Download 1 All cells can harvest energy from organic molecules. To do this

Objective # 28
Module 2E – How Cells Harvest Energy
Identify the following terms and
explain how each is involved in the
transfer of energy within a cell:
a) oxidationoxidation-reduction reactions
b) electron carriers
c) electron transport chain
d) ATP
„ All
cells can harvest energy from organic
molecules. To do this, they catabolize
(break down) organic molecules and use
the energy that is released to make ATP
from ADP and phosphate.
„ In this module, we will look at several
ways cells can catabolize glucose and use
the energy that is released to make ATP.
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Objective 28a
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Objective 28a
„ Redox reactions play a key role in the flow of
energy within cells because electrons have
energy. Therefore, when electrons are
transferred, energy is transferred along with
them.
„ The amount of energy an electron has
depends on the energy level it occupies.
Light and other forms of energy can boost
electrons to a higher energy level. High
energy electrons can be used to make ATP.
„ Reactions
that involve a transfer of
electrons from one atom or molecule to
another are called oxidationoxidation-reduction
(redox) reactions.
reactions.
„ The substance that loses electrons is
oxidized and the substance that gains
electrons is reduced
reduced..
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Objective 28a
„ During
Objective 28a
H+
redox reactions,
ions are
often transferred along with electrons.
„ Since a hydrogen atom consists of one
H+ ion (proton) and one electron, a
substance that loses hydrogen atoms is
oxidized (loses electrons) and a
substance that gains hydrogen atoms is
reduced.
„ Cells
use reduced organic molecules
(molecules rich in hydrogen) as fuel.
„ Cells extract energy from these
molecules by oxidizingg them i.e.
removing hydrogen atoms.
„ Energy stored in the electrons of
removed hydrogen atoms can be used
to make molecules of ATP.
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Objective 28b
Objective 28b
„ Electron
carriers are molecules that
help transfer electrons from one
substance to another within the cell.
They pick up electrons from
substances being oxidized and donate
them to substances being reduced.
„ As mentioned previously, H+ ions are
often transferred along with the
electrons.
„ For
example, during the breakdown of
glucose :
„ enzymes remove 2 H atoms (2 H+ and
2e-) from gglucose
„ both electrons and one H+ are picked
up by NAD+ to form NADH
„ the other H+ is released as a free ion
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Objective 28b
Some Important Electron Carriers
Oxidized Form Add →
Reduced Form
← Remove
NAD+
2e- and 2H+ NADH + H+
FAD
2e- and 2H+ FADH2
NADP+
2e- and 2H+ NADPH + H+
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Objective 28c
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Objective 28c
Electron Transport Chain
„ An
electron transport chain (ETC)
is a series of electron carriers that
are embedded in a membrane and
that p
pass electrons from one carrier
to the next in a specific sequence:
Mitochondrial matrix
Cytochrome
oxidase complex
NADH dehydrogenase
NADH + H+ NAD+
FADH2
bc1 complex
2H+ + 1/2O2
H2O
2 e– FAD
2
e–
2 e–
Q
C
Inner
mitochondrial H+
membrane
H+
H+
Intermembrane space
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Objective 28c
An Electron Transport Chain
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2e–
Energy released from
electrons as they move
along the ETC is used
for ATP synthesis
Electrons from food
As electrons are passed from one carrier to
the next in the ETC, some of their energy
is released. This energy can be used to
make ATP
ATP.
„ In the case of aerobic respiration, the final
electron acceptor is oxygen, which joins
with 2H+ to form water.
„
High energy
The final
electron
acceptor is
oxygen,
which joins
with 2H+ to
form water
Low energy
2H+
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H2 O
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Objective 28d
1/ O
2 2
Structure of ATP
„ ATP:
Adenine
¾ plays
a key role in short
short--term storage
and release of energy within the cell
¾ is sometimes called the “energy
currency” of the cell because it is the
immediate source of energy for most
cell activities
¾ is made by adding a phosphate group
(phosphorylating) ADP
NH2
Triphosphate group
O
O P
O-
O
O
P
O-
N
O
O
P
N
O CH2
N
O-
Electrostatic
repulsion between
phosphates
N
O
OH
OH
Sugar
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Objective # 29
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Objective 29a
Describe the following methods
that are used by cells to make
ATP:
a) substrate
substrate--level phosphorylation
b) electron transport phosphorylation
(also called chemiosmotic
phosphorylation or chemiosmosis)
„ During
substratesubstrate-level
phosphorylation, a high
high--energy
phosphate group is
enzymatically transferred from
an organic molecule (such as the
sugar PEP) directly to ADP to
form ATP:
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Objective 29a
Objective 29b
Enzymatic transfer of a phosphate group
from PEP to ADP, forming pyruvate and
ATP:
Chemiosmotic phosphorylation
occurs in 2 stages:
1) An ETC uses energy from
electrons to pump H+ across a
membrane against their
concentration gradient. This builds
up a store of potential energy.
