 Dear students, in this module, we shall discuss the structure-function relationships of organisms in detail. Dear students, the functions are based on structures. And in living organisms, the structural designs have been developed to match the functional demands. This property are this type of development of structures, enables them to play the role and fulfill their job within an organism. Here, we will take an example in which we will compare frog-feeding mechanisms and the structures involved in them and their functions. And we will see how the structures are related to their functions. In the first step, when the feeding process starts with a frog, a frog leaps for its prey. When it leaps for prey, it contracts its skeletal muscles which are attached to its bones of a limbs. After receiving the food, food reaches the stomach. Stomach has smooth muscles which grind and mix the contents of food. After digestion, nutrients are absorbed and they are absorbed into the blood. Blood flows due to the beating of cardiac muscles of the heart. In all this process, we can see that the structures of muscles have specializations for their functions. For example, the skeletal muscles are evolved and adapted for movement of bones. The smooth muscles of digestive tract are specialized for grinding and mixing of the food contents. Similarly, the cardiac muscles are specialized to pump and circulate blood. Dear students, the structure-function relationships are not restricted to muscle tissue only, but they exist at all levels of biological organization. They are found at the tissue level and down from the tissue level, they are found at the molecular and cellular and molecular levels. We shall take an example again to see that structure-function relationships exist at all levels of biological organization. Beginning with the whole organism, we can see that the level of structure-function relationships comes down to the system level, to the organ level, to the tissue level, to the cellular level and to macromolecular and molecular levels. The muscular system at the system level helps to move the limbs. At the organ level, we see that these muscles provide, are made up of group of muscle tissues. These muscle tissues itself are based on the cellular level. That is, muscle tissue is made up of cells, which are muscle cells and are basically contractile. The contractility in the muscles depends on the contractility of these cells, but these muscle cells have such units or assemblies, which we call sarcomeres, which are the basis of contraction. These sarcomeres are themselves made up of two types of proteins, which are contractile in nature, and these proteins are made up of molecular bases. All these phenomena are made up of these proteins, which are made up of cells, which are made up of tissue contractile and the organs made up of tissue are contractile and as a result, the whole organism is moving. That is our aim. From this whole example, we can conclude that functions are dependent on their structures and the various structures have evolved to fulfill the demands that a physiological process presents.