Biology which is the study of life has been divided into different levels of organization for the convenience to study. The levels or organization can be represented as follows:
Atoms - molecules - cell - Tissue - organ - organ system - Organism - population - community - ecosystem - biosphere.
Cell is the simplest functional unit of living organism. Different cells have different functions and each cell carry the genetic information i.e the DNA. Cells with similar structure and function come together to form tissues. Tissues like epithelial tissue, connective tissue, muscular tissue and nervous tissue. Specific tissues with specialized functions come together to form the organ. In humans there are various organs but all are under the control of brain. Group of organs working together form the organ system. For example digestive system is made of organs such as stomach, small intestine, large intestine, pancreas. All the organ systems together make the organism. Within organism the most complex level of organization is organ system.
Oxidation of pyruvate to acetyl-CoA does not generate energy and glycolysis generates only 2 ATP (2 ATP used and 4 ATP generated). On the contrary TCA generates as much 38 ATP.
