11 2 Introduction to Osmoregulation Approximately 700 million years ago mya after the evolution of eukaryotes and multicellularity the first animals evolved At this point all life existed in the ocean These simple animals had no control over the contents of their extracellular fluid ECF Like modern day sponges each cell in the animal s body was in direct contact with its salty watery environment Each cell contained macromolecules proteins lipids nucleic acids carbohydrates whose structure and function were affected by their chemical environment In order to maintain structure and function of its contents each cell had to independently regulate its balance of water and solutes or intracellular contents Cells do this by controlling traffic across their membranes creating a semi permeable membrane If transport of water and solutes across the membrane were not regulated the inside of the cell would always be at equilibrium with the outside of the cell thus the structure and function of its contents would be wholly dependent on its external environment The figure below illustrates the concept of a semi permeable membrane like that of a cell It then uses this concept to illustrate how a membrane impermeable to solutes but permeable to H2O can result in more water on one side of the membrane than the other If H2O is allowed to diffuse osmosis passively it will move towards higher solute The extent to which the solvent H2O moves is determined by the solute concentration and physical pressure on the solvent like gravity When salt is dissolved in a solvent on the left in the figure below it dissociates into ions that interact with the partial charges on the H2O molecules This binds up H2O reducing its ability to move freely and do work potential energy this is referred to as a reduction in its water potential When solutes are added to water it always lowers the water potential This is relevant to cells because water will move from regions of higher water potential to lower water potential or lower solute to higher solute If solutes are more concentrated on one side of the membrane water will move in that direction Until H2O is able to move from right to left it will exert osmotic pressure on the membrane The water will move across the membrane to the left until the pressure due to osmotic pressure and any physical pressures like gravity on either side of the membrane is equal When solute increases on one side of a semi permeable membrane as it does on the left in the figure below the solution becomes hyperosmotic Its higher solute will draw H2O towards it by passive diffusion A solution with fewer solutes relative to the other side of a membrane is a hypoosmotic solution H2O will leave this solution to go towards areas of higher solute When solute is equal on either side of a membrane the solution is isoosmotic These terms are related to but not synonymous with hypertonic hypotonic and isotonic respectively These refer to the tonicity of a solution which is a measure of only the impermeable solute Permeable solutes can cross the membrane contributing to the process of equilibration Impermeable solutes cannot so the only mechanism of equilibration is via osmosis Thus the concentration of these solutes represents the effective osmotic pressure Similar to the osmotic terms if a solution has more impermeable solutes than on the other side of the membrane it is a hypertonic solution etc These physical properties and terms are used to describe the osmotic and ionic challenges that an animal faces in its environment To maintain cellular function a cell has to maintain levels of water salts other solutes and pH regardless of its external environment As animals have colonized different habitats and done so multiple times they have evolved a wide range of physiological adaptations that allow them to do this Animals use different combinations of tissues to control ion and water balance in various internal and external environments as shown below For example a single animal can include a layer of mucous multiple layers of cells that have apical facing outward and basolateral facing inward sides interstitial space that contains extracellular fluid ECF blood vessels lined with endothelium and blood Each of these represents a different internal environment whose water and ion profile can be regulated We will focus on three processes used to maintain the internal state of an animal homeostasis osmotic regulation pressure of water in body fluids ionic regulation specific ions and nitrogen excretion Recall that animals have evolved to perceive that nitrogenous compounds taste bitter which they find unpleasant Nitrogenous compounds are often toxic but animal bodies make them as end products of protein metabolism as they are simply strings of amino acids where each amino acid has an amine group The animal will use other parts to build its own proteins and feed carbons into cellular respiration but it ends up with nitrogenous compounds leftover which are toxic and must be removed from the cell and the animal s body In water these diffuse down their concentration gradients into the surrounding water but excretion of these toxic nitrogenous compounds presents a challenge on land Based on environmental concentrations of water and salt and physical properties of osmosis each habitat presents different ionic and osmotic challenges Marine environments animals gain salt lose water Freshwater animals lose salt gain water Terrestrial animals lose water Further many animals move between different environments ionic osmotic challenges and must be able to acclimate by altering its homeostatic mechanisms to maintain function After animal life evolved 700 million years ago the next 150 million years of evolution gave rise to multiple tissue layers distinguishing cnidarians like sponges from all other currently living animals The evolution of multiple tissue layers during development resulted in epithelial tissue that could act as a barrier between the inside and outside of the animal Today this epithelial layer includes barriers on an animal s external surface like skin as well as barriers on internal surfaces like digestive tract excretory system Epithelial tissues have physiological functions in respiration digestion and ion and water regulation Over time these outer layers have evolved specialized structures that comprise the integument the organ system that covers the animal like the skin shell hair etc Multiple
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