Multicellular organisms such as humans possess complex body designs. They have specialised cells and tissues for performing various necessary functions of the body such as intake of food and oxygen. Unlike unicellular organisms, multicellular cells are not in direct contact with the outside environment. Therefore, diffusion cannot meet their oxygen requirements.

Any visible movement such as walking, breathing, or growing is generally used to decide whether something is alive or not. However, a living organism can also have movements, which are not visible to the naked eye. Therefore, the presence of life processes is a fundamental criterion that can be used to decide whether something is alive or not.

An organism uses outside raw materials mostly in the form of food and oxygen. The raw materials required by an organism can be quite varied depending on the complexity of the organism and its environment.

Life processes such as nutrition, respiration, transportation, excretion, etc. are essential for maintaining life.

Autotrophic nutrition	Heterotrophic nutrition
(i) Food is synthesised from simple inorganic raw materials such as CO2 and water.	(i) Food is obtained directly or indirectly from autotrophs. This food is broken down with the help of enzymes.
(ii) Presence of green pigment (chlorophyll) is necessary	(ii) No pigment is required in this type of nutrition.
(iii)Food is generally prepared during day time.	(iii) Food can be prepared at all times
(iv) All green plants and some bacteria have this type of nutrition.	(iv) All animals and fungi have this type of nutrition.

The following raw materials are required for photosynthesis:

 The raw material CO₂ enters from the atmosphere through stomata.

 Water is absorbed from the soil by the plant roots.

 Sunlight, an important component to manufacture food, is absorbed by the chlorophyll and other green parts of the plants.

HCl plays following role in our stomach:

(a) Make the medium acidic for action of enzyme pepsin.

(b) Kills the harmful bacteria present in food

(c) Prevents fermentation of food

Enzymes break-down the various complex components of food into simple and soluble components so that they can be absorbed easily.