Ch06 lecture

Chapter 6

Energy Flow in the
Life of a Cell

Lectures by
Gregory Ahearn
University of North Florida

Copyright © 2009 Pearson Education, Inc..

5.1 What Is Energy?
 Energy is the capacity to do work.
• Synthesizing molecules
• Moving objects
• Generating heat and light

Copyright © 2009 Pearson Education Inc.

5.1 What Is Energy?
 Types of energy
• Kinetic: energy of movement
• Potential: stored energy


Fig. 5-1

5.1 What Is Energy?
 First Law of Thermodynamics
• “Energy cannot be created nor destroyed, but
it can change its form.”
• Example: potential energy in gasoline can be
converted to kinetic energy in a car, but the
energy is not lost


5.1 What Is Energy?
 Second Law of Thermodynamics
• “When energy is converted from one form to
another, the amount of useful energy
decreases.”
• No process is 100% efficient.
• Example: more potential energy is in the
gasoline than is transferred to the kinetic
energy of the car moving
• Where is the rest of the energy? It is released
in a less useful form as heat—the total energy
is maintained.

5.1 What Is Energy?
 Matter tends to become less organized.
• There is a continual decrease in useful
energy, and a build up of heat and other nonuseful forms of energy.
• Entropy: the spontaneous reduction in ordered
forms of energy, and an increase in
randomness and disorder as reactions
proceed
• Example: gasoline is made up of an eightcarbon molecule that is highly ordered
• When broken down to single carbons in CO2, it
is less ordered and more random.

5.1 What Is Energy?
 In order to keep useful energy flowing in
ecosystems where the plants and animals
produce more random forms of energy, new
energy must be brought in.


5.1 What Is Energy?
 Sunlight provides an unending supply of
new energy to power all plant and animal
reactions, leading to increased entropy.


Fig. 5-2

5.2 How Does Energy Flow In Chemical
Reactions?
 Chemical reaction: the conversion of one
set of chemical substances (reactants) into
another (products)
• Exergonic reaction: a reaction that releases
energy; the products contain less energy than
the reactants


Reactions?
 Exergonic reaction
energy
released

+
reactants

+
products

(a) Exergonic reaction

Fig. 5-3a

Reactions?
 Endergonic reaction: a reaction that
requires energy input from an outside
source; the products contain more energy
than the reactants


Reactions?
 Endergonic reaction
energy
used

+
+

products

reactants
(b) Endergonic reaction

Fig. 5-3b

Reactions?
 Exergonic reactions release energy.
• Example: sugar burned by a flame in the
presence of oxygen produces carbon dioxide
(CO2) and water
• Sugar and oxygen contain more energy than
the molecules of CO2 and water.
• The extra energy is released as heat.


Reactions?
 Burning glucose releases energy.
energy
released
C6H12O6
(glucose)

+

6 O2
(oxygen)
6 CO2
(carbon
dioxide)


+

6 H2O
(water)

Fig. 5-4

Reactions?
 Endergonic reactions require an input of
energy.
• Example: sunlight energy + CO2 + water in
photosynthesis produces sugar and oxygen
• The sugar contains far more energy than the
CO2 and water used to form it.


Reactions?
 Photosynthesis requires energy.
energy
C6H12O6 + 6 O2
(glucose) (oxygen)
6 CO2
(carbon
dioxide)

+

6 H 2O
(water)


Fig. 5-5

Reactions?
 All reactions require an initial input of energy.
• The initial energy input to a chemical reaction
is called the activation energy.
Activation energy needed
to ignite glucose

high

Energy level of reactants
energy
content
of
molecules

Activation
energy
captured
from
sunlight

glucose

glucose + O2

CO2 + H2O

CO2 + H2O

Energy level of reactants

low
progress of reaction
(a) Burning glucose (sugar): an exergonic reaction


(b) Photosynthesis: an endergonic reaction

Fig. 5-6

Reactions?
 The source of activation energy is the
kinetic energy of movement when
molecules collide.
 Molecular collisions force electron shells of
atoms to mingle and interact, resulting in
chemical reactions.


