Chapter 5 Bioenergetics: The Flow of Energy in the …classpages.warnerpacific.edu/BDupriest/BIO...

© 2012 Pearson Education, Inc.

Lectures by

Kathleen Fitzpatrick Simon Fraser University

Chapter 5

Bioenergetics: The Flow of Energy in the Cell


The 4 needs of every cell


Definition of energy

• The capacity to cause specific physical or

chemical changes

• (The capacity to do work)


Types of work in the cell


Figure 5-2


Figure 5-3


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Figure 5-6

© 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc.

Which of the following is an example of

electrical work?

a. Intestinal epithelial cells absorbing

nutrients.

b. Cardiomyocytes depolarizing.

c. Neurons releasing neurotransmitters

through exocytosis.

d. Lung epithelial cells exchanging gasses.


What do all six forms of biological work have

in common?

a. They all accomplish something that is

intrinsically difficult.

b. They are all used by all cells.

c. They all involve the consumption of

ATP.

d. They all use energy to cause a specific

change.


Energy flow through the biosphere


Figure 5-5


HOW does energy flow?

• Oxidation and reduction (redox reactions)


Human beings are chemotrophs. When a

human being loses fat, most of the mass is

lost to _____.

a. heat

b. the air

c. feces

d. feces and urine


Plants and animals are similar in that _____.

a. both types of cells use aerobic respiration to synthesize ATP in mitochondria

b. both types of cells convert solar energy into chemical bond energy

c. both types of cells rely on heat to provide decreased entropy

d. both types of cells use water to reduce carbon dioxide


True or False: Photosynthesis is an example

of a biochemical pathway that ultimately

reduces carbon containing molecules.

a. True

b. False


Bioenergetics

• Applied thermodynamics


To Understand Energy Flow, We Need

to Understand Systems, Heat, and Work

• Energy can be defined as the ability to cause

change

• Though energy is distributed throughout the

universe, the energy under consideration in any

particular case is called the system, and the rest

of the universe is called the surroundings

• The boundary between the system and

surroundings may be real or hypothetical


Systems can be open or closed

• A closed system is sealed from its environment

and cannot take in nor release energy

• An open system can have energy added to it or

removed from it

• Organisms are open systems, capable of

uptake and release of energy


Figure 5-7


Figure 5-8


The second law of thermodynamics

• The second law of thermodynamics is the law

of thermodynamic spontaneity

• It says that in every physical or chemical change,

the universe tends toward greater disorder or

randomness (entropy)

• It allows us to predict what direction a reaction

will proceed under specific conditions, how much

energy will be released, and how changes in

conditions will affect it


Free Energy

• A measure of spontaneity for a system alone is

called free energy (G)

• G (the free energy change) = Gproducts - Greactants

• G is related to enthalpy and entropy of a reaction

• G = H - T S (T= temperature of the system in

Kelvin, or oC +273)


Figure 5-9


Which of the following is true of

thermodynamically spontaneous reactions?

a. They are all energy-yielding.

b. They are all energy-consuming.

c. They all result in an increase in the

temperature of the system.

d. They all result in a decrease in the

temperature of the system.


Which of the following statements about G

is correct?

a. It is sometimes greater than 0 for spontaneous reactions.

b. It is less than 0 for fast reactions.

c. It is always less than 0 for spontaneous reactions.

d. It can only be found if you know the equilibrium constant and the prevailing concentrations of products and reactants.


Free Energy Change and

Thermodynamic Spontaneity: A

Biological Example

• Consider the oxidation of glucose (a highly

exergonic process)

• C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

• Under standard conditions, H = -673 kcal

/mole glucose and -TS = -13 kcal/mole

• G = -686 kcal/mole


Figure 5-10A


The reverse reaction:

• The reverse reaction is endergonic

• 6CO2 + 6H2O + energy → C6H12O6 + 6O2

• Compared to the oxidation of glucose, H

and -TS have the same magnitude but the

opposite sign

• G = + 686 kcal/mole


Figure 5-10B


A thermodynamically spontaneous

reaction _____.

