Cellular Respiration
What you should know
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The role of ATP in the transfer of energy and the phosphorylation of molecules by ATP.
Metabolic pathways of cellular respiration. The breakdown of glucose to pyruvate in the cytoplasm
in glycolysis, and the progression pathways in the presence or absence of oxygen (fermentation).
The role of the enzyme phosphofructokinase in this pathway.
The formation of citrate.
Pyruvate is broken down to an acetyl group that combines with coenzyme A to be transferred to
the citric acid cycle as acetyl coenzyme A.
Acetyl coenzyme A combines with oxaloacetate to form citrate followed by the enzyme mediated
steps of the cycle.
This cycle results in the generation of ATP, the release of carbon dioxide and the regeneration of
oxaloacetate in the matrix of the mitochondria.
Dehydrogenase enzymes remove hydrogen ions and electrons which are passed to the
coenzymes NAD or FAD to form NADH or FADH2 in glycolysis and citric acid pathways.
NADH and FADH2 release the high-energy electrons to the electron transport chain on the
mitochondrial membrane and this results in the synthesis of ATP.
ATP synthesis — high-energy electrons are used to pump hydrogen ions across a membrane and
flow of these ions back through the membrane synthesises ATP using the membrane protein ATP
synthase. The final electron acceptor is oxygen, which combines with hydrogen ions and electrons
to form water.
Introduction to Cellular
Respiration
• A series of metabolic pathways that
brings about the release of energy from
a foodstuff
• In doing so it also regenerates the highenergy compound Adenosine
Triphosphate (ATP)
ATP
• Adenosine Triphosphate
• Molecule able to provide energy immediately
• Consists of Adenosine & 3 inorganic phosphate
molecules
adenosine
Pi
Pi
Pi
• Energy held within ATP is released when the
terminal phosphate is broken off (by enzymes)
ATP
• Adenosine Triphosphate
• This bond broken to release energy
adenosine
Pi
Pi
Pi
• Adenosine Diphosphate (ADP) and an inorganic
phosphate are produced
• Also, energy is required to regenerate ATP from
ADP & Pi
• ATP acts as the link between catabolic (energy
releasing reactions) and anabolic (energy
requiring reactions)
• At any given moment some ATP molecules are
undergoing breakdown (releasing energy), while
others are being regenerated from ADP + Pi
(using energy)
• This means there is a
relatively fixed quantity
of ATP available
PHOSPHORYLATION
• The addition of a phosphate group to a molecule,
e.g. ADP + Pi = ATP
• Phosphates can also be transferred from ATP to
reactants in the pathway to make them more
ADP
ATP
reactive
e.g. Glucose ------------> Glucose- 6- Phosphate
• Often a step in a pathway can only proceed if a
reactant becomes phosphorylated
Importance of ATP formation
• We all need energy to function
and we get this energy from the
foods we eat
• The most efficient way for cells
to harvest energy stored in food
is through cellular respiration
• a catabolic pathway for the
production of adenosine
triphosphate (ATP)
• ATP, a high energy molecule, is
expended by working cells
• Cellular respiration occurs in
both eukaryotic and prokaryotic
cells
RESPIRATION
• Process by which energy is released
from foods by oxidation.
• It involves the regeneration of ATP
which is a high energy compound.
• Consists of 3 stages
– GLYCOLYSIS
– CITRIC ACID CYCLE (KREBS CYCLE)
– ELECTRON TRANSPORT CHAIN
GLYCOLYSIS
• Takes place in the cytoplasm of the cell.
