Fruits and their seeds may be considered as one sink when compared with other sinks on a plant, but once carbohydrates reach the fruit, some are distributed to the seed(s). The type of fruit and seed determines the ability of the seed to compete for carbohydrates that arrive at the fruit. Seeds are composed of tissues originating in different parts of the ovule. Transfer cells can cope with the anatomical and physiological requirements for unloading of sucrose from the maternal tissue and reloading of sugars by the embryonic tissue. From the biochemical point of view, seeds exhibit heterogeneous sink characteristics. Angiosperm seeds store a high percentage of polysaccharides, proteins, lipids, and sometimes hemicelluloses and organic phosphate. Cell wall polysaccharides are synthesized in the endomembrane system, mainly in the Golgi bodies. Plants invest huge amounts of carbohydrates in fruits and seeds, which are irreversible sinks but are important for the long-term survival of the plant species.