One major obstacle stunting the development and application of mobile ad hoc networks (MANETs) is the lack of a thorough understanding on the throughput capacity of these networks. Preliminary works regarding the exact capacity study of MANETs mainly focus on cell-partitioned networks, an oversimplified network model for practical MANETs. This paper studies the exact throughput capacity under a more realistic and practical network model for MANETs, where network nodes randomly move in a continuous unit square without cell-partition and a slotted ALOHA protocol is adopted for medium access control. For the considered ALOHA MANETs (A-MANETs), we first determine its exact throughput capacity based on the successful transmission probability (STP) and also derive the expected end-to-end delay for a capacity achieving routing algorithm. We then develop efficient closed-form approximations to both the STP and the exact throughput capacity in the concerned A-MANET under a popular local transmission scheme, based on which the corresponding capacity optimization issue is explored. Finally, simulation and numerical results are provided to validate the efficiency of our capacity model and to illustrate our theoretical findings.