In a recent publication, we studied the mechanical stiffening behavior in two-dimensional (2D) cross-linked networks of semiflexible biopolymer filaments under simple shear [Onck, P.R., Koeman, T., Van Dillen, T., Van der Giessen, E., 2005. Alternative explanation of stiffening in cross-linked semiflexible networks. Phys. Rev. Lett. 95, 178102]. These simulations make use of a geometrically nonlinear finite-element technique, taking into account the discreteness of the biopolymer network. As an alternative to the prevalent view, these computations relate the stiffening to nonaffine network reorientations. However, this discrete-network model neglects any interaction of the filaments with the surrounding fluid, which is the origin of entropic stiffening in single filaments and in biopolymer networks, according to MacKintosh et al. For this reason, this article is devoted to a thorough study of the difference between both approaches on the 2D single-filament level. In addition, we investigate the deviation from affine deformation behavior, by comparing the discrete calculations with an affine-network model. (C) 2008 Elsevier Ltd. All rights reserved.