If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
ENE‑2 (EU)
ENE‑2.A (LO)
ENE‑2.A.1 (EK)
ENE‑2.B (LO)
ENE‑2.B.1 (EK)
Sort by:
Biology is brought to you with support from the Amgen Foundation