This lecture covers the importance of reversibility in cells to minimize energy dissipation, the concept of entropy in isolated systems, the second law of thermodynamics, and the different types of free energies related to boundary conditions. It also discusses phase transitions, entropic springs in biological molecules, and the phase transition of entropic springs. The lecture concludes with a detailed analysis of the collapse of an entropic spring and the force-extension curve, highlighting the relationship between free energy, temperature, and polymer contour length.