How Cells Release
Chemical Energy
Chapter 7
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Objectives Today
• Know the meaning of the term "metabolism."
• Understand how cellular energy is produced
from a glucose molecule during cellular
respiration.
• Discuss how the availability of oxygen
determines the pathway by which ATP is
produced.
Metabolism
• is the complete set of chemical
reactions that occur in living cells.
• These processes are the basis of life,
allowing cells to grow and reproduce,
maintain their structures, and respond
to their environments.
Producing the Universal
Currency of Life
All energy-releasing pathways
– require characteristic starting materials - substrates
– yield predictable products and by-products
– produce ATP
– ATP is the universal currency molecule of all life
ATP Is Universal
Energy Source
• Photosynthesizers get energy from the sun
• Animals get energy second- or third-hand
from plants or other organisms
• Regardless, the energy is converted to the
chemical bond energy of ATP
Making ATP
• Plants make ATP during photosynthesis
• Cells of all organisms (including plants)
make ATP by breaking down
carbohydrates, fats, and protein
Main Types of
Energy-Releasing Pathways
Anaerobic pathways
Aerobic pathways
• Evolved first
• Don’t require oxygen
• Start with glycolysis in
cytoplasm
• Completed in cytoplasm
• Evolved later
• Require oxygen
• Start with glycolysis
in cytoplasm
• Completed in
mitochondria
Energy-Releasing Pathways
Overview of Aerobic
Respiration
enzymes
C6H1206 + 6O2
6CO2 + 6H20
glucose
carbon
oxygen
dioxide
water
Main Pathways Start
with Glycolysis
• Glycolysis occurs in cytoplasm
• Reactions are catalyzed by enzymes
• ALL LIFEFORMS ON PLANET EARTH
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 (3rd stage)
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
Glucose
• A simple sugar
(C6H12O6)
• Atoms held
together by
covalent bonds
Glycolysis Occurs
in Two Stages
• Energy-requiring steps (supply energy)
– ATP energy activates glucose and its six-
carbon derivatives
• Energy-releasing steps (gain energy)
– The products of the first part are split into
three-carbon pyruvate molecules
– ATP and NADH form
Net Energy Yield
from Glycolysis
Energy requiring steps:
-2 ATP invested (supply energy)
Energy releasing steps:
2 NADH formed
4 ATP formed (gain energy)
Net yield is 2 ATP and 2 NADH
Second-Stage Reactions
(Stage 2)
• Occur in the
mitochondria
• Pyruvate is broken
down to carbon
dioxide and water
• More ATP is formed
• More coenzymes
are reduced - carry
high energy
inner
mitochondrial
membrane
outer
mitochondrial
membrane
inner
outer
compartment compartment
Two Parts of Second Stage
• Preparatory reactions (Stage 2a)
– Pyruvate is oxidized into two-carbon acetyl
units and carbon dioxide
– NAD+ is reduced
• Krebs cycle (Stage 2b)
– The acetyl units are oxidized to carbon
dioxide
– NAD+ and FAD are reduced
Results of the Second Stage
• All of the carbon molecules in pyruvate
end up in carbon dioxide - released
from cells
• Coenzymes are reduced (they pick up
electrons and hydrogen) - energized
• One molecule of ATP is formed - just
one!
