Download Chapter 7 How Cells Release Chemical energy

Chapter 7
How Cells Release
Chemical energy
Overview of
Carbohydrate Breakdown Pathway
Plants and all other photoautotrophs get
energy from the sun, heterotrophs get by
eating plants and one another
ATP is a common energy currency that
drives metabolic reactions in cells
Pathways of Carbohydrate
Breakdown
Starts with glycolysis in the cytoplasm
– Convert glucose and other sugars to pyruvate
Comparison of main pathways
Fermentation
pathway
Aerobic
Respiration
Produces ATP under
anaerobic conditions
Ends in cytoplasm, do
not use oxygen, yields 2
ATP per molecule of
glucose
Produces ATP by
using oxygen
Ends in mitochondria,
uses oxygen, yields up
to 36 ATP per glucose
molecule
Oxygen acts as the
final acceptor of
electrons used during
these reactions
Overview of Aerobic Respiration
Three main stages of aerobic respiration:
1. Glycolysis
2. Krebs cycle
3. Electron transfer phosphorylation
Summary equation:
C6H12O6 + 6O2 → 6CO2 + 6 H2O
glucose
Cytoplasm
glucose
2 ATP ATP
GLYCOLYSIS
4 ATP ATP
(2net)
2 NADH 2 pyruvate
The first stage, glycolysis, occurs
in the cell’s cytoplasm. Enzymes
convert a glucose molecule to 2
pyruvate for a net yield of 2 ATP.
During the reactions, 2 NAD+ pick
up electrons and hydrogen atoms,
so 2 NADH form.
Mitochondrion
Krebs
Cycle
6CO2
2 ATP ATP
8 NADH, 2 FADH2
ATP
oxygen
Electron Transfer
Phosphorylation
32 ATP
The second stage, the Krebs cycle
and a few steps before it, occurs
inside mitochondria. The 2 pyruvates
are broken down to CO2, which leaves
the cell. During the reactions, 8 NAD+
and 2 FAD pick up electrons and
hydrogen atoms, so 8 NADH and 2
FADH2 form. 2 ATP also form.
The third and final stage, electron
transfer phosphorylation, occurs
inside mitochondria. 10 NADH and 2
FADH2 donate electrons and hydrogen
ions at electron transfer chains.
Electron flow through the chains sets
up H+ gradients that drive ATP
formation. Oxygen
Overview of aerobic respiration
Glycolysis –
Glucose Breakdown Starts
First step – Conversion of glucose to
pyruvate
Enzymes of glycolysis use two ATP to
convert one molecule of glucose to two
molecules of three-carbon pyruvate
Reactions transfer electrons and hydrogen
atoms to two NAD+ (reduces to NADH)
2 ATP is formed by substrate-level
phosphorylation
Products of Glycolysis
Net yield of glycolysis:
– 2 pyruvate, 2 ATP, and 2 NADH per glucose
Pyruvate may:
– Enter fermentation pathways in cytoplasm
– Enter mitochondria and be broken down
further in aerobic respiration
ATP Requiring Steps
Glycolysis
glucose
ATP
ADP
P
glucose–6–phosphate
ATP
ADP
p
P
fructose–1,6–bisphosphate
ATP Generating
steps
2 PGAL
2
NAD+ +
2 Pi
NADH
2 reduced coenzymes
2 PGA
2 ADP
ATP
2 ATP produced
by substrate-level
phosphorylation
2 PEP
2 ADP
ATP
2 pyruvate
to second stage
2 ATP produced
by substrate-level
phosphorylation
Net 2 ATP + 2 NADH
Second Stage of Aerobic
Respiration
Krebs Cycle
Break down of pyruvate to Carbon dioxide
The second stage of aerobic respiration
takes place in the inner compartment of
mitochondria
It starts with acetyl-CoA formation and
proceeds through the Krebs cycle
outer membrane
(next to cytoplasm)
glucose
(glycolysis)
2 pyruvate
OUTER COMPARTMENT
inner membrane
inner
mitochondrial
compartment
outer mitochondrial
compartment (in
between the two
membranes)
a An inner membrane divides a
mitochondrion’s interior into two
compartments. The second and third
stages of aerobic respiration take
place at this membrane.
2 acetyl–CoA
Krebs
Cycle
CO2
ATP
NADH
FADH2
INNER COMPARTMENT
Breakdown of 2 pyruvate to
6CO2 yields 2 ATP. Also, 10
coenzymes are reduced (8
NADH, 2 FADH2). The
coenzymes carry hydrogen
ions and electrons to sites
of the third stage of aerobic
respiration.
b The second stage starts after membrane proteins
transport pyruvate from the cytoplasm, across
both mitochondrial membranes, to the inner
compartment. Six carbon atoms enter these
reactions (in two pyruvate), and six leave (in
six CO2). Many coenzymes form.
