Why is Chlorine so unstable?
First, let's review the Octet Rule of valence electrons. According to this theory, atoms have many electrons surrounding them in layers. The outermost layer is called a "Valence Shell." In terms of determining the stability of an atom, this is the most important shell to look at. Atoms are most stable with 8 electrons in their valence shell (hence the name of the "Octet Rule"), which means they'll do anything to have 8 electrons in its outermost layer. It's willing to either give up or take electrons in order to get closer to that coveted status of having 8 electrons in its outermost shell (the Valence Shell). The closer the atom is to getting to having 8 electrons, the more eager (aka unstable), the atom becomes. For example, if an atom has only 1 or 7 electrons in its valence shell, it'll be DYING to give up/obtain an electron to have 8 electrons. If you look at the periodic table, you see that Chlorine has 7 electrons in its valence shell (let me know if you need help determining the number of valence shells for atoms), and so it's extremely eager to get that 8th electron. Chlorine's eagerness to obtain that last electron and have 8 valence electrons causes it to be an unstable atom.
Why are Proline and Glycine most common in beta turns?
Think of amino acids as beads on a bracelet. You can have different shaped beads on the bracelet, and some types of beads are more conducive towards making tight bends versus other beads. small beads are going to allow the bracelet to make tight bends at its location versus long and straight beads. The amino acids Proline and Glycine are most common in beta turns because by nature, they are molecules (aka "beads") which allow for the amino acid chain to bend easily. More specifically, Proline has a small cyclic structure while Glycine has a small side chain consisting of only a Hydrogen, making these two amino acids very sterically flexible (aka they turn easily).
Could you explain the concept of Osmosis?
Have you noticed that particles like to move from an area of high concentration to low concentration? For example, if you put a drop of food dye into a glass of water, the dye will spread out into the water because the dye particles prefer to move from areas of high to low dye concentration. What happens when you place a semi-permeable membrane (aka a barrier a particle can cross) in between two solutions of different concentrations? The particles will move from the side with a higher concentration of molecules to lower concentration until both sides of the membrane has equal concentrations. I can draw this out for you as well with my Surface Pro.