See uniformitarianism, catecholamine (kat' -uh-k61'-uh???men) Any of a class of neurotransmitters and hormones, including the hormones epinephrine and norepinephrine, that are synthesized from the amino acid tyrosine. cation (cat' -i -on) A positively charged ion. cation exchange A process in which positively charged minerals are made available to a plant whl will hydrogen ions in the soil displace