Q1.
How does the structure of the cell membrane in the cells of alveoli control the movement of substances across it?
Answer
Because the membrane is a lipid bilayer only 7–10 nm thick, small non-polar gases such as O₂ and CO₂ dissolve in it and pass straight through, while ions, glucose and proteins cannot — they need a protein gate.
- The two layers of lipid molecules face water-attracting heads outwards and water-repelling tails inwards. Oxygen and carbon dioxide are small and non-polar, so the oily interior is no barrier to them.
- They cross by diffusion, always from higher to lower concentration. In the alveolus, O₂ is high in the inhaled air and low in the blood, while CO₂ is high in the blood and low in the air — so the two gases move in opposite directions across the same membrane at the same time.
- The proteins embedded in the bilayer act like gatekeepers for everything that cannot dissolve in lipid, which is what makes the membrane selectively permeable rather than simply leaky.
- The alveolar wall is only one cell thick, so the total distance a gas molecule has to cross is tiny — this makes the exchange fast enough to keep up with breathing.
thickness of cell membrane = 7 to 10 nm
1 nm = 0.000001 mm = 10⁻⁶ mm
so the membrane is about 7 × 10⁻⁶ mm to 10 × 10⁻⁶ mm thick
1 nm = 0.000001 mm = 10⁻⁶ mm
so the membrane is about 7 × 10⁻⁶ mm to 10 × 10⁻⁶ mm thick
Why it happens: diffusion slows down sharply as the distance grows, so a thin barrier is not a design accident but a necessity. The lipid bilayer is the thinnest stable sheet that will still hold the cell's contents in, and its oily core is what does the sorting: anything that dissolves in oil crosses freely, anything that dissolves only in water must be let through by a protein. No energy is spent — the concentration gradient does all the work.