Wize University Biology Textbook > Energy & Metabolism
Laws of Thermodynamics
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The Laws of Thermodynamics
The First Law of Thermodynamics
The first law of thermodynamics is that energy cannot be created or destroyed, only transferred from one form to another.
The energy of an isolated system such as the universe is constant.
The Second Law of Thermodynamics
The second law states that a system naturally moves toward more disorder. That is, its entropy always increases.
- Entropy is denoted by the letter S.
- This can also be related to the first law by thinking of the energy transfer. Every transfer of energy from one form to the other increases the universe's entropy.
- If a system is naturally moving toward more disorder, then it will take work to keep or create order. Example: Once you clean your bedroom, it doesn't take long to make a mess again. In order to keep it clean and in order, we need to do work.
Energy In Biology
- Sometimes the energy used in a reaction is not equal to the energy of the structure in the end.
- Where does the energy go? ––> it can be released as heat, light, sound, or movement for example. This means not all the energy is harnessed into the structure of the molecule or in a usable/useful way. Example: ATP hydrolysis reaction results in a lower energy molecule of ADP. The energy generated by breaking phosphate bonds in ATP releases energy that can be used for other reactions.
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Example: Entropy
Consider the following reaction. Did the entropy of this system increase or decrease by creating these products?
AB(l) --> A(g) + B(g)
This may seem like a strange problem, but it's an easy way to think about entropy. In this system, where there was once one molecule, now there are two. This system became more disorganized and there are more possible states for it now that it has two molecules rather than one. Therefore, the entropy of this system increased.
Practice: Laws of Thermodynamics
According to the laws of thermodynamics: