This is a type of nutrition in which one organism obtains food from another organism and during the feeding process one organism is harmed and does not benefit. The organism gaining food, protection and shelter is called parasite while the organism being harmed and providing food, protection and shelter is called host. The association between a host and a parasite is called parasitism. Some parasites live on the surfaces of their hosts. These are called ectoparasites and examples are lice (singular louse), ticks, bed-bugs, fleas and mites while other parasites that live inside the bodies of their hosts are called endoparasites. Examples are Plasmodium, hookworm (Ancylostoma), Ascaris lumbricoides (common roundworm), tape worm (Taenia solium) and liver fluke (Fasciola hepatica).
Parasitic nutrition in flowering plants
Dodder plant (Cassytha filiformis) is a parasitic flowering plant. It has small scale leaves which are not green.
The stem is thin and rope-like and coils round the stem of a host plant. Dodder develops haustoria (suckers) which are sent to the vascular bundles of the host from where it obtains food and water it requires for its metabolism.
Parasitic nutrition in animals
1) Ascaris lumbricoides (intestinal roundworm). This parasitic worm stays in the small intestine of man where it sucks digested food from its host. The oesophagus is muscular and the mouth is equipped with three well developed lips. The muscular oesophagus and the three lips help the parasite to suck digested and soluble food from its host (man).
The body of Ascaris is covered with a thick cuticle to prevent it from being digested by the host’s digestive juice.
It causes abdominal pain, indigestion, anaemia and if they occur in large number, they can coil together and block the intestine leading to death.
2) Hookworm (Ancylostoma duodenale)
Hookworm inhabits the small intestine of its host (man). It has teeth-like structures or cutting plates in its mouth which it uses to cut holes into the intestinal walls and by means of its sucking pharynx, it sucks blood and other fluids from its host. To prevent the blood from clothing, it secretes anticoagulant from its mouth. After meal, it leaves bleeding wounds.
They cause anaemia and abdominal pains in the host.
3) Tapeworm (Taenia solium) is a flat, ribbon or tape-like parasite. The head or scolex has four muscular suckers on the sides. The rostellum is surrounded by hooks. The body consists of a series of proglottides. The long and tape-like body provides a large surface area for absorption of digested food all over the body by diffusion.
The presence of four suckers supported by hooks helps the tapeworm to fasten itself onto the wall of the small intestine.
The host suffers an abdominal disorder and malnutrition and if tapeworm embryos migrate to the brain they damage the brain and may cause epilepsy.
Carnivorous mode of nutrition in plants
A few green plants supplement their nutrients with organic food by trapping insects and other small animals. The trapped insects are digested and the amino acids are absorbed. Often, such carnivorous plants grow in soils poor in nitrogen and therefore, animal proteins are needed to supplement the nitrogenous compounds requirement of the plant.
Sundew (Drosera rotundifolia) and Venus’s flytrap (Dionaea muscipula) are examples of carnivorous or insectivorous plants.
1) Sundew (Drosera rotundifolia). The leaves of sundew have numerous hair like tentacles. Each tentacle has a gland at its tip. The gland secretes sticky substances which help to trap small insects.
The struggling of the trapped insect stimulates the bending of tentacles nearby over the insect. Protein digesting enzymes are secreted thus digesting proteins to amino acids which are absorbed by the plant.
2) Venus’s flytrap (Dionaea muscipula)
The leaf consists of two lobes connected by midrib. In each lobe, a row of teeth-like structures are arranged along the margin of the leaf. On the surface of the leaf, there are small sensitive hairs.
When an insect lands on the sensitive hairs, the lobes change their shape and quickly come together with the teeth-like structures interlocking and thus trapping the insect. The trapped insect is then digested with the enzymes secreted from the glands on the surface of the leaf. The amino acid resulting from the digestion is absorbed.
3) Nepenthes or pitcher plant
Pitcher plant (or Nepenthes) has green leaves and obtains carbohydrates by photosynthesis. It obtains its nitrogen from insects and other small animals. Pitcher plant has pitcher-shaped leaves. (A pitcher is a container for fetching water used by the Romans in the old Roman Empire). The pitcher contains a protein digesting enzyme similar to trypsin. Insects are attracted to the lips of the pitcher by nectar secreted near the brim. In trying to suck the nectar the insect slips into the pitcher and gets drowned in the fluid. The end of the leaf is further modified to form a hood which covers the mouth of the pitcher when an insect falls in.
The insects are further prevented from crawling out by numerous stiff downward pointing hairs. The protein of the trapped insect is digested by the enzymes secreted into the fluid and the product of digestion (amino acid) is absorbed by the inner surface of the leaf.
Other examples of the carnivorous plants include:
4) Bladderwort (Utricularia flexuosa), an aquatic plant, using elaborate sacs in trapping water insects and other small animals.
5) Butterwort (Pinquicula alpina) a terrestrial plant, attracting insects with bright colour and trapping the insects with sticky leaves.
6) Water fly-trap (Aldrovandi vesiclosa) an aquatic plant, trapping insects and other small animals by sensitive hairs and bristles.