What is the alkaloid in coffee?
Straight away the answer is Caffeine.
Now what is the alkaloid in tea?
The answer is little bit time dependent. Time dependent in the sense if you asked the same question 8 or 9 years back the answer is " Teaine ".
But later they found it to be the same as Caffeine. First when they found this alkaloid they put the name as Teaine. Later its structure resembled caffeine.
As a fact tea in its raw form has more caffeine than coffee( in raw form ). But when you prepare tea and coffee coffee has more caffeine than tea. And tea has antioxidants ( which help in fighting cancers and heartattack ) in which many times they are not available naturally.