In molecular communication (MC), molecules can play dual roles, one as information carriers and the other as energy providers based on chemical reactions, the importance of which is self-evident. In this paper, we propose a novel modulation scheme, termed absorption shift keying (AbSK), to harvest unused molecules while boosting system performance. It relies on a third switch-controllable molecule harvesting node in addition to both transmitter and receiver in a conventional point-to-point MC scenario. In this setting, the proposed AbSK encodes information onto the ON/OFF state of the third node, so that it can act as a secondary source while capturing redundant molecules released by the primary source (or transmitter). Two detectors are designed for AbSK, namely ideal maximum likelihood and two-step detectors. Asymptotically tight bounds on the bit error rates of both detectors are derived in closed-form. Simulation results validate our theoretical analysis and show that the proposed AbSK outperforms benchmarks and additionally captures molecules to power future transmissions.