## (5 marks) A human zygote grows by dividing each cell into two identical “daughter” cells. It takes each cell a fixed amount of time (called the cell cycle time) to divide. Five days after fertilisation, when it starts to become a blastocyst, the zygote contains 16 cells. (a) Explain why the number of cells is growing exponentially. (b) Find the doubling time for the number of cells. (c) Using your answer to part (b), find a formula for the number of cells, N, as a function of time t in days from fertilisation. (d) An adult body contains 37 x 1012 cells. How long would it take for the blastocyst to have that many cells? You may assume that the cell cycle time stays the same, though in reality it doesn’t.

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