Thus, simple diffusion will not meet the requirements of all the cells. But what happens when the body size of the organism increases and the body design becomes more complex? In multi-cellular organisms, all the cells may not be in direct contact with the surrounding environment. In the case of a single-celled organism, no specific organs for taking in food, exchange of gases or removal of wastes may be needed because the entire surface of the organism is in contact with the environment. 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 what we call respiration. For this, many organisms use oxygen sourced from outside the body. Oxidising-reducing reactions are some of the most common chemical means to break-down molecules. For this, a series of chemical reactions in the body are necessary. These sources of energy, therefore, need to be broken down or built up in the body, and must be finally converted to a uniform source of energy that can be used for the various molecular movements needed for maintaining living structures, as well as to the kind of molecules the body needs to grow. The outside sources of energy could be quite varied, since the environment is not under the control of the individual organism. Depending on the complexity of these carbon sources, different organisms can then use different kinds of nutritional processes. Since life on earth depends on carbon-based molecules, most of these food sources are also carbon-based. ![]() If the body size of the organisms is to grow, additional raw material will also be needed from outside. So there must be a process to transfer a source of energy from outside the body of the organism, which we call food, to the inside, a process we commonly call nutrition. This energy comes from outside the body of the individual organism. Since these maintenance processes are needed to prevent damage and break-down, energy is needed for them. The processes which together perform this maintenance job are life processes. ![]() Even when we are just sitting in class, even if we are just asleep, this maintenance job has to go on. The maintenance functions of living organisms must go on even when they are not doing anything particular. What are the maintenance processes in living organisms? Let us explore. Since all these structures are made up of molecules, they must move molecules around all the time. So living creatures must keep repairing and maintaining their structures. If order breaks down, the organism will no longer be alive. Because of the effects of the environment, this organised, ordered nature of living structures is very likely to keep breaking down over time. Why are molecular movements needed for life? We have seen in earlier classes that living organisms are well-organised structures they can have tissues, tissues have cells, cells have smaller components in them, and so on. ![]() In fact, viruses do not show any molecular movement in them (until they infect some cell), and that is partly why there is a controversy about whether they are truly alive or not. Is this invisible molecular movement necessary for life? If we ask this question to professional biologists, they will say yes. Movements over very small scales will be invisible to the naked eye – movements of molecules, for example. So using visible movement as the defining characteristic of life is not enough. But a plant that is not visibly growing is still alive, and some animals can breathe without visible movement. In other words, we tend to think of some sort of movement, either growth-related or not, as common evidence for being alive. But what about plants that have leaves of colours other than green? They grow over time, so we know that they are alive, some will say. What about plants? How do we know that they are alive? We see them green, some of us will say. What if the dog or the cow or the man were asleep? We would still think that they were alive, but how did we know that? We see them breathing, and we know that they are alive. ![]() How do we tell the difference between what is alive and what is not alive? If we see a dog running, or a cow chewing cud, or a man shouting loudly on the street, we know that these are living beings.
0 Comments
Leave a Reply. |