Plant species have adapted to suit their environment via natural selection. Plants in dry/arid habitats such as Cacti have cut down the number of stomata, and lowering their evaporating surface areas. In the other direction plants in a rain forest have developed far larger surface areas and greater number of stomata to allow far greater water potential to develop within the plant.
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Air Movement, water vapour will be concentrated around the outside of the stomata. This is known as diffusion shells. The movement of air around the plant will blow away the diffusion shells increasing the rate of evaporation from the leaf.
Plants require a variety of mineral elements. The required minerals are absorbed form the environment in which the plant is occupying. Within aquatic algae concentration of certain ions will be far greater with in the plant cells than in the surrounding water. This means ions must be taken in against the concentration gradient via active transport.
Certain factors contribute to the rate of active uptake. A temperature increase will increase the uptake of minerals within plant, similarly oxygen depletion and other such limiting factors will decrease the uptake of minerals.
One consequence of creature that has to eat is that there are metabolic waste products which cannot be reabsorbed by the body, these must be excreted. The majority of animals have excretory organs of some sort such as Malpighian tubule as is the case in insects or kidneys in vertebrates. These will remove the waste products and will help to maintain and control the volume of water within the body.
Water conservation by controlling the concentrating their urine, is only seen in birds and mammals. Vertebrates have kidneys which are filled with nephrons, these serve to reabsorb salts, water, glucose and around 99% of the fluid filtered from the blood.
The concentration of urine is achieved by means of ‘the loop of Henle’. The ‘loop of Henle’ descends down into the medulla, as this happens the concentration of solutes in the surrounding tissues increases. Due to this increasing salt gradient within the tissue, water diffuses from the filtrate into the tissue so that the tissue and the filtrate have equal osmolarities.
The ascending loop of Henle is permeable to salts but not water. Salts diffuse out into the tissue and cause the filtrate to develop a low osmolarity. The filtrate passes through the collecting duct which is permeable to water and urea. Because the surrounding tissues have an increasing salt gradient, water and urea diffuse into the surrounding tissues. This causes the urine concentration to be increased.
The kangaroo rat is able to excrete urine which is up to eighteen times that of their blood, where humans are only able to excrete urine four times that of their blood. Kangaroo rats concentrate their urine the same way humans do via the ‘loop of Henle’ however within the kangaroo rat ‘the loop of Henle’ is significantly longer allowing the increased concentration. This works as the loop descends far deeper into the medulla and produces a higher concentration gradient within the surrounding tissue, allowing a greater volume of water to be reabsorbed back into the body.
The process of excretion is the elimination of nitrogen from the system. When protein is broken down to carbohydrate to provide energy, the amine group is removed and must be dealt with. In the body the amine group is quickly oxidised to form ammonia. Ammonia is highly toxic and soluble in water, if the organism has a sufficient source of water; ammonia is excreted in the water. The ammonia will diffuse passively out of the repertory organs such as the gills.
Organisms with less fresh water available won’t waste water excreting nitrogen one atom at a time. They will more likely use energy to convert ammonia into urea, as this is less toxic. This only has two nitrogen atoms therefore takes less water to excrete. The reduced toxicity means it can be allowed to build up in the blood far more than ammonia, many organisms have specialized organs to remove urea and other waste products from the blood. Some organisms, such as sharks and snails, allow urea to accumulate in their blood to help with overall osmotic balance. Sharks, for instance, use urea in the blood to make them hyper osmotic in relation to seawater, thus they tend to gain water from the ocean and do not have to worry about dehydration.
Sharks maintain an internal environment which is hypertonic to the surrounding seawater (maintained concentration gradient, allowing the osmosis of water into the shark). They raise the internal osmoticity by raising the level of urea in their blood; due to this they will gain water from the seawater through their gills and the lining of their guts. The excess is excreted as dilute urine.
Marine vertebrates which have evolved on land such as turtles, reptiles and some mammals will have blood hypotonic to seawater. Since they breathe directly from the atmosphere there are no surfaces directly in contact with the seawater, this will reduce surface area on which water loss may occur. Water will still be lost during the excretion of urea in mammals and uric acid in turtles, reptiles and birds, when breathing, eating and drinking. The only sources of water intake for these creatures are the metabolic break down of carbohydrates or the drinking of saltwater (seawater). To overcome the gain of ions from this turtles and birds have evolved to develop salt gland near their eyes which actively transport ions out of the body. Just like those in the eyes of birds, marine fish (or invertebrates) have cells in their gills which allow ion secretion via hypertonic body fluids.
Organisms in marine environments are commonly isotonic in relation to seawater. If they do not have to regulate ion levels they are called osmoconformers. If their environment has a narrow range of solute levels like the ocean where there is little variance in the concentration levels of certain minerals, they are known as stenohaline.
Mammalian kidneys can reabsorb most of the water from urine, leaving urea and salts which need to be removed. They will also remove most of the water from the rectum so little water is lost as food waste is passed.
Fresh water organisms are mostly hypotonic in relation to their environment and face a constant input of water from the surrounding hypotonic solution this may allow the loss of ions to the external solution. This is why most fresh water fish cover their bodies with an impermeable and leave a relatively small number of cells for diffusion. These cells will actively pump ions into the cell using ATP, just like in the turtles but the other way round. The inside of the cell will gain a negative charge and this will cause the uptake of the other ions. Water flows into the body of freshwater organisms moves into the blood and will be excreted as dilute urine. Due to this changing of charge within the cells they are known as osmoregulators.