Necessary.
Important
essential-needed to survive luxury-wanted
Essential nutrients are needed for an organism to survive. Each organism has their own essential nutrients.
Please forgive me, but your question contradicts itself. Essential means that it must be there - no two ways about it. Without it, we cannot proceed. Perhaps if you left the word 'essential' out, or used another, more appropriate one, it might be easier to answer.
needed, necessary, required
labor
needed, necessary, required
labor
Vitamin C is needed to synthesize nonessentail amino acids from essential amino acids.
No, essential means only the ones most needed, so essential characteristics would be a subset of all characteristics.
Rain is essential to growing a crop .
Labor -apex