Cellular bioenergetics and concept of free energy Dr. Samah Kotb Lecturer of Biochemistry 2015...
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Transcript of Cellular bioenergetics and concept of free energy Dr. Samah Kotb Lecturer of Biochemistry 2015...
Cellular bioenergetics and concept of free energy
Dr. Samah KotbLecturer of Biochemistry
2015
Cellular Biochemistry and metabolism 1
CLS 331
2Dr Samah Kotb
Lecturer of Biochemistry
BIOENERGETICS & ATP
• Bioenergetics is basically how living systems make use of free energy
Bioenergetics & ATP
Bioenergetics & ATP
There are 2 types of energy that can be used by systems to do work:-
1. Free Energy2. Heat Energy
Free energy is the kind of energy that can be used
to do work under conditions of constant
temperature & pressure.
• Heat energy can be used to do work only through a change of temperature.
Bioenergetics & ATP:
Heat is not a significant source of energy for living
cells because heat can only do work as it passes from a
zone at one temperature to another at a lower
temperature.
Since living cells have the same temperature
throughout, they cannot make use of heat energy.
Cells use free energy (G) which can work at
constant temperature and pressure. Free energy is
obtained by animal cells from the catabolism of
energy rich nutrient molecules whereas plant cells
obtain it from solar radiant.
What do you know about
Anabolic Pathways
And
Catabolic Pathways
Anabolic Pathways
Reactions that result in
the synthesis of
biomolecules using basic
unit components and
require an input of energy
to take place .
Catabolic Pathways
Reactions through which
energy rich nutrient
molecules are broken down
by chemical reactions into
simple end products. As a
result of catabolic
pathways energy is
produced and released to
the cell.
Standard free energy change (G) of a chemical reaction:
G is the difference between the free energy content
of the reactants and that of the products under
standard conditions of temperature and pressure
(298k & 1 atmospheric pressure).
• When a reaction results in release of energy
It means that the products contain less free energy
than the reactants. Here G for the reaction will have
a negative value and the reaction will be catabolic in
nature.
A reaction is anabolic and will have a positive G
value if the products contain more free energy than
the reactants. Energy has to be put into the reaction
for it to proceed.
The free-energy change of a reaction (ΔG) divided into 3 types:
• 1. A reaction can occur only if ΔG is negative. An output of free energy is required to drive such a reaction, Such reactions are said to be exergonic.
• 2. A system is at equilibrium and no net change can take place if ΔG is zero.
• 3. A reaction can occur if ΔG is positive. An input of free energy is required to drive such a reaction. These reactions are termed endergonic.
It means that the products contain less free energy
than the reactants. Here G for the reaction will
have a negative value and the reaction will be
catabolic in nature.
A reaction is anabolic and will have a positive G
value if the products contain more free energy than
the reactants. Energy has to be put into the reaction
for it to proceed.
Standard free energy change (G) of a chemical reaction:
For every reaction G can be calculated using:-
G = - 2.303 RT log KeqWhile:R = Gas constantT = Absolute Temp.
Keq = Equilibrium constantNote:G indicates constant temperature & pressure and
physiological pH 7.2 for cells.Unites of free energy = calorie (cal) or kilocalorie (kcal) /mole
• A calorie (cal) is equivalent to the amount of heat required to raise the temperature of 1 gram of water from 14.5°C to 15.5°C.
• A kilocalorie (kcal) is equal to 1000 cal.
• A joule (J) is the amount of energy needed to apply a 1-newton force over a distance of 1 meter.
• A kilojoule (kJ) is equal to 1000 J.• 1 kcal = 4.184 kJ.
• The kilocalorie (abbreviated kcal) and the kilojoule (kJ) will be used as the units of energy. One kilocalorie is equivalent to 4.184 kilojoules.
Units of energy
• Consider the reaction
• The ΔG of this reaction is given by
• In which ΔG° is the standard free-energy change, R is the gas constant, T is the absolute temperature, and [A], [B], [C], and [D] are the molar concentrations (more precisely, the activities) of the reactants.