PEP
P
Enzyme
Enzyme
P
ADP
P
ATP
Adenosine
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Objective 29b
2) As the H+ ions move back
through the membrane down
their concentration gradient,
through
g special
p
p
protein
complexes, the stored energy is
released and used to make ATP
from ADP and phosphate.
Objective # 30
Objective 30
„Chemiosmotic
Explain why chemiosmotic
phosphorylation is sometimes
called oxidative phosphorylation.
phosphorylation
phosphorylation is called
oxidative phosphorylation if
the electrons are accepted by
oxygen when they reach the
end of the ETC.
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Objective # 31
Objective 31
„ Aerobic
respiration, anaerobic
respiration, and fermentation refer to 3
different ways of breaking down
glucose ((glucose
glucose catabolism).
catabolism).
„ The chemical bond energy of glucose
(and all other molecules) is stored in
the electrons that make up the
covalent bonds.
Distinguish between aerobic
respiration, anaerobic
respiration,
i i and
d ffermentation.
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Objective 31
glucose catabolism, high
energy elecrons are stripped from
glucose and picked up by the electron
carriers NAD+ and FAD.
„ In most cases, the electrons are passed
to an ETC where their energy is used
to make ATP by chemiosmosis.
„ Eventually, the electrons, along with
H+, are passed to a final acceptor.
Objective 31
„ During
„ If
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Objective 31
molecular oxygen (O2) is the final
electron acceptor, the process is called
aerobic respiration
respiration..
„ If some other inorganic
g
molecule is
the final electron acceptor, the process
is called anaerobic respiration
respiration..
„ If an organic molecule is the final
electron acceptor, the process is called
fermentation..
fermentation
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Objective # 32
„ First
we will examine the
process of aerobic respiration,
then we will take a brief look at
anaerobic respiration
respiration, and finally
we will discuss the process of
fermentation.
Write a summary chemical
equation for aerobic respiration
andd describe
d
ib the
h origin
i i and
d ffate
of each substance involved.
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Objective 32
„ Aerobic
Objective # 33
respiration:
Identify the 4 main stages of aerobic
respiration . For each stage discuss:
a)
b)
c)
d)
the cellular location
the main events that take place
starting substances and end products
the major enzymes involved and the type of
reaction each catalyzes
e) the number of ATPs, NADHs, and FADH2s,
produced (or used up) per glucose molecule
C6H12O6 + 6O2 →
6CO2 + 6H2O + eenergy
e gy
„ some
of the released energy is captured
and stored in molecules of ATP.
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Objective 33
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1. Glycolysis
NADH
The 4 main stages of aerobic
respiration are:
1) Glycolysis
2) Pyruvate
P
t Oxidation
O id ti
3) The Krebs Cycle (Citric Acid Cycle)
4) Oxidative Phosphorylation (ETC)
Stages of Aerobic
Respiration
Glucose
Pyruvate
2. Pyruvate
Oxidation
NADH
CO2
Acetyl-CoA
NADH
e–
CO2
3. Krebs
Cycle
H+
32 ATP
FADH2
2 ATP
e–
2H+
H2O 1 +
/2O2
e–
4. Oxidative Phosphorylation
Q
C
H+
H+
H+
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1st Stage - Glycolysis:
„ Glucose
(6C) is broken down into 2
molecules of pyruvate (3C each).
„ 2 ATP are used for activation energy
and 4 ATP are produced by substrate
level phosphorylation resulting in a net
gain of 2 ATP.
„ The oxidation of glucose releases 4
high energy electrons which are used
to reduce 2 NAD+ to form 2 NADH.
