This refers to the method of feeding. There are two types of nutrition:

Auto-tropic nutrition: This is where organisms build up organic substances from simple inorganic ones. For instance, plants need only carbon dioxide, water and salts to make proteins, carbohydrates, etc. The autotrophs are thus producers.

Autotrophs are divided into two:

Photo-autotrophs: These are organisms that have pigments that absorb energy from sunlight for example chlorophyll; the energy is used in the synthesis of organic substances. Examples of photo-autrophs include green plants, green algae and some bacteria. Photo-autotrophism may be called photosynthesis.

-  Chemo-autotrophs: These are usually bacteria which get energy from simple reactions of inorganic chemicals; the energy is used to build up organic substances. For instance, nitrifying bacteria in the soil work on ammonium compounds changing them to nitrates. Chemo-autotrophism may be called chemosynthesis.

Heterotrophic nutrition: This is where organisms use complex organic compounds as a source of food. These organic substances are made by autotrophs. The heterotrophs digest the organic substances and absorb the produced fluid. So heterotrophs are called consumers or decomposers.

There are different modes of heterotrophic nutrition.

-  Holozoic nutrition. This means a feeding method where animals eat other animals or plants and digest them internally. Examples include:

Carnivores: which eat other animals. If they kill and eat other animals, they are called predators for example lions, leopards, cats etc. If they eat animals killed by other animals they are called scavengers for example vultures.

Herbivores: these eat plants and plant products. Grazing animals eat grass and small plants for example cows. Browsing animals eat leaves and branches of trees and shrubs for example elephants, giraffes.

Omnivores: these eat both plants and animals materials for example man.

Parasitism: This is feeding relationship between two organisms of different species in which one called the parasite gets advantages from the relationship with the other called the host which as result suffers injurious effects.

Symbiotism: This is a relationship between two organisms of different species where each organism benefits from the other.

Saprophytism: This is a feeding relationship where organisms secrete enzymes into dead decaying remains of animals or plants and absorb the digested products back to their bodies. Such organisms are called saprophytes or decomposers for example fungi and bacteria.

-  Commensalism: This is where two unrelated organisms live together but with no obvious benefit or harm to each other; or one of the organisms may benefit without necessarily harming the other. One example of commensals are the hermit crab and the sea anemone which live in abandoned shells of molluscs. The sea anemone has stings which protect the hermit crab from predators.

At this stage it is hard to tell relationships. For instance, some bacteria live in man's colon (large intestine). Are they parasites or commensals? They produce vitamin K which man needs, so they might be symbionts.  But there is evidence that man does not use this vitamin, so the bacteria might be commensals.

N.B. The bacteria in man's colon are safe but if they get to the urinary tract they cause severe infection.

PARASITISM:

This is feeding relationship between two organisms of different species, in which one called the parasite gets advantages from the relationship with the other called the host which as result suffers injurous effects.

Parasitism may be between one animal and another for example a tick and a cow, between a plant and another for example blight fungus on a tomato or potato; a parasitic animal on a plant host for example mites or aphids on cabbage; or a parasitic plant on an animal host for example ringworm on man. In rare cases in animals one sex may be parasitic on the other within the same species for example the anglerfish males which parasitize the females.

Types of Parasites:

There are two types of parasites:

• Ectoparasites- these are situated externally upon the host. They may be fixed upon the outer tissues for example a tick on a cow or may wander about the surface for example a flea on a dog.
• Endoparasites- these live entirely surrounded by tissues of the host for example roundworms, malaria parasites in man.

Some parasites live permanently on or in the host i.e. obligate parasites; others called facultative parasites may have other means of survival.

Parasitic micro-organisms that cause disease are called pathogens.

Some parasites are vectors which carry other parasites from one host to the other.

An animal where a parasite spends part of its life cycle is called a secondary host.

Characteristics of parasites:

Some parasites have reduced powers of locomotion for example tapeworms.

Some parasites lack the alimentary canal or feeding organs for example the tapeworms which absorbs partly digested food from the alimentary canal.

They have well-developed mouth parts especially in ectoparasites like the flea.

They have well-developed organs for attachment by which they cling securely for example the books and suckers of a tapeworm.

They have a thick resistant covering especially for parasitic animals like the round worm.

They have very rapid means of escape for example the flea.

They produce anti-coagulant enzymes especially the blood-feeding parasitic animals for example mosquitoes.

They have extra sensitivity to chemicals in order to reach the right place for example the liver fluke.

They produce anti-enzymes so that they are not digested for example the tapeworm.

They have ability to respire in low oxygen concentrations for example tapeworms.

They possess hermaphrodite condition which enables self-fertilisation for example the tapeworm.

They have association of sexes where the female carries the male for example Bilharzia worms.

They lay enormous numbers of eggs for example the tapeworm produces over 70 million eggs per year.

They use secondary hosts or vectors.

Control of parasites:

• By avoiding contact between parasite and host.
• Use of artificial immunity.
• Quarantine laws may be used where there is restriction of movement of people, animals, vehicles or planes to and fro disease areas.
• Restrict breeding of parasites.
• Spray or burn parasites' habitats.
• Biological control i.e. control of pests by use of natural enemies for example the ladybird to control aphids.
• Destroy the vectors.

 

SYMBIOTISM

This is a relationship between two organisms of different species where each organism benefits from the other. It is also called mutualism.

Examples of symbiotism:

In the rumen of ruminants there are anaerobic bacteria. The ruminant and the anaerobic bacteria are symbionts.

Benefits of the ruminant:

the bacteria digest cellulose of the cell walls in the herbage so that the ruminants enzymes can have access to the carbohydrates inside the cells.

the bacteria ferment carbohydrates to organic acids which are absorbed by the ruminant for respiration to release energy.

the bacteria are finally digested in the abomasum so that they are a source of proteins and vitamins B and K.

Benefits of the anaerobic bacteria:

            -  the bacteria gain carbohydrates from the herbage.

            -  the bacteria gain ideal environment for growth i.e. warm and moist conditions.

In the rabbit, the bacteria are found in the caecum which is enlarged.

• In the root nodules of legumes, there are bacteria called Rhizobium which enter roots through the root hairs.

The Rhizobium fixes nitrogen into the soil for plants to absorb nitrates and they also form amino acids which are absorbed by legumes for protein.

The legumes provide carbohydrates and accommodation for the bacteria.

Lichens are algae and fungi permanently living together on tree barks, roof tiles and stones. The alga is green and it photosynthesizes in light to produce carbohydrates used by the fungus and also the alga absorbs nitrogen from the atmosphere i.e. it is a nitrogen fixer. The fungus protects the alga from drying up (desiccation).

In the head of termites there are protozoans which produce the enzyme cellulase which digests cellulose of plant cell walls so that the termite's enzymes can gain access to the carbohydrates inside plant cells. The termites provide carbohydrates and accommodation to the protozoa.