience and personal familiarity to his study. He is able to appraise the foreign-policy proclivities of the People's Republic from the perspective of an insider seeped in Chinese history. Jian's assessment reflects on much more constrained Chinese objectives than those that bother many outside critics. As a country bent in earnest on certifying for itself a leading role in the global community, Chinese decision makers are certainly cognizant of the imperative to appreciably accommodate current national policies in apparent deference to international standards.2 According to Professor Jian, even though China may boast currently of a ten percent annual economic growth, widespread poverty still haunts its rural interior. The regime can employ its extensive masses in the service of its growing

Shamma Al Rathy Essay Example | Topics and Well Written Essays - 750 words

Shamma Al Rathy - Essay Example ill be charged for the architects' services instead of at cost, significantly increase the company's assets and, correspondingly, the shareholders equity; Such "unbilled" receivables may mislead if they include also uncompleted stages of the projects. It would be recommended to reflect the projects at cost. At the end of reporting period ongoing projects should be evaluated, percentage of their completion estimated, and corresponding revenue recognized. 6. Premium Coupons: From consumers' point of view these coupons add value and promote purchase of coffee, so cost of redeemed coupons should apply to the sales revenue of coffee; Given that the company can reasonably estimate from previous experience percentage of the coupons that will be redeemed in the future and that the sale of promotion coffee has already been maid, allowance for the 10% of outstanding coupons should be applied to the 2004 sales revenues for coffee. 7. Travelers Checks: Bank records 1.5% fee as its revenue; American Express records increase in the checks outstanding and unearned revenue. 8. Product Repurchase Agreement: Neither of the manufacturers has revenue in 2004: Manufacturer A should not recognize AED 600,000 as revenue because of possible repurchase of the product in the future, Wholesaler B does not have revenue because compensation for its services will be paid only in July of the following year. 9. Franchises: The initial services (training, introduction to the referral system, and marketing aids) are provided during the year when the agreement is signed. 75% of the receipts come from the annual fees. The company should recognize the initial franchise fee as revenue in the same year the agreement is signed, or allocate it between two or three first years. If the market becomes saturated,...Trees left to grow for one more year are equivalent to work-in-process inventory for manufacturing companies. On-going projects reflected at the rates at which the customers will be charged for the architects' services instead of at cost, significantly increase the company's assets and, correspondingly, the shareholders equity; It would be recommended to reflect the projects at cost. At the end of reporting period ongoing projects should be evaluated, percentage of their completion estimated, and corresponding revenue recognized. Given that the company can reasonably estimate from previous experience percentage of the coupons that will be redeemed in the future and that the sale of promotion coffee has already been maid, allowance for the 10% of outstanding coupons should be applied to the 2004 sales revenues for coffee. The company should recognize the initial franchise fee as revenue in the same year the agreement is signed, or allocate it between two or three first years. If the market becomes saturated, the company's profits are likely to drop 25% in comparison with the previous year and then be kept at the same level.