Animals are therefore said to be heterotrophic ,while plants are autotrophic .Hetero means ‘different’ or ‘other’, whereas means ‘auto’ or ‘self’. Animals are depend on food produced by other organism, but plants can make our own food.
Animals are not only group of organism which feed heterotrophically. Fungi also are hetrotrophs, as are many protoctist and prokaryotes. All of them are dependent on food made by autotrophs.
Types of heterotrophic nutrition:
Heterotrophic organisms have evolved many different ways of obtaining the organic nutrients they need.
Is the method by which humans and other mammals feed, as well as many other animals such as insects .food, in solid or liquid form is taken into a tube called the alimentary canal where it is digested and absorded into the body.
Is the method by which fungi and many prokaryotes feed. They live and grow on their substance , which can be anything organic, such as milk bread or dead body .They secrete enzymes from their bodies, and digest the food material around them before absorbing it.
Is the method of feeding which has evolved in many different groups of organisms, including various kinds of worms and fungi and a few plants. Parasites fed on, and live in close association with, a living organism of a different species, called their host. They can feed either holozoically biting their host or sucking fluids from it, or they can feed saprotrophically ,absorbing soluble food into their bodies.
Holozoic nutrition-feeding in humans:
Humans like all other animals are heterotrophs .This means that we need to eat food containing organic molecules. These organic molecules includes carbohydrates, fats and proteins. This all are our only source of energy. In contrast autotrophs such as green plants do not need to take in any organic molecules at all. They obtain their energy from sunlight, and can use their energy to built organic molecules from inorganic one. They produce carbohydrates from carbon dioxide and water, by photosynthesis and can them use these carbohydrates, plus inorganic ions such as nitrate, phosphate and magnesium , to manufacture all the organic molecules that they require .