BIOENERGETICS

Objectives

- To gain an understanding of concepts used to deal with energy flow in living systems
- To understand the following terms and concepts:
1. Enthalpy
2. Entropy
3. Free energy
4. Bioenergetic coupling of chemical reactions
5. Additivity of free energy changes
6. Relationship between standard free energy change and equilibrium constants
7. Role of ATP as energy currency of the cell

Bioenergetics – Biochemical thermodynamics

- Quantitative study of the energy transductions that occur in living cells and of the nature and
function of the chemical processes underlying these transductions
- Provides underlying principles to explain why some reactions may occur while others do not
- N.B:
- Non-biological systems may use heat energy to perform work, whereas biological systems are
essentially isothermic and use chemical energy to power living processes

Biomedical Importance of Bioenergetics

- Fuel is required to provide energy for normal processes, so understanding energy production
and utilization is fundamental to understanding normal nutrition and metabolism

Starvation
- Occurs when available energy are depleted
- Certain forms of malnutrition are associated with energy imbalance e.g . marasmus – a wasting
disease due to insufficient energy and protein intake

Obesity
- Excess storage of surplus energy results in obesity which can have negative effects on health

Gibbs Free Energy Change

- Gibbs free energy change is that portion of the total energy change in a system that is available
for doing work
- It is the useful energy
- When a reaction proceeds with a release of free energy, the system changes so as to possess less
free energy.
- In this case the free energy change has a negative value.
- And the reaction is said to be exergonic

- In endergonic reactions, the system gains energy and the free energy change is positive.

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Units of free energy change

- joules/mole (J/mol)

- Calories/mole (Cal/mol)

Enthalpy, H

1. Enthalpy is the heat content of the reacting system

2. It reflects the number and kinds of chemical bonds in the reactants and products
3. When a chemical reaction releases heat, it is said to be exothermic.
4. The heat content of the products is less than that of the reactants
5. And by convention, enthalpy change a negative value
6. Reacting systems that take up heat from their surroundings are endothermic.
7. And have positive value of enthalpy change
8. Units of enthalpy:
- joules/mole (J/mol)
- calories/mole (Calories/mole)

Entropy, S

1. Entropy is a quantitative expression for the randomness or disorder of a system

2. When the products of a reaction are less complex and more disordered than the reactants,
the reaction is said to proceed with a gain in entropy
3. Units of entropy change:

– J/mol.K

- Cal/mol.K

K = units of absolute temp (i.e. 25 degrees Centigrade = 298K)

Example:

- Oxidation of glucose
- Increase in number of molecules or when a solid is converted to liquid or gas, generates
molecular disorder.
- Entropy increases

Relationship between free energy change, enthalpy change, and entropy change:

- Under conditions existing in biological systems (constant temp and pressure), changes in free
energy, changes in enthalpy, and changes in entropy are related to each other quantitatively:
Change in free energy = enthalpy change-entropy change x temperature (K)

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 - Change in enthalpy has a negative sign when heat is released by the system to the surroundings
- Change in entropy has a positive sign when entropy increases
- In a favorable (exergonic) process which releases heat and increases entropy
e.g oxidation of glucose
Free energy change = (negative value of enthalpy) minus (T x positive value of entropy)
Free energy change = negative value

For favorable or spontaneous processes, free energy change has a negative value

Free energy

- We must subtract the energy lost to increasing entropy of the system from the total enthalpy
change to obtain the amount energy left over (available for useful work)
- At equilibrium in a closed system no net change in free energy can occur,

Free energy change = 0 and

Enthalpy change = T X Entropy change

Example

1. Heat water in a tea cattle

2. Steam is produced and potentially capable of doing work
3. Allow to cool, no work is done
4. Temperature of surroundings increases by an infinitesimal amount until equilibrium is
reached
5. Kettle and surroundings are at the same temp, the free energy that was once in kettle has
disappeared
6. No free energy available to do work
7. Irreversible

N.B:

When a reaction system is not at equilibrium, the tendency to move toward equilibrium represents a
driving force, the magnitude of which can be expressed as the free energy change for the reaction.

At 25 degrees centigrade (298K) and at (1M) for participants, the driving force = standard free energy

Actual free energy changes depend on reactant and product concentrations:

- The standard free energy change tells us which direction and how far a given reaction will go to
reach equilibrium when the initial concentration of each compound is 1.0 M, the pH is 7.0, the
temp is 25 deg C, the pressure is 1.0 atmosphere.

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Biological Energy Transduction

1. Life depends on constant energy transduction mechanisms

2. In most organisms, from prokaryotes to eukaryotes the energy obtained whether from light,
inorganic or organic compounds is transduced into a transmembrane difference of :
- Electrochemical potential across the prokaryotic cytoplasmic membrane
- Or the eukaryotic mitochondrial membrane
3. For example, non-photosynthetic organisms obtain energy by the degradation of proteins,
carbohydrates, lipids and nucleic acids
4. Which feed electrons to the respiratory chain.
5. Here, the electron transfer is coupled to the translocation of ions across the membrane
6. and the energy released as result of the favorable electron transfer is transduced to the form of
a transmembrane difference of electrochemical potential
7. This transmembrane potential is vital for solute/nutrient cell import, synthesis of ATP and
motility
8. Peter Mitchell proposed the existence of such a potential for the first time in his Chemiosmotic
Theory

9. Key words:
- Energy transduction mechanisms – are constant in living organisms
- Prokaryotes and eukaryotes
- Transmembrane difference
- Electrochemical potential
- Prokaryotic cytoplasmic membranes
- Eukaryotic mitochondrial membranes
- Non-photosynthetic organisms
- Biosynthesis of foodstuffs
- Degradation of foodstuffs
- Respiratory chain (Electron transport chain)
- Solute/nutrient import into cells
- Motility
- Transmembranne potential – vital for solute/nutrient import into a cell
- Synthesis of ATP
- Chemiosmotic Theory – Peter Mitchell

(Show diagram)

Biological Energy Transduction

1. ATP as the currency of energy exchange

2. When the terminal phosphate group is removed from ATP by hydrolysis, two negatively
charged products are formed, ADP and the phosphate group (inorganic phosphate)
3. These products are electrically more stable than the parent molecule and do not readily
recombine
4. The total free energy (G ) of the products is much less than that of ATP; hence, energy is
liberated i,e the reaction is exergonic

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 The Standard Free Energy Change:
5. The amount of energy liberated under strictly defined conditions is called the standard free
energy
6. This value for the hydrolysis ATP is relatively high (-8 kilocalories per mole)

Definition of a kilocalorie

- One kilocalorie is the amount of heat required to raise the temperature of 1000 grams of water
one degree Celsius
7. Conversely, the formation of ATP from ADP and inorganic phosphate is an energy – requiring
( i.e. endergonic) reaction with a standard free energy change of +8 kilocalories per mole
8. The hydrolysis of the remaining phosphate-to-phosphate bond of ADP is also accompanied
by a liberation of energy (the standard free energy change)
Structure and Functions of cellular organelles