UNIT 2: Metabolic Processes
Chapter 4: Cellular Respiration
pg. 166 - 209
4.1: Introduction to Cellular Respiration and Fermentation
pg. 168 – 171
The energy which keeps our planet alive comes from the solar energy
radiating from the Sun. When sunlight enters our biosphere, it is not usable
by all organisms. The sunlight must be transformed into usable chemical
potential energy, this is achieved by a process called photosynthesis. Green
plants and certain algae are capable of photosynthesis.
Heterotrophs must eat plants to obtain this chemical potential energy to
survive.
All living organism must use this chemical potential energy to survive. The
energy is released through a process called cellular respiration, all living
thing cellular respire.
Figure 1: The flow of energy from sunlight to ATP, pg. 168
Aerobic Respiration
Aerobic respiration is the breakdown of glucose in the presence of oxygen
producing water, carbon dioxide and ATP. The ATP is then used by the cells
to perform their cellular metabolic functions.
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
∆G = -2870 KJ/mol
Aerobic Cellular Respiration – is a process that uses oxygen to harvest
energy from organic components.
Obligate aerobe – is an organism that cannot live without oxygen.
Most eukaryotes, plants and animals, and some prokaryotes are obligate
aerobes, requiring oxygen to live. They oxidize glucose to obtain free energy,
but they can also use lipids and proteins to obtain energy.
There are four stages of cellular respiration that take place to produce ATP,
either by substrate-level or oxidative phosphorylation.
1. Glycolysis (in cytosol). Enzymes break down one molecule of glucose
into two molecules of pyruvate. Some high-energy ATP (via
substrate-level Phosphorylation) and NADH are synthesized.
2. Pyruvate oxidation (in mitochondria). Each of the two molecules of
pyruvate produced in glycolysis is transported to the mitochondria and
is oxidized, resulting in the production of CO2 (a waste molecule),
NADH, and an acetyl group that is initially attached to coenzyme A
(acetyl-CoA).
3. Citric acid cycle (in mitochondria). Acetyl-CoA molecules from
pyruvate oxidation enter a metabolic cycle, where the acetyl group is
completely oxidized to CO2. In the process, ATP (via substrate-level
Phosphorylation) and the electron carriers NADH and FADH2 are
synthesized.
4. Electron transport and oxidative Phosphorylation (in
mitochondria). The NADH and FADH2 (synthesized during glycolysis,
pyruvate oxidation, and citric acid cycle) are oxidized. Their highenergy electrons and hydrogens are passed from one oxidizing agent
to the next until they are transferred to O2, producing water. The free
energy released during electron transport is indirectly used to
synthesize a large amount of ATP by oxidative Phosphorylation.
Figure 2: Cellular Respiration, a simplified flow diagram shows the four stages of Cellular
respiration. pg. 169
Substrate-level phosphorylation – is the formation of ATP by the direct
transfer of a phosphate group from a substrate to ADP.
Oxidative phosphorylation – is a process that forms ATP using energy
transferred that forms ATP using energy transferred indirectly from a series
of redox reactions.
Glycolysis – is a series of reactions in which a glucose molecule is broken
into two pyruvate molecules and energy is released.
Pyruvate oxidation - is a reaction in which pyruvate is oxidized by NAD+,
and CO2 is removed, forming an acetyl group and releasing NADH.
Citric Acid Cycle – is a cyclic series of reactions that transfer energy from
organic molecules to ATP, NADH, and FADH2 and releases carbon atoms
as CO2.
The Mitochondrion
The mitochondrion is a membrane bound organelle, and is the power house
of the cell, producing ATP. There are four steps to cellular respiration. Three
of the four occur in the mitochondrion (pyruvate oxidation, citric acid cycle,
and electron transport) and one in the cytosol (glycolysis).
The mitochondrion is composed outer membrane, inner membrane (cristae),
inter-membrane space, and the matrix.
Figure 3: Mitochondrion, the membranes and compartments, pg. 170
In prokaryotes, the four processes of cellular respiration take place in the
cytosol. Prokaryotes do not have membrane bound organelles.
Anaerobic Pathways
There are more then one biochemical pathways to extract energy from food
molecules. Aerobic respiration requires oxygen, and anaerobic respiration
does not require the presence of oxygen to obtain chemical potential energy.
There are two pathways, anaerobic respiration (bacteria) and fermentation
(plant).
Anaerobic Respiration – is a process that uses a final inorganic oxidizing
agent other than oxygen to produce energy.
Fermentation – is a process that uses an organic compound as the final
oxidizing agent to produce energy.
Obligate anaerobic – is an organism that cannot survive in the presence of
oxygen.
Facultative anaerobe – is an organism that can live or without oxygen.