This lecture explores the control of membrane fluidity, the impact of temperature and lipid composition on fluidity, and the functions of membrane proteins. It also delves into the relationship between habitat and lipid composition, the role of unsaturated fats in hibernating species, and the collective roles of membrane lipids as impermeable barriers. Additionally, it covers the functions of membrane proteins, the modeling of cell membranes, and the study of molecule mobility using Fluorescence Recovery After Photobleaching.