Download CELLULAR RESPIRATION

CELLULAR
RESPIRATION
HARVESTING CHEMICAL
ENERGY
Four Features of Enzymes
3) The same enzyme sometimes works
for both the forward and reverse
reactions, but not always
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
two
substrate
molecules
substrates
contacting
active site
of enzyme
Induced-Fit Model

active sight

TRANSITION
STATE
(tightest
binding but
least stable)

end
product
enzyme
unchanged
by the
reaction

Substrate molecules
are brought together
Substrates are
oriented in ways that
favor reaction
Active sites may
promote acid-base
reactions
Active sites may shut
out water
Factors Influencing
Enzyme Activity
Temperature
pH
Salt concentration
Allosteric regulators
Coenzymes and cofactors
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 shape
of active site
Effect of pH
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
Producing the Universal
Currency of Life
All energy-releasing pathways



require characteristic starting materials
yield predictable products and byproducts
produce ATP
ATP Is Universal
Energy Source

Photosynthesizers get energy from the
sun

Animals get energy second- or thirdhand from plants or other organisms

Regardless, the energy is converted to
the chemical bond energy of ATP
A review of how ATP drives cellular work
Making ATP

Plants make ATP during photosynthesis

Cells of all organisms make ATP by
breaking down carbohydrates, fats, and
protein
Redox Reactions


The loss of electrons is called oxidation.
The addition of electrons is called
reduction
Ae- + B  A + Be-
Overview of Aerobic
Respiration
C6H1206 + 6O2
glucose
oxygen
6CO2 + 6H20
carbon
dioxide
water
Electrons “fall” from organic molecules to
oxygen during cellular respiration





In cellular respiration, glucose and other fuel
molecules are oxidized, releasing energy.
In the summary equation of cellular respiration:
C6H12O6 + 6O2 -> 6CO2 + 6H2O
Glucose is oxidized, oxygen is reduced, and
electrons loose potential energy.
Cellular respiration does not oxidize glucose in a
single step that transfers all the hydrogen in
the fuel to oxygen at one time.
Rather, glucose and other fuels are broken
down gradually in a series of steps, each
catalyzed by a specific enzyme
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
An overview of cellular respiration (Layer 1)
An overview of cellular respiration (Layer 2)
An overview of cellular respiration (Layer 3)
Glycolysis Occurs
in Two Stages

Energy-requiring steps

ATP energy activates glucose and its six-
carbon derivatives

Energy-releasing steps

The products of the first part are split
into three-carbon pyruvate molecules

ATP and NADH form
Energy-Requiring Steps
glucose
ATP
ADP
P
glucose-6-phosphate
P
fructose-6-phosphate
ATP
ADP
P
fructose-1,6-bisphosphate
2 ATP invested
EnergyReleasing
Steps
PGAL
PGAL
NAD+
NADH
Pi
P
P
NADH
Pi
P
1,3-bisphosphoglycerate
ADP
NAD+
ATP
P
1,3-bisphosphoglycerate
ADP
ATP
substrate-level
phosphorylation
2 ATP invested
P
P
3-phosphoglycerate
3-phosphoglycerate
P
P
2-phosphoglycerate
H2
O
2-phosphoglycerate
H2
O
PEP
PEP
P
ADP
ATP
P
ADP
ATP
substrate-level
phosphorylation
2 ATP invested
pyruvate
pyruvate
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
Second-Stage
Reactions




Occur in the
mitochondria
Pyruvate is
broken down to
carbon dioxide
More ATP is
formed
More coenzymes
are reduced
PREPARATORY
STEPS
pyruvate
coenzyme A (CoA)
NAD+
(CO2)
NADH
CoA
Acetyl–CoA
KREBS CYCLE
CoA
oxaloacetate
citrate H O
2
NADH
H2O
NAD+
malate
NAD+
H2O
isocitrate
NADH
fumarate
FADH2
FAD
a-ketogluterate
CoA
NAD+
NADH
succinate
CoA
succinyl–CoA
ATP
ADP + phosphate
group (from GTP)
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
What is Acetyl-CoA?

A two-carbon acetyl group linked to
coenzyme A
CH3
Acetyl group
C=O
Coenzyme A
The Krebs Cycle
Overall Reactants




Acetyl-CoA
3 NAD+
FAD
ADP and Pi
Overall Products





Coenzyme A
2 CO2
3 NADH
FADH2
ATP
A summary of the Krebs cycle
Results of the Second Stage




All of the carbon molecules in pyruvate
end up in carbon dioxide
Coenzymes are reduced (they pick up
electrons and hydrogen)
One molecule of ATP is formed
Four-carbon oxaloacetate is
regenerated
Coenzyme Reductions During
First Two Stages

Glycolysis
Preparatory
reactions
Krebs cycle
2 NADH
2 FADH2 + 6 NADH

Total
2 FADH2 + 10 NADH


2 NADH
Electron Transport
Phosphorylation




Occurs in the mitochondria
Coenzymes deliver electrons to electron
transport systems
Electron transport sets up H+ ion
gradients
Flow of H+ down gradients powers ATP
formation
Electron Transport




Electron transport systems are embedded in
inner mitochondrial compartment
NADH and FADH2 give up electrons that they
picked up in earlier stages to electron
transport system
Electrons are transported through the
system
The final electron acceptor is oxygen
Creating an H+ Gradient
OUTER COMPARTMENT
NADH
INNER COMPARTMENT
Making ATP:
Chemiosmotic Model
ATP
INNER
COMPARTMENT
ADP
+
Pi
Importance of Oxygen


Electron transport phosphorylation
requires the presence of oxygen
Oxygen withdraws spent electrons from
the electron transport system, then
combines with H+ to form water
Summary of Energy Harvest
(per molecule of glucose)

Glycolysis


Krebs cycle and preparatory reactions


2 ATP formed by substrate-level
phosphorylation
2 ATP formed by substrate-level
phosphorylation
Electron transport phosphorylation

32 ATP formed
Energy Harvest from
Coenzyme Reductions

What are the sources of electrons
used to generate the 32 ATP in the
final stage?
4 ATP - generated using electrons
released during glycolysis and carried by
NADH
 28 ATP - generated using electrons
formed during second-stage reactions
and carried by NADH and FADH2

Energy Harvest Varies




NADH formed in cytoplasm cannot
enter mitochondrion
It delivers electrons to mitochondrial
membrane
Membrane proteins shuttle electrons to
NAD+ or FAD inside mitochondrion
Electrons given to FAD yield less ATP
than those given to NAD+
Efficiency of
Aerobic Respiration

686 kcal of energy are released

7.5 kcal are conserved in each ATP

When 36 ATP form, 270 kcal (36 X 7.5) are
captured in ATP

Efficiency is 270 / 686 X 100 = 39 percent

Most energy is lost as heat
Anaerobic Pathways

Do not use oxygen

Produce less ATP than aerobic
pathways

Two types

Fermentation pathways

Anaerobic electron transport
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+
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
Alcoholic
Fermentation
GLYCOLYSIS
C6H12O6
2
ATP
energy input
2 NAD+
2 ADP
2
4
NADH
ATP
2 pyruvate
energy output
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
Anaerobic Electron Transport




Carried out by certain bacteria
Electron transport system is in bacterial
plasma membrane
Final electron acceptor is compound from
environment (such as nitrate), NOT oxygen
ATP yield is almost as good as from aerobic
respiration
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
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
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 transport