5.2 How Does Energy Flow in Chemical
Reactions?
 Exergonic reactions may be linked with
endergonic reactions.
• Endergonic reactions obtain energy from
energy-releasing exergonic reactions in
coupled reactions.
• Example: the exergonic reaction of burning
gasoline in a car provides the endergonic
reaction of moving the car
• Example: exergonic reactions in the sun
release light energy used to drive endergonic
sugar-making reactions in plants

5.3 How Is Energy Carried Between
Coupled Reactions?
 The job of transferring energy from one
place in a cell to another is done by energycarrier molecules.
• ATP (adenosine triphosphate) is the main
energy carrier molecule in cells, and provides
energy for many endergonic reactions.


Coupled Reactions?
 ATP is made from ADP (adenosine
diphosphate) and phosphate plus energy
released from an exergonic reaction (e.g.,
glucose breakdown) in a cell.
energy
A
A

P
ADP


P

+

P

phosphate

P

P

P

ATP

Fig. 5-7

Coupled Reactions?
 ATP is the principal energy carrier in cells.
• ATP stores energy in its phosphate bonds and
carries the energy to various sites in the cell
where energy-requiring reactions occur.
• ATP’s phosphate bonds then break yielding
ADP, phosphate, and energy.
• This energy is then transferred to the energyrequiring reaction.


Coupled Reactions?
 Breakdown of ATP releases energy.
energy
A

P
ATP

P

P
A

P
ADP


P

+

P

phosphate

Fig. 5-8

Coupled Reactions?
 To summarize:
• Exergonic reactions (e.g., glucose breakdown)
drive endergonic reactions (e.g., the
conversion of ADP to ATP).
• ATP moves to different parts of the cell and is
broken down exergonically to liberate its
energy to drive endergonic reactions.


Coupled Reactions?
 Coupled reactions
glucose
A
exergonic
(glucose breakdown)

P

P

P
protein

endergonic
(ATP synthesis)
exergonic
(ATP breakdown)

CO2 + H2O + heat
A

P

P

+

endergonic
(protein synthesis)

P
amino
acids


Fig. 5-9

Coupled Reactions?
 A biological example of coupled reactions
• Muscle contraction (an endergonic reaction) is
powered by the exergonic breakdown of ATP.
• During energy transfer in this coupled
reaction, heat is given off, with overall loss of
usable energy.


Coupled Reactions?
 ATP breakdown is coupled with muscle
contraction.
Exergonic reaction:

ATP

Endergonic reaction:
+

20 units
energy

relaxed
muscle

contracted
muscle

100 units
+ ADP + P
energy
released
Energy released from ATP
breakdown exceeds the
energy used for muscle
contraction, so the overall
coupled reaction is exergonic

Coupled reaction:
+
relaxed
muscle


ATP

+ 80 units
energy
contracted
released
muscle
as heat

+

ADP +

P

Fig. 5-10

Coupled Reactions?
PLAY

Animation—Energy and Chemical Reactions


Coupled Reactions?
 Electron carriers also transport energy
within cells.
• Besides ATP, other carrier molecules
transport energy within a cell.
• Electron carriers capture energetic electrons
transferred by some exergonic reaction.
• Energized electron carriers then donate these
energy-containing electrons to endergonic
reactions.


