The Mass Effect games have gone to great lengths to design fictional worlds and races that are plausible:
Alien blood colors: The red, yellow, blue, green, and purple alien blood seen in-game exists in-real life:
- Red - Most vertebrates. The color is due to the presence of iron.
- Yellow - Beetles, sea squirts and cucumbers. Their blood contains high concentrations of vanabin, which contain vanadium that gives the blood its color. Vanabin does not transport oxygen and its purpose is still a mystery.
- Green - Most annelids (worms/leeches) and select lizard species. The color is due to the presence of a chemical called chlorocruorin. For certain lizards the color is due to high concentrations of bile that would be lethal in most other lifeforms.
- Blue - Crustaceans, octopus/squid, and mollusk. The color is due to the presence of copper.
- Purple - Brachiopods and select marine worms. The color is caused by the protein hemerythrin used to transport oxygen.
Asari reproduce is similar to Bdelloidea, a class of female only rotifers found in freshwater habitats all over the world. They are often referred to as "ancient asexuals" because they rely exclusively on parthenogenetic reproduction - development of an embryo from an unfertilized egg cell.
Extra Krogan organs:
Many animals have "extra" organs, like squid's three hearts or the three jaws and 32 brains (physiologically; not anatomically) of a leech.
Levo vs Dextro:
Molecules based on carbon atoms can have up to four atoms attached to a central carbon. If you imagine this as a tetrahedron (d4, if you're a tabletop gamer) then you'll notice that you can actually arrange those four atoms in two different ways that cannot be reconciled by rotating the molecule. If they're arranged one way, it's (arbitrarily) named a levo- or left-handed molecule. If it's arranged the other way, it's a dextro- (right handed) molecule. Enzymes are built to attach to specific molecular geometries, and if they don't match the molecule, they don't work. So imagine if you will an enzyme that is designed to break a protein down into its constituent amino acids, so they can be re-used to build bodily proteins. This protease is shaped to grab onto L-amino acids and ignore everything else. When confronted by a D-amino acid, it quite simply won't grab on, which means the protein doesn't get digested, and is quite useless to the organism.
Out of the major food molecules, protein and carbohydrates exhibit chirality, fats do in triglyceride (but not free fatty acid) form. Minor food molecules such as volatile flavor compounds and vitamins vary based on the particular molecules. Interestingly, ethanol is non-chiral, so Tali's need for Turian brandy is a curious case, and is either a case of the writers not being biochemists, or the possibility that Quarians and Turians do not have ethanol dehydrogenase, an enzyme to break down ethanol, and instead rely on some other form of alcohol for intoxication.