What makes a phospholipid bilayer semi-permeable, what can through it, and how does this relate to our body?

The phospholipid bilayer is essentially composed of phosphate heads with lipid tails. The heads are not completely compact meaning there are small pores between them. Water and small uncharged ions can usually diffuse through the membrane without the help of integral membrane proteins. Larger, charged ions, such as sodium and potium need these membrane channels to enter the cell. This is why is it semi-permeable. some things that permeate and some cannot. Facilitated diffusion, like mentioned earlier, is the use of proteins that are embedded in the membrane to move an ion that would normally not diffuse through the membrane DOWN its concentration gradient. So, like the name implies, diffusion is facilitated by membrane channels. Active transport is the movement of ions AGAINST their concentration gradient. This is important because keeping an ion gradient is what drives reactions in the body. The most prominent concentration gradient in the body is that of sodium and potium, generated by the Na/K pump. This allows for the cell to undergo action potentials, once the concentration gradient is temporarily equalized. Another example you can discuss is how glucose is transported into the cell via facilitated diffusion. Hope this helps!!!