Chapter 9 Cellular Respiration
Cellular
respiration
– Is the most prevalent and efficient catabolic pathway
– Consumes oxygen and organic molecules such as glucose
– Yields ATP
Outline
•
Energy flow and
chemical recycling in ecosystems
•
ATP review
•
Oxidation/Reduction
reactions
•
Cellular
Respiration
–
Glycolysis
–
Citric Acid Cycle
–
Oxidative
Phosphorylation
•
Fermentation
•
Catabolism of Food
Molecules
•
Control of
Cellular Respiration
Figure 9.1
Life
requires a constant source of energy to do work.
Fig 9.2 Energy flow and chemical recycling in
ecosystems
Energy Flows and Matter is Cycled
-
The
ultimate source of most energy in living things is sunlight. Solar energy is converted to chemical energy
in complex organic molecules by the process of photosynthesis.
-
The
process of cellular respiration converts the chemical energy in complex organic
molecules to ATP which is used for cellular work. Some energy is lost as heat, so solar energy
must be continually supplied – energy flow.
-
Matter
is continually cycled. In photosynthesis,
carbon dioxide and water are combined to make glucose and oxygen. In cellular respiration, glucose is combusted
in the presence of oxygen to produce carbon dioxide and water, which in turn
are used in photosynthesis
ATP – The
energy currency of the Cell
•
Transfers energy
captured during cellular respiration to the sites where energy is used in the
cell.
ATP – The
energy currency of the Cell
•
Energy is stored
in high-energy phosphate bonds.
How does
ATP Drive Endergonic Reactions?
•
ATP → ADP + P liberates 7 kcal of chemical energy which is coupled to
the endergonic reaction
How Cells
Use ATP
•
Coupled to movement
– Actin and myosin
in muscle
– Spindle fibers
during cell division
•
Drives endergonic reactions
– Anabolic
reactions
•
Transport
– active transport, endocytosis, exocytosis
Figure 8.11 A review of how
ATP drives cellular work
The Flow
of Energy in Living Things: Oxidation - Reduction
•
Ultimate source of energy for life is the sun.
•
Potential energy is stored in chemical bonds.
•
During chemical reactions, energy is passed
from molecule to molecule in the form of electrons.
– Oxidation-reduction (redox) reactions.
Redox
reactions
•
Oxidation: Loss of electrons
•
Reduction: Gain of electrons
•
OIL RIG (oxidation is loss; reduction is gain)
•
Energy is transferred from one molecule to
another via redox reactions.
A typical
redox reaction
•
Some redox
reactions do not completely exchange electrons.
•
A change the
degree of electron sharing in covalent bonds results in a change of the
potential energy of the electrons (Fig. 9.3).
Aerobic Respiration
C6 H12 O6 +
6O2 → 6H2O + 6 CO2
•
A series of oxidation reduction reactions
•
Glucose becomes oxidized to carbon dioxide,
and oxygen becomes reduced to water.
Electron Carrier Molecules
•
Oxidizes glucose
in a series of enzyme catalyzed steps
•
Electrons from
organic compounds are usually first transferred to a coenzyme, which functions
as an electron carrier molecule.
•
Two types:
–
NAD+ +
H + e- → NADH
–
FAD + 2H → FADH2
Electron
Transport Chain
•
The electron
transport chain is a series of redox reaction that breaks the fall of electrons
into glucose in to a several energy releasing steps instead of one big
explosion.
•
This flow of
electrons is coupled to a process that regenerates ATP from ADP.
Figure 9.5 An introduction
to electron transport chains
Electron
Transport Chain
•
Electrons are
extracted from organic molecules in food (i.e. glucose) and are carried via the
electron shuttle, NADH, to the electron transport chain.
•
Electrons cascade
down a chain of electron carriers, losing a small amount of energy in each
step, until they reach oxygen, the final electron acceptor.
•
Oxygen is very
electronegative, and pulls the electrons so that they always go ‘down’ the
chain, rather than reversing back ‘uphill’.
•
The energy
released from this exergonic electron fall is used to regenerate ATP using a
mechanism that will be revealed later in this lecture!
An
Overview of Glucose Catabolism
•
Stages of aerobic respiration:
•
1. glycolysis
•
2. The Citric Acid Cycle (Krebs Cycle)
•
3. Electron Transport chain (Oxidative
Phosphorylation)
Figure 9.6 An overview of
cellular respiration
An
Overview of Glucose Catabolism
•
Two ways of making ATP from
the catabolism of organic molecules.
Figure 9.7 Substrate-level
phosphorylation
Oxidative phosphorylation
•
Energy from
electrons harvested from catabolism drives the production of ATP.
