1-Up Hearts basically perform the same function as 1-Up Mushrooms, awarding one Extra Life to the player in Super Mario Land, Super Mario Land 2: 6 Golden Coins and Donkey Kong on the Game Boy. Most likely, the reason 1-Up Hearts are used in these games rather than 1-Up Mushrooms is that the black-and-white palette of the original Game Boy would make it impossible to distinguish between 1-Up Mushrooms and Super Mushrooms.
Unlike most Mushrooms, in Super Mario Land, a 1-Up Heart will remain in one place until Mario grabs it. However, in Super Mario Land 2: 6 Golden Coins, the 1-Up Heart moves like the 1-Up Mushroom and slides away from Mario unless he can quickly catch it. Also in Super Mario Land 2, the 1-Up Hearts can be caught by Pumpkin Zone's witch boss, who then runs away from Mario with them.
In Donkey Kong on the Game Boy, 1-Up Hearts floats in mid-air at spots that are hard for Mario to reach without special moves. They reappear if the player lost a life and had to repeat the level.
1-Up Hearts appear in minigames in both Super Mario Land titles, and similar items appear in "Modern" versions of Game & Watch games in Game & Watch Gallery and its sequels. These Hearts replenish a life that was lost, and only appear when a player reaches 200, 500, and 700 points in most games; if a player does not have any Misses when they pass these points, a Heart does not appear. In this game, they are pink and have a two-toned color scheme similar to the one suggested by the white and gray pattern of the Super Mario Land 1-Up Hearts.