Final answer:
Conditionally essential amino acids are those that become essential only under specific circumstances, such as medical conditions like PKU where phenylalanine cannot be converted into tyrosine due to a mutation. Normally, the nine essential amino acids, which cannot be synthesized by the body, must be consumed through the diet.
Step-by-step explanation:
Amino acids that are only essential due to medical conditions or lifespan period are known as conditionally essential amino acids. These amino acids become essential under certain physiological states or due to health conditions. For instance, in the medical condition phenylketonuria (PKU), individuals have a mutation in the enzyme phenylalanine hydroxylase. This enzyme typically converts the amino acid phenylalanine into tyrosine. However, due to the mutation, individuals with PKU cannot properly synthesize tyrosine from phenylalanine, making tyrosine an essential amino acid for them.
Normally, the human body cannot synthesize nine amino acids, and these are termed the essential amino acids, which include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. These need to be consumed through the diet. In contrast, there are non-essential amino acids which the body can produce on its own and do not require dietary intake. Examples of non-essential amino acids are glycine, alanine, serine, proline, and others.
It is important to note that whether an amino acid is classified as essential, non-essential, or conditionally essential can vary depending on the specific needs and circumstances of an organism or individual.