The green spot:
We all know the potato, Solanum tuberosum, as a delicate and cheap food supply. Normally you can eat tons of potatoes without feeling anything. But when a potato turns green they are not recommended as edible.
The potato belongs to the Solanaceae family, also called the Nightshade- or Potato family. One of the chemically most interesting characteristics about the Solanaceae family is the alkaloid synthesising ability.
Alkaloids are bittertasting, nitrogen containing compounds, often structured as rings (nitrogen heterocycles). Most of the alkaloids are toxic, but some of them also has pharmaceutical usage.
Two of these toxics are Solanine and Chaconine. The reason why they synthesise these compounds is to protect themselves against herbavores, insects and diseases. The toxic part of these compounds are called Solanidine. These compounds can be found in every part of the plant, but in higher concentrations in parts with surfaces exposed tp sunlight, while the synthesis is catalysed by exposure to sunlight.
Parts of the plant which is exposed to sunlight also produces chloroplasts, which is green. That is why leafs and so on is green, and why potatoes turn green when they are exposed to sunlight!
The compounds are water soluble so they are going to be transported throughout the whole potato. Therefore these compounds can be found in both the green and white parts of the potato, but will still contain the highest concentration in the green part.
My advice: Either throw the whole potato out, or cut the green part off. But do not eat it! The most common symptoms of intoxication is diarrhea, headaches, vomitting and nausea, so you do not want that!