Essential Concepts of
Metabolism
Chapter 5
Microbiology 130
Metabolism: An Overview
MetabolismAnabolism
Catabolism- electron transfer
OxydationReduction-
How do microbes obtain energy?
Autotrophs- self feeders, use CO2 to
sysnthesis
organic compounds
- Photoautotrophs- use sunlight for
energy
- Chemoautotrophs- use inorganics such as
sulfides and nitrites for energy
Heterotrophs- other feeders, use organic
molecules,
- Photoheterotrophs-obtain chemical energy
from
light
- Chemoheterotrophs- obtain energy from
What Is Energy?
Capacity to do work
Forms of energy
– Potential energy,
– Kinetic energy
– Chemical energy
– What Can Cells Do With Energy?
Cells use energy for:
– Chemical work
– Mechanical work
– Electrochemical work
One-Way Flow of Energy
The sun is life’s primary energy
source
Producers trap energy from the sun
and convert it into chemical bond
energy
All organisms use the energy stored
in the bonds of organic compounds
to do work
Endergonic Reactions
Energy input
required
glucose, a
high energy
product
Product has more
energy than
+
6O2
starting substances
ENERGY IN
6
low energy
starting
substances
6
6
6
Exergonic Reactions
Energy is
released
Products have
less energy
than starting
substance
glucose, a high
energy starting
substance
+ 6O2
ENERGY OUT
low energy products
6
6
The Role of ATP
Cells “earn” ATP in exergonic
reactions
Cells “spend” ATP in endergonic
base
reactions
three
phosphate
groups
sugar
ATP/ADP Cycle
When adenosine triphosphate (ATP)
gives up a phosphate group, adenosine
diphosphate (ADP) forms
ATP can re-form when ADP binds to
inorganic phosphate or to a phosphate
group that was split from a different
molecule
Regenerating ATP by this ATP/ADP cycle
helps drive most metabolic reactions
Participants in
Metabolic Reactions
Reactants
Energy carriers
Enzymes
Intermediates
Cofactors
Products
Transport
Chemical Equilibrium
At equilibrium, the energy in the
reactants equals that in the products
Product and reactant molecules
usually differ in energy content
Therefore, at equilibrium, the
amount of reactant almost never
equals the amount of product
Chemical Equilibrium
Redox Reactions
Cells release energy efficiently by
electron transfers, or oxidationreduction reactions (“redox”
reactions)
One molecule gives up electrons
(is oxidized) and another gains
them (is reduced)
Hydrogen atoms are commonly
released at the same time, thus
becoming H+
Electron Transfer Chains
Arrangement of enzymes,
coenzymes, at cell membrane
As one molecule is oxidized, next is
reduced
Function in aerobic respiration and
photosynthesis
Uncontrolled vs. Controlled
Energy Release
H2
1/2 O2
H2
2H+
Explosive
release of
energy as
heat that
cannot be
harnessed
for cellular
work
H2O
2e-
Energy input
splits hydrogen
Into protons (H+)
and electrons
1/2 O2
Some
released
energy is
harnessed
for cellular
work (e.g.,
making ATP)
Electrons
transferred
through electron
transfer chain
2e2H+
Spent electrons
and free oxygen
form water.
1/2 O2
H2O
Metabolic Pathways
Defined as enzymemediated sequences
of reactions in cells
– Biosynthetic (anabolic)
–
ex: photosynthesis
– Degradative
(catabolic) –
ex: aerobic
respiration
ENERGY IN
ENERGY IN
photosynthesis
organic
compounds,
oxygen
carbon
dioxide,
water
aerobic
respiration
ENERGY OUT
Enzyme Structure
and Function
Enzymes are catalytic molecules
They speed the rate at which
reactions approach equilibrium
Four Features of Enzymes
1) Enzymes do not make anything
happen that could not happen on its
own. They just make it happen
much faster.
2) Reactions do not alter or use up
enzyme molecules.
Four Features of Enzymes
3) The same enzyme usually works
for both the forward and reverse
reactions.
4) Each type of enzyme recognizes
and binds to only certain
substrates.
