UNIT 2: Metabolic Processes
Chapter 4: Cellular Respiration
pg. 166 - 209
4.3: The Efficiency and Regulation of Cellular Respiration
pg. 183 – 189
The Efficiency of Cellular Respiration
The efficiency of oxidative phosphorylation is difficult to calculate.
It has been determined that for every NADH that enters the electron
transport chain there are 3 ATP molecules synthesized.
Also for every FADH2 that enters, only 2 ATP molecules are synthesized.
In the first 3 stages of cellular respiration there are 4 ATP synthesized by
substrate-level phosphorylation. There are 10 NADH molecules created, and
2 molecules of FADH2 that will travel into the electron transport chain.
The first two NADH produced in glycolysis only have the potential to
synthesize 2 ATP, they lose energy as they cross the outer and inner
membranes to get to the electron transport chain.
(NADH = 3 ATP and FADH2 = 2 ATP)
Prokaryote (malate-aspartate Eukaryote (glycerol-phosphate
shuttle)
shuttle)
Glycolysis:
2 ATP
2 ATP
Citric Acid Cycle:
2 ATP
2 ATP
Electron Transport:
34 ATP
32 ATP
Total
38 ATP
36 ATP
Figure 1: Total ATP Yield, total ATP yield by aerobic cellular respiration from the complete
oxidation of one molecule of glucose. This tally assumes an energy-efficient transfer of energy from
an NADH to the electron transport chain, including the use of the malate-aspartate shuffle.
pg. 184
The efficiency of ATP phosphorylation of ATP yields about 31 KJ/mol.
When assuming the complete oxidation of glucose produces 38 ATP, then
the total amount of potential energy stored is 1178 KJ/mol. If glucose
releases 2870 KJ/mol of energy and the amount stored is 1178 KJ/mol, then
the rest is lost as thermal energy. This expresses 41.0% efficiency.
Creatine Phosphate
Maintaining a constant supply of ATP is important to the survival of the cell.
Some cells require different amounts of ATP at different times to function
properly, muscle and brain cells. The cells use Creatine phosphate at times
of fluctuation. Extra ATP at low times of energy needs is used to
phosphorylate Creatine phosphate. The Creatine phosphate is then stored in
the cell and at times of need the Creatine phosphate is used to produce ATP.
creatine + ATP → creatine phosphate + ADP
creatine phosphate → creatine + ATP
Metabolic Rates
Metabolic rate – is the amount of energy that is expended per unit time in an
organism.
Basal Metabolic Rate (BMR) – is the metabolic rate of an organism at rest.
The amount of energy that a person or cell uses depends on their activities at
a given time. Energy demands will either increase or decrease depending on
the work being done at a given time.
A human at rest usually uses energy to breath, maintain body temperature,
digest food, think, and produce wastes. The basal metabolic rate (BMR) of
an individual is the amount of energy used at a state of rest. The BMR is
approximately 60 – 70% of the total daily energy used by the body. The unit
of measure of BMR is KJ/m2/h.
BMR varies from individual to individual and throughout a person’s lifetime.
Regulating Cellular Respiration
ATP synthesis is regulated by enzymes and transport systems, and it ability
to meet the cells requirements. This is based on supply and demand. The cell
does not waste valuable resources, and to control its supply and demand it is
reliant on a feedback system. For cellular respiration, the cell uses feedback
inhibition, where the end products of the metabolic pathway will inhibit an
enzyme earlier in the pathway preventing the metabolic reaction from
continuing.
ATP, NADH, and citrate allosterically inhibit the enzyme
phosphofructokinase in glycolysis.
Figure 2: Feedback Inhibition, pg. 186
Alternative to Glucose
Alternative energy sources can be used to produce ATP. Carbohydrates are
the primary energy source used by cells in cellular respiration, but lipids and
proteins can also enter the cellular respiratory pathways to produce ATP.
Lipid Catabolism is the break down of triglycerides into a glycerol group
and three fatty acid chains.
The glycerol portion (3 carbons) is converted into glucose by a process
called gluconeogenesis, or a molecule of dihydroxyacetone (DHAP). The
DHAP is then converted to 3-phosphoglycerate (G3P), which then enters
glycolysis at step 6.
The fatty acids (12 carbons) are transported into the mitochondrial matrix
where acetyl groups (2 carbons) are removed and combined with coenzyme
A, by a series of enzymes in a process called beta-oxidation.
Beta-oxidation – is a process in which fatty acids are broken down into
acetyl-CoA through catabolism.
The acetyl-CoA molecules can enter the citric acid cycle in the matrix.
Protein Catabolism is the break down of proteins into individual amino acids.
Through a process of deamination, amino groups are removed from the
amino acid molecules. The amino groups are converted to ammonia (NH3)
which is a waste product that is eventually removed via the Excretory
System in urine. The remaining portion, 2 carbon chain, and the radical
group, are converted into various components of glycolysis and the citric
acid cycle.
Deamination – is the first step in protein catabolism, involving the removal
of the amino group of an amino acid as ammonia.
For example; leucine is converted into acetyl-CoA, alanine is converted into
pyruvate, and proline is converted into α-ketoglutarate.
Figure 3: Alternative Pathways, this diagram shows the major pathways that oxidize carbohydrates,
fats, and proteins. CoA funnels the products to many oxidative pathways into the citric acid cycle. pg.
187