Cell Respiration
3.7, 8.1
Assessment statements
3.7.1 Define cell respiration.
3.7.2 State that, in cell respiration, glucose in the
cytoplasm is broken down by glycolysis into
pyruvate, with a small yield of ATP.
3.7.3 Explain that, during anaerobic respiration,
pyruvate can be converted in the cytoplasm into
lactate, or ethanol and carbon dioxide, with no
further yield of ATP.
3.7.4 Explain that, during aerobic cell respiration,
pyruvate can be broken down in the
mitochondrion into carbon dioxide and water
with a large yield of ATP.
Cell respiration
• The controlled release of energy from
organic compounds in cells to form ATP
1st step of cell respiration:
Glycolysis
1. Glucose enters cell through membrane and
floats in cytoplasm
2. An enzyme modifies the glucose slightly, then
a second enzyme modifies this molecule even
more.
3. Series of reactions cleave the 6-carbon
glucose into two 3-carbon molecules called
pyruvate
4. Some of the energy released from the
breaking of covalent bonds in the glucose is
used to form 4 ATP molecules
http://library.thinkquest.org/27819/media/glycolysis.gif
Anaerobic respiration
(Fermentation)
•
Alcoholic
–
–
–
–
Glycolysis
Pyruvates converted to ethanol
CO2 released
Ex. yeast
• Lactic acid
– Glycolysis
– Pyruvates converted into lactic acid
– CO2 produced
– Allows glycolysis to continue in absence of
oxygen
– Benefit?
Aerobic respiration
• Most efficient
• Performed by cells with mitochondria and
oxygen
Steps of aerobic respiration
1. Begins with glycolysis
2. Pyruvates enter mitochondrion
3. Each pyruvate loses a CO2 molecule and
becomes acetyl-CoA
4. Acetyl-CoA enters into series of reactions
called the Krebs cycle
5. 2CO2 produced from each pyruvate
6. Series of other reactions through electron
transport chain
7. Water and large amt. of ATP produced
8. More efficient b/c glucose is completely
oxidized
Assessment statements
8.1.1 State that oxidation involves the loss of
electrons from an element whereas reduction
involves a gain of electrons; and that oxidation
frequently involves losing oxygen or gaining
hydrogen.
8.1.2 Outline the process of glycolysis, including
phosphorylation, lysis, oxidation and ATP
formation.
8.1.3 Draw and label a diagram showing the
structure of a mitochondrion as seen in electron
micrograph
8.1.4 Explain aerobic respiration, including
the link reaction, the Krebs cycle, the role
of NADH + H+, the electron transport chain
and the role of oxygen.
8.1.5 Explain oxidative phosphorylation in
terms of chemiosmosis.
8.1.6 Explain the relationship between the
structure of the mitochondrion and its
function.
Oxidation and reduction
Oxidation
Reduction
Loss of electrons
Gain of electrons
Gain of oxygen
Loss of oxygen
Loss of hydrogen
Gain of hydrogen
Many C-O bonds
Many C-H bonds
Results in a compound
with lower potential
energy
Results in a compound
with higher potential
energy
Oxidation and reduction cont.
• Cellular respiration is a catabolic pathway
which contains both oxidation and
reduction reactions
– Glucose is oxidized b/c electrons are
transferred from it to oxygen; protons follow
the electrons to produce water
– Oxygen atoms that occur in the oxygen
molecules on the reactant side of the equation
are reduced; large drop in the potential
energy on the product side
Oxidation and reduction cont.
• Always occur together
• Referred to as redox reactions
• Reduced form always has more potential
energy than the oxidized form of the
molecule
Glycolysis – “sugar splitting”
• Thought to have been one of the first
biochemical pathways to evolve
• Uses no oxygen
• No required organelles
• Occurs in both prokaryotic and eukaryotic
cells
• A hexose, generally glucose, is split in the
process
Three stages of glycolysis
1. Two molecules of ATP are used to begin
glycolysis. The phosphates from the
ATPs phosphorylate glucose to form
fructose-1, 6-bisphosphate
6-carbon glucose
2 ATP
2 ADP
P
P
2. The 6-carbon phosphorylated fructose is
split into two 3-carbon sugars called
glyceraldehyde-3 (G3P). This process
involves lysis.
P
P
Fructose-1, 6-bisphosphate
P
Glyceraldehyde-3-phosphate
P
Glyceraldehyde-3-phosphate
Step 3
a.
b.
c.
d.
e.
f.
Once the two G3P molecules are formed, they enter
an oxidation phase involving ATP formation and
production of the reduced coenzyme NAD.
