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First…watch this video. Seriously.
http://www.khanacademy.org/video/introduction-
to-cellular-respiration?playlist=Biology
2 NSCC BIOL211
In this lecture
• Cellular respiration
• Redox reactions
• Glycolysis
– Pyruvate oxidation
• Krebs Cycle
• Electron Transport Chain
3 NSCC BIOL211
Why do we do respiration?
• Cellular respiration provides most of our ATP
• The components of our diet provides the
reactants for cellular respiration
– Glucose is what we’ll study today
– Lipids and protein breakdown will be briefly
covered, and studied more in depth in another
course
4 NSCC BIOL211
Biochemical pathways are:
Exergonic and endergonic reactions
Oxidation and reduction reactions
Enzymatic reactions
5 NSCC BIOL211
From food to ATP • Amylase in saliva starts to break down starches
to disaccharides
• Stomach acid breaks apart large structures such
as cells and intercellular structures
• Amylase in the small intestine completes the
breakdown of all carbohydrates to disaccharides
• Maltases, lactases, and sucrases break down
disaccharides into monosaccharides
• Glucose is brought to all the cells in the body
through the circulatory system
7 NSCC BIOL211
Energy production sites in the cell
• Glucose is brought inside the cell by
cotransport with sodium
• The mitochondria are where ATP is produced
8 NSCC BIOL211
Electron energy levels
• An electron loses potential energy when it
shifts to a more electronegative atom
NSCC BIOL211 9
C6H12O6 + 6O2 6CO2 + 6H2O + energy
Here, electrons are transferred from carbon and hydrogen to
oxygen
The electrons in this reaction lose a LOT of potential energy
Carbohydrates and fats are high-energy foods because they
have a lot of electrons associated with hydrogen
Figure 9.5
(a) Uncontrolled reaction (b) Cellular respiration
Explosive release of
heat and light energy
Controlled release of energy for
synthesis of ATP
Fre
e e
nerg
y, G
Fre
e e
nerg
y, G
H2 1/2 O2 2 H 1/2 O2
1/2 O2
H2O H2O
2 H+ 2 e
2 e
2 H+
ATP
ATP
ATP
(from food via NADH)
10 NSCC BIOL211
The chemical reactions in respiration
Oxidation and reduction OIL RIG
Oxidation is “losing” Reduction is “gaining”
What is being lost and gained? Electrons!
Electrons are usually lost to oxygen
Oxygen is
highly
electro-
negative
The chemistry definition:
Oxidation and reduction reactions often occur in a pair,
and together are called redox reactions
11 NSCC BIOL211
The chemical reactions in respiration
Oxidation and reduction
Losing a hydrogen atom Gaining a hydrogen atom
In biochemical reactions, hydrogen is what usually gets
swapped around
Hydrogen almost always bonds to an atom that is more
electronegative (C, O, N, P), and so loses its electron
The biology definition:
12 NSCC BIOL211
The chemical reactions in respiration
• Redox reactions
• Phosphorylation/dephosphorylation
– Carried out by kinases and phosphatases
– Phosphorylation increases chemical potential
energy and “primes” the molecule for work
14 NSCC BIOL211
New players in the enzyme game
• NAD+ and NADH
• FAD and FADH2
NAD+ is derived
from niacin
NAD+ shuttles electrons through
the various stages of cellular
respiration
NAD+ is a coenzyme
15 NSCC BIOL211
FAD is derived from riboflavin
NAD+ and FAD
• NAD+ accepts electrons and becomes NADH
• NAD+ is reduced into NADH
• NADH is a reducing agent, and is recycled back to
NAD+ through oxidation
Each NADH (the reduced form of NAD+) represents
stored energy that is tapped to synthesize ATP.
Each NADH produces 3 ATPs in the e- transport chain
NAD+ NADH
reduction
oxidation Oxidized
form
Reduced
form
16 NSCC BIOL211
What is cellular respiration?
