Chapter 6 - Cell Respiration
Metabolism - the sum of all the chemical
reactions that occur in the body. It is
comprised of:
 anabolism – synthesis of molecules, requires
input of energy
 catabolism – break down of molecules,
releases energy..
 aerobic – occurs in the presence of oxygen
 anaerobic – occurs in the absence of oxygen..
Energy Flow And Chemical Cycling In
The Biosphere
• Fuel molecules in food represent solar energy
– Energy stored in food can be traced back to the sun
• Animals depend on plants to convert solar
energy to chemical energy
– This chemical energy is in the form of sugars and
other organic molecules..
• Those organisms that convert sun energy
into food energy are producers.
– autotrophs - the most common carry out
photosynthesis
• Those organisms that consume the
autotrophs are consumers.
– heterotrophs..
Chemical Cycling Between Photosynthesis And
Cellular Respiration
• The ingredients for photosynthesis are CO2 and H2O
– CO2 is obtained from the air by a plant’s leaves
– H2O is obtained from the damp soil by a plant’s roots
• Chloroplasts rearrange the atoms of these ingredients to
produce sugars (glucose) and other organic molecules
– O2 is a by-product of photosynthesis
• Both plants and animals perform cellular respiration
– Cellular respiration is a chemical process that harvests energy
from organic molecules and occurs in mitochondria
• The waste products of cellular respiration, CO2 and H2O,
are used in photosynthesis..
Sunlight
energy
Ecosystem
Photosynthesis
(in chloroplasts)
Glucose
Carbon dioxide
Oxygen
Water
Cellular respiration
(in mitochondria)
for cellular work
Heat energy
We have been designed to liberate energy from
food molecules by aerobic cellular respiration.
This process is described as aerobic because
oxygen is required. Why is oxygen required?
During cellular respiration, hydrogen and its
bonding electrons change partners from
glucose to water.
Oxygen is the final e- acceptor.
Glucose
Oxygen
Carbon
dioxide
Water
Energy
• Chemical reactions that transfer electrons from
one substance to another are called oxidationreduction reactions.
– the loss of electrons (and hydrogens) is called
oxidation
– the gain of electrons (and hydrogens) is called
reduction
Oxidation
[Glucose loses electrons (and hydrogens)]
Glucose
Oxygen
Carbon
dioxide
[Oxygen gains electrons (and hydrogens)]
Reduction
Water
When NAD (nicotinamide adenine dinucleotide)
is reduced, a pair of hydrogen atoms donates a
pair of e-, one of which then binds one proton
and the other proton follows along = NADH +
H+. We simplify this with NADH2..
Aerobic cellular respiration occurs in four
stages:
glycolysis
transition reaction
Krebs cycle
electron transport pathway..
• Glycolysis – glucose must be “activated” by the
addition of two phosphate groups P . The
addition of the P also traps glucose within the
cell. This process occurs in the cytosol.
2ADP + Pi
2ATP
C6H12O6
glucose
2 C3H4O3
pyruvic acid
2NAD + 4H
2NADH2
2 Pyruvic acid
Glucose
• In animals if oxygen is not present to take the efrom NADH2, then the e- will be donated to pyruvic
acid = Lactic acid pathway (anaerobic respiration).
• The final product is lactic acid. This metabolic
pathway only yields 2 ATP/molecule.
2 ADP+ 2
Glycolysis
2 NAD
2 NAD
Glucose
2 Pyruvic
acid
+ 2 H
2 Lactic
acid
• Various types of microorganisms perform
fermentation
– Yeast cells carry out a slightly different type of
fermentation pathway = alcoholic fermentation
– This pathway produces CO2 and ethyl alcohol
2 ADP+
2
2 ATP
2 CO2 released
Glycolysis
2 NAD
2 NAD
Glucose
2 Pyruvic
acid
+ 2 H
2 Ethyl
alcohol
• Transition reaction = pyruvic acid moves into
the matrix of the mitochondrion. CO2 is
cleaved off and at the same time Coenzyme A
is added.Coenzyme A is derived from the
vitamin pantotenic acid.
NAD + 2H
2C3H4O3 + 2CoA
pyruvic acid
coenzyme A
NADH2
2C2H3O-CoA + 2CO2
acetyl-CoA
carbon
dioxide
CoA
Acetic
acid
Pyruvic
acid
CO2
Acetyl-CoA
(acetyl-coenzyme A)
Coenzyme A
Krebs Cycle
• Acetic acid (2C) is added to oxaloacetic
acid (4C) to form citric acid (6C). CO2 is
enzymatically released. This occurs in the
matrix of the mitochondria.
3NAD+6H
3NADH2
2C2H3O-CoA
FAD+2H FADH2 2ADP+P 2ATP
4CO2
Input
Output
Acetic acid
2 CO2
ADP
Krebs
Cycle
3 NAD
FAD
Electron Transport System
• e- are passed along a chain of molecules to O2,
which acts as the final e- acceptor.
– The chain functions as a chemical machine that
uses energy released by the “fall” of electrons to
pump hydrogen ions across the inner mitochondrial membrane
– These ions store potential energy
– When the hydrogen ions flow back through the
membrane, they release energy
34 ADP+Pi
2 H+ + 2e- + ½ O2
34 ATP
H2O
• If the last member of the chain remained in a reduced
state, it would be unable to accept more e-. Etransport would then progress only to the next-to-last
molecule. This process would continue until all of the
elements of the chain remained in the reduced state. At
this point, the system would stop and no ATP could be
produced in the mitochondrion. With the system
incapacitated, NADH2 and FADH2 could not become
oxidized by donating their electrons to the chain and,
through inhibition of Krebs cycle enzymes, no more
NADH2 and FADH2 could be produced in the
mitochondrion. The Krebs cycle would stop and
respiration would become anaerobic..
• Lipids and proteins can also be used in aerobic
Food
respiration.
Polysaccharides
Sugars
Glycerol
Fats
Fatty acids
Proteins
Amino acids
Amino groups
Glycolysis
AcetylCoA
Krebs
Cycle
Electron
Transport
Lipogenesis
• Excess glucose does not complete respiration
but instead is converted into glycerol and
fatty acids. The acetyl-CoA subunits from
the transition reaction are added together to
produce fatty acids. This occurs primarily in
adipose tissue and the liver..
Lipolysis
• Triglycerides are hydrolyzed into glycerol
and free fatty acids (FFA) by lipolysis.
• In some tissues glycerol can be converted
into phosphoglyceraldehyde.
• FFAs are a major energy source and are
metabolized by b-oxidation..
Amino Acids
• Excess amino acids (a.a.) in the diet are not simply
stored as additional protein – instead they are
deaminated and the carbon skeleton is either respired
or converted to carbohydrates or fats.
• Adequate amounts of amino acids are required for
growth and repair. Some a.a. can be make by
rearranging parts of carbohydrates and essential a.a.
A new amino acid can be obtained by transamination.
– Amine group (NH2) transferred from one amino acid to
form another amino acid and a keto acid.
– Catalyzed by a specific enzyme (transaminase)..
• Excess amino acids are processed for
excretion by oxidative deamination. The
amine group is removed and converted to
urea, which is then excreted by the kidneys.
• Not all cells can use glucose as the energy
source.
• Blood contains a variety of energy sources:
– Glucose and ketone bodies, fatty acids, lactic
acid, and amino acids.
• Different tissues preferentially use different
energy molecules.
– Blood [glucose] maintained as many organs
spare glucose.
• Why??