Why is diffusion insufficient to meet the oxygen requirements of multi-cellular organisms like humans?

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.

Related Questions Life Processes

The kidneys in human beings are a part of the system for
(a) nutrition.

(b) respiration.

(c) excretion.

(d) transportation.

Ans: (c) excretion.

The amount of urine produced depends on the amount of excess water and dissolved wastes present in the body. Some other factors such as habitat of an organism and hormone such as Anti-diuretic hormone (ADH) also regulates the amount of urine produced.

Gastric glands secrete hydrochloric acid, pepsin and mucus. They have following functions:
(a) Hydrochloric acid kills any germ which may be present in food. It makes the environment in the stomach acidic; which is essential for functioning of gastric enzyme.
 (b) Pepsin digests protein.
(c) Mucus prevents the inner lining of stomach from getting damaged by hydrochloric acid. 

Green plants are the main conducts through which solar energy is trapped and channeled to all other organisms. Green plants are the main source of food (directly or indirectly) for most of the heterotrophs. Without green plants, no animal shall be able to get food and thus no animal would survive. Hence, almost all life forms would be wiped out if green plants disappear from earth.

Nutrition is required by every organism for various purposes. An organism needs energy to carry out various activities, and this energy comes from food. An organism also needs raw materials for growth and repair. These raw materials come from food. This shows that nutrition is a necessity for an organism.