Drag the persistence-length slider to watch the Marko–Siggia force–extension curve stiffen and soften. A short gives a floppy chain with a large entropic restoring force; a long gives a stiffer chain that is already nearly straight, so less force is needed to extend it.
The curve is the Marko–Siggia interpolation formula
which captures both the low-force entropic regime and the steep divergence as the extension approaches the contour length . The prefactor is why the whole curve scales inversely with persistence-length.