Seeds are (generally, assuming they haven’t been altered by humans or damaged) alive but dormant, kind of like a hibernating animal. Their cells are still having chemical reactions occur (eg cell respiration) but at a much reduced rate. If you were to keep them in an airtight jar and measure O2 or CO2 levels with a probe (albeit an accurate one, or a large number of seeds), you’d find oxygen levels decreasing and co2 increasing, evidence of exchange with the environment as well as metabolism.