In this section, the structure of cell, cell organelles, and cell membrane is described clearly with supporting diagrams. The cell organelles such as endoplasmic reticulum, Golgi apparatus, secretary vesicles, lysosomes, Mitochondria, Ribosomes, microfilaments, microtubules, centrioles, their structure and functions are described adequately. Cell membrane structure, membrane transport are clearly explained. The role of carrier mechanisms and involvement of different types of ATP are described. Resting membrane potential, Action potential and their genesis are given in an understandable way. Different gating mechanisms and their relationship to different phases of action potential are described clearly with the held of diagrams. Cell division different stages are described. The classic physiologic concept of milieu interior promulgated by Claude Bernard in 1859 was later very well developed by Walter B. Cannon who coined the term “Homeostasis” to signify these steady states maintained by co-ordinated complex physiological processes. All our body organ systems function towards only one aid, i.e. to keep the internal environment always constant. The reader should keep this fact always in mind. To mention a few homeostatic conditions of the body: body temperature, pH of the body fluids, composition of the body fluids, Blood pressure, heart rate, respiratory rate, etc.