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Chapter 16
Respiration
Cellular (internal or tissue) Respiration - the
metabolic processes within cells which
release energy from glucose.
Gaseous Exchange - the processes involved
in obtaining the oxygen for respiration and
the removal of gaseous wastes.
Cellular Respiration can
be divided into 3 stages:
1 glycolysis
2 Krebs (tricarboxylic
acid) cycle
3 Electron (hydrogen)
transport system
16.1 Adenosine triphosphate (ATP)
ATP is the short-term energy store of all
cells. It is easily transported and is therefore
the universal energy carrier.
Computer graphics
representation of ATP
16.1.1 Structure of ATP
Adenine + P  AMP + P
 ADP + P
 ATP
P=inorganic
phosphate
= H3PO4
16.1.2 Importance of ATP
The energy stored in ATP is released when ATP is
hydrolysed by enzymes into ADP
Further hydrolysis of ADP into AMP releases a
similar quantity of energy
BUT hydrolysis of AMP releases much less energy
than the other two
The energy is stored in the molecule as a whole,
although the breaking of the bonds initiates its
release
AMP & ADP may be reconverted to ATP by adding
phosphate group(s) through the process called
phosphorylation, e.g.
1.Photosynthetic phosphorylation - occurs
during
photosynthesis
in
chlorophyll
containing cells
2.Oxidative phosphorylation - occurs during
cellular respiration in all aerobic cells
The addition of each phosphate molecule requires
30.6 kJ of energy.
If less than this, energy cannot be stored as ATP but
lost as heat.
ATP is a means of transferring free energy from
energy-rich compounds to cellular reactions
requiring it.
It is by far the most abundant and the most important
energy transferring compound used by cells.
16.1.3 Uses of ATP
An active cells may use as many as 2 million
molecules of ATP every second !
ATP is a source of energy for:
1. Anabolic processes - build up macromolecules from
component units,
e.g. polysaccharide from monosaccharides,
proteins from amino acids and DNA replication
2. Movement - e.g. muscle contraction, ciliary action
and spindle formation in cell division
3. Active transport- move materials against a
concentration gradient e.g. Na/K pumps
4. Secretion - form vesicles necessary in cell secretions
5. Activation of chemicals - make chemicals more
reactive, e.g. phosphorylation of glucose
16.2 Glycolysis
Break down glucose into 2 molecules of
pyruvate (pyruvic acid), 2ATP & 2H atoms
It occurs in all cells.
In anaerobic respiration it is the only stage
of respiration
Stages of glycolysis:
(in the cytoplasm)
Stages of glycolysis: (in the cytoplasm)
Each glucose molecule produces two
molecules of glycerate 3-phosphate
The energy yield is a net gain of 2 ATP and
2 pairs of H atoms
Each pair of H atoms gives 3 ATP
Total number of ATP is 8
Krebs Cycle
16.3 Krebs (tricarboxylic acid) cycle
(in the mitochondrion)
Majority of the energy from glucose is locked up in
the pyruvate.
Pyruvate enters the mitochondria and in the presence
of oxygen, they are broken down into water and
carbon dioxide.
A total of 4 pairs of H atoms and 1 ATP are
produced:
3 pairs of H atoms give 9 ATP
1 pair of H atoms give 2 ATP
Total number of ATP = 38
3 pairs of H atoms give 9 ATP
1 pair of H atoms give 2 ATP
9 + 2 + 1 = 12 ATP
+ 3 ATP
= 15 ATP
3ATP x 2 = 6 ATP
+ 2ATP (glycolysis) = 8 ATP
8 + 15x2 = 38 ATP
16.3.1 Importance of Krebs Cycle
1. It brings about the degradation of
macromoelcules
2. It provides the reducing power for the
electron transport system
3. It is an interconversion centre for making
fatty acids, amino acids, chlorophyll, etc.
16.4 Electron transport system
The electron transport system is the means by
which energy from the Krebs cycle, in the
form of hydrogen atoms, is converted to ATP
The H atoms attached to the H-carriers NAD
& FAD are transferred to a chain of other
carriers at progressively lower energy levels.
As the H atom pass from one carrier to the
next, the energy released is used to produce
ATP.
