The most air-tight substance that an organism can produce is likely a combination of natural polymers, similar to those found in some biological structures. One such substance is chitin, which is a complex carbohydrate found in the exoskeletons of arthropods (like insects and crustaceans) and the cell walls of fungi. Chitin is both strong and flexible, making it an ideal candidate for forming a hermetic membrane.
In addition to chitin, the organism could produce proteins and lipids that help create a more air-tight seal. For example, the organism could secrete a natural "glue" made of proteins and lipids that fill gaps and create a tight seal between the chitin structures. This would be similar to the way some mollusks produce proteins and calcium carbonate to create their shells.
To create an air-tight membrane, the organism would need to combine these substances in a way that creates a layered structure, similar to how layers of keratin and lipids create water-resistant barriers in human skin. This multi-layered structure would make the membrane more effective at preventing gas exchange, while still allowing for flexibility and movement.
By looking at these examples, one can imagine a hypothetical creature that combines these elements to create an air-tight hermetic membrane. It could produce chitin as the primary structural component, with proteins acting as a flexible "glue" to fill gaps and provide additional strength, and lipids creating an effective barrier against gas exchange. This combination would allow the organism to maintain optimal internal conditions even in environments with a low atmospheric pressure.
In summary, an air-tight substance suitable for creating a hermetic membrane in an organism would likely be a combination of chitin, proteins, and lipids. These substances would be layered to form a flexible, strong, and air-tight barrier. The organism would need to ingest and process the necessary building blocks to produce this membrane through its diet.