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

[Q]: Passage: Heat waves have increased in frequency and duration in recent years. The summer 2011 North American heat wave brought record temperatures across the Midwestern and Eastern United States. Many states and localities broke records for temperature and for the most days above 100 o F. Answer: cell membrane
[A]: What are all cells surrounded by?

[Q]: Passage: The menstrual cycle includes events that take place in the ovary, such as ovulation. Answer: ovary
[A]: Ovulation takes place in what organ and is part of the menstrual cycle?

[Q]: Passage: The immune system naturally becomes less effective as people get older. This is why older people are generally more susceptible to disease. Answer: immune system
[A]:
What system gets less effective with age, that generally makes older people more disease susceptible?