1 11 2 Osmoregulation Introduction BIO 361T Fall 2014 1 Explain why water will diffuse across a semi permeable membrane in the direction of higher solute concentration Solutes dissolve by hydrogen bonding to water which reduces the potential energy of the water water potential and prevents it from moving The free water on the other side will diffuse towards the bound water until the osmotic pressure on either side is equal 2 Most of the water that crosses the membrane does so via facilitated diffusion a What membrane protein is involved in this Aquaporin increases osmosis across cell membrane by up to 100x b Why is there a membrane protein for diffusion of water into cells but not O2 Small non polar molecules like O2 diffuse freely across the non polar tails of the phospholipid bilayer Small polar molecules like H2O diffuse but more slowly c Why can t ions diffuse freely across the membrane like H2O Because they are charged As molecules get larger and more electronegative they are excluded more by the crowded non polar tails of the phospholipid bilayer d Give an example of a cell type that would be debilitated if ion concentrations were at equilibrium across the membrane Explain your answer Neuron difference in ion concentration across membrane is actively maintained because it creates a voltage which can be used to send fast electrical signals 3 The blue crab is euryhaline It can move from freshwater to seawater and has cellular mechanisms to cope with this change in salinity a What osmotic ionic challenges are present in freshwater Why Has low osmolarity so will exert osmotic pressure on cells and enter cells pull solutes out b What osmotic ionic challenges will be present in seawater Why Has high osmolarity so will pull water out c The osmotic pressure of seawater is 1100 milliosmoles L mOsm and freshwater is less than 10 mOsm How is this related to solute concentration molarity M More solutes will attract more water If the solutes dissolve they hydrogen bond to the partial charges on polar H2O molecules d Given your answer to c explain why 1 M of urea exerts 1 Osm of osmotic pressure but 1 M of NaCl exerts 2 Osm Urea is non polar but NaCl will dissociate into Na and Cl each of which will attract H2O and exert osmotic pressure 2 e Blue crab blood is 975 mOsm broken down as follows total 975 Na 472 K 10 Ca 16 2 Mg 7 2 Cl 470 What is the intracellular osmolarity of a blue crab total 222 01 f Na 5 K 105 2 Ca 01 Mg 2 2 Cl 110 Is the inside of the blue crab s cells hyperosmotic hypoosmotic or isosmotic to its blood Hypo osmotic g How will this affect the crab if it is not regulated Water will leave the cells causing them to shrivel and die if severe h Once a blue crab has acclimatized to seawater its blood will be similar to seawater and its cells will be at 941 mOsm However the intracellular concentrations of the ions listed above will not change significantly How is this possible What do you think accounts for the increase in intracellular osmotic pressure Other solutes besides those ions must have increased Blue crabs use other ions and sugars to acclimate a little bit short term but long term they increase urea a bit and amino acids and glycerol a lot Uncharged amino acids and glycerol are compatible solutes so they will not negatively affect function of enzymes like ions do i Why didn t the crab acclimatize by increasing its intracellular ion concentrations Enzymes are sensitive to ion function as proteins can be ionized and denatured If ion concentrations change inside the cell it will affect cellular function j How else could the crab have relieved the osmotic pressure on its cells Let H2O out of the cell which would cause the cells to shrivel This changes surface area of the cell for exchange and transport changes the shape of the cell and increases concentrations of contents affecting enzyme function and rate of chemical reactions k Is the blue crab osmoconforming or osmoregulating Explain your answer Osmoconforming the total solute concentration of its ECF blood is comparable to seawater It is only regulating its intracellular environment l Is the blue crab ionoconforming or ionoregulating Explain your answer Ionoconforming the solute profile of its ECF blood is also comparable to seawater It is only regulating its intracellular solute profile
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