Each step of this process is its own reaction and thus has its own options to bleed energy out of the system, but that's fine so long as the energy of the original photons is enough. If the excited chlorophyll is energetic enough, there will be enough energy in the system such that even though we lose some to entropy, heating up the cell, there is enough energy to drive all the reactions and deliver the electrons back to the chlorophyll to be used again.
You can also find examples of the reverse reaction in biology, ATP to excited photons:
The reaction: Luciferase + Luciferin + ATP + O2 -------------------> oxyluciferin + AMP + emitted photon
This reaction is actually used quite extensively in the lab.
Re: Entropy - The statistical mechanics viewpoint.
Hi Son Goku,
I understand the microstate / macrostate definition, thanks for that.
But I'm struggling with the link to 'usable work' and I'm hoping you'll be able to help me out.
* Which of these concepts is actually the 'definition' of entropy
The definition I gave is "the" definition of entropy. The thermodynamic version related to usable work is an idealisation. For example, the thermodynamic definition of entropy and the associated second law says entropy never decreases. The statistical mechanical definition says that entropy is extremely unlikely to decrease.
However the idealisation involved in the thermodynamic definition is the second most accurate idealisation in all of science. More on this below.
* The link you describe between the two views is quite conceptual - can it be demonstrated rigorously with maths? (I'm not asking you to actually show the math, btw)
Good question. Yes, it can be rigorously shown, although it takes an incredible amount of work. Physicists always went by the conceptual link and assumed they were the same. However it was only in 1960s with work by David Ruelle that people began to check if it was rigorously true. Basically you get the "usable work" definition back when you take the limit of an infinite number of particles. In fact statistical mechanics turns into thermodynamics in the infinite particle limit.
Of course the real world doesn't have an infinite number of particles, so the thermodynamic definition is not the correct one and is really an idealisation. However the error of this idealisation is so small (and I mean really small), that it doesn't matter at all in practical terms.
* I actually think there is quite a strong link with disorder here - do you think people are wrong to say that entropy is a measure of disorder ? If so, why are they wrong?
They are wrong in the technical sense that entropy and disorder are not the same. In a lot of situations increase in entropy would correspond to increase in disorder. However there are cases where they don't really have any relation, for example increasing entropy might lead to what humans would call increasing order.
The real problem is that disorder is related to our intuitive notions of chaos and order, so it leads to a misconception that our ideas of chaos and order are somehow a principle of the physical world.
However in a mechanical setting, or in everyday applications (car rusting, fridge breaking, e.t.c.) it's perfectly fine to think of entropy as disorder.
quote:So energy lost in one reaction is not necessarily unusable in another reaction.
Yes. But you must remember that we talk about a "system." That is, we assume that we have accounted for all players in the game: We have the pot of hot water, the ice cube, and some sort of "engine" that can do work as the heat flows from the hot water to the frozen water.
Eventually, all of those things will be the same temperature, there will be no more heat flow, and the engine grinds to a halt.
We call this system "closed."
Now, suppose we could add something to this system. Perhaps we heat some of the water so that it's even hotter or take some of the water and refreeze it. Then we can run the engine again, but notice that it required something outside the system to do this.
We call this system "open."
Note, a "closed" system can be quite complicated. For example, suppose the engine between the hot water and the ice cube is used to freeze some water. That'll certainly keep things going for a while but since we can never convert all the heat into work, eventually everything equalizes.
This is why the thermodynamic concept of entropy is dependent upon temperature. Mathematically, entropy is defined as energy ("Q") divided by temperature ("T"). Thus, the entropy of a reaction is related to the temperature at which the reaction takes place.
There's a connected concept: Enthalpy. It's sort of the opposite of entropy: Where entropy is the energy unavailable to do work, enthalpy is the energy that is available to do work. This is why some reactions result in a decrease of entropy. If the amount of energy used to run the reaction is sufficiently large such that the total energy of the system reduces, then we can reduce entropy, too. Remember that example I gave above where we use an engine to create ice? That's an example of energy being used to reduce entropy.
You take some coal and burn it, producing heat. That heat is used to heat up water into steam. The steam is used to turn a turbine. The turbine generates electricity. The electricity is used to power a refrigerator. The refrigerator is used to make ice.
Now, the water going from liquid to solid is a decrease in entropy. But the amount of energy that was used and processed in this system is much more than the entropic decrease.
quote:Why would a layperson need to understand entropy?
To a certain extent, they don't. There is the joy of simply being an educated person, but it isn't like the typical person needs to be able to make a thermodynamic calculation in their daily lives.
Around here, however, it's important because one of the common complaints made by creationists is that evolution somehow violates the Second Law. They can never explain how, but they are certain that it does. It's because they don't understand what entropy is.
There are multiple failures in their claim connected to their misconception of entropy. The first is their assumption that a "more complex" organism (and note that what makes it "more complex" is never defined) represents less entropy than a "more primitive" one (again, notice the lack of definition as to what the means.)
Why? Why would this necessarily be the case? If we assume the concept of going from a single cell to a multicellular organism is what we mean by "more complex," it would seem that entropy has actually increased: There are so many more reactions taking place that the biological functions have plenty of opportunities to bleed off energy as entropy.
But even so, why would a "more complex" process be a problem? If the new process is more efficient, isn't that a more favored reaction? It can make use of the available resources better, supporting more organisms than one that is wasteful. This doesn't stop entropy from happening. It just happens more slowly.
And on top of that, there's this thing called the sun. If you take a look at the way life on this planet works, tracing back the energy, you find that most life traces back their energy to the sun. Suppose that refrigerator example from above was powered by solar cells rather than coal. Well, we've got a ton of sunlight available: That refrigerator is going to be working for quite some time, generating a lot of ice. The reason why this works is because the huge thermodynamic reaction of the sun and its entropic increase far outweighs the puny entropic decrease that we have here on this planet.
The idea that evolution is a violation of physics is laughable and shows a fundamental misunderstanding of both biology and physics.
Rrhain Thank you for your submission to Science. Your paper was reviewed by a jury of seventh graders so that they could look for balance and to allow them to make up their own minds. We are sorry to say that they found your paper "bogus," specifically describing the section on the laboratory work "boring." We regret that we will be unable to publish your work at this time. Minds are like parachutes. Just because you've lost yours doesn't mean you can use mine.