The series of carriers is called the respiratory
chain. The carriers include:
NAD (nicotinamide adenine dinucleotide),
FAD (Flavine adenine dinucleotide,
coenzyme Q and
iron-containing proteins (cytochromes)
Oxidative phosphorylation: generation of ATP by
H atoms in an electron transport system in which
H atoms link with O atoms to form water.
Oxygen is necessary as the final H acceptor.
In its absence, anaerobic glycolysis can continue
to provide energy for the cell.
Cyanide poisons the enzyme cytochrome oxidase
to stop aerobic respiration.
16.4.1 Mitochondria and oxidative phosphorylation
Mitochondria are present in all eukaryotic cells.
Highly active cells have numerous large
mitochondria packed with cristae, e.g.
Liver cells – for driving chemical reactions
inside the body
Striated muscles – muscle contraction
Sperm tails – provide energy to propel the
sperm
Nerve cells – numerous mitochondria are present
to provide energy needed for synapses to function
Intestinal epithelial cells – mitochondria beneath
the microvilli to release energy for the absorption
of digested food by active transport
Inner membrane of the mitochondrion is folded to
form cristae which are lined with stalked particles
for oxidative phosphorylation while the enzymes
of the Krebs Cycle are mostly found in the matrix
of the mitochondrion.
Portions of a mitochondrion
matrix
Outer membrane
Intermembrane space
Inner membrane
Stalked particle
Intermembrane space
The synthesis of ATP according to the chemi-osmotic
theory of Mitchell
16.5 Anaerobic respiration
Obligate anaerobes:
Organisms must respire anaerobically, they
die in the presence of oxygen
Facultative anaerobes:
Organisms respire aerobically, but can carry
out anaerobic respiration if lack of oxygen
Without oxygen, H atoms accumulate and
prevent glycolysis to continue. H atoms are
accepted by the pyruvate formed at the end
of glycolysis to give either alcohol or lactate,
in a process called fermentation.
16.5.1 Alcoholic fermentation
pyruvate  ethanal + CO2
ethanal + 2H (from NAD+)  ethanol
16.5.2 Lactate fermentation
pyruvate + 2H (from NAD+)
 lactic acid (lactate)
Lactic acid can be oxidised by oxygen to
release energy
OR resynthesized into carbohydrate
OR excreted
Examples: muscles during heavy exercise,
baby during & immediately following birth
Oxygen Debt:
Lactic acid causes muscle fatigue,
Oxygen debt is the extra oxygen to
remove all the lactic acid after heavy
exercise to CO2, H2O or glucose in the
liver, and
To replenish the depleted stores of ATP &
oxygen in the tissue
16.6 Comparison of energy yields
Aerobic Respiration:
Two sources
1 direct phosphorylation
2 indirect oxidative phosphorylation of
hydrogen ions from glucose breakdown
TABLE 16.1 ATP yield during aerobic respiration of
one molecule of glucose
Respiratory
process
No.of reduced H No.of ATP
carrier
molecules
molecules formed formed from
reduced
H carriers
No. of
Total No.
ATP
of ATP
molecules molecules
Total
ATP
38
TABLE 16.1 ATP yield during aerobic respiration of
one molecule of glucose
Respiratory
process
No.of reduced H No.of ATP
carrier
molecules
molecules formed formed from
reduced
H carriers
No. of
Total No.
ATP
of ATP
molecules molecules
Glycolysis
Pyruvate 
acetyl CoA
Krebs
Cycle
Total
ATP
38
TABLE 16.1 ATP yield during aerobic respiration of
one molecule of glucose
Respiratory
process
No.of reduced H No.of ATP
carrier
molecules
molecules formed formed from
reduced
H carriers
Glycolysis
2NADH2+
No. of
Total No.
ATP
of ATP
molecules molecules
Pyruvate 
acetyl CoA
Krebs
Cycle
Total
ATP
38
TABLE 16.1 ATP yield during aerobic respiration of
one molecule of glucose
Respiratory
process
No.of reduced H No.of ATP
carrier
molecules
molecules formed formed from
reduced
H carriers
Glycolysis
2NADH2+
No. of
Total No.
