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Chapter 4

BIO204 CH42 (Lecture 14/15)

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Sanja Hinic- Frlog

CH42 (14/ 15) November-05-13 2:46 PM Electrolyte - a compound that dissociates into ions when dissolved in water 4 ways animals gain water: 1. By absorbing it via osmosis 2. By drinking 3. By eating 4. As a by-product of cellular respiration Lose water: via urine & feces, evaporation, or osmotic loss Osmoregulation and osmotic stress • Diffusion - occurs anytime a solute is at higher [ ] in one location than another (along concentration gradient) Selectively permeable membrane - membrane that some solutes can cross more easily than other solutes can • Osmosis - involves a selectively permeable membrane; movement of water from areas of higher water concentration to areas of lower water concentration • Osmolarity - concentration of dissolved substance in solution (measured in mol/L) IT WILL SWELL • Osmoregulation - process by which living organisms control the [] of water and salt in their bodies Not every marine fish osmoregulate the same way • Tissues of sponges, jellyfish, and flatworms are isotonic relative to seawater: solute [] inside and outside these animals are equal • Most marine invertebrates are osmoconformers (do not actively regulate the osmolarity of their tissues) • Marine fish are osmoregulators (actively regulate osmolarity inside their bodies to achieve homeostasis) homeostasis) ○ Their tissues are hypotonic relative to seawater (solution inside body contain less solutes than outside) □ Osmoregulation - replace what is lost • Crazy fish - comes from unusual behaviour (predates on an angle) • Gill epithelium of freshwater fish is hypertonic relative to surrounding water (solution inside cells have more solutes than outside), so epithelial cells gain water through osmosis • Evaporation (not osmosis) Water and electrolyte balance in aquatic environments How do sharks excrete salt? - Proteins and processes involved also occur in other marine animals - Cells don’t transport ions against their electrochemical gradient directly but instead cells move ions indirectly (use active transport to set up strong electrochemical gradient for a different ion -- typically Na+ in animals and H+ in plants, sodium proton gradient is used to transport without further expenditure of energy) How sharks osmoregulate - Rectal gland secretes a concentrated salt solution The role of Na+/K+-ATPase - Sodium potassium pump - pumps ions out of extracellular fluid toward lumen of gland where they are excreted. Molecular model for salt excretion - Cotransporters - membrane proteins that transport more than one type of ion or molecule at a time (symporter in shark) The salt-secreting system discovered in sharks is found in marine birds and reptiles that drink salt water and excrete NaCl via glands in their nostrils, marine fish that excrete salt from their gills and mammals that transport salt in their kidneys. The shark rectal gland secretes a concentrated salt solution. Ions can be concentrated only if they are actively transported against a concentration gradient. Epithelial cells along the inner surface, or lumen, of the shark rectal gland contain sodium-potassium pumps. Salt excretion is a multistep process: Na+/K+- ATPase creates an electrochemical gradient favouring the diffusion of Na+ into the cell, allowing the cell to transport other ions without additional energy expenditure; Na+, Cl–, and K+ enter the cell, powered by the Na+ gradient; Chloride channels allow Cl– to diffuse down its concentration gradient into the lumen of the gland; Na+ diffuses into the lumen of the gland, following their charge and concentration gradient. YUP. Sharks slowly lost some weight and expelled urea into their surroundings, level or urea in blood and overall osmolarity remained unchanged. Shark is consuming its own proteins to replace urea. How do salmon osmoregulate? Many species of salmon are anadromous (young develop from eggs laid in freshwater, then migrate to the ocean where they spend several years feeding and growing, then return to freshwater to breed) How do they go from hypotonic to hypertonic environment? They increase Na+/K+-ATPase activity in gills Location of channels will change depending on environment Animals that move between environments with dramatically different osmotic stresses such as marine to freshwater environments have a gill epithelium with specialized cells called chloride cells which are capable of moving salt. When such animals are in salt water, these cells are abundant and active. This also implies that freshwater salt water, these cells are abundant and active. This also implies that freshwater version of a chloride cell imports salt (rather than excrete it). Evidence for the existence of this cell is supported by the following observations: osmoregulatory cells may be in different locations; different forms of Na+/K+-ATPase may be activated; the orientation of key transport proteins “flips.” When osmotic stress changes from marine to freshwater environments, the nature of the gill epithelium changes. Specifically, active pumping of ions takes place in a different population of cells in seawater versus freshwater. Additionally, different forms of sodium-potassium pumps may be activated when individuals are in salt water versus freshwater Answer: E, this is a general rule, there are exceptions Q#1: Desiccation is probably the biggest issue, especially important for small animals on land. Terrestrial animals with a large surface area relative to their size (volume) have a more difficult time with water homeostasis Q#2: Structure for exchange of solutes and wastes on land must be protected from desiccation. One solution may be a water-‐impermeable coating, which minimizes evaporation form the body surface. This indeed exist in many terrestrial animals. In terrestrial insects, the surface area of the body is covered with the exoskeleton that consists of chitin, a tough polysaccharide, and layers of protein, or collectively, the cuticle. The cuticle is also covered with waterproof wax. Additional water loss is also a result (by-product) of respiration. Insects obtain oxygen through small openings on their cuticle, which can be closed to prevent water loss to the external environment during respiration (more on respiration in Lectures 18 and 19) How do insects minimize water loss from the body surface? Gas exchange occurs across the membranes of epithelial cells that line an extensive system of tubes called tracheae Spiracles = openings that close/ open due to environmental changes Cuticle = chitin + protein Types of nitrogenous wastes: impact on water balance Ammonia - strong base, a by-product of catabolic reactions and it can readily form ammonium ion, which can be toxic to cells - Converted to urea (less toxic compound, excreted in urine) in humans - Converted to uric acid in birds, reptiles, terrestrial arthropods Uric acid has low water solubility Type of nitrogenous waste produced by an animal correlates with its lineage - its evolutionary history, but also correlates with the amount of osmotic stress a species endures (depends on the habitat of the species) There is a fitness trade-off between the energetic cost of excreting urea or uric acid and the benefit of conserving water Fish detoxify ammonia by diluting it to a low concentration and excrete it as watery urine. In freshwater and saltwater fish, ammonia diffuses across the gills into the surrounding water along a concentration gradient. Birds, reptiles, and terrestrial arthropods convert ammonia to uric acid, which can be excreted as a dry paste. Humans convert ammonia to less toxic urea and excrete it in urine. Key point is that there is a fitness trade-off between the benefit of conserving water and energetic cost of producing nitrogenous waste! (make sure you understand what this means – if not, ask me) Maintaining homeostasis: the excretory system - insects must carefully regulate the composition of a blood-like fluid called hemolymph - Malpighian tubules, and on their hindgut—the posterior portion of their digestive tract (excretory organs that maintain water and electrolyte balance) In order to balance water intake and nitrogenous waste, insects regulate the composition of a bloodlike fluid called hemolymph. Insects have to: remove nitrogenous before they build up to toxic concentrations; excrete excess electrolytes before they create osmotic stress; regulate water balance constantly. To accomplish this, insects Malpighian tubules (an excretory o
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