BIO 203 Water & Ion Balance Summary PDF

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Summary of the water and ion balance lectures given in BIO 201, written in paragraph form with figures and highlights.
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3. Water & Ion Balance Recall that all life is aquatic, and the basic environment in all internal environments of animals is a relatively similar solution of salt water (chapter 1). Thus, organisms need to keep a relatively constant water and ion (salts) balance through homeostasis. This process is tightly associated with temperature regulation because evaporative cooling, which reduces water, is an important mechanism in most endotherms. In a human body, most of the water is in the ICF, and red blood cells makes up 40% of the blood supply. Solutes, like heat, will move down their concentration gradient through diffusion. Thus, a similar equation to the rate of heat transfer equation that determines membrane permeability is called the membrane flux equation. The membrane of most cells consist of a phospholipid bilayer, with a polar head and a hydrophobic tail. Hydrophobic substances can easily diffuse through the lipid bilayers, while hydrophilic substances cannot simply diffuse through, and require a transmembrane protein, which are pores that provide pathways for the movement of water and ions across the lipid bilayer. Transmembrane proteins are highly regulated because it is the mechanism for which the cell receives and exports various substances. One type of transmembrane protein is the ion channel, which can be either in an open or closed state. Ion channels are selective. For instance, if an ion channel is permeable to K+ but impermeable to Na+, it is selective and is called a K+ channel. All channels, including ion channels, are always passive transport. There are two mechanisms of moving substances between membranes: passive transport and active transport. Passive transport does not require energy and requires a simply concentration gradient (diffusion). Other factors like electrical forces (ions) can also influence passive transport. Active transport requires energy and is when a substance moves against its concentration gradient. An example of an active transport transmembrane protein would be the important Na+-K+ ATPase. This protein is the primary mechanism for maintaining the concentration gradients of Na+ and K+. No mechanism actively transports water across membranes; the only mechanism for water movement is osmosis (diffusion of water). Osmosis is an colligative property, which are properties of a solution whose magnitude depends only upon the solute concentration and is independent of the nature of the solute (e.g. boiling point elevation, freezing point depression). Osmotic pressure is the pressure that must be applied to oppose osmosis (i.e. the hydrostatic pressure produced by osmosis). Osmolarity is an index of water concentration. The higher the concentration of solutes, the lower the concentration of water and the higher the osmolarity of the solution. Water will always move from a region of low osmolarity to high osmolarity. Note that solutes like NaCl that dissociate will contribute more to osmolarity (i.e. it will decrease the osmolarity more than solutes that do not dissociate).

When comparing osmolarity between solutions, the terms isosmotic, hypoosmotic, and hyperosmotic is relative. While both refer to the concentration of water, osmolarity is different than tonicity, the latter defined as the effect a solution has on the cell. The difference is that osmolarity does not initially define a frame of reference while the tonicity’s frame of reference is always the solution outside the cell. For instance, a hypotonic solution will always refer to the solution outside the cell that is hypoosmotic with respect to the ICF (i.e. a solution outside the cell that will make the cells get larger and burst). Cells always want to maintain an environment that is relatively isotonic (and therefore isosmotic).

Maintaining water balance in an organism directly depends on maintaining ion balance. The external environment’s water/ion concentration directly affects the internal environment of the organism (ECF), which in turn affects the cells (ICF). When discussing water/ion balance, we are referring mostly to the relationship between the external environment of the organism and the ECF. Thus, if an organism is termed hyperosmotic, it is implied that the organism is hyperosmotic relative to its environment.

Organisms can be classified as an osmoregulator, osmoconformer, eurohaline, and setnohaline. They can also be defined as an ionoconformer or an an ionoregulator. An organism can be both an osmoconformer and an ionoregulator, but all organisms that are osmoregulators are also ionoregulator, because in order to regulate water organisms need to regulate ions (i.e. there is no active way to regulate water). It is interesting to note that all vertebrates have a relatively constant osmolarity (300 mOsm) and a relatively similar concentration of Na+ and Cl-. A hypothesis for this is that this distribution of water and ions reflected the solutions of the oceans during when vertebrate ancestors evolved into land animals.