Glucose Energy
C6H12O6 + 6O2 6CO2 + 6H2O + energy Cellular respiration
38 ATP Heat
1 molecule of glucose produces 28 ATPs
Cellular respiration is the step-by-step
release and harness of the chemical
potential energy in glucose 17 NSCC BIOL211
- 686kcal/mol
Redox Reactions in Cellular Respiration
During cellular respiration, the fuel (such as glucose) is
oxidized, and O2 is reduced
becomes oxidized
becomes reduced
18 NSCC BIOL211
Two types of cellular respiration:
• Aerobic respiration – takes place in the
presence of oxygen
• Anaerobic respiration – takes place in the
absence of oxygen
– Fermentation is a type of anaerobic respiration
where sugars are partially degraded
– Consumes compounds other than oxygen
The breakdown of organic
molecules is always exergonic
Cellular respiration includes both aerobic and anaerobic respiration but is
often used to refer to aerobic respiration
19 NSCC BIOL211
The stages of aerobic respiration
Glycolysis Breaks glucose in
half
Citric Acid Cycle*
Electron Transport
Chain
Consumes two ATP
Generates 4 ATP
Net 2 ATP
Generates 2 ATP Rearranges the half-
glucose molecule
Generates 34 ATP Electrons from
glucose are passed
around
*AKA the Krebs cycle and the TCA cycle 21 NSCC BIOL211
Figure 9.6-1
Electrons
carried
via NADH
Glycolysis
Glucose Pyruvate
CYTOSOL MITOCHONDRION
ATP
Substrate-level
phosphorylation
Glycolysis
23 NSCC BIOL211
• The players:
– Glucose
– Pyruvate
– ADP/ATP
– Enzymes
• The processes:
– Substrate-level phosphorylation
• The locations
– Cytoplasm
Glycolysis
24 NSCC BIOL211
The production of
ATP from ADP by
direct transfer of a
phosphate group
from a
phosphorylated
protein
Glycolysis
Broken down into two stages:
• The “investment” phase (uses 2 ATP)
• The “payoff” phase (produces 2 ATP)
Beginning structure: End structures:
- Breaking down of glucose
- Can be done with or without oxygen “glyco” = glucose
“lysis” = breaking
apart
“You have to spend money to make money” 25 NSCC BIOL211
• Begin:
– Glucose
– NAD+
– ADP
• End:
– 2 pyruvate
– 2 NADH
– 2 ATP
36 NSCC BIOL211
Glycolysis
Most of glycose’s original energy
is still present in pyruvate!
• In the presence of O2, pyruvate enters the
mitochondrion where the oxidation of glucose
is completed during TCA cycle
• Without O2, pyruvate undergoes fermentation
into either ethanol or lactic acid
Glycolysis
37 NSCC BIOL211
• Before pyruvate can be fed into TCA cycle, it
must become acetyl-CoA (acetyl-coenzyme A)
• It does this through pyruvate oxidation
– Produces one NADH from NAD+
– Three-carbon pyruvate is converted into two-
carbons + acetyl-CoA
40 NSCC BIOL211
Pyruvate Oxidation
Think of acetyl-
CoA as a
transporter for
the carbon
atoms from
pyruvate
• Acetyl-CoA couples with oxaloacetate, the
first molecule in TCA cycle
• Acetyl-CoA + oxaloacetate = citrate
NSCC BIOL211 42
Pyruvate Oxidation
• The players: – Acetyl-CoA
– Oxaloacetate
– Plus many more carbon skeleton intermediates
– Enzymes
• The processes: – Hydrolysis
– Redox reactions
• The locations: – Mitochondrial matrix
NSCC BIOL211 43
The Citric Acid Cycle
44 NSCC BIOL211
The Citric Acid Cycle
Begin:
Oxaloacetate
End:
Oxaloacetate
The citric acid cycle, also called the Krebs cycle,
completes the break down of pyruvate to CO2
1 ADP
3 NAD+
1 FAD
1 ATP
3 NADH
1 FADH2
The citric acid cycle has eight steps, each catalyzed by a specific enzyme
NSCC BIOL211 47
The Citric Acid Cycle
CoA is recycled
here to go back
to pyruvate
oxidation
Step 1 and 2: In detail
• The citric acid cycle is the entry point for other
catabolic pathways
• Acetyl-CoA can be derived from carbohydrates,
proteins, and fats
NSCC BIOL211 53
The Citric Acid Cycle
• Begin:
– Acetyl-CoA
– Oxaloacetate
– 3NAD+
– 2 FAD
– 1ADP
• End:
– 3 CO2s
– Oxaloacetate
– 3 NADH
– 2 FADH2
– 1 ATP
NSCC BIOL211 54
The Citric Acid Cycle
The whole point of
TCA cycle is to
produce NADH
and FADH2
This is for one pyruvate. Remember, one glucose molecule produces two pyruvates!
The electron transport chain
• The whole cellular process is about producing
ATP. Why do we care about NADH and
FADH2?
– These molecules then get oxidized in the electron
transport chain
– Every NADH will produce 3 ATP
– Every FADH2 will produce 2 ATP
NSCC BIOL211 56
NSCC BIOL211 57
End:
ATP
10 NAD+
2 FAD
Begin:
ADP
10 NADH
2 FADH2
The electron transport chain
The electron transport chain breaks the large free-energy drop from food to O2
into smaller steps that release energy in manageable amounts
Electrons are passed along at lower and lower energy levels to release their energy
• The players:
– FADH2, NADH, ADP
– ATP Synthase
– Cytochromes and membrane proteins
• The processes:
– Chemiosmosis
• The location:
– Intermembrane space of the mitochondria
NSCC BIOL211 58
The electron transport chain
• The electron transport chain is a series of
proteins that pass along electrons
– Electrons come from NADH and FADH2
– Proteins are embedded in the matrix membrane
– Each time an electron is passed, it releases energy
– That energy is used to drive protons across the
membrane into the intermembrane space
– This creates an electrochemical gradient
NSCC BIOL211 59
The electron transport chain
NADH NAD+ + H+ + 2e-
NSCC BIOL211 60
The electron transport chain
CoQ
CytC
CytB
O2
Energ
y in
creas
es
Release of energy
Release of energy
Release of energy
CoQ, CytC, and
CytB are all
membrane proteins
on the inner matrix
membrane
NSCC BIOL211 61
The electron transport chain
FADH2’s electrons are
lower energy than
NADH, and so enter
the electron transport
chain at a protein
further along in the
chain
The transport proteins alternate
reduced and oxidized states as they
accept and donate electrons
Oxygen accepts those now
very-low energy electrons.
