Answer:
Plasma membrane
Step-by-step explanation:
The plasma membrane is selectively permeable bilayer of phospholipids in which proteins are embedded. Being selectively permeable, it allows only certain non-polar, uncharged and small substances to pass through it. Oxygen and carbon dioxide have smaller structures that can pass easily through the phospholipid tails of the bilayer. Hence, these gases can pass the plasma membrane by passive movement without any energy expenditure.