Living organisms must continually capture/acquire, store, and use energy to carry out the functions of life.

Capturing/acquiring, storing and using energy are accomplished through cellular chemical processes.

Cells are chemical machines that obey fundamental chemical and physical laws of nature.

The sum of a cells chemical reactions is called metabolism.


Energy is the ability to do work.

Work is the transfer of energy from one body or place to another.

Cells use catabolic reactions to convert chemical potential energy into kinetic energy

¨ the hydrolysis of ATP transfers chemical potential energy (stored energy) to a protein pump to move particles through a membrane (kinetic energy)

Cells use anabolic reactions to convert kinetic energy into chemical potential energy

¨ photosynthesis captures light energy and stores it as chemical potential energy

During these reactions, energy is not created or destroyed but simply transferred from one place to another.  This is The First Law of Thermodynamics:

“The total amount of energy in the universe is constant.  Energy cannot be created or destroyed but only converted from one form into another…”

Cellular Respiration and Bond Energies

Biological molecules (carbohydrates, lipids, proteins) have energy stored in their chemical bonds.

Cellular respiration is the process by which a cell enzymatically breaks these energy containing bonds to release their energy and harness it to fulfill a cellular function.

Bond energy; – is a measure of the stability of a covalent bond.

– is equal to the minimum amount of energy required to break the bond

– is equal to the amount of energy released when a bond is formed

In a chemical reaction:

– bonds between reactants must be broken (energy absorbed)

– bonds between products form (energy is released)

Potential Energy Diagrams

The amount of energy needed to strain and break reactant bonds causing a reaction to proceed is called the activation energy.

If product bonds are more stable that those in the reactants, energy is released.

If product bonds are less stable that those in the reactions, energy has been absorbed.


Atoms form covalent bonds to achieve greater stability.  Then why do endothermic reactions proceed when they make products that are less stable?

Energy is not the only factor that determines whether or chemical or physical change will occur, entropy must also be taken into consideration.


Entropy is the measure of the randomness or disorder in energy or in a collection of objects.

In chemical reactions, entropy increases when:

–  solid reactants become liquid or gaseous products

–  fewer moles of reactant molecules form a greater number of moles of product molecules

–      complex molecules react to form simpler subunits (polymers into monomers)

– solutes move from an area of high concentration to an area of lower concentration until they are uniformly distributed

The universe favours an increase in entropy.  This is The Second Law of Thermodynamics

“The entropy of the universe increases with any change that occurs…”

Exothermic reactions

– are favoured because product molecules are more stable than reactant molecules.

– also accompanied by an increase in entropy

Endothermic reactions:

– are not favoured because product molecules are less stable than reactant molecules.

– will occur of entropy increases after the reaction

Living Systems and Entropy

PROBLEM: Living organisms SEEM to violate the second law of thermodynamics through their anabolic processes, building ordered structures like proteins and DNA, and other processes.

REALITY: Anabolic processes that decrease entropy are accompanied by energy-yielding catabolic processes that contribute an even greater increase in entropy.

OBEYING the 2nd Law: Living organisms are able to make highly ordered structures by coupling FREE ENERGY-yielding catabolic processes (increase entropy) with energy-requiring anabolic processes (decrease entropy).

Free energy

Free energy is the energy that can do useful work.

Not all energy transformations can do useful work.

Exothermic and Endothermic terms are used to describe the change in total energy that takes place in a chemical reaction.

Exergonic reaction is a chemical reaction that is accompanied by a decrease in free energy.

Endergonic reaction is a chemical reaction that is accompanied by an increase in free energy.

Adenosine Triphosphate (ATP)

ATP is the primary source of free energy in living cells.

“HIGH ENERGY” BOND: The terminal phosphate is relatively unstable (high energy) because of the concentration of negative charge in the triphosphate tail of the molecule.

When free energy is needed an enzyme (ATPase) hydrolyzes the terminal phosphate from the ATP molecule à 31 kJ/mol free energy released

Energy not released directly as heat since this would increase the temperature of the cell.

Free energy is used to attach a phosphate to another molecule directly associated with the work the cell needs to do, which causes the molecule to be more reactive à perform its function

Phosphorylation is the process where a phosphate group is attached to a molecule.

Redox Reactions

Oxidation is the process of losing electrons; oxidized = lose electrons

Reduction is the process of gaining electrons; reduced = gain electrons

Redox reactions are reactions that involve the transfer of electrons between two substances, where one substance is OX-idized and the other substance is RED-uced.

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