Bitter gourd, cucumber, etc. are bitter to taste. Which chemical causes bitterness in them?

I found these when I searched online: [this](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2768385/) says that it is because of calcium, while [this](http://ajcn.nutrition.org/content/72/6/1424.full) and [this](https://www.gardeningknowhow.com/edible/vegetables/cucumber/what-causes-bitter-cucumber.htm) talk about physical effects.