|
Parasitic nutrition is a mode of heterotrophic nutrition where an organism (known as a parasite) lives on the body surface or inside the body of another type of organism (known as a host). The parasite obtains nutrition directly from the body of the host. Since these parasites derive their nourishment from their host, this symbiotic interaction is often described as harmful to the host. Parasites are dependent on their host for survival, since the host provides nutrition and protection. As a result of this dependence, parasites have considerable modifications to optimise parasitic nutrition and therefore their survival. Parasites are divided into two groups: endoparasites and ectoparasites. Endoparasites are parasites that live inside the body of the host, whereas ectoparasites are parasites that live on the outer surface of the host and generally attach themselves during feeding. Due to the different strategies of endoparasites and ectoparasites they require different adaptations in order to acquire nutrients from their host. Parasites require nutrients to carry out essential functions including reproduction and growth. Essentially, the nutrients required from the host are carbohydrates, amino acids and lipids. Carbohydrates are utilised to generate energy, whilst amino acids and fatty acids are involved in the synthesis of macromolecules and the production of eggs. Most parasites are heterotrophs, so they therefore are unable to synthesise their own 'food' i.e. organic compounds and must acquire these from their host. ==Endoparasitism== Endoparasites are parasites which live inside the body of the host. This group includes helminths, trematodes and cestodes. Endoparasites are two groups of parasites: intercellular and intracellular parasites. Intercellular parasites live in spaces within the host e.g. the alimentary canal, whereas intracellular parasites live in cells within the host e.g. erythrocytes. Intracellular parasites typically rely on a third organism, a vector, to transmit the parasite between hosts.〔〔 〕 Rather than requiring adaptations to penetrate the host, as ectoparasites do, endoparasites are in a nutrient-rich location so they instead have adaptations to maximise nutrient absorption. Endoparasites have a readily available and renewable supply of nutrients inside the host, which in some cases is pre-digested by the host, so mechanisms of nutrient absorption across their body surface is a common feature.〔 As part of their life cycle strategy, endoparasites must also be able to transmit from within the host body and survive the hostile environment within the host. Only by achieving this can they benefit from acquiring nutrition in this way. Endoparasites have various anatomical and biochemical adaptations, typically at the host-parasite interface, to maximise nutrient acquisition. One such adaptation is the tegument, a metabolically active external cover which plays an important role in the acquisition of nutrients from the host.〔 The parasite tegument is permeable to various organic solutes and has transporters for the facilitated or active uptake of nutrients. Various studies have attempted to characterise these transporters in a number of parasites e.g. the amino acid transporter molecules in protozoa.〔〔 〕 Cestodes do not have a gut so the tegument is therefore critical for nutrient uptake. In cestodes the tegument is highly efficient with spine-like microtriches, similar to microvilli, to increase the surface area available for nutrient acquisition.〔 〕 In many parasites the tegument structure has folds or microvilli to maximise the surface area available for diffusion and uptake of nutrients. The tegument also commonly has additional organelles and features with important functions in metabolism including the glycocalyx. The glycocalyx is a carbohydrate-rich layer which enhances nutrient absorption and secretes enzymes to aid primary digestion.〔 〕 Another important adaptation of endoparasites is the gut, which digests host macromolecules into soluble utilisable products.〔 This feature is particularly important in endoparasites which are not located in the alimentary canal and therefore the supply of nutrients is not pre-digested by the host. The gut lining typically has a layer of endodermal cells which secrete proteolytic enzymes to aid digestion. Some endoparasites have both a gut and anus, some lack an anus and some have neither i.e. those residing in the alimentary canal which instead diffuse pre-digested host nutrients across their body surface.〔 The relative importance of the tegument, gut and other adaptations involved in nutrient acquisition varies between different endoparasitic species. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「parasitic nutrition」の詳細全文を読む スポンサード リンク
|