Activation Energy
For a reaction
to occur, an
energy barrier
must be
surmounted
Enzymes make
the energy
barrier smaller
activation energy
without enzyme
starting
substance
activation energy
with enzyme
energy
released
by the
reaction
products
How Catalase Works
Induced-Fit Model
Substrate molecules are brought
together
Substrates are oriented in ways
that favor reaction
Active sites may promote acidbase reactions
Active sites may shut out water
Factors Influencing
Enzyme Activity
Temperature
pH
Salt concentration
Allosteric regulators
Coenzymes and cofactors
Enzyme Helpers
Cofactors
– Coenzymes
NAD+, NADP+, FAD
Accept electrons and hydrogen ions;
transfer them within cell
Derived from vitamins
– Metal ions
Ferrous iron in cytochromes
Allosteric Activation
allosteric
activator
vacant
allosteric
binding
site
active site
altered,
can bind
substrate
enzyme active site
active site cannot
bind substrate
Allosteric Inhibition
allosteric inhibitor
allosteric
binding
site vacant;
active site
can bind
substrate
active site altered,
can’t bind substrate
Feedback Inhibition
enzyme 2
enzyme
1
SUBSTRATE
enzyme 3
enzyme 4
enzyme 5
A cellular change, caused by a
specific activity, shuts down the
activity that brought it about
END
PRODUCT
(tryptophan)
Effect of Temperature
Small increase in
temperature
increases
molecular
collisions,
reaction rates
High
temperatures
disrupt bonds
and destroy the
Effect of pH
Producing the Universal Currency
of Life
All energy-releasing pathways
– require characteristic starting materials
– yield predictable products and byproducts
– produce ATP
Main Types of
Energy-Releasing Pathways
Anaerobic pathways
Evolved first
Don’t require oxygen
Start with glycolysis in cytoplasm
Completed in cytoplasm
Aerobic pathways
Evolved later
Require oxygen
Start with glycolysis in cytoplasm
Completed in mitochondria
Energy-Releasing Pathways
Main Pathways Start
with Glycolysis
Glycolysis occurs in cytoplasm
Reactions are catalyzed by enzymes
Glucose
(six carbons)
2 Pyruvate
(three carbons)
The Role of Coenzymes
NAD+ and FAD accept electrons and
hydrogen from intermediates during
the first two stages
When reduced, they are NADH and
FADH2
In the third stage, these coenzymes
deliver the electrons and hydrogen to
the transfer chain
Anaerobic Pathways
Do not use oxygen
Produce less ATP than aerobic pathways
Two types of fermentation pathways
– Alcoholic fermentation
– Lactate fermentation
Fermentation Pathways
Begin with glycolysis
Do not break glucose down completely to
carbon dioxide and water
Yield only the 2 ATP from glycolysis
Steps that follow glycolysis serve only to
regenerate NAD+
Alcoholic Fermentation
glycolysis
C6H12O6
2
ATP
energy input
2 ADP
2 NAD+
2
4
NADH
ATP
energy output
2 pyruvate
2 ATP net
ethanol
formation
2 H2O
2 CO2
2 acetaldehyde
electrons, hydrogen
from NADH
2 ethanol
Yeasts
Single-celled fungi
Carry out alcoholic fermentation
Saccharomyces cerevisiae
– Baker’s yeast
– Carbon dioxide makes bread dough rise
Saccharomyces ellipsoideus
– Used to make beer and wine
Lactate Fermentation
Carried out by certain bacteria
Electron transfer chain is in bacterial
plasma membrane
Final electron acceptor is compound
from environment (such as nitrate),
not oxygen
ATP yield is low
Lactate Fermentation
glycolysis
C6H12O6
2
ATP
energy input
2 NAD+
2 ADP
2
4
NADH
ATP
energy output
2 pyruvate
2 ATP net
lactate
formation
electrons, hydrogen
from NADH
2 lactate
Carbohydrate Breakdown
and Storage
Glucose is absorbed into blood
Pancreas releases insulin
Insulin stimulates glucose uptake by cells
Cells convert glucose to glucose-6phosphate
This traps glucose in cytoplasm where it
can be used for glycolysis
Glycolysis Occurs
in Two Stages
Energy-requiring steps
– ATP energy activates glucose and its sixcarbon derivatives
Energy-releasing steps