• Is a series of enzyme controlled steps
• Glucose (6C) is broken down into two molecules
of Pyruvic Acid (3C) (Pyruvate)
• Does not require oxygen
• Net gain of 2 ATP
Energy investment and payoff during
glycolysis
• The first half of the
chain makes up the
energy investment
phase- where 2 ATP are
used per glucose
molecule
• The second half of the
chain makes up the
energy payoff phasewhere 4 ATP are
produced per glucose
molecule
Phosphorylation
occurs twice. The 2nd
time by
phosphofructokinase
Phosphorylation during energy
investment stage
• The first phosphorylation of intermediates
leads to a product that can continue to a
number of other pathways
• (E.g. fermentation in the absence of oxygen)
• The second phosphorylation catalysed by
phosphofructokinase is irreversible and leads
only to the glycolytic pathway
Energy payoff stage
• Hydrogen ions are released by the action of a
dehydrogenase enzyme
• Co-enzymes NAD and FAD pick up the H+ ions to
form NADH or FADH in glycolysis and the citric
acid pathways
• NADH and FADH release high energy electrons
to the electron transport chain on the
mitochondrial membrane
• Resulting in the synthesis of ATP
GLYCOLYSIS
GLUCOSE
2 ATP
2 NAD
2 NADH2
2 ADP+Pi
4 ADP+Pi
4 ATP
PYRUVIC ACID
MITOCHONDRIA
• Citric Acid Cycle
takes place in the
matrix
• Electron
Transport Chain
takes place on the
cristae
CITRIC ACID CYCLE
• Takes place in the matrix of the mitochondria.
• Requires oxygen
• Pyruvate/Pyruvic acid converted to Acetyl which
then combines with Coenzyme A (2C)
• Further Hydrogen ions are released and bind to
NAD, forming NADH
• Acetyl CoA combines with oxaloacetate a 4C
compound to form 6C citrate
• This stage involves the regeneration of
oxaloacetate
CITRIC ACID CYCLE
• Citrate is converted back to oxaloacetate
by a series of enzyme controlled reactions.
• During the cycle, carbon is released in the
form of carbon dioxide, hydrogen is
released and binds to NAD/FAD and ATP
is formed.
CITRIC ACID CYCLE
PYRUVIC ACID (3C)
2NAD
CO2
2NADH2
ACETYL COA (2C)
CITRIC ACID (6C)
CO2
4C oxaloacetate
2NAD
2NADH2
2NADH2
2NAD
5C COMPOUND
4C COMPOUND
2FADH2
2FAD
2NAD
2NADH2
4C COMPOUND
CO2
ADP+Pi
ATP
ELECTRON TRANSPORT CHAIN
• Takes place on the cristae of the mitochondria.
• The reduced NAD/FAD transfer the high energy electrons
to a chain of carriers called the cytochrome system
• Energy from the electrons is used to pump H+ from the
inner matrix to the intermembrane space
• This maintains a higher conc of hydrogen ions in the
intermembrane space, so..
• The return flow of H+ ions rotates part of the membrane
protein ATP synthase and ATP is generated
• The final electron acceptor is oxygen which combines with
hydrogen ions and low energy electron to form water
ELECTRON TRANSPORT CHAIN
• The transfer of one H molecule
releases 3 ATP molecules
• This is called oxidative
phosphorylation
ELECTRON TRANSFER SYSTEM
ADP +Pi
ADP +Pi
ADP +Pi
NADH2
WATER
SERIES OF HYDROGEN CARRIERS
NAD
OXYGEN
ATP
ATP
ATP
ATP PRODUCTION
SUMMARY
• Each NADH2 molecule produces 3
ATP
• 12 NADH2 = 36ATP from Kreb’s
cycle
• 2 ATP from glycolysis
• 38 ATP in total
What you should know
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Substrates for respiration. Starch and glycogen, other sugar molecules,
amino acids and fats.
Regulation of the pathways of cellular respiration by feedback inhibition —
regulation of ATP production, by inhibition of phosphofructokinase by ATP
and citrate,
synchronisation of rates of glycolysis and citric acid cycle.
Energy systems in muscle cells.
Creatine phosphate breaks down to release energy and phosphate that is
used to convert ADP to ATP at a fast rate. This system can only support
strenuous muscle activity for around 10 seconds, when the creatine
phosphate supply runs out. It is restored when energy demands are low.
Lactic acid metabolism. Oxygen deficiency, conversion of pyruvate to lactic
acid, muscle fatigue, oxygen debt.
Types of skeletal muscle fibres
Slow twitch (Type 1) muscle fibres contract more slowly, but can sustain
contractions for longer and so are good for endurance activities. Fast twitch
(Type 2) muscle fibres contract more quickly, over short periods, so are
good for bursts of activity.
