Energy Production In A Cell
(Chapter 25 Metabolism)
Large food molecules contain a lot of potential energy in the form of chemical bonds but it
requires a lot of work to liberate the energy. Cells need a quick easy way to get energy for
anabolism: this is done with ATP. ATP is an unstable molecule, the bonds of which are easy to
break making it a useful source of energy for cells.
ATP → ADP + P + free energy from food
Food energy + ADP + P → ATP
Catabolic reactions generate energy to make ATP, and the ATP energy is used to drive anabolic
reactions, such as metabolic turnover (replacement of cell parts), growth and cell division, and
special functions (such as secretion, absorption, contraction, or signaling).
Metabolism = the sum of all chemical reactions in the body; catabolism + anabolism
All energy production begins in the cytosol of the cell. Large molecules are catabolized into
smaller molecules, but very little energy is produced:
Proteins → amino acids
Triglycerides → fatty acids and glycerol
Carbohydrates → short carbon chains
These smaller molecules are then absorbed and processed in reactions inside the mitochondria.
40% of the energy is captured to produce ATP from ADP and the remaining 60% escapes as heat
(used to maintain body temperature).
Oxidation-Reduction Reactions (Redox Rxns)
Oxidation = the removal of electrons
(or addition of oxygen)
Reduction = the addition of electrons
These reactions are always coupled: one molecule must be oxidized while another is reduced.
A-e’ + B → A + B-e’
(A is oxidized while B is reduced)
The reduced molecule gains energy while the oxidized molecule loses energy.
Cells more commonly perform dehydrogenation reactions where a hydrogen (1 proton + 1
electron) is exchanged instead of a free electron, but this is still a redox reaction.
Catabolism of large molecules results in reduced carrier compounds, which are then oxidized to
generate ATP.
Amy Warenda Czura, Ph.D.
1
SCCC BIO130 Chapter 25 Cellular Respiration Handout
ATP Production
Generation of ATP involves the addition of a phosphate to ADP and can be accomplished one of
two ways:
1. Substrate Level Phosphorylation: a high-energy phosphate is transferred directly from a
substrate to ADP thus forming ATP.
2. Oxidative Phosphorylation: electrons are transferred from an organic compound to a cofactor
carrier molecule (e.g. NAD+). The electrons are passed through other carriers (the electron
transport chain) to a final acceptor (oxygen) and the passing of the electrons releases energy
that is harvested to add a phosphate to ADP in a process called chemiosmosis.
Carbohydrate Catabolism
Carbohydrates are the primary source of cellular energy for most organisms. Glucose is the most
commonly used carbohydrate and will always be used first. Fatty acids will be used second
when glucose is in short supply, and then more rarely amino acids can be utilized. Glucose can
be catabolized for ATP production in two ways:
1. Cellular respiration: requires oxygen to serve as the final electron acceptor in a series of
redox reactions that generate ATP by oxidative phosphorylation. This is the most efficient
method of ATP production (1 glucose generates 36 ATP) and involves reaction performed
inside the mitochondria.
2. Fermentation: requires an organic molecule to serve as the final electron acceptor and can be
done in the absence of oxygen. ATP is synthesized using substrate level phosphorylation,
which is less efficient (1 glucose generates 2 ATP). In humans this results in the production
of lactic acid. Fermentation reactions are carried out in the cytoplasm.
Aerobic Respiration Of Glucose
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
(energy from 1 glucose → 36 ATP)
Three stages of aerobic respiration:
1. Glycolysis: oxidation of glucose to pyruvic acid with some ATP and NADH produced.
2. Citric Acid Cycle: oxidation of acetyl to carbon dioxide with some ATP, NADH and FADH2
produced.
3. Electron Transport Chain: NADH and FADH2 are oxidized providing electrons for redox
reactions ultimately reduce oxygen to generate ATP. The majority of the ATP is produced at
this step.
