Your third sentence ended very abruptly, you could fuse the the third sentence into the next sentence following by removing the period and re-structuring the sentence so the paragraph flows more smoothly. I would split up the two definitions in the second sentence into two different sentences for clarity. I would also add a summary final sentence. Each week, post your own Perfect Paragraph and comment on three Perfect Paragraphs.
Suggest improvements. Don't just say "Looks good. The difference is the the energy absorbed by an endothermic reaction or released by an exothermic reaction is heat. Endergonic and exergonic reactions may release other kinds of energy besides heat, such as light or even sound. For example, a glow stick is an exergonic reaction that releases light. It is not an exothermic reaction because it does not release heat. If a reaction is endergonic in one direction, it is exergonic in the other direction and vice versa.
For this reaction, endergonic and exergonic reactions may be called reversible reactions. The amount of free energy is the same for both the forward and reverse reaction, but the energy is absorbed positive by the endergonic reaction and released negative by the exergonic reaction. For example, consider the synthesis and degradation of adenosine triphosphate ATP. The reverse process, the hydrolysis of ATP, is an exergonic process with a Gibbs free energy value equal in magnitude, but opposite in sign of Chemical reactions proceed in both the forward and reverse direction until chemical equilibrium is reached and the forward and reverse reactions proceed at the same rate.
At chemical equilibrium, the system is in its most stable energy state. Equilibrium is bad news for biochemistry, because cells need metabolic reactions to occur or else they die. It is important to note that the term spontaneous - in the context of thermodynamics - does NOT imply anything about how fast the reaction proceeds. The change in free energy only describes the difference between beginning and end states NOT how fast that transition takes. This is somewhat contrary to the everyday use of the term which usually carries the implicit understanding that something happens quickly.
However, an iron nail exposed to air does not rust instantly - it may take years. These chemical reactions are called endergonic reactions , and they are NOT spontaneous. An endergonic reaction will not take place on its own without the transfer of energy into the reaction or increase of entropy somewhere else. Exergonic and endergonic reactions result in changes in Gibbs free energy.
In exergonic reaction the free energy of the products is lower than that of the reactants; meanwhile in endergonic the free energy of the products is higher than that of the reactants. The building of complex molecules, such as sugars, from simpler ones is an anabolic process and is endergonic. On the other hand, the catabolic process, such as the breaking down of sugar into simpler molecules is generally exergonic. Remember, the terms endergonic and exergonic only refer to the difference in free energy between the products and reactants - they don't tell you about the rate of reaction how fast it happens.
The issue of rate will be discussed in later sections. An important concept in the study of metabolism and energy is that of chemical equilibrium.
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