Transcript chapter5_Sections 1-4 2012 - (per 3) and wed 4/24 (per 2,6)

Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 5
Ground Rules of Metabolism
(Sections 5.1 - 5.4)
Albia Dugger • Miami Dade College
5.1 A Toast to
Alcohol Dehydrogenase
• Metabolic processes build and break down organic molecules
such as ethanol and other toxins
• Alcohol breakdown directly damages liver cells, and interferes
with normal processes of metabolism
• Currently the most serious drug problem on college
campuses is binge drinking
Alcohol Metabolism
• The enzyme
alcohol
dehydrogenase
helps the liver
break down toxic
alcohols (ethanol)
5.2 Energy and the World of Life
• There are many forms of energy:
• Kinetic energy, potential energy
• Light, heat, electricity, motion
• Energy cannot be created or destroyed (first law of
thermodynamics)
• Energy can be converted from one form to another and thus
transferred between objects or systems
Energy Disperses
• Energy tends to disperse spontaneously (second law of
thermodynamics)
• A bit disperses at each energy transfer, usually as heat
• Entropy is a measure of how dispersed the energy of a
system has become
Key Terms
• energy
• The capacity to do work
• kinetic energy
• The energy of motion
• entropy
• Measure of how much the energy of a system is dispersed
Key Terms
• first law of thermodynamics
• Energy cannot be created or destroyed
• second law of thermodynamics
• Energy tends to disperse spontaneously
Kinetic Energy
Entropy
• Entropy tends to
increase, but the total
amount of energy in any
system always stays the
same
Entropy
Entropy
heat
energy
Time
Stepped Art
Fig. 5.3, p. 76
Work
• Work occurs as a result of an energy transfer
• A plant converts light energy to chemical energy in
photosynthesis
• Most other cellular work occurs by transfer of chemical energy
from one molecule to another (such as transferring chemical
energy from ATP to other molecules)
Energy’s One-Way Flow
• Living things maintain their organization only as long as they
harvest energy from someplace else
• Energy flows in one direction through the biosphere, starting
mainly from the sun, then into and out of ecosystems
• Producers and then consumers use energy to assemble,
rearrange, and break down organic molecules that cycle
among organisms throughout ecosystems
Energy Conversion
• It takes 10,000 pounds
of feed to raise a 1,000pound steer
• About 15% of energy in
food builds body mass;
the rest is lost as heat
during energy
conversions
Energy Flow
• Energy flows from the
environment into living
organisms, and back to
the environment
• Materials cycle among
producers and
consumers
Energy Flow
sunlight
energy
Producers
plants and other
self-feeding organisms
nutrient cycling
Consumers
animals, most fungi,
many protists, bacteria
Fig. 5.5, p. 77
Potential Energy
• Energy’s spontaneous dispersal is resisted by chemical
bonds
• Energy in chemical bonds is a type of potential energy,
because it can be stored
• potential energy
• Stored energy
Key Concepts
• Energy Flow
• Organisms maintain their organization only by continually
harvesting energy from their environment
• ATP couples reactions that release usable energy with
reactions that require it
5.3 Energy in the Molecules of Life
• Every chemical bond holds energy – the amount of energy
depends on which elements are taking part in the bond
• Cells store and retrieve free energy by making and breaking
chemical bonds in metabolic reactions, in which reactants
are converted to products
Key Terms
• reaction
• Process of chemical change
• reactant
• Molecule that enters a reaction
• product
• A molecule that remains at the end of a reaction
Chemical Bookkeeping
• In equations that represent chemical reactions, reactants are
written to the left of an arrow that points to the products
• A number before a formula indicates the number of molecules
• The same number of atoms that enter a reaction remain at
the reaction’s end
Chemical Bookkeeping
Chemical Bookkeeping
2H2
(hydrogen)
O2
(oxygen)
2H2O
(water)
Reactants
Products
4 hydrogen atoms
+ 2 oxygen atoms
4 hydrogen atoms
+ 2 oxygen atoms
Stepped Art
Fig. 5.6, p. 78
Energy In, Energy Out
• In most reactions, free energy of reactants differs from free
energy of products
• Reactions in which reactants have less free energy than
products are endergonic – they will not proceed without a
net energy input
• Reactions in which reactants have greater free energy than
products are exergonic – they end with a net release of free
energy
Key Terms
• endergonic
• “Energy in”
• Reaction that converts molecules with lower energy to
molecules with higher energy
• Requires net input of free energy to proceed
• exergonic
• “Energy out”
• Reaction that converts molecules with higher energy to
molecules with lower energy
• Ends with a net release of free energy
Energy In, Energy Out
Energy In, Energy Out
Free energy
2H2
O2
2
energy out
1
energy in
2H2 O
Fig. 5.7, p. 78
Why Earth Does Not Go Up in Flames
• Earth is rich in oxygen—and in potential exergonic reactions;
why doesn’t it burst into flames?
• Luckily, energy is required to break chemical bonds of
reactants, even in an exergonic reaction
• activation energy
• Minimum amount of energy required to start a reaction
• Keeps exergonic reactions from starting spontaneously
Activation Energy
Activation Energy
Reactants:
2H2
O2
Free energy
Activation energy
Difference between
free energy of
reactants and products
Products: 2H2O
Fig. 5.8, p. 79
ATP—The Cell’s Energy Currency
• ATP is the main currency in a cell’s energy economy
• ATP (Adenosine triphosphate)
• Nucleotide with three phosphate groups linked by highenergy bonds
• An energy carrier that couples endergonic with exergonic
reactions in cells
ATP
ATP
adenine
three phosphate
groups
ribose
A Structure of ATP.
