A bean is actually a seed that contains the embryo of a future plant. The embryo is packaged with a food source to use during its early stages of growth, and is covered with a protective seed coat. Soaking the bean in water activates germination of the seed, and the embryo inside begins to grow and unfold.
When you removed the seed coat, you probably noticed a small, white, root-like structure sticking out from one side of the bean. This structure, called the radicle, is the future root of the plant.
When you split the bean open, you may notice tiny white leaves on the end of the radicle. These are the first true leaves of the bean plant.
The bulk of the inside of the bean is the cotyledon, or seed leaf. Each bean contains two cotyledons that separate when you split apart the bean. When the bean sprouts, the cotyledons turn green and have the appearance of leaves (hence their alternate name, seed leaves), but they’re actually not (yet) photosynthetic. Filled with stored starches, fats, and proteins, the cotyledon supplies energy to the developing embryo during the early stages of its growth. Later, once the embryo has used the food stored in the cotyledon, the plant will begin to make its own food through photosynthesis, which occurs in both the seed leaves and the true leaves.