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

Haemoglobin transports oxygen molecule to all the body cells for cellular respiration. The haemoglobin pigment present in the blood gets attached to four O2 molecules that are obtained from breathing. It thus forms oxyhaemoglobin and the blood becomes oxygenated. This oxygenated blood is then distributed to all the body cells by the heart. After giving away O2 to the body cells, blood takes away CO2 which is the end product of cellular respiration. Now the blood becomes de-oxygenated.

Since haemoglobin pigment has less affinity for CO2, CO2 is mainly transported in the dissolved form. This de-oxygenated blood gives CO2 to lung alveoli and takes O2 in return.

Transportation of O2 and CO2 in blood

Blood flow through the human heart takes place in following steps: 

• Deoxygenated blood from different organs comes to the right atrium through the vena cava.
• From the right atrium, blood goes to the right ventricle. The tricuspid valve between the right atrium and right ventricle prevents the backflow of blood.
• From the right ventricle, blood goes to the lungs through pulmonary artery. Inside the lungs, carbon dioxide is removed from the blood and oxygen enters the blood.
• From the lungs, blood goes to the left atrium through pulmonary vein.
• From the left atrium, blood goes to the left ventricle.
• From the left ventricle, blood is pumped into the aorta so that it can be supplied to different organs. 

If plant is releasing carbon dioxide and taking in oxygen during the day, it means that respiration is happening in plant. But it does not mean that photosynthesis is not happening. Carbon dioxide released after respiration comes out of stomata. For photosynthesis, the plant takes in carbon dioxide from atmosphere. In other words, plant does not depend on respiration for carbon dioxide for photosynthesis.

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.

Match the words of Column (A) with that of Column (B)

Column (A)  

Column (B)
(a) Phloem (i) Excretion
(b) Nephron (ii) Translocation of food
(c) Veins (iii) Clotting of blood
(d) Platelets (iv) Deoxygenated blood

Ans. (a)-(ii), (b)-(i), (c)-(iv), (d)-(iii)