MET - Energetics [LESSON]
Energetics
Energy is a property of objects which can be transferred to other objects or converted into different forms, but cannot be created or destroyed. Organisms use energy to survive, grow, respond to stimuli, reproduce, and for every type of biological process.
First Two Laws of Thermodynamics
Recall that the first law of thermodynamics says that energy cannot be created nor destroyed, but can only be converted from one form into another. The second law of thermodynamics is really a conceptually difficult topic, but it can be simplified into the idea that as energy is transferred or transformed, some is lost as heat. Both of these laws will appear in our lesson about energy in biological systems.
Metabolism
The processes of making and breaking down carbohydrate molecules illustrate two types of metabolic pathways. A metabolic pathway is a step-by-step series of interconnected biochemical reactions that convert a substrate molecule or molecules through a series of metabolic intermediates, eventually yielding a final product or products. For example, one metabolic pathway for carbohydrates breaks large molecules down into glucose. Another metabolic pathway might build glucose into large carbohydrate molecules for storage. The first of these processes requires energy and is referred to as anabolic. The second process produces energy and is referred to as catabolic. Consequently, metabolism is composed of these two opposite pathways:
- Anabolism (building molecules)
- Catabolism (breaking down molecules)
Anabolic pathways are those that require energy to synthesize larger molecules. One example of an anabolic pathway is the synthesis of sugar from CO2. Other examples include the synthesis of large proteins from amino acid building blocks and the synthesis of new DNA strands from nucleic acid building blocks. Catabolic pathways are those that generate energy by breaking down larger molecules. Some catabolic pathways can capture that energy to produce ATP, the molecule used to power all cellular processes. Other energy-storing molecules, such as lipids, are also broken down through similar catabolic reactions to release energy and make ATP. Both types of pathways are required for maintaining the cell's energy balance.
Free Energy (Gibbs)
A measurement of free energy is used to quantify energy transfers. Free energy is called Gibbs free energy (G) after Josiah Willard Gibbs, the scientist who developed the measurement. Recall that according to the second law of thermodynamics, all energy transfers involve the loss of some amount of energy in an unusable form such as heat, resulting in entropy. Gibbs free energy specifically refers to the energy associated with a chemical reaction that is available after accounting for entropy. In other words, Gibbs free energy is usable energy or energy that is available to do work.
What is the relationship between free energy and stability?
Things that have more free energy are more unstable. Therefore the lower the energy, the more stable the compound.
The calculation for ∆G using the equation ∆G=∆H-T∆S is no longer in the AP Biology curriculum and you will NOT have to calculate ∆G this way. However, you will need to look at a reaction diagram and use the y-axis to calculate ∆G. For the purposes of this course, ∆G is the change in free energy between the products and the reactants of a chemical reaction and can be calculated by simply subtracting the two: ∆G = Gproducts-Greactants
Endergonic 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. Exergonic means energy is exiting the system. These reactions are also referred to as spontaneous reactions because they can occur without the addition of energy into the system. It’s important to realize that the word spontaneous has a very specific meaning here: it means a reaction will take place without added energy, but it doesn't say anything about how quickly the reaction will happen. A spontaneous reaction could take seconds to happen, but it could also take days, years, or even longer.
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; they are non-spontaneous. An endergonic reaction will not take place on its own without the addition of free energy.
Does it take energy input to break a bond or is energy released from a bond when broken?
It takes energy input to break a bond. Look at the increase in energy from the reactants in EVERY reaction diagram. Energy is NOT released from a bond when bonds break.
Now, use the Endergonic and Exergonic Reactions tool below to practice learning about endergonic and exergonic reactions.
Later in this module, we will learn about cellular respiration which is a coupling of an extremely exergonic reaction to an endergonic one and photosynthesis which is an endergonic reaction.
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