Oxygen is the terminal
electron acceptor
• The release of energy is used to pump H+
across the matrix membrane into the
intermembrane space
NSCC BIOL211 62
The electron transport chain
• H+ are pumped against their gradient using the
energy released from passing electrons to lower and
lower energy states
• This creates an electrochemical gradient
NSCC BIOL211 63
The electron transport chain
We can then
couple the
potential energy
in the
electrochemical
gradient to
another
biochemical
reaction
NSCC BIOL211 65
The electron transport chain
What is that electrochemical gradient used for?
To create ATP!
How is that done?
Through a protein called ATP synthase
ATP synthase translates the potential energy in the electrochemical
gradient into the potential energy in the phosphate bonds of ATP
The flow of H+ with its electrochemical gradient is an exergonic reaction
ATP synthase couples an exergonic reaction with an endergonic reaction
NSCC BIOL211 66
The electron transport chain
• ATP synthase is a turbine that
connects the flow of protons
to ADP ATP
phosphorylation
• This is called chemiosmosis
Electrochemical energy Kinetic energy Chemical energy
NSCC BIOL211 67
The electron transport chain
As the turbine turns with
the “current” of protons
flowing past, it
phosphorylates ADP into
ATP
NSCC BIOL211 70
The electron transport chain
• The energy stored in a H+ gradient across a
membrane couples the redox reactions of the
electron transport chain to ATP synthesis
• The H+ gradient is referred to as a proton-motive
force, emphasizing its capacity to do work
NSCC BIOL211 71
Energy flows in this direction:
glucose NADH electron transport chain proton-motive force ATP
Fermentation
• Anaerobic respiration uses an electron
transport chain with a final electron acceptor
other than O2, for example sulfate
• Produces much less energy than aerobic
respiration
– Only source of ATP is substrate-level
phosphorylation
NSCC BIOL211 73
• Two common types of fermentation:
– Lactic acid fermentation • Lactic acid fermentation by some fungi and bacteria is used to
make cheese and yogurt
• Human muscle cells use lactic acid fermentation to generate ATP
when O2 is scarce
– Alcohol fermentation
• Alcohol fermentation by yeast is used in brewing,
winemaking, and baking
NSCC BIOL211 74
Fermentation
• Pyruvate is converted
to ethanol in two steps
– NADH produced in
glycolysis is oxidized
to NAD+
– Glucose is not
conpletely digested
NSCC BIOL211 75
Alcohol Fermentation
• Pyruvate is
converted to
lactate in one
step
– NADH produced
during glycolysis
is oxidized to
NAD+
NSCC BIOL211 76
Lactic Acid Fermentation
Comparing Aerobic and Anaerobic
Respiration
Aerobic Respiration Anaerobic Respiration
Glycolysis Yes Yes
Krebs Cycle Yes No
Electron Transport
Chain
Yes No
ATP Production 32 per glucose 2 per glucose
NADH production Yes Yes
FADH2 production Yes No
Terminal electron
acceptor
O2 Pyruvate or acetaldehyde
NSCC BIOL211 77
Who uses what pathway?
NSCC BIOL211 78
• Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2
• Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration
• We require oxygen to live, and are obligate aerobes
• In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes
Figure 9.18
Glucose
CYTOSOL Glycolysis
Pyruvate
No O2 present:
Fermentation
O2 present:
Aerobic cellular
respiration
Ethanol,
lactate, or
other products
Acetyl CoA
MITOCHONDRION
Citric
acid
cycle
Catabolism of other biomolecules
NSCC BIOL211 80
• Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle
• Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA)
• Fatty acids are broken down by beta oxidation and yield acetyl CoA
• An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate
Controlling Respiration
NSCC BIOL211 81
ATP and citrate inhibit
phosphofructokinase
AMP is a positive allosteric
regulator of phosphofructokinase
If you have a lot of ATP or citrate (that
means a lot of energy) glycolysis is shut
down
If you have a lot of AMP (very little
energy is present) glycolysis is
stimulated
If ATP concentration begins to drop,
respiration speeds up; when there is
plenty of ATP, respiration slows down
The Evolutionary Significance of
Glycolysis
NSCC BIOL211 82
• Ancient prokaryotes are thought to have used
glycolysis long before there was oxygen in the
atmosphere
• Very little O2 was available in the atmosphere until
about 2.7 billion years ago, so early prokaryotes likely
used only glycolysis to generate ATP
• Glycolysis is a very ancient process