9.4 Water Balance The bodys internal environment of extracellular fluld must mai

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9.4 Water Balance The bodys internal environment of extracellular fluld must maintain a constane solute content, and often temperature. Humans and many other terrestridwe ute uarrequirea stable aqarous environment to suarvive. They carry this aqueous solutioniide their body, and they must continuously replenish and maintain i Aquatic organiams must also maintain their internal environment, since the ditions of the external environment change in terma Atlantic salmon (Salmo salar) spends part of its life cycle in both freshwates saltwater emvironments.For the fint two or three years of its life, the salmon freshwater rivers and lakes where the solute concentration (about Q.1 ) is le the concentration wihin its body (about LO %). This oncentration gradient the upeake of water into the salmon's internal environment, and therefore mechanisms must expel the excess water from the body. As the salmon abandons the freshwater environment for the salt water of the Atlantic Ocean the concentration of solutes is about 35 In this saltwater environment. the of solutes and temperature. The molecules Ives lesw then sugar solutorn water less concentrated selectvely xperiences a continuous los of water from its body which it must replenish permeabie In addition to changing solute concentrations, there are other membrane ences betnween terrestrial and aquatic environments. Because water is not as to conserve water and maintain a homeostatic balance of solutes within their bodh In this section, you will consider the following questions: Why do animals need maintain a water balancet Why do animals need an excretory system? Osmosis In Chapter 2, you learned about the role of osmosis in the homeostatic water balance of cells. During osmosis, water molecules move from a region where they are highly concentrated to a region where their concentration is lower. This movement occurs across a selectively permeable membrane, which allows water but very few solute of water molecules to flow through. The different water concentrations on the two sides of Figure 1 () Two solutions have dfent membrane are produced by diferent numbers of solute molecules (Figure Ifa). The side with a lower concentration of solutes has a higher concentration of water mol- concentrations. (b) Osmosis occurs when water moves across a selectvety ecules. The water moves osmotically across the membrane to the other side, where permeable membrane from an area of the water concentration is lower (Figure 1(b)). Selective permeability is a key factor igh concentradion to an area of low in osmosis because it helps to maintain differences in solute concentration on the two sides of biological membranes, such as cell membranes. Proteins are among the most important solutes in establishing the conditions that produce osmosis osmotic pressure the pressure h results from a difference in solute Osmotic pressure is the pressure that results from a difference in water concentra tion, or a water concentration gradient, between the two sides of a selectively perme concentration between f a selectively permeatle membranepressure hyperosmotic the property of the the two sides able membrane. The greater the water concentration gradient, the greater the osmotic difference between the two sides. A solution with a higher concentration of solute molecules on one side of a selectively permeable membrane is said to be byperosmotic (hypertonic) to a solution with a lower concentration of solutes on the other side. Water tends to move to the hyperosmotic side. The solution with a lower solute concentration is said to be hypoosmotic (hypotonic) to the solution with the higher solute concentration. Water tends to move from the hypoosmotic solution Solutions with the same solute and water concentrations are lsoosmotic (isotonic) solution on one side of a selectively permeable membrane that has the lower concentration of water hypoosmotic the property of the solution on one side of a selectively permeable membrane that has the higher concentrations. This movement tends to continue until the two solutions are isoos Water moves across the membrane when the two solutions have different water motic. Water still moves across the membrane even when the solutions are isoosmotic lsoosmotic the property of two solutions but the water movement is equal in both directions so there is no net movement that have equal water concentrations Another factor that determines whether osmosis will occur is hydrostatic, or wates. pressure. If the hydrostatic pressure on one side of a membrane (such as a plant cell membrane) is equal to the osmotic pressure on the other side of the membrane, there s no net flow of water. As water continues to cross the membrane, the internal 442 Chapter 9 Homeostasis: A Fine Balance

Explanation / Answer

About 73% of our body is water. Water acts as a medium and forms the bases of conduction in the body. it is essential for survival. Terrestrial organisms ofterm have various mechanisms to conserve the water in their body. A water balance is made by the body through homeostasis. Even in aquatic organisms, maintaining this water balance is crucial when and if the salinity of the water changes. Water is present in both extracellular and intracellular environment in the body. the movement of the water molecules across a selectively permeable membrane along a concentration gradient is called osmosis. This mainatins a concentration gradient for different solutes across the membrane. The concentration gradient of water also gives rise to osmotic pressure which is higher on the side of higher water concentration. Such solution will have low solute concentration as well and can also be called as hypotonic. A solution with higher solute concentration or low water concentration is called hypertonic, if however, the potential is same on both sides of the membrane there is not net flow of water and both solutions Are called isotonic/isoosmotic. Hydrostatic pressure also influence osmosis, if the hydrostatic pressure is more one one side than the osmotic pressure then there will not be any water movement. Hydrostatic pressure increases upon movement of water and is important to maintain turgitdity inside them due to the pressure exerted by water on cell walls. This cannot happen in animal cells since they donot have a cell wall. upon increasing the pressure inside them they will burst. Reciprocably, if the pressure decreases they will shrink and may collapse. When osmosis is applied in regulation of osmotic pressure, water balance and electrolyte balance in the body fluids it is referred to as osmoregulation. Although, most of the times the cells in the body are maintained at isosmotic potential. waste products which increase in concentration upon metabolism disrupts the isosomosis whihch is why they are continuosly removed from the body.

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