ATP
of ATP
molecules molecules
Pyruvate  2NADH2+
acetyl CoA
Krebs
Cycle
Total
ATP
38
TABLE 16.1 ATP yield during aerobic respiration of
one molecule of glucose
Respiratory
process
No.of reduced H No.of ATP
carrier
molecules
molecules formed formed from
reduced
H carriers
Glycolysis
2NADH2+
No. of
Total No.
ATP
of ATP
molecules molecules
Pyruvate  2NADH2+
acetyl CoA
Krebs
Cycle
3 NADH2+ x 2
1 FADH2+ x 2
Total
ATP
38
TABLE 16.1 ATP yield during aerobic respiration of
one molecule of glucose
Respiratory
process
No.of reduced H No.of ATP
carrier
molecules
molecules formed formed from
reduced
H carriers
Glycolysis
2NADH2+
No. of
Total No.
ATP
of ATP
molecules molecules
2x3=6
Pyruvate  2NADH2+
acetyl CoA
Krebs
Cycle
3 NADH2+ x 2
1 FADH2+ x 2
Total
ATP
38
TABLE 16.1 ATP yield during aerobic respiration of
one molecule of glucose
Respiratory
process
No.of reduced H No.of ATP
carrier
molecules
molecules formed formed from
reduced
H carriers
Glycolysis
2NADH2+
2x3=6
Pyruvate  2NADH2+
acetyl CoA
2x3=6
Krebs
Cycle
No. of
Total No.
ATP
of ATP
molecules molecules
3 NADH2+ x 2
1 FADH2+ x 2
Total
ATP
38
TABLE 16.1 ATP yield during aerobic respiration of
one molecule of glucose
Respiratory
process
No.of reduced H No.of ATP
carrier
molecules
molecules formed formed from
reduced
H carriers
Glycolysis
2NADH2+
2x3=6
Pyruvate  2NADH2+
acetyl CoA
2x3=6
Krebs
Cycle
No. of
Total No.
ATP
of ATP
molecules molecules
3 NADH2+ x 2 6 x 3 = 18
1 FADH2+ x 2 2 x 2 = 4
Total
ATP
38
TABLE 16.1 ATP yield during aerobic respiration of
one molecule of glucose
Respiratory
process
No.of reduced H No.of ATP
carrier
molecules
molecules formed formed from
reduced
H carriers
No. of
Total No.
ATP
of ATP
molecules molecules
Glycolysis
2NADH2+
2x3=6
2
Pyruvate  2NADH2+
acetyl CoA
2x3=6
Krebs
Cycle
3 NADH2+ x 2 6 x 3 = 18
1 FADH2+ x 2 2 x 2 = 4
Total
ATP
38
TABLE 16.1 ATP yield during aerobic respiration of
one molecule of glucose
Respiratory
process
No.of reduced H No.of ATP
carrier
molecules
molecules formed formed from
reduced
H carriers
No. of
Total No.
ATP
of ATP
molecules molecules
Glycolysis
2NADH2+
2x3=6
2
Pyruvate  2NADH2+
acetyl CoA
2x3=6
0
Krebs
Cycle
3 NADH2+ x 2 6 x 3 = 18
1 FADH2+ x 2 2 x 2 = 4
Total
ATP
38
TABLE 16.1 ATP yield during aerobic respiration of
one molecule of glucose
Respiratory
process
No.of reduced H No.of ATP
carrier
molecules
molecules formed formed from
reduced
H carriers
No. of
Total No.
ATP
of ATP
molecules molecules
Glycolysis
2NADH2+
2x3=6
2
Pyruvate  2NADH2+
acetyl CoA
2x3=6
0
Krebs
Cycle
3 NADH2+ x 2 6 x 3 = 18 1 x 2
1 FADH2+ x 2 2 x 2 = 4
Total
ATP
38
TABLE 16.1 ATP yield during aerobic respiration of
one molecule of glucose
Respiratory
process
No.of reduced H No.of ATP
carrier
molecules
molecules formed formed from
reduced
H carriers
No. of
Total No.