Fig. 5.9a, p. 79
Phosphorylation
• When a phosphate group is transferred from ATP to another
molecule, energy is transferred along with the phosphate
• Phosphate-group transfers (phosphorylations) to and from
ATP couple exergonic reactions with endergonic ones
• phosphorylation
• Addition of a phosphate group to a molecule
• Occurs by the transfer of a phosphate group from a donor
molecule such as ATP
ATP and ADP
ATP and ADP
adenine
AMP
ADP
ATP
ribose
B After ATP loses one phosphate group, the nucleotide is
ADP (adenosine diphosphate); after losing two phosphate
groups, it is AMP (adenosine monophosphate)
Fig. 5.9b, p. 79
ATP/ADP Cycle
• Cells constantly use up ATP to drive endergonic reactions, so
they constantly replenish it by the ATP/ADP cycle
• ATP/ADP cycle (which is endergonic , which is
exergonic?)
• Process by which cells regenerate ATP
• ADP forms when ATP loses a phosphate group, then ATP
forms again as ADP gains a phosphate group
ATP/ADP Cycle
ATP/ADP Cycle
energy in
energy out
ADP + phosphate
C ATP forms by endergonic reactions. ADP forms again
when ATP energy is transferred to another molecule
along with a phosphate group. Energy from such
transfers drives cellular work.
Fig. 5.9c, p. 79
5.4 How Enzymes Work
• Enzymes makes a reaction run much faster than it would on
its own, without being changed by the reaction
• catalysis
• The acceleration of a reaction rate by a molecule that is
unchanged by participating in the reaction
• Most enzymes are proteins, but some are RNAs
Substrates
• Each enzyme recognizes specific reactants, or substrates,
and alters them in a specific way
• substrate
• A molecule that is specifically acted upon by an enzyme
Active Sites
• Enzyme specificity occurs because an enzyme’s polypeptide
chains fold up into one or more active sites
• An active site is complementary in shape, size, polarity, and
charge to the enzyme’s substrate
• active site
• Pocket in an enzyme where substrates bind and a reaction
occurs
An Active Site
An Active Site
Fig. 5.10a, p. 80
An Active Site
active site
enzyme
A Like other enzymes,
hexokinase’s active sites bind
and alter specific substrates. A
model of the whole enzyme is
shown to the left.
Fig. 5.10a, p. 80
An Active Site
Fig. 5.10b, p. 80
An Active Site
reactant(s)
B A close-up shows glucose
and phosphate meeting inside
the enzyme’s active site. The
microenvironment of the site
favors a reaction between the
two substrate molecules.
Fig. 5.10b, p. 80
An Active Site
Fig. 5.10c, p. 80
An Active Site
product(s)
C Here, the glucose has
bonded with the phosphate.
The product of this reaction,
glucose-6-phosphate, is
shown leaving the active site.
Fig. 5.10c, p. 80
Lowering Activation Energy
• Enzymes lower activation energy in four ways:
• Bringing substrates closer together
• Orienting substrates in positions that favor reaction
• Inducing the fit between a substrate and the enzyme’s
active site (induced-fit model)
• Shutting out water molecules
• induced-fit model
• As the substrate begins to bind to the enzyme, the active
site actively changes shape to accommodate the incoming
substrate. This improves the fit between the two.
Lowering Activation Energy
Lowering Activation Energy
Free energy
Transition state
Activation energy
without enzyme
Activation energy
with enzyme
Reactants
Products
Time
Fig. 5.11, p. 80
Effects of Temperature, pH, and Salinity
• Each type of enzyme works best within a characteristic range
of temperature, pH, and salt concentration:
• Adding heat energy boosts free energy, increasing
reaction rate (within a given range)
• Most human enzymes have an optimal pH between 6 and
8 (e.g. pepsin functions only in stomach fluid, pH 2)
• Too much or too little salt disrupts hydrogen bonding that
holds an enzyme in its three-dimensional shape
Enzymes and Temperature
Enzymes and Temperature
Enzyme activity
normal
tyrosinase
temperaturesensitive
tyrosinase
20°C (68°F) 30°C (86°F) 40°C (104°F)
Temperature
Fig. 5.12, p. 81
Enzymes and pH
Enzymes and pH
glycogen
phosphorylase
Enzyme activity
trypsin
pepsin
1
2
3
4
5
6
7
8
9
10
11
pH
Fig. 5.13, p. 81
Help From Cofactors
• Most enzymes require cofactors, which are metal ions or
organic coenzymes in order to function
• cofactor
• A metal ion or a coenzyme that associates with an
enzyme and is necessary for its function
• coenzyme
• An organic molecule that is a cofactor
Coenzymes and Cofactors
• Coenzymes may be modified by taking part in a reaction
• Example: NAD+ becomes NADH by accepting electrons
and a hydrogen atom in a reaction
• Cofactors are metal ions
• Example: The iron atom at the center of each heme
• In the enzyme catalase, iron pulls on the substrate’s
electrons, which brings on the transition state
Antioxidants
• Cofactors in some antioxidants help them stop reactions with
oxygen that produce free radicals (harmful atoms or
molecules with unpaired electrons)
• Example: Catalase is an antioxidant
• antioxidant
• Substance that prevents molecules from reacting with
oxygen
Key Concepts
• How Enzymes Work
• Enzymes tremendously increase the rate of metabolic
reactions
• Cofactors assist enzymes, and environmental factors such
as temperature, salt, and pH can influence enzyme
function