Table of Contents
- Osmoregulation Energetics
- Osmoregulators in Freshwater and Seawater
- Land Osmoregulators
- Lesson Summary
Every organism operates within a fluid environment, from cells and tissues to organs and organ systems. For these organisms to function properly, water and solutes must be maintained within a narrow margin. This can be challenging for animals due to stress from the external environment. For example, a freshwater organism that lives in an external environment that creates challenges by threatening to flood and dilute body fluids must possess adaptations that reduce water uptake, conserve solutes, and absorb salts from its surroundings. Organisms must utilize the homeostatic process of osmoregulation in order to regulate solute concentrations and balance the gain and loss of water. The kidney is an organism's greatest balancing act. The main role of the kidney is to maintain osmotic balance by regulating water and salts within the body. This is essential for the body to function correctly.
Water enters and leaves an organism through osmosis. Osmosis occurs any time two solutions are separated by a membrane that differ in osmotic pressure, or osmolarity. If two organisms separated by a selectively permeable membrane have the same osmolarity, they are classified as isosmotic. In this instance, there is no net movement of water between the solutions. Water will move at equal rates in both directions. If two solutions differ in osmolarity, the one with the greater concentration of solutes is said to be hyperosmotic and the one that has a lesser concentration of solutes is said to be hyposmotic. Water will flow from the hyposmotic to the hyperosmotic one.
Marine animals can be isosmotic to their surroundings. These animals do not actively adjust their internal osmolarity and are referred to as osmoconformers. Because an osmoconformer is isosmotic (the osmolarity internally is equal to the surrounding environment), there is no need to gain or lose water. These organisms live in water that has a stable composition, therefore allowing them to maintain a constant osmolarity. One example of an osmoconformer is a jellyfish. Jellyfish maintain a body fluid concentration isotonic (equal) to their environment. Osmoconformers, like the jellyfish, have no need to osmoregulate because seawater has a stable composition and closely matches the concentrations found within these marine organisms.
In contrast to osmoconformers, osmoregulators are animals that need to control their internal osmolarity because their body fluids are not isosmotic with their environment. Osmoregulators must release excess water into the outside environment if they live in a hyposmotic (freshwater) environment, or they need to take in water to offset water loss if they live in a hyperosmotic environment. Osmoregulation enables organisms to live in environments that osmoconformers cannot. Additionally, it allows marine organisms to maintain an osmolarity different from seawater. For example, sharks have an internal salt concentration much lower than that of seawater. As a result, salt diffuses into the shark's body from the seawater across their gills. The shark's kidneys filter the excess salt and excrete it.
Energetics is the study of the amount or type of energy used in a particular process, such as osmoregulation. Any time an animal has to maintain an osmolarity difference between the body and the external environment, there is an energy cost. Diffusion of water and salts will occur to establish equilibrium of the system; however, osmoregulators must expend energy to regulate the osmotic gradients that cause water to flow in and out of the system. Active transport, the process of pumping molecules against the concentration gradient, occurs to manipulate the solute concentrations within the animal's body.
|live in environments with very stable compositions||not isosmotic to their environment|
|have a constant internal osmolarity||must control their internal osmolarity|
|no need to gain or lose water||discharge water if the animal lives in a hypotonic environment or takes in water if the animal lives in a hypertonic environment|
|regulates naturally through osmosis and diffusion||requires energy to regulate|
|example: jellyfish||example: human|
Humans are osmoregulators. If trapped at sea, a human could not drink seawater to survive. The regulation of osmotic pressure is controlled by the kidney. The kidney filters the blood to regulate the concentrations by regulating the amount of water and salts within the blood. Any excess is filtered out and excreted in the urine. Producing excessive amounts of urine is a way to remove excess water while retaining solutes.