Stage 1: Glycolysis
•
10 enzyme
catalyzed reactions that convert six carbon glucose to three
carbon pyruvate.
•
2 ATP are produced
for each molecule of glucose, by substrate level phosphorylation.
•
2 NAD+ are reduced
to 2 NADH with H and electrons from the glucose.
•
Occurs
in the cytoplasm.
•
All cells use glycolysis.
Figure 9.8
The energy input and output of Glycolysis
Figure 9.9 A closer look at glycolysis: energy investment phase
The Key Events of Glycolysis
•
Glucose priming
•
Cleavage and rearrangement.
•
Substrate level
phosphorylation.
The
end products are 2 three carbon pyruvate, 2 ATP and 2 NADH.
Stage
2: The Citric Acid Cycle
(Krebs Cycle)
•
Oxidation of
Pyruvate
–
Pyruvate
+ NAD+ + CoA → Acetyl-CoA + NADH + CO2.
•
The two carbon
acetyl group is then transferred to four carbon oxaloacetic
acetate in the Citric Acid cycle and the combustion of what was formerly
glucose is completed.
•
The end products
are 6 CO2,
6 NADH, 2 FADH2, and 2 ATP.
Figure 9.10 Conversion of
pyruvate to acetyl CoA, the junction between glycolysis and the Krebs cycle
Figure 9.11 A closer look at
the Krebs cycle
Stage 3:
The Electron Transport Chain
•
Electrons are passed through the Electron
Transport.
•
An electrochemical gradient forms across the
inner mitochondrial membrane.
•
The process of chemiosmosis is used to make
ATP.
Figure 9.5 An introduction
to electron transport chains
Stage 3:
The Electron Transport Chain
•
A collection of
electron carrier molecules embedded in the inner mitochondrial membrane.
•
NADH and FADH2
bring e- from the Krebs cycle to the ETC.
•
Are
passed form carrier to carrier, until they reach oxygen, the final electron
acceptor, forming water.
•
Electronegative
oxygen pulls the electrons down the chain.
•
Energy released is
coupled to the regeneration of ATP!!!
Figure 9.13 Free-energy
change during electron transport
Figure 9.14 ATP synthase, a
molecular mill
Figure 9.15 Chemiosmosis
couples the electron transport chain to ATP synthesis
Electron
Transport Chain, Summary
•
NADH and FADH2 bring electrons from the Krebs cycle
to the ETC.
•
Electrons are passed down the ETC.
•
H+ are passed through the membrane to the
mitochondrial intermembrane space.
•
O2
is the final electron receptor: O2 + 4H+ + 4e- → 2 H20.
•
Purpose: to
build an electrochemical gradient across the mitochondrial inner membrane.
Chemiosmosis
•
H+ in high concentration within the
intermembrane space.
•
They can only reenter the mitochondrial space
via the ATP synthase channel protein.
•
Their flow through ATP synthase is coupled to
the production of ATP, similar to water through an electrical generator.
Figure 9.16 Review: how each
molecule of glucose yields many ATP molecules during cellular respiration
Fermentation
•
Catabolism of Food
Molecules
•
Control of
Cellular Respiration
Fermentation
•
Glycolysis can
occur in the absence of oxygen,
•
However, NAD+ must
be regenerated.
•
Types of
fermentation
–
Ethanol
–
Lactic acid
Figure 9.17a Fermentation
Figure 9.17b Fermentation
What Types
of Cells Can Undergo Fermentation?
|
Types of fermentation |
Cells where it occurs |
|
|
|
|
|
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Figure 9.18 Fermentation
If oxygen
is present, pyruvate enters the mitochondria, is oxidized and enters the Krebs Cycle.
If oxygen is not present, pyruvate is reduced to ethanol or lactate and NAD+ is
regenerated.
Glycolysis
is an ancient metabolic pathway.
•
Bacteria existed
the better part of a billion years before oxygen was present on earth. These bacteria must have been anaerobic and
relied on glycolysis and fermentation.
•
Glycolysis is the
most widespread metabolic pathway, found in virtually all cells, which suggests
that it evolved early in the history of life.
•
It is located in
the cytosol – it does not require membrane bound
organelles.
Figure 9.19 The catabolism
of various food molecules
Proteins
are deaminated and then feed directly into the Krebs
cycle or are converted to pyruvate.
Fats are
converted to acetyl CoA by beta-oxidation, then enter the Krebs cycle.
The
control of cellular respiration
Fig 20
Phosphofructokinase (PFK), the second enzyme in glycolysis
is an important control point for cellular respiration.
•
Excess
ATP inhibits PFK.
•
Excess
citrate inhibits PFK.
•
AMP
stimulate PFK