how does the anatomy and chemical make up of the cell membrane make it selectively permeable?

Answer 1

The cell membrane is a lipid bilayer made from phospholipids. As a lipid, it is permeable to other lipids (e.g. steroid hormones) but not to polar molecules (e.g. potassium or glucose). Polar molecules require various types of membrane proteins (e.g. potassium channels) or endocytosis (glucose) for absorption into the cell.

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Answer 2

It is a fluid, phospholipid bi-layer. Go to " animal cell membrane ", on wikipedia for the full story; I am going off-line soon.