Chapter 1: NUTRITION IN ANIMALS  
Animals get food from plants or other animals. They exhibit heterotrophic  
mode of nutrition. Animal nutrition involves nutrients requirement, mode of  
taking food and its utilization in the animal body.  
▪
▪
Nutrition is the study of the relationship between diet, health and diseases. OR  
Nutrition is a process of feeding (taking in) food through mouth so that the body can  
acquire energy for different body activities (metabolisms). For example; energy release,  
growth, repair and keeping the body healthy. OR  
▪
Nutrition is the process of acquiring nutrients from the environment and processing  
them into a form that can be used by the body  
.
People who study nutrition are called nutritionists or dieticians. They advise people on  
how nutrition affects their health and well-being.  
TYPES OF NUTRITION  
There are two types of nutrition namely;  
Autotrophic nutrition.  
Heterotrophic nutrition.  
Autotrophic nutrition is the mode of nutrition in which the organisms acquire  
energy from sunlight and utilize it to synthesize their food using carbon dioxide  
(퐶푂2) and water.  
HETEROTROPHIC NUTRITION  
Heterotrophic nutrition is the type of nutrition in which an organism feeds by  
taking in organic substances made by other organisms. OR  
Heterotrophic nutrition is the type of nutrition in which organism is unable to  
make its own food and depends on food which is already made by other  
organisms.  
▪
Organisms that obtain organic food substances from other organisms are termed as  
heterotrophs. “Hetero” means different and “trophy” means feeding. Therefore,  
heterotroph means different feeding.  
▪
Examples of heterotrophs include: all animals, fungi, most bacteria and some  
protoctists.  
Types of Heterotrophic Nutrition  
There are three forms of heterotrophic nutrition, namely;  
(a)  
(b)  
(c)  
Holozoic Nutrition.  
Symbiotic Nutrition (symbiosis).  
Saprophytic Nutrition.  
A. HOLOZOIC NUTRITION: It is the type of heterotrophic nutrition whereby  
the organism feeds on complex food substances, digests, absorbs and  
assimilates/uses the nutrients. The undigested and indigestible food  
remains are egested as faeces.  
Holozoic mode of nutrition is divided into three modes of feeding:  
(i) Herbivorous. This is the mode of feeding in which an organism feeds  
on plants only. Animals which feed on plants are called herbivores.  
Examples: grasshoppers, cattle, rabbits, goats, giraffes, antelopes,  
zebras, and elephants.  
(ii) Carnivorous. This is the mode of feeding in which an organism feeds  
on other living animals only. They are organisms which feed on  
meat/flesh only. They are called carnivores. Examples: lions, tigers,  
leopards.  
(iii) Omnivorous. This is the mode of feeding in which an organism feeds  
on both animals and plants food sources. They also feed on fungi and  
algae.  
They  
are  
called  
omnivores  
.
They  
are  
also  
known  
as  
opportunistic feeders because they feed on variety of food sources.  
Examples: human beings, chimpanzees, bears, birds, pigs, turtles,  
lizards and rats.  
NOTE  
:
Insectivores feed on insects. e.g. ant, shrews, birds like finches. Frugivores  
feed on fruits and seeds, e.g. birds like quelea and organisms that feed on liquid  
are called Fluid feeders, e.g. butterflies (feed on nectar), mosquitoes (feed on  
blood) and aphids (feed on sap).  
B. SYMBIOTIC  
NUTRITION  
(SYMBIOSIS):  
Symbiosis  
means  
“living  
together.” This is a natural feeding relationship or an association between  
two different species in which one or both benefit. There are three common  
types of symbiotic relationships which are;  
(i)  
Mutualism.  
This is the feeding relationship between two living  
organisms of different species in which both benefit. For example:  
Leguminous plants and nitrogen-fixing bacteria show mutualism where  
the bacteria benefit by getting shelter and synthesized nutrients from  
the plants while the plants benefit by getting nitrate f ixed by the  
bacteria from nitrogen.  
