Summary Bioenergetics and Thermodynamics

Bioenergetics: Thermodynamics
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Learning Objectives
1. Describe what metabolism is.
2. Explain the first and second laws of thermodynamics
3. Understand what free energy is

Metabolism
A collection of chemical reactions that occur within the cells of living organisms in order to
maintain life
Enzyme-catalysed reactions are connected in series: the product of one reaction becomes the
starting material, [or substrate], for the next.

TWO CATEGORIES OF METABOLISM: opposing streams of chemical reactions occur in cells…

Catabolism [Catabolic pathways]
• Breaks down foodstuffs into smaller
molecules.
• Generates energy for the cell and small
molecules the cell needs as building
blocks.

Anabolism [Anabolic/biosynthetic
pathways]
• Uses energy harnessed by
catabolism.
• Drive synthesis of many molecules
that form the cell.

Thermodynamics
• Thermodynamics → The principles that govern energy flow
• ‘Thermo’ meaning heat is the historical origin of thermodynamics.
o These principles consider other forms of energy and the processes the convert one
form of energy into another i.e. ENERGY CHANGE
• Bioenergetics → “applied thermodynamics”
o Applies the principles of thermodynamics to reactions ad processes in biological
systems.
• There exist 3 laws of thermodynamics…

First Law of Thermodynamics
• “The law of conservation of energy”
• The first law of thermodynamics states that in every physical or chemical change, the total
amount of energy in the universe remains constant, although the form of the energy may
change

, • BASICALLY! Energy can be converted from one form to another but can never be created or
destroyed.

Applied to the whole universe or a closed system, the 1st law means that:
The total amount of energy [present in all forms] MUST BE THE SAME before and after any
process/reaction occurs

Applied to an open system e.g. cell, the 1st law says that:
During the course of any reaction/process, Energy out = Energy in - Energy stored

1. Internal energy
• The total energy stored within a system
• Represented by the symbol E
• Cannot measure E for a system directly
• It is possible to measure the change in internal energy, ΔE

ΔE is the difference in internal energy of the system before the process and after the process:

ΔE = E2 - E1 same as ΔE = Eproducts - Ereactants

2. Enthalpy
• AKA heat content
• Represented by symbol H
• Dependent on pressure [P], volume [V] and Internal energy [E] as changes in heat content
following a reaction affects the total energy as well as pressure and volume.

H = E + PV
• In biological systems pressure and volume change a little or not at all (usually are zero)
• So, for biological reactions, P and V are usually zero (or at least negligible)…
• Changes in the value of enthalpy heat content for reactions are valid estimates of ΔE

ΔH = ΔE meaning ΔH = ΔHproducts – ΔHreactants
ΔH may be either positive or negative…

ΔH NEGATIVE
- Heat content of products is LESS THAN heat content of reactants
- Heat is RELEASED
- EXOTHERMIC REACTION!!!!
- Example → the burning of gasoline in a car
o the heat content of the products (CO2 and H2O) is less than the heat content of the
reactants (gasoline and O2)

ΔH POSITIVE
- Heat content of the products is GREATER THAN heat content of reactants
- Heat energy is ABSORBED
- ENDOTHERMIC REACTION!!!!
- Example → melting of an ice cube
o the heat content of the resulting liquid water is greater than the heat content of the
ice before melting