ATP
of ATP
molecules molecules
Glycolysis
2NADH2+
2x3=6
2
Pyruvate  2NADH2+
acetyl CoA
2x3=6
0
Krebs
Cycle
8
3 NADH2+ x 2 6 x 3 = 18 1 x 2
1 FADH2+ x 2 2 x 2 = 4
Total
ATP
38
TABLE 16.1 ATP yield during aerobic respiration of
one molecule of glucose
Respiratory
process
No.of reduced H No.of ATP
carrier
molecules
molecules formed formed from
reduced
H carriers
No. of
Total No.
ATP
of ATP
molecules molecules
Glycolysis
2NADH2+
2x3=6
2
8
Pyruvate  2NADH2+
acetyl CoA
2x3=6
0
6
Krebs
Cycle
3 NADH2+ x 2 6 x 3 = 18 1 x 2
1 FADH2+ x 2 2 x 2 = 4
Total
ATP
38
TABLE 16.1 ATP yield during aerobic respiration of
one molecule of glucose
Respiratory
process
No.of reduced H No.of ATP
carrier
molecules
molecules formed formed from
reduced
H carriers
No. of
Total No.
ATP
of ATP
molecules molecules
Glycolysis
2NADH2+
2x3=6
2
8
Pyruvate  2NADH2+
acetyl CoA
2x3=6
0
6
Krebs
Cycle
3 NADH2+ x 2 6 x 3 = 18 1 x 2
1 FADH2+ x 2 2 x 2 = 4
24
Total
ATP
38
The total of 38 ATP produced represents the
maximum possible yield;
the actual yield may be different depending upon
the conditions in any one cell at the time.
For example, the NADH + H+ in glycolysis may
enter the mitochondria in two, indirect ways.
This may give only 4 ATP instead of 6 ATP.
Total ATP = 36
Efficiency = energy by glycolysis & Krebs Cycle /
glucose molecule
= 40 %
Anaerobic Respiration:
All NADH + H+ are not available for
oxidative phosphorylation.
Only 2 ATPs are released.
Efficiency = 2%
BUT energy is locked up in lactate or
ethanol
16.7 Alternative respiratory substrates
Sugars are not the only material which can
be oxidised by cells to release energy.
Both fats and proteins may, in certain
circumstances, be used as respiratory
substrates, without first being converted to
carbohydrate.
16.7.1 Respiration of fat
FAT  fatty acids + glycerol
Glycerol  glyceraldehyde 3-phosphate
16.7.1 Respiration of fat
FAT  fatty acids + glycerol
Glycerol  glyceraldehyde 3-phosphate
 glycolysis
16.7.1 Respiration of fat
FAT  fatty acids + glycerol
Glycerol  glyceraldehyde 3-phosphate
 glycolysis
Fatty acids  broken into parts of 2-C compound
- 2C
16.7.1 Respiration of fat
FAT  fatty acids + glycerol
Glycerol  glyceraldehyde 3-phosphate
 glycolysis
Fatty acids  broken into parts of 2-C compound
 acetyl CoA in mitochondria.
16.7.1 Respiration of fat
FAT  fatty acids + glycerol
Glycerol  glyceraldehyde 3-phosphate
 glycolysis
Fatty acids  broken into parts of 2-C compound
 acetyl CoA in mitochondria
Fat oxidation produces a lot of H atoms which
release ATP through the electron carrier system.
It is twice as much as energy as the same mass of
carbohydrates.
16.7.2 Respiration of protein
Protein is only used in cases of starvation.
Proteins  amino acids
 carbohydrate + amino group
16.7.2 Respiration of protein
Protein is only used in cases of starvation.
Proteins  amino acids
 carbohydrate + amino group
aspartic acid  oxaloacetate
glutamic acid   -ketoglutarate
alanine  pyruvate
16.8 Respiratory quotients (not required)
R.Q. = CO2 evolved / O2 consumed
a measure of the ratio of carbon dioxide
evolved by an organism to the oxygen
consumed, over a certain period
For glucose oxidation: R Q = 1
C6H12O6 + 6O2  6CO2 + 6H2O + energy
For fat oxidation:
RQ  1
For protein oxidation: R Q = 0.9
Organisms rarely, if ever, respire a single
food substance, nor are substances always
completely oxidized. Experimental R Q
values therefore do not give the exact nature
of the material being respired. Most resting
animals have R Qs between 0.8 and 0.9 (a
mixture of carbohydrate & fat metabolism,
assuming proteins are only respired during
starvation).