|MadSci Network: Anatomy|
Thanks for submitting your question to the MadSci Network. I realize that you asked for someone to write you a new answer, but it seems silly to have one of our volunteer experts replicate the work that someone has already done; we have answers in our archives that address your primary questions, like what makes the heart beat, or what constitutes a neuron's signals. For example, take a look at this answer (1068057226.Bc) about neurons.
For more information about neuron function, you can also use our search engine to locate answers containing the words muscle and neuron.
I think that your questions will eventually lead you to a question like, "what makes cells work?" because when you get down to it, organ and tissue functions like the heart-beat and neuron signalling are based in the function of the cells that make up those tissues.
The thing that ultimately makes a cell work is the cell membrane. The cell membrane separates the internal contents of a cell (of all cells, including bacteria, archea and eukaryotes) form the exterior environment. This allows the cell to maintain an internal environment of LOW entropy, by creating an external environment that has higher entropy.
This issue is also addressed in our archives. For example, take a look at these answers; (859247407.Ev), and (942797163.Gb) , and also this answer (961007835.Bc).
All of the stuff that goes on inside of a cell -- DNA, RNA, proteins, other membranes (in the case of eukaryotes), gradients -- and all of the interactions that occur between cells (like the heart beating, and neuronal signalling) could not happen if the cell membrane was not intact, and of course, when the cell membrane is disrupted, the cell dies. A great many anti-microbial agents work just this way, by destroying the integrity of the cell membrane (921165350.Mi).
In fact, it seems quite likely that the cell membrane predated all of the other complex intra-cellular systems that are found in all cells today; this includes DNA, ribosomes, and proteins. The earliest cells were likely RNA-based organisms, in which RNA carried out the roles taken by DNA and proteins today, and of course, they did it inside of a membrane. These RNA-based organisms all went extinct billions of years ago, but we've inherited their membranes. You can read this answer (1085611161.Sh) for more information on why we think that RNA-based organisms predated DNA-RNA-protein based organisms, and you can search our archives for the term 'RNA world' for even more information.
All cell membranes are made by pinching-off a bubble from a pre-existng cellular membrane, all the way back to the first cell. Rudolf Carl Virchow realized this in 1855, when he said, "Omnis cellula e cellula", or "all cells arise from other cells". It is the origin of that first cell membrane, and therefore the first cell, that is still a mystery. So, that would be the question that we can't yet answer -- how did the first cell come into existance?
We have evidence that cell membranes (lipid bilayers) can form spontaneously, as discussed in this answer (940611612.Bc). Here, you can see that it is entropy again that drives the spontaneous formation of lipid bi-layers. So ultimately, the answer to the question, "what makes biological systems work" is the second law of thermodynamics.
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