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: