Physics of Complex Organisms - Introduction

There is nothing in the Universe that does not obey the laws of nature underpinned by the theories of Quantum Mechanics and of Relativity.  Living organisms are no exception - their organization, function and size are all subject to constraints defined by the laws of nature.

There are two distinct regimes that one must distinguish here.  The function of organisms at the molecular level falls in the realm of quantum mechanics and mainly the science of chemical reactions determines this.  Evolution has fine-tuned the chemistry to an extent that it might be fair to say that all organisms share a basic optimized unit of life.  The basic unit of life is the biological cell.  Cells come in a large variety in animals and plants but they all appear to have some fundamental properties that make them self-sufficient like houses are.  (There are some fascinating websites that compare different parts and functions of a house with the organelles of a cell).  A cell is essentially a self-sustaining organism that embodies life as it is defined.  A cell has limitations in what it can achieve but these may be overcome by cells functioning together in the form of multicellular organisms.  There are many different ways that cells can join together and the numbers can get very large indeed - a human body has of the order of a million billion cells! Multi-cellular organisms are like cities where a larger number of houses exist but operate in a well-defined regulated manner to cooperate and flourish.

How cells organize themselves in multi-cellular organisms can, fortunately, be described by classical laws of nature which are much easier to visualize and understand and the physics involved is quite straightforward - it is a macroscopic system.  There are many different ways multi-cellular organisms can be constructed. With time, their design in terms of energy efficiency, strength, functionality etc. would have been optimized by natural selection working within the constraints of the laws of physics and this is what we see in today's successful animals and plants.  Of course, they all will not follow the same design - it is a question of engineering - just as an engineer works with basic materials and/or designs to construct different structures - organisms use a variety of designs and materials too.

A good example is the solution for providing oxygen more efficiently to cells over the whole body of an organism.  Diffusion, convection, circulation, hemoglobin are methods that progressively made it easier to transport oxygen over greater distances to more distant part of the body and making possible the growth in animal size. Artificial red blood cells, recpirocytes, have been proposed that will increase the oxygen carrying capacity of normal red blood cells by 236 times with corresponding increase in human endurance. This is an example where modern technology can enhance animal functionality but still working within the laws of physics.

What appears fundamental to our discussion is the size of the organism. Unfortunately, there is not a unique size for all animals or plants and they come in vastly different shapes and sizes.  For animals, the body density is very nearly 1 gm/cc and instead of the volume and shape, it is equally good to work with organism mass.  In nature we indeed have a very large range in masses; mammals alone range from a few grams for a shrew to many tons for a whale.  What is surprising is that even with this vast range of sizes, most body functions scale smoothly as some power law of body mass (allometric scaling). The relative size of the skeleton, metabolic rates, size of the brain, heart beat rate, life span and many more parameters depend on body mass and vary more or less smoothly from the tiniest to the largest of the mammals.

An interesting example to allude to here is that the heart beat rate varies as -0.25 to the power of mass while the life span varies as +0.25 to the power of the mass.  This means the smaller animal heart beats faster but they do not live as long. The total number of heart beats, equal to the product of heart beat rate and life span, for all mammals is therefore independent of their mass (size). Actually the numbers work out such that we all, from shrews to whales, are allowed about 2 billion heart beats in a lifetime!

To stretch the analogy of cells and houses further, we can think of complex organisms as cities. For efficient operation, cities too have to deliver essential commodities like water, food, energy, information etc. to all buildings just as complex organisms need to supply chemical molecules, oxygen, nerve impulses, remove toxic wastes etc. from each cell, however remote.  A system of some sort of networks must operate to maximize efficiency and smooth operation of the city or the organism.  Networks play a fundamental role in the development of complex organisms and go some way to explain their behaviour. 

How size affects animal structure and function is a fascinating subject and in future blogs, I shall look at many aspects of this topic and attempt to understand the physics involved. 

This I shall do but the next few publications will be on the question of how do we measure the age of the Earth and the Universe and where did all the elements come from!!

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