In a cell, biomolecules are maintained in a state of ‘dynamic’ or ‘steady state’ equilibrium. Cell organelles can be separated by density gradient ultracentrifugation. All cells in the body contain nucleus except mature erythrocytes. Endoplasmic reticulum is involved in protein synthesis and also detoxification of various drugs. Golgi apparatus is primarily involved in glycosylation, protein sorting, packaging and secretion. Lysosomes are the ‘suicide’ bags, which contain many hydrolyzing enzymes. Mitochondria, the ‘power house’ of the cell has its own DNA, can synthesize its own proteins. It is sometimes referred to as ‘mini cell’. Antibiotics inhibiting bacterial protein biosynthesis can inhibit mitochondrial protein biosynthesis also. Membranes are mainly composed of lipids (phospholipids), proteins and a small percentage of carbohydrates. Phospholipids, which are amphipathic in nature, are arranged as bilayers. Cholesterol content and nature of the fatty acid of the membrane, influences the fluidity. Membrane proteins can be integral, peripheral or transmembrane. Transmembrane proteins serve as receptors, tissue specific antigens, ion-channels etc. Transport of molecules across the plasma membrane could be energy dependent (active) or energy independent (passive). Ion-channels function for the transport of the ions such as Ca2+, K+, Cl-, Na+ etc. Ionophores or transport antibiotics increase permeability of membranes by acting as channel formers. They could be mobile ion carriers (e.g. Valinomycin) or Channel formers (e.g. Gramicidin). Na+-K+ ATPase (Sodium pump) is an example of Active transport. Cardiotonic drugs like Digoxin and Ouabain competitively inhibit K+ ion binding. The property is used to enhance contractility of the cardiac muscle. Transport systems may be Uniport, Antiport or Symport.