Coenzyme Reductions during
First Two Stages - high energy
molecules
• Glycolysis (stage 1)
2 NADH
• Preparatory
reactions (stage 2a)
2 NADH
• Krebs cycle
2 FADH2 + 6 NADH
(stage 2b)
• Total
2 FADH2 + 10 NADH
Electron Transfer
Phosphorylation (chain)
• Occurs in the mitochondria
• Coenzymes deliver electrons to electron
transfer chains
• Electron transfer sets up H+ ion
gradients
• Flow of H+ down gradients powers ATP
formation (from between membranes to
inner matrix of mitochondria)
Second Stage of
Aerobic Respiration
Acetyl-CoA
Formation
pyruvate
coenzyme A
(CO2)
NAD+
NADH
CoA
acetyl-CoA
Krebs Cycle
CoA
oxaloacetate
citrate
NAD+
NADH
NADH
NAD+
FADH2
NAD+
FAD
NADH
ATP
ADP +
phosphate
group
Electron Transfer
Phosphorylation
glucose
GLYCOLYSIS
pyruvate
• Electron transfer chains
are embedded in inner
mitochondrial
compartment
• NADH and FADH2 give up electrons that they picked up
in earlier stages to electron transfer chain
KREBS
CYCLE
ELECTRON TRANSFER
PHOSPHORYLATION
• Electrons are transferred through the chain
• The final electron acceptor is oxygen
Creating an H+ Gradient
OUTER COMPARTMENT
NADH
INNER COMPARTMENT
ATP Formation
-as the H+ rush back into the
mitochondrial matrix they spin a protein
that makes lots of ATP
ATP
INNER
COMPARTMENT
ADP
+
Pi
Summary of Transfers
glucose
ATP
2 PGAL
ATP
2 NADH
2 pyruvate
glycolysis
2 CO2
2 FADH2
e–
2 acetyl-CoA
2 NADH
H+
H+
2
ATP
6 NADH
Krebs
Cycle
KREBS
CYCLE
ATP
2 FADH2
4 CO2
H+
H+
ATP
36 ATP
electron
transfer
phosphorylation
H+
H+
ADP
+ Pi
H+
H+
H+
Importance of Oxygen
• Electron transfer chains require the
presence of oxygen
• Oxygen combines with spent electrons
and H+ to form water - which is used by
the cell - it is not released from your
lungs
Summary of Energy Harvest
(per molecule of glucose)
• Glycolysis step
– 2 ATP formed by substrate-level
phosphorylation
• Krebs cycle and preparatory reactions
– 2 ATP formed by substrate-level
phosphorylation
• Electron transfer phosphorylation
– 32 ATP formed
Efficiency of
Aerobic Respiration
• 686 kcal is the about of energy in a glucose
molecule
• Of the above, 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 (over 60%) is lost as heat
What if a cell does not use
oxygen?
How does it get energy?
Anaerobic Pathways
• Do not use oxygen
• Produce less ATP than aerobic pathways
• Two types of fermentation pathways
– Alcoholic fermentation - great for students
– Lactate fermentation - hurts me
Fermentation Pathways
• Begin with glycolysis
• Do not break glucose down completely to
carbon dioxide and water
• Yield only the 2 ATP from glycolysis (stage 1)
• Steps that follow glycolysis serve only to
regenerate NAD+ - no energy is produced
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
• No mitochondria, so where does this take place?
• 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 (a hormone)
• Insulin stimulates glucose uptake by cells
• Cells convert glucose to glucose-6-phosphate
• This traps glucose in cytoplasm where it can
be used for glycolysis
Making Glycogen - (this is not glucose)
• If glucose intake is high, ATP-making
machinery goes into high gear
• When ATP levels rise high enough, glucose6-phosphate is diverted into glycogen
synthesis (mainly in liver and muscle)
• Glycogen is the main storage polysaccharide
in animals
Using Glycogen
• When blood levels of glucose decline,
pancreas releases glucagon (a hormone)
• 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) - selfish ba**ards!!!!!
Energy Reserves
• Glycogen makes up only about 1 percent of the
body’s energy reserves - sugars
• Proteins make up 21 percent of energy reserves
- proteins
• Fat makes up the bulk of reserves (78 percent) fats
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
1. Life started without OXYGEN
2. Earliest organisms used anaerobic
pathways
3. Then came OXYGEN from some Bacteria
4. The came organisms with aerobic pathways
Cycle of life
Aerobic Respiration
Photosynthesis
• Reactants
• Reactants
– Sugar
– Carbon dioxide
– Oxygen
– Water
• Products
• Products
– Carbon dioxide
– Sugar
– Water
– Oxygen