Key Concepts Freshwater marine and terrestrial habitats pose different challenges to animals with regard to maintaining water and electrolyte balance In marine animals specialized epithelial cells have membrane proteins that remove excess salt NaCl from the body so that it can be excreted The same types of cells are found in the kidneys of mammals 2011 Pearson Education Inc Key Concepts In terrestrial insects the hindgut and Malpighian tubules are responsible for excreting water soluble waste products and achieving homeostasis with respect to water and electrolyte concentrations In terrestrial vertebrates the kidney is responsible for excreting water soluble waste products and achieving homeostasis with respect to water and electrolyte concentrations 2011 Pearson Education Inc Introduction The chemical reactions that make life possible occur in an aqueous solution If the balance of water and solutes in the solution is disturbed those chemical reactions and life itself may stop An electrolyte is a compound that dissociates into ions when dissolved in water Because cells require precise concentrations of Na Cl K and Ca2 to function normally maintaining electrolyte balance is crucial Water balance electrolyte balance and excretion of waste products are tightly integrated processes 2011 Pearson Education Inc Osmoregulation and Osmotic Stress Electrolytes and water move through organisms by diffusion and osmosis Diffusion is the movement of substances from regions of higher concentration to regions of lower concentration along their concentration gradients Osmosis is the diffusion of water through a selectively permeable membrane from areas of higher water concentration to areas of lower water concentration 2011 Pearson Education Inc Osmoregulation and Osmotic Stress Dissolved substances or solutes move down their concentration gradients across a selectively permeable membrane via diffusion The concentration of dissolved substances in a solution measured in moles per liter is referred to as its osmolarity Diffusion and osmosis affect animals differently in marine freshwater and terrestrial habitats As a result these environments pose different challenges to animals in maintaining water and electrolyte balance 2011 Pearson Education Inc 2011 Pearson Education Inc What Is Osmotic Stress Osmotic stress occurs when the concentration of dissolved substances in a cell or tissue is abnormal Living organisms control the concentration of water and salt in their bodies through osmoregulation Organisms such as sponges and jellyfish do not need to osmoregulate because seawater is a fairly constant ionic and osmotic environment and nearly matches the electrolyte concentrations found within these animals Relative to seawater their tissues are isotonic Such animals are called osmoconformers 2011 Pearson Education Inc Osmotic Stress in Seawater In contrast to osmoconformers most marine fish are osmoregulators Fish actively regulate osmolarity inside their bodies to achieve homeostasis Osmoregulation is required in marine vertebrates because their tissues are hypotonic to salt water the solution inside the cells contains fewer solutes than the solution outside They lose water by osmosis and gain electrolytes by diffusion 2011 Pearson Education Inc 2011 Pearson Education Inc Osmotic Stress in Freshwater Marine fish are under osmotic stress because they lose water and gain salt freshwater animals are under osmotic stress because they gain water and lose salt The tissues of freshwater fish are hypertonic to the surrounding water the solution inside the cells contains more solutes than the solution outside Thus their cells gain water through osmosis and lose electrolytes by diffusion 2011 Pearson Education Inc 2011 Pearson Education Inc Osmotic Stress on Land Land animals constantly lose water to the environment just as many marine animals do but they lose it by evaporation rather than osmosis Land animals also lose water when they produce urine sweat or pant 2011 Pearson Education Inc 2011 Pearson Education Inc How Do Cells Move Electrolytes and Water Solutes move across membranes by passive or active transport Passive transport is driven by diffusion along an electrochemical gradient and does not require an expenditure of energy in the form of ATP Facilitated diffusion is passive transport of solutes via proteins called channels or carriers Active transport occurs when ATP powers the movement of a solute against its electrochemical gradient Based on membrane proteins called pumps 2011 Pearson Education Inc 2011 Pearson Education Inc Active Transport Uses Pumps The sodium potassium pump Na K ATPase is the most important type of pump in animals Once a pump establishes a concentration gradient secondary active transport can occur The energy released when a solute is transported along its concentration gradient can be used by a cotransporter to transport another molecule against its concentration gradient Symporters move solutes in the same direction Antiporters move solutes in opposite directions 2011 Pearson Education Inc Water and Electrolyte Balance in Aquatic Environments Virtually all living fish experience osmotic stress Sharks have been used as a model organism in researching osmoregulation The salt secreting system discovered in sharks is found in a wide array of species including Homo sapiens Plant and animal cells use active transport to set up a strong electrochemical gradient for one ion typically Na in animals and H in plants which is then used to transport other substances without further expenditure of energy 2011 Pearson Education Inc A Common Molecular Mechanism of Salt Excretion In many animals epithelial cells that transport sodium and chloride ions contain the same combination of membrane proteins found in the shark rectal gland These species include Marine birds and reptiles that drink salt water and excrete NaCl via glands in their nostrils Marine fish that excrete salt from their gills Mammals that transport salt in their kidneys 2011 Pearson Education Inc A Common Molecular Mechanism of Salt Excretion Research on the shark rectal gland also had an unforeseen benefit for biomedical research A human protein called cystic fibrosis transmembrane regulator CFTR was identified and found to be 80 percent identical to the shark chloride channel Subsequent studies supported the hypothesis that cystic fibrosis results from a defect in a chloride channel 2011 Pearson Education Inc How Do