The mechanical properties of a micrometer-sized giant liposome are studied by deforming it from the inside using dual-beam optical tweezers. As the liposome is extended, its shape changes from a sphere to a lemon shape, and finally, a tubular part is generated. The surface tension sigma and the bending rigidity k of the lipid membrane are obtained from the measured force-extension curve. In a one-phase liposome, it was found that sigma increases as the charged component increases but k remains approximately constant. In a two-phase liposome, the characteristic deformation and the force-extension curve differ from those observed for the one-phase liposome. (c) 2009 Elsevier B.V. All rights reserved.