Multicellular organisms may have many organ systems. Different populations inhabiting an area at the same time make up a community. A community and its physical environment constitutes an ecosystem. Each level of organization has emergent properties not accounted for by a sum of the parts. Maintaining organization and carrying on life requires outside source of energy.
Food provides nutrient molecules used as building blocks or for energy.
Metabolism is all chemical reactions that occur in a cell. Ultimate source of energy for life on earth is the sun through photosynthesis. Organisms must remain homeostatic or keep themselves stable in temperature, moisture level, acidity, and other physiological factors. Response often results in movements of plant or animal. Ability to respond helps organism survive.