– The products of the first part are split into
three-carbon pyruvate molecules
– ATP and NADH form
Energy-Requiring Steps
ENERGY-REQUIRING STEPS
OF GLYCOLYSIS
glucose
ATP
2 ATP invested
ADP
P
glucose–6–phosphate
P
fructose–6–phosphate
ATP
ADP
P
fructose–1,6–bisphosphate
P
DHAP
Energy-Releasing Steps
ENERGY-RELEASING STEPS
OF GLYCOLYSIS
PGAL
PGAL
NAD+
Pi
NADH
P
P
NADH
P
1,3-bisphosphoglycerate
ADP
NAD+
Pi
ATP
P
1,3-bisphosphoglycerate
ADP
ATP
substrate-level
phosphorylation
2 ATP invested
P
P
3-phosphoglycerate
3-phosphoglycerate
P
P
2-phosphoglycerate
H2O
2-phosphoglycerate
H2O
P
P
PEP
PEP
ADP
ATP
ADP
ATP
substrate-level
phosphorylation
2 ATP invested
pyruvate
pyruvate
to second set of reactions
Net Energy Yield
from Glycolysis
Energy requiring steps:
2 ATP invested
Energy releasing steps:
2 NADH formed
4 ATP formed
Net yield is 2 ATP and 2 NADH
Overview of Aerobic Respiration
Overview of Aerobic Respiration
C6H1206 + 6O2
glucose
oxygen

6CO2 + 6H20
carbon
dioxide
water
Overview of Aerobic Respiration
glucose
cytoplasm
2
ATP
ATP
GLYCOLYSIS
energy input to
start reactions
e- + H+
(2 ATP net)
2 pyruvate
2 NADH
mitochondrion
2 NADH
8 NADH
2 FADH2
e-
e- + H+
2 CO2
e- + H+
4 CO2
e- + H+
Krebs
Cycle
2
ELECTRON
TRANSPORT
PHOSPHORYLATION
H+
32
ATP
ATP
water
e- + oxygen
TYPICAL ENERGY YIELD: 36 ATP
Second-Stage Reactions
Occur in the mitochondria
Pyruvate is broken down to carbon dioxide
More ATP is formed
More coenzymes are reduced
Two Parts of Second Stage
Preparatory reactions
– Pyruvate is oxidized into two-carbon
acetyl units and carbon dioxide
– NAD+ is reduced
Krebs cycle
– The acetyl units are oxidized to carbon
dioxide
– NAD+ and FAD are reduced
Preparatory Reactions
pyruvate + coenzyme A + NAD+
acetyl-CoA + NADH + CO2
One of the carbons from pyruvate is released in
CO2
Two carbons are attached to coenzyme A and
continue on to the Krebs cycle
Using Glycogen
When blood levels of glucose decline,
pancreas releases glucagon
Glucagon stimulates liver cells to convert
glycogen back to glucose and to release it
to the blood
(Muscle cells do not release their stored
glycogen)
Energy Reserves
Glycogen makes up only about 1 percent
of the body’s energy reserves
Proteins make up 21 percent of energy
reserves
Fat makes up the bulk of reserves (78
percent)
Energy from Fats
Most stored fats are triglycerides
Triglycerides are broken down to glycerol and fatty acids
Glycerol is converted to PGAL, an intermediate of glycolysis
Fatty acids are broken down and converted to acetyl-CoA,
which enters Krebs cycle
Energy from Proteins
Proteins are broken down to amino acids
Amino acids are broken apart
Amino group is removed, ammonia forms,
is converted to urea and excreted
Carbon backbones can enter the Krebs
cycle or its preparatory reactions
Reaction Sites
FOOD
fats
fatty
acids
glycogen
glycerol
complex
carbohydrates
proteins
simple sugars
(e.g., glucose)
amino acids
NH3
glucose-6phosphate
urea
carbon
backbones
PGAL
2
glycolysis
ATP
4 ATP
(2 ATP net)
NADH
pyruvate
Acetyl-CoA
NADH
NADH,
FADH2
CO2
Krebs
Cycle
2 ATP
CO2
e–
ATP
ATP
ATP
H+
e– + oxygen
many ATP
fats
Evolution of Metabolic Pathways
When life originated, atmosphere had little
oxygen
Earliest organisms used anaerobic
pathways
Later, noncyclic pathway of photosynthesis
increased atmospheric oxygen
Cells arose that used oxygen as final
acceptor in electron transfer
Aerobic Respiration
Reactants
– Sugar
– Oxygen
Products
– Carbon dioxide
– Water
Photosynthesis
Reactants
– Carbon dioxide
– Water
Products
– Sugar
– Oxygen
Processes Are
Linked
Life Is System
of Prolonging Order
Powered by energy inputs from sun, life
continues onward through reproduction
Following instructions in DNA, energy
and materials can be organized,
generation after generation
With death, molecules are released and
may be cycled as raw material for next
generation