Well, there is none! it is the same molecule discovered in 1819 by Runge, German chemist, in the coffee beans, and in 1827 by Oudry, French researcher in the tea leaves. It will nevertheless take until 1838 to realize that it was the same molecule. The caffeine is part of the family of alkaloids and it represents 3% of the dry weight of the leaf of the tea plant. It contributes to the growth of the plant, and that is why it is found in larger amounts in the buds and young leaves.
Caffeine is part of the family of stimulants. Although theine and caffeine are the same molecule, their action is different depending on the carrier.
In coffee, 100% of the caffeine is in the free state and therefore passes rapidly into the blood, while in tea, caffeine is captured by polyphenols, which makes ist pass into the blood more slowly during digestion. Caffeine is released very slowly into the blood when drinking tea.
It will be even slower if the tea infuses long because it will contain a higher concentration of polyphenols.
The amount of caffeine found in a cup of coffee is not the same as that found in a cup of tea : between 50 and 175mg for 10cl of coffee, against 20 to 70mg for the same amount of tea.