Organisms have a water budget that maintains a constant balance of water/ions. Most vertebrates are osmoregulators with an osmolarity of around 300 mOsm because they are, of course, living on land, where the concentration gradient of water is much different. It is important to note that water volume and osmolarity is different. An animal can have the same osmolarity as another animal but still be dehydrated because osmolarity is a ratio. Some pathways of water intake are through drinking, food, metabolic water, and uptake from body surface. Pathways for water loss include evaporation and respiration, which are large problems for terrestrial animals. Factors to consider in terms of water/ion balance include: 1. Availability of water and salts (aquatic vs terrestrial, seawater vs freshwater, arid vs humid habitats) 2. Respiration and temperature (applies only to terrestrial animals) 3. Permeability of skin (frogs/amphibians have highly permeable skins, while cows have highly impermeable ones) 4. Diet: can greatly affect salt uptake (seal eating marine invertebrates (1000 mOsm) vs fish (300 mOsm)) 5. Excretion: all osmoregulators have some mechanism to excrete excess water/ions (kidney, salt glands in marine birds/reptiles, gut/ gills in fish, skin/bladder in amphibians, Malpighian tubules in insects) Because all vertebrates have a body fluid osmotic concentration (BFOC) of 300 mOsm, fresh water bony fish are hyperosmotic while salt water bony fish are hypoosmotic. Thus, freshwater and salt water fish face very different osmotic challenges. Water is constantly trying to enter the hyperosmotic freshwater fish, while water is constantly trying to leave the hyperosmotic salt fish. In both cases, this osmosis primarily happens through the gills. Various strategies that fresh and salt water fish use to maintain there concentration of water/ions is shown in the table below. Transport epithelial cells are cells that are capable of transferring substances through the body of the cell. They typically have the following characteristics: 1. An asymetrical distribution of transporters in the apical and basal membranes 2. Are connected by tight junctions, which are physical connections that prevent solutes from moving between cells 3. There are many types of epithelial cells 4. Because these cells rely on active transport, they have abundant mitochondria to meet energy demands of ion transport Transcellular transport is the movement of solutes or water, usually involving active transport and transmembrane proteins, through epithelial cells. Paracellular transport is the passive movement of solutes or water between adjacent epithelial cells. Although the latter always involves passive transport (diffusion), they are also tightly regulated by tight junctions. There are many different types of transmembrane proteins in epithelial and other cells, shown in the diagram on the next page. Carrier proteins can be both active and passive transport, while channel proteins (i.e. ion channels) are passive transport (i.e. facilitated transport).

Epithelial cells take advantage of the laws of chemistry and physics and combine many different transmembrane proteins to transport substances across the cell. The gills of freshwater fish import ions like chloride, while the saltwater fish export ions. However, fish like salmon who migrate from freshwater to seawater undergo a process called smoltification, which generates new epithelial cells (does not modify old epithelial cells) that meet the demands of the new environment. Some terrestrial animals that live in very arid habitats, such as the kangaroo rat, can obtain most or all or their water by metabolism and food ingestion (i.e. they do not need to drink water). These animals are very efficient at conserving water (e.g. very concentrated urine, temporal counter-current exchange when respiring). Because animals like the kangaroo rat are very small, one of the challenges they face is that they have a high MR, which means their bodies will heat up quickly. On the other end, large desert animals like the camel have a low MR. Because their body temperature will change slowly, one survival strategy of large animals use is to fluctuate their body temperature throughout the day when dehydrated. Large desert animals can also afford to be active during the day, while smaller animals are usually nocturnal to prevent overheating. Note that, especially in arid habitats, water/ion balance is directly related to temperature regulation....