a. happens quickly, with no addition of energy to the system

b. happens quickly whether or not there is an addition of energy to the system

c. has the capacity to happen, but might not happen quickly

d. has the capacity to happen, but only in the presence of a catalyst


The Meaning of Spontaneity

• The term spontaneous tells us that a reaction

can take place, not that it will

• Whether an exergonic reaction will proceed

depends on a favorable (negative) G but also

the availability of a mechanism

• Usually an input of activation energy is required

as well


The Equilibrium Constant is a Measure

of Directionality

• The equilibrium constant Keq, the ratio of

product concentrations to reactant

concentration at equilibrium

• At equilibrium there is no net change in the

concentrations of reactants or products

• A B Keq = [B]eq / [A]eq


The equilibrium constant

• If you know the equilibrium constant for a

reaction, you can tell whether a particular

mixture of products and reactants is in

equilibrium

• The tendency toward equilibrium provides the

driving force for every chemical reaction


Concentration ratio

• A concentration ratio (products to reactants)

less than Keq means that the reaction will

proceed to the right to generate more product

• A concentration ratio greater than Keq means

that the reaction will proceed to the left


Figure 5-11


ΔG Can Be Calculated Readily


General calculation of ΔG

• G is free energy change in cal/mol

• R is the gas constant (1.987 cal/mol x K)

• T is the temperature in kelvins

• Keq is equilibrium constant at standard temperature

of 298 K (25oC)

• ln is the natural log


General calculation of G

• For the generalized reaction

• G can be calculated as


Limitations on G

• G is a thermodynamic parameter that tells us

whether a reaction is thermodynamically possible

as written

• It also tells us how much free energy would be

liberated if the reaction took place

• However, it tells us nothing about rate or

mechanism of the reaction


The Standard Free Energy Change is G

Measured Under Standard Conditions

• Because G may vary under different conditions, we

must identify the conditions for which a given

measurement of G is made

• Biochemists have agreed on conditions to define the

standard state (G0)

• These conditions are 25oC (298K), 1 atmosphere

pressure, and all products and reactants at a

concentration of 1M


Standard conditions

• Biochemists usually specify a standard pH = 7.0,

therefore the concentration of H+ and OH- ions is

10-7

• The concentration of water is not included in

calculating free energy change

• Keq and G are written with a ′ to indicate standard

pH


Standard change

• In any thermodynamic parameter, the standard

change refers to

– The conversion of one mole of a specified reactant to

product, or

– The formation of one mole of a specified product from

the reactants

• Under standard conditions; e.g., the standard free

energy change (G0′)


G0′ and K′eq

• There is a linear relationship between G0′

and ln K′eq

•

• This means that G0′ can be calculated

directly from the equilibrium constant,

provided Keq was determined under the same

standard conditions


The most useful formulas

• Under standard conditions, RT = 592 cal/mol


Figure 5-12


G0′ is different from G in that _____.

a. G0’ is calculated under conditions common in cells, whereas G is used for biochemical reactions in a test tube It is sometimes greater than 0 for spontaneous reactions.

b. G0’ might be positive for a reaction that occurs spontaneously under conditions prevailing in a cell, whereas G is always negative for a reaction that occurs spontaneously under conditions prevailing in a cell

c. G0’ is a measure of the direction of a reaction under standard conditions, whereas G is a measure of the direction of a reaction under prevailing conditions


If G′ for a reaction in a cell is positive, _____.

a. the reaction can never occur because it is not spontaneous

b. a change in temperature will have no effect on the reaction

c. the addition of a catalyst is required for the reaction to occur

d. a change in the prevailing concentrations of products and reactants could make the reaction spontaneous


Summing Up: The Meaning of G′

and G0’

• If K′eq is greater than 1.0 then G0′ will be negative and the reaction can proceed to the right (towards the products) under standard conditions

• If K′eq is less than 1.0 then G0′ will be positive and the reaction will tend toward the left (toward the reactants) under standard conditions


G0′

• G0′ values are convenient

• They are easily determined from the equilibrium constant and provide a uniform convention for reporting free energy changes

• But, G0′ is an arbitrary standard, referring to impossible conditions for most biological systems


G′

• For real life situations, G′ is the most useful measure of thermodynamic spontaneity

• G′ provides a measure of how far from

equilibrium a reaction is, under the conditions

in a cell

• Under the special case where G′ = 0, the

reaction is in equilibrium; however, reactions

in living cells are rarely in equilibrium


Table 5-1


Life and the Steady State:

Reactions That Move Toward

Equilibrium Without Ever Getting

There

• At equilibrium the forward and reverse rates of a

reaction are the same, so there is no net flow of

matter, nor energy produced

• Living cells are characterized by continuous

reactions and maintain themselves in states far

from equilibrium


Steady state

• Cells maintain a steady state in which

reactants, products, and intermediates are

kept at levels far from equilibrium

• This is possible because cells are open

systems that receive large amounts of energy

from the environment

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