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2nd Stage - Pyruvate Oxidation:
3rd Stage - Krebs Cycle:
„ Each
acetyl group (2C) from pyruvate
oxidation is broken down into 2
molecules of carbon dioxide (1C)
„ The breakdown of each acetyl group
releases enough energy to form 1 ATP
by substrate level phosphorylation
„ The oxidation of each acetyl group
releases enough high energy electrons
to form 3 NADH and 1 FADH2
„ Each
pyruvate (3C) from glycolysis is
broken down into 1 CO2 (1C) and 1
acetyl group (2C)
„ The acetyl g
group
p jjoins with coenzyme A
to form acetyl
acetyl--CoA
„ The oxidation of each pyruvate releases
2 high energy electrons which are used
to reduce 1 NAD+ to form 1 NADH.
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4th Stage - Oxidative Phosphorylation:
NADH and FADH2 (produced during
glycolysis, pyruvate oxidation, and the Krebs
cycle) donate high energy electrons to an
electron transport chain
„ As the electrons are passed along the ETC,
their energy is used to make ATP by
chemiosmosis
„ At the end of the ETC, electrons join with
oxygen and 2H+ to form water
„
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Objective 33
„ The
theoretical yield of ATP from the
breakdown of one molecule of glucose
by aerobic respiration is 38.
„ In eukaryotes, this is reduced to 36
ATP because NADH generated by
glycolysis in the cytoplasm has to be
actively transported into the
mitochondria. This costs the cell 1
ATP per NADH transported.
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Objective 33
Theoretical ATP Yield from Aerobic Respiration
Glucose
2 ATP
Glycolysis
Pyruvate
2
ATP
2 NADH
6
ATP
2 NADH
6
ATP
2
ATP
Chemiosmosis
Pyruvate oxidation
Krebs
Cycle
6 NADH
18 ATP
2 FADH2
4
Chemiosmosis
ATP
Total net ATP yield = 38 (36 in eukaryotes)
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Objective 33
„
The actual yield for eukaryotes is reduced
to approximately 30 ATP per glucose
molecule for 2 reasons:
¾ The inner mitochondrial membrane is
y allowingg some protons
p
to pass
p
“leaky”
through without passing through ATP
synthase.
¾ Energy stored in the proton gradient is
used for other purposes besides
generation of ATP
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Objective # 34
Objective 34
„ During
aerobic respiration, cells use
O2 as the final electron acceptor at the
end of the electron transport chain.
„ Many bacteria can respire
p without O2,
using CO2, SO4, nitrates, or other
inorganic compounds as the final
electron acceptor in place of O2. This
process is called anaerobic respiration.
Describe the process of
p
anaerobic respiration.
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Objective # 35
Objective 35
„ In
some cases, the high energy
electrons picked up by NAD+ during
glycolysis are not donated to an ETC.
„ Instead, NADH donates its extra
electrons and H+ directly to an organic
molecule, which serves as the final
electron acceptor. This process is
called fermentation.
fermentation.
Describe the process of
fermentation and distinguish
b
between
alcoholic
l h li ffermentation
i
and lactic acid fermentation.
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Objective 35
Objective 35
„ Alcoholic
fermentation:
¾ occurs
in singlesingle-celled fungi called yeast
¾ a terminal CO2 is removed from the
pyruvic acid (3C) produced during
glycolysis, producing acetaldehyde (2C)
¾ acetaldehyde accepts 2 ee- and a H+
from NADH, producing ethanol and
NAD+
Although bacteria can carry out
more than a dozen kinds of
f
fermentation,
i eukaryotes
k
are
capable of only a few:
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Objective 35
Alcohol Fermentation in Yeast
H
Glucose
H
2 ADP
G
L
Y
C
O
L
Y
S
I
S
2 ATP
O–
2 NAD+
„ Lactic
Acid Fermentation:
¾ used by most animal cells when O2 is
not available
¾ NADH
N DH do
donates
ates 2 ee- aand
d a H+ ddirectly
ect y
to the pyruvate (3C) produced during
glycolysis, producing lactate (3C) and
NAD+
OH
CH3
2 Ethanol
2 NADH
H
C O
C O
CO2
C O
CH3
C
2 Pyruvate
CH3
2 Acetaldehyde
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Objective 35
Lactic Acid Fermentation in Muscle Cells
2 ADP
2 ATP
Glucose
O–
G
L
Y
C
O
L
Y
S
I
S
C
O
C
OH
H
2 NAD+
Alcoholic fermentation and lactic acid
fermentation each generate 2 ATP per
glucose molecule (generated by
substrate level phosphorylation during
glycolysis)) compared to the theoretical
glycolysis
maximum of 36 ATP per glucose
during aerobic respiration in eukaryotes.
CH3
2 Lactate
O–
2 NADH
C
O
C
O
CH3
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2 Pyruvate
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