Biology Chapter 44 I

Biology Chapter 44 I - Chapter 44 Osmoregulation and...

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Chapter 44 Osmoregulation and Excretion Overview: A Balancing Act Metabolism also poses the problem of disposal of wastes. ° The breakdown of proteins and nucleic acids is problematic because ammonia, the primary metabolic waste from breakdown of these molecules, is very toxic. An organism maintains a physiological favorable environment by osmoregulation , regulating solute balance and the gain and loss of water and excretion , the removal of nitrogen-containing waste products of metabolism. Concept 44.1 Osmoregulation balances the uptake and loss of water and solutes Water enters and leaves cells by osmosis, the movement of water across a selectively permeable membrane. ° Osmosis occurs whenever two solutions separated by a membrane differ in osmotic pressure, or osmolarity If two solutions separated by a selectively permeable membrane have the same osmolarity, they are said to be isoosmotic. There is no net movement of water by osmosis between isoosmotic solutions, although water molecules do cross at equal rates in both directions. ° When two solutions differ in osmolarity, the one with the greater concentration of solutes is referred to as hyperosmotic, and the more dilute solution is hypoosmotic. ° Water flows by osmosis from a hypoosmotic solution to a hyperosmotic one. Osmoregulators expend energy to control their internal osmolarity; osmoconformers are isoosmotic with their surroundings. There are two basic solutions to the problem of balancing water gain with water loss. ° One—available only to marine animals—is to be isoosmotic to the surroundings as an osmoconformer. They have a very constant internal osmolarity. In contrast, an osmoregulator is an animal that must control its internal osmolarity because its body fluids are not isoosmotic with the outside environment. ° An osmoregulator must discharge excess water if it lives in a hypoosmotic environment or take in water to offset osmotic loss if it inhabits a hyperosmotic environment. ° Osmoregulation enables animals to live in environments that are uninhabitable to osmoconformers ° It also enables many marine animals to maintain internal osmolarities different from that of seawater. Whenever animals maintain an osmolarity difference between the body and the external environment, osmoregulation has an energy cost. ° Because diffusion tends to equalize concentrations in a system, osmoregulators must expend energy to maintain the osmotic gradients via active transport. ° The energy costs depend mainly on how different an animal’s osmolarity is from its surroundings, how easily water and solutes can move across the animal’s surface, and how much membrane-transport work is required to pump solutes. Most animals, whether osmoconformers or osmoregulators, cannot tolerate substantial changes in external osmolarity
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Biology Chapter 44 I - Chapter 44 Osmoregulation and...

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