Energy and Living Systems
Energy is the ability to do work. Two types of Energy:
1.
Potential is stored energy. Most important form is that stored in bonds of molecules.
2.
Kinetic is energy of movement. Heat and light are examples of kinetic energy
Two Laws of Thermodynamics: Explains the flow and behavior of energy in the universe
1.
Law of Conservation of Energy: energy is neither created nor destroyed but is converted from one
form to the other
2.
Law of Entropy. Entropy is disorder (heat is a form of entropy). Every energy change results in an
increase in entropy. Burning fossil fuels releases its potential energy (in bonds) causing a flash of light
and a burst of heat (energy escaping from the system). Energy changes are accompanied by an
increase in entropy.
A chemical reaction
Reactants  Products
Chemical Reactions do one of two things: Store or Release heat
Endergonic Reactions : Requires a net input of energy to get the reaction started. Photosynthesis is endergonic
requires solar energy to drive the reaction. Stores the energy in the chemical bonds of the end product, glucose.
Typically they store the energy input in the form of high energy bonds
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Potential energy of products (sugar) is greater than the potential energy of the reactants (CO 2).
Exergonic Reactions Release energy. The amount of energy released in this reaction is equal to the difference in the
potential energy of the reactants and the products. Exergonic reactions are energy-releasing.
 Potential energy of reactants (sugar) is greater than the potential energy of the products (CO 2).
Cell Respiration is exergonic: Burns food substances (lipids, carbohydrates, proteins) to form ATP and
heat. Burning wood and fuel is exergonic burns carbon skeletons to form light and heat.
Metabolism is the sum of all chemical reactions in the body, both endergonic and exergonic. Cell reactions require
small amounts of energy, ATP is preferred form of energy. The energy released in the burning of glucose by the
body is trapped in the high energy bonds in ATP.
ATP = Adenosine triphosphate
[ADP + P i  ATP] same as Adenine- Pi -Pi -Pi
(this is an endergonic reaction coupled to an exergonic reaction)
Hydrolysis of ATP is breaking the bond between the last two P i releases the kinetic energy.
Energy Barrier
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All chemical reactions have to overcome an energy barrier (energy of activation) to yield the products.
An enzyme is a protein molecule which serves as a biological catalyst, in lowering the energy of activation,
thus speeding up the rate of reaction, without itself being chemically involved.
ENZYMES: these are proteins that function as catalyst. This means they speed up the rate of a chemical rxn in the
body without being consumed in the process.
By lowering “activation energy” the reaction speeds up.
The name of an enzyme ends in ase. In general
Lipids  glycerol and fatty acids (lipase)
Proteins  amino acids (proteases)
Maltose  glucose + glucose (maltase)
Environmental Effects on Enzyme Activity
Temperature, pH, Salt concentration, Cofactors (i.e. Mg ) and Coenzymes (organic cofactors, i.e. vitamins)
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All enzymes have a site on their surface known as the ‘active site’ which is where the substrate binds to the
enzyme. This site can bind a molecule of a specific shape by ‘induced fit’.
Enzyme Inhibitors are substances that bind to the active site of an enzyme and inhibit the enzyme. There are of two
types:
1) Competitive: resembles the enzyme’s normal substrate and competes with it for binding to the enzyme’s
active site.
2) Non-competitive: Binds outside the active site but changes the shape of the enzyme so that its active
site no more fits the substrate.
Negative feedback: the end product of the reaction inhibits the enzyme
Antibiotic penicillin interferes with an enzyme that helps to build bacterial cell walls
When a reaction is thermodynamically favorable it proceeds spontaneously, and requires no energy input. However
the process can proceed slowly. Enzymes evolved over time to speed up thermodynamically favorable reactions.
Examples of Spontaneous Processes (Thermodynamically Favorable Reactions)
 Passive and Facilitative Transport. (i.e. O2 diffuses into red blood cells and CO2 moves out of RBC in the
lungs)
 Hydrolysis of ATP to ADP + Pi
 A ball rolling down hill
 Hydrolysis of Food (digestion) to monomers and small molecules
When a high energy molecule is broken down to a small low energy molecule energy is released from the system.
This energy is captured and uses to drive endergonic reaction (those requiring energy).
Cells Expend Energy for Active Transport
Active transport involves the aid of a transport protein in moving a solute up a concentration gradient (from
low concentration to high concentration).
Energy for active transport is obtained by the hydrolysis of ATP (ATP  ADP + Pi ).
The Cell and organelles such as Chloroplast and Mitochondria use coupled reactions to make energy available for
cellular work using enzymes and membrane mediated processes.
Photosynthesis and Cell Respiration are coupled:
Photosynthesis uses solar energy to build high potential energy molecules (endergonic). During cell
respiration these high potential energy molecules are broken down to release their energy to form ATP
The Reactants in Photosynthesis are the Products in Mitochondria
CO2 + H2O  Glucose and O2
Synthesis of ATP from ADP + Pi is coupled with the hydrolysis of ATP