Osmoregulation is essential for an organism to maintain homeostasis. Organisms must balance water in order to survive. More aquatic animals are found in the sea than anywhere else. Most marine animals are osmoconformers; however, some require osmoregulation. Saltwater fish that osmoregulate do so by gaining water and salts from drinking seawater. To osmoregulate, the saltwater fish will filter excess salt ions across the gills and through the kidney. A small amount of water is also excreted.
The osmoregulatory issues of freshwater fish differ greatly compared to those of marine animals. Fish that live in freshwater constantly gain water through osmosis, not drinking, and lose salts by diffusion for osmolarity, because their internal solute concentration is higher than their surroundings. Many freshwater fish maintain balance by excreting large amounts of diluted urine.
The threat of water loss is a significant regulatory problem for land animals and plants. Humans will die if they lose roughly 12% of their body's water. In order to prevent this, humans must maintain a balanced internal environment of water and solutes. The kidneys of vertebrates function in both osmoregulation and excretion. The kidneys regulate the water concentration within the bloodstream, filtering out excess water into urine and holding onto it if an individual is dehydrated.
Animals that live in dry environments, such as camels, can withstand about twice that level of dehydration due to adaptations. Plants have a waxy cuticle to prevent water loss. The eggs of birds and reptiles are protected by a hard, waterproof shell. These adaptations are essential to these organisms' survival.
Energetics is the study of the amount or type of energy used in a particular process, such as osmoregulation. Every organism operates within a fluid environment. For these organisms to survive, water and solutes must be maintained within a narrow margin. This can be challenging for animals due to stress from their external environments. The way animals regulate this differs. All animals face the same central problem of osmoregulation, the homeostatic process to regulate solute concentrations and balance the gain and loss of water. Osmosis occurs any time two solutions are separated by a membrane that differ in osmotic pressure, or osmolarity. Some animals are classified as osmoconformers and others are osmoregulators. Osmoconformers, such as the jellyfish, live in environments with very stable compositions and also have a constant internal osmolarity. These organisms are isosmotic to their environment. Therefore, there is no tendency to gain or lose water due to their stable composition and constant internal osmolarity. Freshwater fish gain water through osmosis rather than drinking and remove salts through simple diffusion, which requires no energy.
In contrast, osmoregulators must control their internal osmolarity because they are not isosmotic to their external environment. Marine animals are mostly osmoconformers; however, some must osmoregulate. As a result, they must excrete excess water if they live in a hyposmotic environment or take in water if they live in a hyperosmotic environment. One example of an osmoregulator is a human. Humans must regulate water and solutes in order to survive. Additionally, marine animals can excrete only a little fluid in order to keep their bodies balanced. The kidneys play a critical role in maintaining homeostasis within the body by maintaining osmotic balance. Their job is to regulate the amount of water, ions, and other nutrients within the blood, filtering out what the body doesn't need. This excess is excreted out as urine. The body, specifically the kidneys, controls the concentration of urine depending on the needs of the person. If the individual is dehydrated, the body will excrete a more concentrated (less water) urine, resulting in a dark yellow color. In times of excess fluid, the body will excrete a diluted (more water) urine, resulting in a lighter, more clear color. In addition to humans, some organisms have special adaptations in order to survive. Animals like the camel are adapted to live in dry environments by being able to withstand about two times the level of dehydration as humans. Plants are equipped with a waxy cuticle to prevent water loss, and the eggs of birds and reptiles are protected by a hard, waterproof shell. These adaptions are critical in the survival of these organisms.
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Humans are osmoregulators. If trapped at sea, a human could not drink seawater to survive. The regulation of osmotic pressure is controlled by the kidney. The kidney filters the blood in order to remove excess water and solutes, which are excreted as urine.
Organisms must utilize the homeostatic process of osmoregulation in order to regulate solute concentrations and balance the gain and loss of water. Osmoregulators are animals that must control their internal osmolarity because they are not isosmotic to their environment. These animals must expend energy to regulate their internal environment by either discharging water if the animal lives in a hypotonic environment or taking in water if the animal lives in a hypertonic environment.
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