Buffalo Feathers!!! Physics can explain any phenomna that occurs in the natural world...breathing, blood circulation, whatnot. The problem that arrises is that by using something overly complex to explain a basic principle is that it becomes cumbersome, thus the birth of the other sciences. Basic Biology and Chemistry are cake walk classes Physics kicked me around...it ended up alright, but it kicked me around.
Perhaps I should supply some context for my statements. I have a Ph.D. in physics (earned it in August), and have also taken college level introductory biology and AP chemistry (plus there is a certain amount of overlap between statistical physics and inorganic chemistry). While you may have found biology and chemistry classes easier than physics, in my opinion this does not relate to how complex the fields are for the purposes of this discussion.
First of all, physics is actually an older discipline, in terms of its scientific development, than biology. This means that part of the reason physics is harder is because we have had time to get to the really hard problems, whereas biology is still scratching the surface. Biology spent a long time flailing around to find even the basic principles and is only now really beginning to find the proper quantitative methods for getting the same kind of depth already available in physics. If you try to solve truly comparable problems, in terms of predictive power, biology is much more difficult than physics at every step. In your biology classes, you solved enormously simpler problems than you did in your physics classes, but this is not inherent to the disciplines but rather a side-effect of the way we teach them.
Part of the reason this lack of relative depth is not obvious is because biology is something we experience much more directly than we do physics. It is part of our life, and we have well-developed intuition to deal with things like animals and other people. Things just happen. Thus the true complexity of the system is masked by the fact that we ourselves are correspondingly complex and know how to deal with it. We don't realize that explanations and descriptions given in daily life hide a lot of complexity that would come out in a full quantitative treatment on par with the normal physics descriptions.
In addition, the world is fairly regular, even in biology. However, the complexity in biology allows exceptions to "bubble up" that physics can't tolerate. In physics, an electron is an electron and all electrons are absolutely identical (this can be stated mathematically and has noticeable effects on the world around us. Like that it exists.) In physics there are on the order of 32 fundamental particles, maybe a few more. Their behavior is unintuitive and difficult to understand, but has been almost completely solved in real-world situations barring interactions (which is where biology comes from).
The number of species in biology on the other hand, is enormous. I don't even know where to start. Even within species, we get exceptions. A cat, is not just a cat. Many cats behave similarly, until you get the cat with a mental disorder. The sheer richness of biology dwarfs that of physics. The behavior is enormously subtle and cannot be predicted with anything like the precision physics has. The biosphere is orders of magnitude more complex than anything mentioned in a physics textbook; this is glossed over in biology classes simply because you know what the biosphere is already and don't demand that your teacher make predictions for the biosphere that are as good as the predictions we have for the orbit of Mars.
This is being addressed, of course, by biologists or physics who dabble in biology, or whatever. However, it is fairly simple to make an introductory course in biology and get handed only easy problems while *feeling* like the material is comparable to the hard problems presented in physics. Hence the illusion.