Umami is a Japanese word that doesn't translate into English very well. The closest we can get to explaining it is "yummy" or "savory" or "delicious". It is the flavor of richness that comes from foods high in protein... "meatiness", the reason why a grilled mushroom tastes like a good substitute for steak. Also the reason why real cheese is more flavorful than the processed variety.
do you crave sweets?
A "sour" receptor allows in a flood of H+ (hydrogen) ions. These ions are given off by chemicals that are acids. Acids have a sour taste. Think about citric acid and ascorbic acids which are high in fruits.
a sour expression
The "bitter" receptor is triggered by many different chemicals and signals an unpleasant taste to your brain. Many poisons trigger this response and make you want to spit out whatever food you just ate that tasted bitter. Alkaloids are in this category.
many greens like arugula and dandelion produce bitter alkaloid compounds that taste foul.But the "umami" flavor receptor is triggered by the amino acid "glutamate" or "glutamic acid". This chemical is a building block of protein (as are all amino acids) and is known more commonly by the chemical name "monosodium glutamate" or "MSG".
Glutamate naturally occurs in foods rich with savoriness and meatiness. There is nothing artificial about MSG.