Biology · Breathing and Exchange of Gases · NEET
The NCERT order is fixed: (1) Breathing or pulmonary ventilation, (2) Diffusion of O2 and CO2 across the alveolar membrane, (3) Transport of gases by the blood, (4) Diffusion of O2 and CO2 between blood and tissues, (5) Utilisation of O2 by the cells (cellular respiration). Air must first come in, then move blood-ward, travel in blood, reach tissues, and finally be used. Any other order is wrong in NEET.
Breathing (pulmonary ventilation) is always the FIRST step. It draws atmospheric air into the alveoli and pushes CO2-rich air out. Students sometimes place cellular respiration first because it is the reason we breathe, but NCERT sequences by the physical path of air, so breathing comes first and cellular respiration comes last.
Both are diffusion of O2 and CO2, but at different sites. Step 2 is diffusion across the alveolar membrane, between alveolar air and blood in the lungs (O2 enters blood, CO2 leaves blood). Step 4 is diffusion between blood and body tissues (O2 leaves blood into cells, CO2 enters blood). Alveolar diffusion loads oxygen; tissue diffusion unloads it. They are steps 2 and 4, with transport by blood (step 3) in between.
Because gases must first cross the alveolar membrane into the blood (step 2) before the blood can carry them (step 3). You cannot transport a gas the blood has not yet picked up. So the order is: diffuse into blood at alveoli, then transport, then diffuse out at tissues.
Yes. The 5th and final step is utilisation of O2 by cells for catabolic reactions, releasing CO2 - this is cellular respiration. NEET 2026 phrased step 5 simply as 'Cellular respiration'. So do not treat cellular respiration as separate; in this chapter it is the last of the five steps.
Select the sequence of steps in Respiration. A. Diffusion of gases (O2 and CO2) across alveolar membrane. B. Diffusion of O2 and CO2 between blood and tissues. C. Transport of gases by the blood. D. Pulmonary ventilation by which atmospheric air is drawn in and CO2 rich alveolar air is released out. E. Utilisation of O2 by the cells for catabolic reactions and resultant release of CO2.
In humans, respiration occurs in the following steps. Arrange these steps in the correct order. A. Diffusion of O2 and CO2 between blood and tissues. B. Diffusion of O2 and CO2 across alveolar membrane. C. Pulmonary ventilation by which atmospheric air is drawn in and CO2 rich alveolar air is released out. D. Cellular respiration. E. Transport of gases by the blood.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
NCERT describes 5 steps: breathing, diffusion at alveoli, transport by blood, diffusion at tissues, and utilisation of oxygen by cells.
The first four steps (breathing, both diffusions, and transport) are physical and give no ATP. ATP is produced only in the fifth step, cellular respiration, where cells use O2 in catabolic reactions.
No. Breathing (pulmonary ventilation) is only the first step of respiration. Respiration is the whole process of all 5 steps, ending with oxygen use inside cells. See the concept page comparing breathing and respiration.
No. At both the alveoli (step 2) and the tissues (step 4), O2 and CO2 move only by simple diffusion, following partial pressure gradients. No energy is spent to move the gases across the membranes.
Actual gas exchange happens at two diffusion sites: the alveolar membrane (step 2) and the blood-tissue interface (step 4). The alveoli are the exchange surface in the lungs; the conducting part only carries air.