▪
▪
Also, bacteria living in the gut of herbivores (goat, sheep or cow) get  
habitat in a ruminant’s gut and feed on food digested by animals. The  
bacteria in turn help the animals to digest cellulose by releasing  
cellulase enzymes that digest cellulose into simple compounds which  
can be absorbed and assimilated by the ruminants.  
(ii)  
Commensalism: This is the feeding relationship between two living  
organisms of different species in which one species benefits while the  
other neither benefits nor is harmed. For example:  
▪
Cattle egrets follow herds of cattle or buffalo and feed on insects  
disturbed by the animals. The cattle or buffalo are not harmed by the  
feeding activities of cattle egrets. Hence, the cattle egret benefits while  
cattle or buffalo neither benefits nor is harmed.  
▪
Also, epiphytes (moses and algae) which are small plants that grow on  
trees but do not harm the trees. The epiphytes get access to nutrients,  
exposure to sunlight and support from the trees while the trees are  
neither harmed nor benefitted.  
(iii)  
Parasitism: This is the feeding relationship between two different  
species where one species benefits while the other is harmed. The  
species which benefits is called a parasite and the one which is  
harmed is called a host. There are two types of parasites, namely;  
Endoparasites: Parasites that live and obtain their food inside the  
body of the host. E.g. tapeworms, Plasmodium sp. and liver flukes.  
Ectoparasites: parasites that live outside the body of their living host  
and suck fluid from them, for example; ticks, fleas, lice and bedbugs.  
▪
▪
C. SAPROPHYTIC NUTRITION: This is the feeding relationship in which an  
organism feeds on dead or decaying bodies of other organisms, mainly  
plants and animals. The organisms that feed on decaying organic matters  
are called saprophytes  
.
In this mode of feeding, a saprophyte releases digestive enzymes externally  
on the food substance. The released enzymes digest/convert complex  
organic molecules such as starch into simple molecules such as glucose.  
Digestion which takes place by the release of enzymes outside the cell is  
also called extracellular digestion. For example;  
▪
A bread mould growing on the surface of decaying bread to obtain the  
nutrients.  
▪
The mushroom growing on log releases enzymes externally through  
rhizoids which decompose and decay the log from which the mushroom  
gets its nutrients.  
IMPORTANCE OF NUTRITION IN LIVING THINGS  
Nutrition provides necessary nutrients which have several functions for maintaining  
proper body health. Therefore, the importance of nutrition includes the following;  
(i)  
It provides nutrients for the body to produce energy needed for various life  
processes. Example; carbohydrates and lipids.  
(ii)  
It protects (defend) the body against infections and diseases. Example; vitamins.  
It replaces and repairs the damaged or worn out cells and tissues of the body.  
Example; proteins.  
(iii)  
(iv)  
It promotes general body health so as to reduce the risk of getting chronic  
diseases such as heart attack, stroke, blood pressure. Example; vitamins.  
It maintains a constant body temperature in endotherms (warm blooded animals).  
It helps the body to produce important chemical substances such as enzymes  
and hormones.  
(v)  
(vi)  
(vii)  
It is required for reproduction. example; formation of gametes.  
(viii) It is required for growth and development of cells, tissue and organs.  
(ix)  
(x)  
It brings about contraction and relaxation of muscles (movement).  
It is used in production of light by living organisms such as fireflies and glowing  
worms, (bioluminescence).  
NUTRIENTS  
Nutrients are food substances which are essential to life and health. There are several  
types of substances that are needed by the human body for its proper functioning. The  
basic food substances include; Carbohydrates, Proteins, Lipids (Fats and Oils),  
Vitamins, Mineral salts, Water and Roughages.  
Depending on the quantity needed by the body, nutrients can be divided into two  
categories which are;  
▪
Macronutrients: Nutrients that are needed by the body in large amount. Example;  
carbohydrates, lipids and proteins.  
▪
Micronutrients: these are needed by the body in small amount but are very important  
for carrying out body functions. Examples; vitamins and minerals. They assist in  
producing energy, digesting food substances and making macromolecules.  
NOTE: Food is any substance which is required in the body for growth, repair, supply of  
energy and keeping the body healthy.