Substrates for respiration
• Starch and glycogen (carbohydrates) are broken
down to glucose
• Maltose and sucrose (carbohydrates) can be
converted to glucose or glycolysis intermediates
• Proteins can be broken down to amino acids and
converted to intermediates of glycolysis and the
citric acid cycle
• Fats can be broken down into fatty acids and
glycerol. Glycerol is converted to a glycolytic
intermediate and fatty acids converted for use in
the citric acid cycle
Regulation of Cellular Respiration
• The cell conserves its resources by
only producing ATP when required
• Feedback inhibition regulates and
synchronises the rates of the
glycolytic and citric acid cycle
pathways
• If more ATP than the
cell needs is produced
the ATP inhibits
phosphofructokinase
slowing glycolysis
• High concentrations of
citrate also inhibit
phosphofructokinase
• When citrate
concentration drops
the enzyme is no longer
inhibited
Energy systems in muscle cells
• During strenuous muscle
activity the cell breaks
down its reserves of ATP
and releases energy
• Muscle cells can only store
enough ATP for a few
muscle contractions
• Muscle cells have an
additional source of energy
Energy systems in muscle cells
• Creatine phosphate acts
as a high energy reserve
available to muscle cells
during strenuous exercise
• During strenuous exercise
creatine phosphate
breaks down releasing
energy and phosphate
which are used to convert
ADP to ATP by
phosphorylation
• This system can only
support strenuous muscle
activity for around 10
seconds before the
supply of creatine
phosphate runs out
• When ATP demand is low,
ATP from cellular
respiration restores the
levels of creatine
phosphate
• If strenuous exercise continues the
cells respire anaerobically as they
do not get enough oxygen
• Neither the citric acid cycle nor
electron transport system can
generate the ATP required
• Only glycolysis is able to provide
more ATP
• This results in pyruvate being
converted to lactic acid
• It involves the transfer of hydrogen
from NADH produced during
glycolysis to pyruvic acid to produce
lactic acid
• NAD is regenerated to maintain ATP
production during glycolysis
• Only 2 molecules of ATP are
produced from each molecule of
glucose
Lactic acid
metabolism
• As lactic acid builds in the muscles it
causes fatigue
• An oxygen debt builds up
• When the oxygen debt is repaid, the
lactic acid is converted back to
pyruvic acid which then enters the
aerobic pathway
ANAEROBIC RESPIRATION
Oxygen debt
builds up
Glucose
Pyruvic Acid
Lactic Acid
(6C)
(2 X 3C)
(2 X 3C)
Oxygen debt
repaid
AEROBIC V ANAEROBIC
Number of ATP
molecules per
glucose
molecule
Products of
reaction (other
than ATP)
Location in cell
Aerobic
Respiration
Anaerobic
Respiration
38
2
Carbon
dioxide and
water
Lactic acid
mitochondrion
cytoplasm
Types of skeletal muscle
• Skeletal muscles bring about movement of the body
• Two types of skeletal muscle fibres
– Type 1- Slow Twitch Muscle Fibres
– These contract slowly, but sustain contractions for
longer
– Good for endurance activities
– Rely on aerobic respiration to generate ATP
– Have many mitochondria
– Have a large blood supply
– Have a high concentration of myoglobin which is
good at storing oxygen (myoglobin also extracts
oxygen from the blood)
– Major storage fuel is fats
• Type 2- Fast Twitch Muscle Fibres
– Muscle fibres contract quickly
– Over short periods of time
– Good for bursts of activity
– Generates ATP through glycolysis
– Have only a few mitochondria
– Lower blood supply
– Major storage fuels are glycogen and creatine
phosphate
• Most human muscle tissue contains both slow and
fast twitch fibres
• Athletes show distinct patterns of muscle fibres
that reflect their sporting activities
•Slow twitch fibres are responsible
for stamina and suppleness. The
action of slow twitch fibres is
dependent on aerobic respiration. So,
the supply of oxygen is important.
The presence of large quantity of
myoglobin is necessary. Therefore,
slow twitch fibres give the
characteristic red colour.
• Fast twitch fibres are
responsible for strength.
Sports requiring sudden bursts
of maximum activity, such as in
sprinting, throwing, jumping and
lifting rely on fast twitch
fibres.