|MadSci Network: Biochemistry|
Hi Jeremy, Thanks for this interesting question. It is of course one of the three big philosophical questions in biology, along with "how did life arise?" and "what is consiousness?". The questions of what life is and how it arose are very closely related, because at some point non-living material has to take on living characteristics.
A curious observation concerning life is that it may be hard to define, but we all recognise it when we see it. In fact there is a list of characteristics that define a living organism-people may object to any of these in isolation, but taken together they constitute a fairly complete description. Living things move, they feed, they excrete, they respire (generate energy) they grow, they sense their environment, they reproduce and they evolve. It's true to say that a flame does many of these things, but we know instinctively that a flame isn't the same as a living thing. It could be said to excrete CO2 for instance, by "respiring" combustible materials with oxygen, but no-one would argue that this is the same type of process as cellular respiration.
In one way, the characteristics of life are quite easy to define, because the only life we know is life on earth. Then the question becomes "what is life as we know it?". Life on earth is defined by the biochemicals that we identify in living things, so my simple definition of a living thing is one in which DNA and RNA are used as the basis for genetic information transfer and proteins are used as structural components and catalytic agents.
This isn't really answering your question of course, all we have done so far is defined the components of a living cell. You used the word "organised" in your question and the key to life is organisation, in particular self-organisation. Living cells are "eddies of order in a stream of chaos". In chemical terms, they are regions in which entropy is decreased and non-equilibrium conditions are maintained. This is often misunderstood as going against the 2nd law of thermodynamics, but remember that the 2nd law refers to a closed system. The universe is an open system and it's fine for local areas to decrease their entropy so long as the overall trend is upwards.
A hot research topic at the moment is therefore the notion of "self- organising systems", in other words how does something which is essentially a soup of molecules organise itself into a complex, functioning system? We are beginning to see that this is not so great a problem as it first sounds. Even quite simple chemical systems can display organisation, simply as a consequence of the properties of the molecules involved and the way in which they physically interact. But what I feel these studies are missing is the fact that when we observe a cell, we are seeing the product of billions of years of evolution. True, if we view the components of a cell in isolation as molecules, it's hard to see how they work. But life isn't about taking these components, mixing them together and saying "well I've just got molecule soup, how can that work?" As with cooking you need ingredients and time. What we observe today came about gradually as more and more components were brought together, mixed, interacted, used or discarded as needed and without knowing the sequence of events, we may never fully understand how a cell came to be.
I hope these thoughts don't sound too vague and wishy-washy! My personal view is that "what is life?" isn't a very useful question, it's a bit like saying "what is reality?" or "what is the number eleven?" Science should really be aimed at asking well-defined questions with yes or no answers. It's a difficult area, but I'd say we have defined the characteristics of life and we have some notion of how a cell is organised. Understanding how the complex properties of a cell emerge is in many ways a problem for mathematicians, provided they don't forget about evolution!
If you haven't already, try the famous book "What is Life?" by Erwin Schrodinger-it's still thought provoking, although somewhat dated and naive now. And if you think you can demonstrate how life arose, you can win big money at http://www.us.net/life/!
Try the links in the MadSci Library for more information on Biochemistry.