Each G3P undergoes oxidation to from a reduced
molecule of NAD+, which is NADH.
As NADH is being formed, released energy is used to
add an inorganic phosphate to the remaining 3-carbon
compound.
This results in a compound with two phosphate
groups.
Enzymes then remove the phosphate groups so they
can be added to ADP to produce ATP.
The end result is the formation of four molecules of
ATP, two molecules of NADH and two molecules of
pyruvate (the ionized form of pyruvic acid)
P
2
G3P
2
NAD+
2P
2 NADH
P
P
2
4 ADP
4 ATP
2
pyruvate
Summary of glycolysis
1. 2 ATPs are used to start process
2. Total of 4 ATPs are produced (net gain of 2
ATPs)
3. 2 molecules of NADH are produced
4. Involves substrate-level phosphorylation, lysis,
oxidation and ATP formation
5. Occurs in the cytoplasm of the cell
6. Metabolic pathway controlled by enzymes;
when ATP is high, feedback inhibition will block
first enzyme slowing or stopping the process
7. 2 pyruvate molecules are present at the end of
the pathway
Mitochondria
• Place where the rest
of cell respiration
takes place in the
presence of oxygen
The link reaction
1. Pyruvate enters the matrix of the mito.
via active transport
2. Pyruvate is decarboxylated to form the 2carbon acetyl group
3. Removed carbon is released as CO2
4. The acetyl group is then oxidized with
the formation of reduced NAD+
5. The acetyl group combines with
coenzyme A (CoA) to form acetyl CoA
Krebs cycle (tricarboxylic acid
cycle)
• If cellular ATP levels are low, the acetyl
CoA enters the Krebs cycle
• Occurs within the matrix of the mito.
• Called a cycle b/c it begins and ends with
the same substance
• Step 1
– Acetyl CoA combines with a 4-carbon
compound called oxaloacetate to form a 6carbon compound called citrate
• Step 2
– Citrate is oxidized to form a 5-carbon
compound
– Carbon combines with oxygen to form CO2
– NAD+ forms NADH
• Step 3
– 5-carbon compound is oxidized to form a 4carbon compound
– Carbon combines with oxygen to form CO2
• Step 4
– 4-carbon compound undergoes various
changes resulting in another NADH, FADH2,
and ATP
• The 4-carbon compound is changed
during these steps to re-form the starting
compound of the cycle, coxaloacetate
• The Krebs cycle will run twice for each
glucose molecule entering cellular
respiration
Krebs Cycle outcomes
•
•
•
•
2 ATP
6 NADH
2 FADH2
4 CO2
Electron Transport Chain
• Occurs on the inner mitochondrial membrane
and on the membranes of the cristae
• Embedded in the membranes are molecules that
are easily reduced and oxidized
• These carriers of electrons are close together
and pass the electrons from one to another due
to an energy gradient
• Each carrier molecule has a slightly different
electronegativity and a different attraction for
electrons
• Most of these carriers are proteins with heme
groups and are referred to as cytochromes.
ETC cont.
• In the process, small amts. of energy are
released
• Sources of the electrons are the coenzymes
NADH and FADH2 from the Krebs cycle and link
reactions
• Electrons step down in potential energy as they
pass from one carrier to another
• At the end of the chain, the de-energized
electrons combine with available oxygen (final
electron acceptor)
• Two hydrogen ions from the aqueous surrounds
combine as well forming water
Chemiosmosis and Oxidative
Phosphorylation
• Chemiosmosis involves the movement of
protons (hydrogen ions) to provide energy
so that phosphorylation can occur
• Because this type of phosphorylation uses
an electron transport chain, it is called
oxidative phosphorylation
Review of interior structure of
mitochondrion
• Matrix – place where
Kreb’s cycle takes
place
• Cristae – large
surface area for ETC
to function
• Membranes – barrier
allowing for proton
accumulation on one
side; ATP synthase
So, what just happened?
1. Electrons provide energy needed to
pump protons from matrix to
intermembrane space
2. Difference in concentration of hydrogen
ions exists
3. Ions passively move into the matrix
through a channel in ATP synthase
4. The enzyme harnesses available energy
and phosphorylates ADP
Summary of ATP production in
cellular respiration
• Glucose → NADH/FADH2 → ETC →
chemiosmosis → ATP
Process
ATP used
ATP produced
Net ATP gain
Glycolysis
2
4
2
Krebs cycle
0
2
2
ETC and chem.
0
32
32
Total
2
38
36
• Only about 30 ATP is generated in reality
• Accounts for 30% of energy present in the chemical
bonds of glucose
• Where does the rest go?