Acetyl-CoA Formation
Two pyruvates from glycolysis are
converted to two acetyl-CoA
Two CO2 leave the cell
Acetyl-CoA enters the Krebs cycle
Krebs Cycle
In each turn of the Krebs cycle, one acetylCoA is converted to two molecules of CO2
After two cycles
– Two pyruvates are dismantled
– Glucose molecule that entered glycolysis is
fully broken down
Energy Products
Reactions transfer electrons and hydrogen
atoms to NAD+ and FAD
– Reduced to NADH and FADH2
ATP forms by substrate-level
phosphorylation
– Direct transfer of a phosphate group from a
reaction intermediate to ADP
Net Results
Second stage of aerobic respiration results in
Six CO2, two ATP, eight NADH, and two FADH2
for every two pyruvates
Adding the yield from glycolysis, the total is
– Twelve reduced coenzymes and four ATP for
each glucose molecule
Coenzymes deliver electrons and hydrogen to
the third stage of reaction
Acetyl–CoA
Formation
pyruvate
NAD+
coenzyme A
NADH
CO2
acetyl–CoA
coenzyme A
Krebs
Cycle
oxaloacetate
citrate
CO2
NAD+
Krebs
Cycle
NADH
NADH
NAD+
NAD+
FADH2
CO2
NADH
FAD
ADP + Pi
ATP
Third Stage: Aerobic Respiration’s
Big Energy Payoff
Coenzymes deliver electrons and hydrogen ions
to electron transfer chains in the inner
mitochondrial membrane
Energy released by electrons flowing through
the transfer chains moves H+ from the inner to
the outer compartment
H+ gradient builds up across the inner
membrane
H+ ions flow by concentration gradient back to
the inner compartment through ATP synthases
The Aerobic Part of Aerobic
Respiration
At the end of electron transfer chain
oxygen and H+ , forming water
Overall, aerobic respiration yields up to 36
ATP for each glucose molecule
glucose
Glycolysis
you
are
here
Krebs
Cycle
Electron Transfer
Phosphorylation
INNER
COMPARTMENT
H+
NADH
FADH2
H+
H+
H+
H2O
INNER
MITOCHONDRIAL
MEMBRANE
H+
OUTER
COMPARTMENT
H+
H+
H+
1/2 O2
ATP
ADP + Pi
H+
H+
H+
H+
H+
glucose
2 ATP
2 NAD+
Glycolysis
2 NADH
ATP (2 net)
2 pyruvate
CYTOPLASM
OUTER MITOCHONDRIAL
COMPARTMENT
2 NADH
2 CO2
2 NADH
6 NADH
2 FADH2
INNER MITOCHONDRIAL
COMPARTMENT
2 acetyl-CoA
4 CO2
Krebs
Cycle
2 ATP
ADP + Pi
Electron Transfer
Phosphorylation
H+
water
+
H+ H
H+
oxygen
32 ATP
H+
Anaerobic
Energy-Releasing Pathways
Fermentation Pathway
Begins with glycolysis and ends in the
cytoplasm
Do not use oxygen or electron transfer
chains
Two ATP is formed from glycolysis
Final steps do not produce ATP; only
regenerate oxidized NAD+ required for
glycolysis to continue
glucose
Glycolysis
you are
here
Fermentation
Pathway
Anaerobic Pathways
Alcoholic fermentation
End product: Ethyl alcohol (or ethanol)
Pyruvate(3 carbon) is broken down
Enzyme splits pyruvate into two - carbon
acetaldehyde and carbon dioxide
Alcoholic fermentation
Role of Yeast in fermentation
They are unicellular fungi
Sacchromyces cerevisiae (Baker’s yeast)
Cells release carbon dioxide in
fementation and dough expands
Other strains are used in the production of
wine
Alcoholic Fermentation
Anaerobic Pathways
Lactate fermentation
End product: Lactate
Conversion of pyruvate into three carbon
lactate (lactic acid)
Fermenters such as lactobacillus
acidophillus can ferment milk, butter milk,
cheese, yogurt
Glycolysis
glucose
2 NAD+
2
ATP
2 NADH
4
ATP
pyruvate
Lactate
Fermentation
2 NADH
2 NAD+
lactate
The Twitchers
Slow-twitch and fast-twitch skeletal muscle
fibers can support different activity levels
Aerobic respiration and lactate
fermentation proceed in different fibers of
muscles
These pathways yeilds ATP for muscles
Alternative Energy Sources
Complex Carbohydrate break down
They are broken down into simple sugars like
glucose
Glucose gets converted to glucose-6-phosphate.
If the body doesn’t need glucose for
energy,glucose-6-phosphate can be converted
to glycogen for storage
When blood sugar drops, glycogen is converted
to glucose-6-phosphate and and enters the
glycolysis pathway
Alternative Energy Sources;
Energy from Fats
Most fat in the body are triglycerides
Enzymes convert fat into glycerol and fatty
acid
Glycerol is converted into PGAL, an
intermediate of glycolysis
The carbon back bones of the fatty acid
tail is broken apart, and fragments are
converted into acetyl CoA, which can enter
the Krebs cycle
Energy from Proteins
Enzymes split proteins into amino acid
subunits
The amino group is removed and
becomes ammonia, then urea
Urea is excreted
Carbon Back bones can enter at several
different points of Krebs cycle
FOOD
fats
fatty acids
COMPLEX CARBOHYDRATES
glycerol
glucose, other simple sugars
PROTEINS
amino acids
acetyl-coA
acetyl-coA PGAL
Glycolysis
NADH pyruvate
oxaloacetate
or another
intermediate
of the Krebs
Krebs
Cycle
NADH, FADH2
Electron Transfer
Phosphorylation
Life’s Unity
Photosynthesis and aerobic respiration are
interconnected on a global scale
In its organization, diversity, and continuity
through generations, life shows unity at
the bioenergetic and molecular levels
sunlight
energy in
Photosynthesis
Driven by energy input from
the sun, electrons and hydrogen
are used to form ATP. ATP
energy drives the synthesis of
glucose from hydrogen, electrons
(delivered by coenzymes), and
carbon dioxide’s atoms.
energy out (heat)
glucose
(stored
chemical
energy)
carbon
dioxide,
water
oxygen
Aerobic Respiration
Energy input from two ATP
initiates three stages of
reactions. Many ATP form
during thecomplete
breakdown of
glucose to carbon dioxide
and water.
energy out (heat)
chemical energy in
many ATP available
to drive nearly all
cellular tasks