Coupled Reactions?
 Common electron carriers are NAD+ and
FAD.
high-energy
reactants

energized
e–

NADH

depleted
low-energy
products


e–

high-energy
products

NAD+ + H+

low-energy
reactants

Fig. 5-11

Coupled Reactions?
PLAY

Animation—Energy and Life


5.4 How Do Cells Control Their Metabolic
Reactions?
 Cell metabolism: the multitude of chemical
reactions going on at any specific time in a
cell
 Metabolic pathways: the sequence of
cellular reactions (e.g., photosynthesis and
glycolysis)
Initial reactant

PATHWAY 1

A

B
enzyme 1

D

C
enzyme 2

enzyme 3

E
enzyme 4

G

F

PATHWAY 2
enzyme 5

Final products

Intermediates

enzyme 6

Fig. 5-12

Reactions?
 At body temperature, many spontaneous
reactions proceed too slowly to sustain life.
• A reaction can be controlled by controlling its
activation energy (the energy needed to start
the reaction).
• At body temperature, reactions occur too
slowly because their activation energies are
too high.
• Molecules called catalysts are able to gain
access to energy that is not produced
spontaneously.

Reactions?
 Catalysts reduce activation energy.
• Catalysts are molecules that speed up a
reaction without being used up or
permanently altered.
• They speed up the
reaction by reducing
the activation
energy.
high

Activation energy
without catalyst

energy
content
of
molecules

Activation energy
with catalyst

reactants

products
low


Fig. 5-13

Reactions?
 Three important principles about all
catalysts
• Catalysts speed up a reaction.
• They speed up reactions that would occur
anyway, if their activation energy could be
surmounted.
• Catalysts are not altered by the reaction.


Reactions?
 Enzymes are biological catalysts.
• Almost all enzymes are proteins.
• Enzymes are highly specialized, generally
catalyzing only a single reaction.
• In metabolic pathways involving multiple
reactions, each reaction is catalyzed by a
different enzyme.


Reactions?
 The structure of enzymes allows them to
catalyze specific reactions.
• Enzymes have an active site where the
reactant molecules, called substrates, enter
and undergo a chemical change as a result.
• The specificity of an enzyme reaction is due to
the distinctive shape of the active site, which
only allows proper substrate molecules to
enter.


Reactions?
 How does an enzyme catalyze a reaction?
• Both substrates enter the enzyme’s active
site.
• Substrates enter an enzyme’s active site,
changing both of their shapes.
• The chemical bonds are altered in the
substrates, promoting the reaction.
• The substrates change into a new form that
will not fit the active site, and so are released.


Reactions?
 The cycle of enzyme–substrate interactions
substrates
active site
of enzyme

enzyme

1 Substrates enter
the active site in a
specific orientation

3 The substrates, bonded
together, leave the enzyme;
the enzyme is ready for a
new set of substrates

2 The substrates and
active site change shape,
promoting a reaction
between the substrates

Fig. 5-14

Reactions?
PLAY

Animation—Enzymes


Reactions?
 Cells regulate metabolism by controlling
enzymes.
• Allosteric regulation can increase or decrease
enzyme activity.
• In allosteric regulation, an enzyme’s activity
is modified by a regulator molecule.
• The regulator molecule binds to a special
regulatory site on the enzyme separate
from the enzyme’s active site.


Reactions?
 Binding of the regulator molecule modifies
the active site on the enzyme, causing the
enzyme to become more or less able to
bind substrate.
 Thus, allosteric regulation can either
promote or inhibit enzyme activity.


Reactions?
 Enzyme structure
substrate
active site

Many enzymes have
both active sites and
allosteric regulatory
sites

enzyme

(a) Enzyme structure

allosteric
regulatory site
Fig. 5-15a

Reactions?
 Allosteric inhibition
An allosteric regulator
molecule causes the
active site to change
shape, so the substrate
no longer fits

(b) Allosteric inhibition

allosteric
regulator
molecule
Fig. 5-15b

Reactions?
 Competitive inhibition can be temporary or
permanent.
 Some regulatory molecules temporarily bind
directly to an enzyme’s active site,
preventing the substrate molecules from
binding.
 These molecules compete with the
substrate for access to the active site, and
control the enzyme by competitive inhibition.

Reactions?
 Competitive inhibition

A competitive inhibitor molecule
occupies the active site and
blocks entry of the substrate


Fig. 5-16

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