Learn French: Le noyau est entouré d'une membrane. - The nucleus is surrounded by a membrane.

In French, the passive voice usually uses par to mean "by". However, verbs describing a continuous state, physical condition, or emotion—such as being surrounded, covered, loved, or respected—take de instead. Since being surrounded by a membrane is a constant physical state rather than a specific action, we use de (which becomes d' before the vowel in une).

Why is entouré not spelled with an extra "e" at the end?

In the passive voice, the past participle must agree in gender and number with the subject. Because the subject le noyau is masculine singular, entouré stays in its base form. If the subject were feminine, like la planète (the planet), it would be est entourée.

Is the word noyau only used for biology or physics?

Not at all! While it means "nucleus" in science, in everyday life le noyau commonly refers to the "pit" or "stone" of a fruit, like a peach or an avocado. It can also mean the "core" or center of something, such as a "core group" of people (un noyau dur).

How would this sentence change if it were in the active voice?

To make it active, you would make the membrane the subject doing the action: Une membrane entoure le noyau (A membrane surrounds the nucleus). The passive voice is just a stylistic choice here to keep the focus on the nucleus.