Living creatures must keep repairing and maintaining their structures all the time which might break down due to environmental effects. Since all these structures are made up of molecules, they must move molecules around all the time.
The processes which together perform the maintenance functions of living organisms even when they are not doing anything particular job are life processes.
Nutrition is a process of intake and utilization of nutrients by an organism as an energy source or for biosynthesis of body constituents. It is a process to transfer a source of energy from outside the body of the organism, which we call food, to the inside. Different organisms use different kinds of nutritional processes.
The bio-catalysts used by the heterotrophs for the break down of complex substances into simpler ones before they can be used for the upkeep and growth of the body are called enzymes.
- The process of acquiring oxygen from outside the body, and to use it in the process of break-down of food sources for cellular needs, is called respiration.
- It is a biochemical process taking place in the mitochondria and involves exchange of gases oxygen and carbon dioxide and oxidation of food in cells to release energy.
These are some of the most common chemical means to break-down molecules. For this, many organisms use oxygen sourced from outside the body.
In multi-cellular organisms, simple diffusion will not meet the oxygen requirements. Since the food and oxygen are now taken up at one place in the body of the organisms, while all parts of the body need them, this creates a need for a transportation system for carrying food and oxygen from one place to another in the body.
The biological process of removal of excess or toxic wastes from the body is called excretion. When chemical reactions use the carbon source and the oxygen for energy generation, they create by¬products that are not only useless for the cells of the body, but could even be harmful. These waste by products are therefore needed to be removed from the body and discarded outside by a process called excretion.
Why is diffusion insufficient to meet the oxygen requirements of multicellular organisms like humans?
Single-celled organisms do not have specific organs for taking in food, exchange of gases or removal of wastes because the entire surface of the organism is in contact with the environment. Whereas, in multi-cellular organisms like humans, all the cells may not be in direct contact with the surrounding environment due to which simple diffusion is insufficient to meet the requirements of all the cells.
What criteria do we use to decide whether something is alive?
The criteria we should use to decide whether an organism is alive or not is the molecular movements within the organisms, which are needed for repairing and maintaining their structures which keep breaking down over time due to the effects of the environment.