The brain coordinates the physical changes that occur during puberty and the plays an especially important role in this process.

Hypothalamus

The brain coordinates the physical changes that occur during puberty and the hypothalamus plays an especially important role in this process.

Explanation:

Since the hypothalamus controls the secretion of hormones from the pituitary gland, and the pituitary gland, also known as master gland, controls the activities of all other glands like the following:

- gonads (ovaries and testes), responsible for producing many of the sex hormones and hence, development of sexual characterictics

- thyroid, responsible for producing thyroxine, responsible for body metabolism and development.

Thus, hypothalamus plays important role in puberty

cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!