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Δ G = Δ H T Δ S

The standard free energy change of a chemical reaction is expressed as an amount of energy per mole of the reaction product (either in kilojoules or kilocalories, kJ/mol or kcal/mol; 1 kJ = 0.239 kcal) under standard pH, temperature, and pressure conditions. Standard pH, temperature, and pressure conditions are generally calculated at pH 7.0 in biological systems, 25 degrees Celsius, and 100 kilopascals (1 atm pressure), respectively. It is important to note that cellular conditions vary considerably from these standard conditions, and so standard calculated ∆G values for biological reactions will be different inside the cell.

Endergonic reactions and exergonic reactions

If energy is released during a chemical reaction, then the resulting value from the above equation will be a negative number. In other words, reactions that release energy have a ∆G<0. A negative ∆G also means that the products of the reaction have less free energy than the reactants, because they gave off some free energy during the reaction. Reactions that have a negative ∆G and consequently release free energy are called exergonic reactions . Think: ex ergonic means energy is ex iting the system. These reactions are also referred to as spontaneous reactions, because they can occur without the addition of energy into the system. Understanding which chemical reactions are spontaneous and release free energy is extremely useful for biologists, because these reactions can be harnessed to perform work inside the cell. An important distinction must be drawn between the term spontaneous and the idea of a chemical reaction that occurs immediately. Contrary to the everyday use of the term, a spontaneous reaction is not one that suddenly or quickly occurs. The rusting of iron is an example of a spontaneous reaction that occurs slowly, little by little, over time.

If a chemical reaction requires an input of energy rather than releasing energy, then the ∆G for that reaction will be a positive value. In this case, the products have more free energy than the reactants. Thus, the products of these reactions can be thought of as energy-storing molecules. These chemical reactions are called endergonic reactions , and they are non-spontaneous. An endergonic reaction will not take place on its own without the addition of free energy.

Let’s revisit the example of the synthesis and breakdown of the food molecule, glucose. Remember that the building of complex molecules, such as sugars, from simpler ones is an anabolic process and requires energy. Therefore, the chemical reactions involved in anabolic processes are endergonic reactions. On the other hand, the catabolic process of breaking sugar down into simpler molecules releases energy in a series of exergonic reactions. Like the example of rust above, the breakdown of sugar involves spontaneous reactions, but these reactions don’t occur instantaneously. [link] shows some other examples of endergonic and exergonic reactions. Later sections will provide more information about what else is required to make even spontaneous reactions happen more efficiently.

Questions & Answers

A closed circulatory system is a closed-loop system, in which blood is not free in a cavity. Blood is separate from the bodily interstitial fluid and contained within blood vessels. In this type of system, blood circulates unidirectionally from the heart around the systemic circulatory route, and th
Peter Reply
food
Ramatulai
what is meiosis
Ram Reply
meiosis is a reduction Division in which four hapliod off springs are produced
Nandanee
what is electrochemical gradient?
Joneth Reply
what is mitosis?
Aliness Reply
(cytology) the division of cell nucleus in which the genome is copied and separated into two identical halves.it is normally follow by cell division
Adekunle
it Is the division of the mother cell into two daughter cells,genetically identical to each other and to their parent cell
Joneth
it is a study of living organisms
Prudence Reply
What are other types of cell
ATAMA Reply
plant and animal cell
Jessy
prokaryotic and eukaryotic cell
Joneth
meaning inside and outside cells
Joneth
external feature of earthworm
Ajas Reply
what is the reason behind that before F2 generation is no segregation?
asmamaw Reply
what is mitosis and meiosis
Rabiu Reply
hypothesis theory law
Tamba Reply
what is hypothesis theory law
Tamba
Briefly describe the process of mitosis and meiosis.
Lilian Reply
what is chromosome
Deborah Reply
Chromosome is the thread-like structure containing DNA and found in the nucleus of a cell. DNA contains gene which has genetic information.
Lilian
if a cell is killed by strain why then is it use
uchenna Reply
what is an atom
Finda Reply
an atom is the must smallest element in the world.
Young
please help me friends. I wont a good example of lab report based on carbohydrate,lipid,reducing and non reducing sugar.
Ng
that can't be split by any chemical means
uchenna
No an atom is the smallest particles of an element which can take part in a chemical reaction
Solayemi
An atom is the smallest indivisible particle of an element which can take part in a chemical reaction.
Samuel
sure atoms can't be splited
Solayemi
atom can't be split
john
what is molecule
john
Molecules consists of one or more tiny particles called atom
Rofiah
atoms joined together by covalent bonds
Bad
an atom is a particle that cannot be divided further by a chemical reaction
Bad
how do earthworms excrete
Ndlovu
what is genotype
Precious Reply

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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