11 3 Aquatic Osmoregulation Animal cells are permeable to water and can regulate the extent of osmosis by inserting and removing aquaporin proteins from their membranes Regulating intracellular inside the cell concentrations of ions and other solutes is crucial as the rate of chemical reactions and the function of enzymes depend on their chemical environment Even the simplest animals are able to regulate intracellular environment using a semi permeable phospholipid bilayer and combinations of membrane proteins As you saw in the previous lesson cells have a variety of membrane proteins that facilitate diffusion of polar charged or larger substances as these cannot diffuse efficiently through the phospholipid bilayer to enter or exit the cell Cells can also use ATP to actively pump substances in or out of the cell against their concentration gradients allowing cells to build up some substances and keep others out In addition to regulating the levels of individual solutes this also regulates cell volume as water will diffuse towards areas of higher solute concentration via osmosis What are the osmotic and ionic challenges that an animal s cells face First of all the extracellular fluid ECF the chemical environment that each cell lives in will be affected by the animal s environment In aquatic animals solutes and water can potentially diffuse between the water the ECF and the inside of their cells Thus the chemical composition of their aquatic environment presents a challenge to cellular levels of solutes and water Second each cell needs oxygen and nutrients to respire and stay alive and produces CO2 and nitrogenous waste products as a result of metabolism The amounts of each of these substances must be regulated so that each cell can actively respire and not build up toxic waste products We have already learned about how animal bodies get oxygen to their cells and CO2 out of their cells via the circulatory and respiratory systems Osmoregulation is primarily concerned transporting ions waste products and other solutes Many of these need to be removed from the cells but others like ions are necessary for cellular function and their availability and balance are crucial This lesson will cover some of the osmoregulatory processes in aquatic animals both marine and freshwater As you can see in the table below some marine animals do not regulate their ECF contents allowing it to osmoconform and ionoconform to their environment All other marine animals and all freshwater animals osmoregulate and ionoregulate with the exception of hagfish to varying degrees In the simplest marine invertebrates like sponges there is no ECF The entire animal is only a few cell layers thick so the cells are directly bathed in seawater Each cell uses the previously mentioned regulatory mechanisms which is sufficient to prevent water loss and maintain cellular function On the other hand freshwater sponges must prevent their cells from swelling and bursting as their environment is hypoosmotic to their cells The cells of freshwater sponges have contractile vacuoles compartments that can expand and contract using cellular motor proteins The cell can fill this vacuole with water and even concentrate toxic substances inside of it sequestering them from the rest of the cell then expel its contents into the surrounding water This allows the cell to take on water without disrupting cellular function then bail it out to prevent the cell from bursting Contractile vacuoles are also common in freshwater protozoans a diverse group of unicellular amoeboid non animal eukaryotes such as Paramecium as shown below When these simple animals metabolize proteins and produce amine groups as nitrogenous waste products they simply release the ammonia into the surrounding water Although ammonia is toxic because these animals are aquatic it becomes quickly diluted Almost all aquatic animals are ammoniateles meaning they do not process their nitrogenous waste to make it less toxic because it can be released into the water and diluted Almost all other aquatic animals regulate both the total osmolarity of their ECF and blood and the specific ion contents They do this using three processes filtration reabsorption and excretion The cells of the animal will produce waste which will diffuse from cells to ECF and blood The circulatory system will carry these waste molecules as well as many ions and useful byproducts of metabolism to be filtered out of the blood Some of these molecules will be reabsorbed back into the body the remainder will be excreted Most animals have multiple tissue layers including an epithelial layer that covers the external and internal surface of the animal separating its interstitium from its environment The epithelial cells are particularly adapted to regulating solute and water balance via these three processes forming a regulatory barrier between the environment and the rest of the animal s cells These cells form a variety of filtering reabsorbing and excreting structures in animals that vary with osmotic challenge of the animal s habitat as well as lineage In some worms there are epithelial structures specialized for osmoregulation called protonephridia During development some of the epithelial cells that make up the outermost layer of the animal the epidermis invaginate towards the center of the body These epithelial cells secrete a substance that forms the basal lamina which forms a filtration sheet on their basolateral side One end of each protonephridia is open to the aquatic environment the other ends in a flame cell This is a flagellated cell that draws fluids from the interstitium out the protonephridum through the nephridial tubule Cellular waste diffuses down its concentration gradient into the interstitial space where it is drawn towards the protonephridia and filtered through the basal lamina The resulting filtrate is then driven out of the body through the nephridiopore The tubule is lined with cells that have convoluted apical surfaces These increase surface area allowing the worm to re absorb some molecules instead of excreting them More complex aquatic animals that have circulatory systems like molluscs and annelid worms have metanephridia These animals have hemolymph in their circulatory system and a separate fluid filled body cavity called the coelom Their blood vessels are lined with podocytes which form a leaky epithelial layer that allows paracellular transport Due to a blood pressure that is
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