ExcretionExcretion is the process whereby an organism eliminates metabolic wastes and unwanted chemicals from its system. Metabolism is the sum total of all the chemical reactions occurring in the cells and body. Some products of these metabolic reactions are toxic and so must be processed or eliminated from the body. Others are simply materials that are present in excess and so must be eliminated as waste. The process of excretion is quite different to defecation, which is the removal of undigested food wastes from the gut. However, the gut of many animals also has a role in excretion as some materials may be excreted into the gut and eliminated with the faeces. In insects most excretory products are excreted into the gut lumen and eliminated along with faecal matter. Excretion is also important in eliminating excess water and other unwanted chemicals that may be ingested and enter the body fluids, such as plant poisons and excess salts.One of the main functions of excretion is to remove excess nitrogen. Nitrogen enters the diet in the form of amino acids, nucleic acids and certain salts. One of the main products of excretion in aquatic organisms is ammonia. Ammonia contains nitrogen and is a small molecule which dissolves readily in water. This allows it to be easily excreted into the surrounding water. However, this becomes a problem for terrestrial organisms. Ammonia is toxic to cells and so must be quickly ejected from the body, however, being water-soluble it is typically ejected in solution, which requires water. The mammalian solution is to convert the ammonia into a less toxic substance called urea. This conversion takes place in the liver: the ammonia produced by cells enters the bloodstream where the liver removes it, converts it into urea which again enters the bloodstream to be excreted by the kidneys. Being less toxic, the urea can be temporarily stored and excreted in a concentrated solution, requiring less water.Birds and reptiles have a better water-conserving system; they excrete uric acid (or urate salts). Uric acid is not readily soluble in water and is of low toxicity and so can be excreted with very little water. The dry excreta of birds is a mixture of faecal matter and uric acid crystals and when water is scarce birds can produce very dry excreta.Arthropods, including insects, have adopted similar solutions. Woodlice, which are not insects but crustaceans, are only partially adapted to terrestrial conditions, preferring moist habitats, but they do excrete ammonia. Interestingly they can vent off ammonia gas, rather than relying on the wastage of water to remove the ammonia in solution. Insects are better adapted to dry conditions, although aquatic insects and some insect larvae excrete ammonia, most terrestrial forms excrete uric acid (or salts of uric acid called urates, such as ammonium urate).If one considers how small an insect is and how rapidly a small drop of water may evaporate, then one realises that insects have outstanding water-conserving systems. Bedbugs (Rhodnius) can survive for weeks without ingesting any water! Some insects can tolerate extremely dry conditions and may excrete uric acid as a dry crystalline powder, along with bone-dry faeces! Insects generally produce only trace amounts of urea. Malpighian tubules The main excretory organ of the insect is the Malpighian tubule. Insects contain anything from 2 to 150 or more Malpighian tubules depending on the genus. Malpighian tubules are tubular outgrowths of the gut. They typically develop as pouches emerging from the junction between the midgut and the hindgut, though there actual final position varies — they may be attached to the midgut, hindgut or the midgut-hindgut junction as is the case with our ant above.Each Malpighian tubule is a blind-ending tube whose lumen is continuous with the lumen of the gut. Each consists of a single layer of epithelial cells, forming the tubule wall, enclosed by an elastic membrane (basement membrane — a fibrous and porous protein mesh). In most insects there is a thin layer of striated muscle around this membrane. Typically muscle cells spiral around the distal end (the end furthest from the gut) of the tubule, causing it to twist and turn in gentle writhing movements as the muscles contract. The proximal end (near the gut) may be coated in circular and longitudinal muscle fibres, giving rise to peristalsis or squeezing movements which empty the contents of the tubule into the gut. In some cases, such as in caterpillars, the Malpighian tubules on each side (3 on each side in this case) empty into a small bladder, which then empties into the gut. In this case only the bladder may be muscular and its lumen is lined by cuticle (suggesting that the bladder is an extension of the hindgut).The tubules do not just hang around in the air! The body cavity of the insect is filled with a fluid, usually colourless, called haemolymph. This fluid bathes the organs and tissues and is circulated around the insect body. The tubules are also typically loosely or firmly anchored in place by the tracheae which attach to them.The twisting and turning of the Malpighain tubules presumably keeps them in contact with fresh haemolymph (perhaps by circulating the heamolymph around the tubule). Metabolic wastes and other unwanted chemicals that entered the insect system pass into the haemolymph, or are excreted into the haemolymph by the cells. These include nitrogenous waste and plant toxins such as alkaloids. It is the job of the Malpighian tubules to keep the haemolymph cleansed of these wastes — they remove wastes from the haemolymph and then excrete them into the gut lumen.Outside the muscle layer is a ‘peritoneal covering’ of cells with embedded tracheoles, which carry oxygen to the Malpighian tubules which their mitochondria use to generate the needed ATP by aerobic respiration. How do Malpighain tubules work? Waste materials and excess water pass from the haemolymph into the Malpighain tubules, by crossing the epithelial wall of these blind-ended tubes. Recent evidence shows that these cells contain pumps, proteins called proton-secreting V-ATPase. These proteins use energy in the form of ATP (see respiration) to pump protons into the lumen of the Malpighian tubule. Protons are positively charged and to maintain charge balance the removal of protons from the epithelial cells, into the tubule lumen, is balanced by the inward movement of potassium ions, which move from the haemolymph, into the epithelial cells and then out into the tubule lumen also. The diagram below shows a section through a segment of a Malpighian tubule.
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The epithelial cells have microvilli (fingerlike projections) projecting into the tubule lumen and are rich in mitochondria (green stripy rods) which produce the ATP required by the pumps. A model of how ion transport across the epithelium is thought to take place is illustrated.