Given a scientific passage and an answer, generate a question for the given answer.

Passage: Membranes Biological membranes surround cells and serve to keep the insides separated from the outsides. They are formed of phospholipid bilayer1 s, which by definition are a double layer of fatty acid2 molecules (mostly phospholipid3 s, lipids containing lots of phosphorus). Proteins4 serve very important functions in cellular membranes. They are active transports in and out of the cell, acting as gatekeepers. They relay signals in and out of the cell. Proteins are the site of many enzymatic reactions in the cell, and play a role in regulation of cellular processes. Answer: biological membranes
What is it that surrounds cells and serve to keep the insides separated from the outsides?

Passage: 22.2.1 Lytic Cycle 1. Attachment: The virus binds to specific receptors on the host cell. Entry: There are two ways in which a virus can enter cells. Firstly, the virus can inject its nucleic acid into the host cell. Secondly, if a virus is contained in an envelope, the host cell can phagocytosise the entire virus particle into a vacuole. When the virus breaks out of the vacuole, it then releases its nucleic acid into the cell. Replication: The virus's nucleic acid instructs the host cell to replicate the virus's DNA or RNA. Assembly: New virus particles are assembled. Lysis and Release: The virus directs the production of an enzyme which damages the host cell wall, causing the host cell to swell and burst. The newly formed virus particles are now released. Answer: nucleic acid
What does a virus inject into the host cell?

Passage: Acids and bases have had many uses throughout history. In this chapter we want to explore the properties of acids and bases and the reactions in which they take part. Answer: bases
Acids are the opposites of what?