• ΔG° is the free energy change for this reaction under standard conditions that is, when each of the reactants A, B, C, and D is present at a concentration of 1.0 M (for a gas, the standard state is usually chosen to be 1 atmosphere).
• Thus, the ΔG of a reaction depends on the nature of the reactants (expressed in the ΔG° term of equation 1) and on their concentrations (expressed in the logarithmic term of equation 1).
• The ΔG of a reaction depends only on the free energy of the products (the final state) minus the free energy of the reactants (the initial state).
• The ΔG of a reaction is independent of the path (or molecular mechanism) of the transformation. The mechanism of a reaction has no effect on ΔG.
• The ΔG provides no information about the rate of a reaction.
G values of pathways can be calculated
The G value of an overall pathway can be calculated as
the algebraic sum of the G values of the individual
reactions making the pathway:-
Gpathway = G1 + G2 + G3 + G4 + G5
Chemistry of ATP (Adenosine – tri – phosphate):
ATP is a nucleotide type molecule made of the following
components:-1. The nitrogenous base adenine2. The pentose sugar ribose3. Three phosphate groups
Chemistry of ATP (Adenosine – tri – phosphate)
Thus:-ATP ADP + Pi G = -7.3 kcal/mole
ADP AMP + Pi G = -7.2 kcal/mole
AMP Adenosine + Pi G = -3.2 kcal/mole
ATP, ADP and AMP are present in all forms of life. They occur
not only in the cytosol of cells but also in the mitochondria
& nucleus. In normal respiring cells ATP makes up 80% of
the three ribonucleotides. ADP & AMP account for 20%.
Chemistry of ATP (Adenosine – tri – phosphate):
At pH 7, ATP occurs as the multiply charged anion ATP4-
whereas ADP occurs as ADP3-. This is because their phosphate
groups are completely ionized at the intracellular PH. ATP and
ADP occur inside cells as magnesium complexes:-
ATP4- + Mg2+ (ATP-Mg)2-
ADP3- + Mg2+ (ADP-Mg)ــ
Chemistry of ATP (Adenosine – tri – phosphate):
Inside cells the concentration of ATP remains normally relatively
constantly high. It’s rate of formation equals it’s rate of
hydrolysis. Thus the terminal phosphate group of ATP undergoes
continuous removal & replacement from the pool of inorganic
phosphate during cell metabolism.
G values for some characteristic reactions
Super high energy compounds are compounds generated during catabolism. They are phosphorylated compounds. Once formed along a catabolic pathway, they undergo immediate hydrolysis (dephosphorylation). As a result a large amount of energy is released this is used by the cell to synthesize ATP from ADP and the hydrolyzed inorganic phosphate.
G values for some characteristic reactions
The Bioenergetics of Muscle Contraction
The contraction of muscle requires a large amount of
energy that cannot be fulfilled by the ATP stored
inside muscle tissue. In addition to ATP there is a
super-high energy compound stored in muscle cells
that plays a major role in the energetics of muscle.
This super-high energy compound is also present in
large concentrations in other contractile tissues such
as brain & nerve tissue.
The Bioenergetics of Muscle Contraction
This compound is PHOSPHOCREATINE. It serves as a storage form
of high energy phosphate groups. The G value for the hydrolytic
reaction of phosphocreatine is highly negative (-10.3 kcal/mole).
This is greater than that of ATP. The energy released is sufficient to
allow coupled synthesis of ATP from ADP:-
The Bioenergetics of Muscle Contraction
Phosphocreatine thus functions to keep the ATP
concentration in muscle cells at constantly high level
whenever some of the ATP of muscle cells is used for
contraction, ADP is formed. Through the action of
creatine kinase phosphocreatine is quickly hydrolyzed
and donates its phosphate group to ADP to form ATP.
The phosphocreatine level inside muscle is 3-4 times
greater than that of ATP and thus stores enough high
energy phosphate groups to keep the ATP level
constantly high during short periods of intense
muscular contraction.
The Bioenergetics of Muscle Contraction
The Bioenergetics of Muscle Contraction: