We investigated the thermodynamic, structural, and dynamics changes in dendrimer-membrane systems during dendrimer adsorption to biological membrane systems by combining atomistic molecular dynamics simulations with umbrella sampling techniques to understand the atomistic interactions between the dendrimer and biological membranes. An ethylenediamine core polyamidoamine dendrimer (generation 3) with amine terminal groups and both zwitterionic dipalmitoyl-phosphatidyl-choline (DPPC) and anionic palmitoyl-oleoyl-phosphatidyl glycerol (POPG) lipid bilayer membranes were used as the model dendrimer and biological membranes, respectively, in this study. The free energy of the dendrimer adsorption onto two model membranes with different charge states was quantitatively determined. For the zwitterionic DPPC membrane, the dendrimer has a minimum free energy of approximately 50 kcal/mol, which is 15 kcal/mol higher than that observed in previous studies. The dominant contribution to the adsorption potential energy is the van der Waals attraction between the dendrimer and the DPPC membrane. However, the anionic POPG membrane pulls the positively charged dendrimer with an attractive mean force of about 200 pN, finally positioning the dendrimer in the membrane headgroup region. As a result of these strong attractive dendrimer and membrane interactions, the dendrimer structurally undergoes the transition from spherical to a pancake conformation, which slows its lateral mobility, especially in the presence of the POPG membrane. The bilayer lipid membranes are also perturbed by the dendrimer adsorption.