*(NAD (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) are
coenzymes that function to transport electrons in the form of hydrogen: NAD+ carries 2 electrons
but only one proton, FAD carries 2 complete hydrogen atoms)
Amy Warenda Czura, Ph.D.
2
SCCC BIO130 Chapter 25 Cellular Respiration Handout
Glycolysis
a.k.a the Embden-Myerhof Pathway
Glycolysis is an anaerobic process (does not involve oxygen) that occurs in the cytoplasm. It
consists of a 10-step metabolic pathway that catabolizes and oxidizes 1 glucose molecule into 2
pyruvic acid molecules and generates 2 molecules of ATP by substrate level phosphorylation.
Many cells can survive on glycolysis alone (called fermentation) but it is not very efficient, and
fermentation generates lactic acid as a waste product, which will need to be removed and
processed to prevent a drastic alteration in pH and loss of homeostasis.
Two stages:
1. Preparatory Stage: two ATP molecules are used to phosphorylate one 6-carbon glucose and
catabolize it into two 3-carbon molecules.
2. Energy Conservation Stage: The two 3-carbon molecules are oxidized to generate two 3carbon pyruvic acid molecules. At the same time two NAD+ molecules are reduced to two
NADH molecules and four ATP molecules are produced by substrate level phosphorylation.
Summary of glycolysis:
1 Glucose + 2 NAD+ + 2 ADP + 2 P → 2 Pyruvic acid + 2 NADH + 2 H+ + 2 ATP
Decarboxylation (Preparation for the Citric Acid Cycle)
This is the first step in the aerobic process of glucose metabolism. This and all subsequent steps
(Citric Acid Cycle and Electron Transport) will occur only when oxygen is available and will
take place inside the mitochondria. The 3-carbon pyruvic acid is decarboxylated into carbon
dioxide and a 2-carbon acetyl. The acetyl is attached to Coenzyme A (which serves only as a
carrier) and one NAD+ is reduced to NADH (this will happen twice: 1 glucose generates 2
pyruvic acid molecules). Decarboxylation reactions occur in the matrix of the mitochondria.
Summary of decarboxylation:
2 Pyruvic acid + 2 NAD+ + 2 CoA → 2 Acetyl CoA + 2 CO2 + 2 NADH
Citric Acid Cycle
a.k.a. The Kreb’s Cycle or The Tricarboxylic Acid Cycle
This part of the aerobic metabolism of glucose involves 8 enzymatic reactions occurring in the
mitochondrial matrix that function to reduce the coenzymes NAD+ and FAD. The 2-carbon
acetyl is attached to a 4-carbon oxaloacetic acid creating a 6-carbon citric acid. Oxidation and
decarboxylation reactions occur which catabolize the 6-carbon citric acid back into a 4-carbon
oxaloacetic acid and two carbon dioxide molecules. At the same time three NAD+ and one FAD
are reduced into three NADH and one FADH2 respectively, and one ATP is produced by
substrate level phosphorylation. (Remember: 1 glucose → 2 pyruvic acid → 2 acetyl so this
cycle will run twice.)
Summary of the Citric Acid Cycle:
2 Acetyl Co A + 6 NAD+ + 2 FAD + 2 ADP + 2 P + 4 H2O →
2 CoA + 4 CO2 + 6 NADH + 4 H+ + 2 FADH2 + 2 ATP
Amy Warenda Czura, Ph.D.
3
SCCC BIO130 Chapter 25 Cellular Respiration Handout
Electron Transport Chain
This is the truly aerobic part of the aerobic metabolism of glucose as this is where the oxygen is
utilized. Oxidative phosphorylation occurs on a membrane, the mitochondrial cristae, to
generate most of the ATP produced from glucose. Coenzymes from the previous reactions pass
electrons to a series of electron carrier molecules, which carry out redox reactions resulting in the
chemiosmotic generation of ATP.
There are three classes of carrier molecules:
1. FMN (flavin mononucleotide): protein + flavin coenzyme
2. CoEnzyme Q : nonprotein
3. Cytochromes: protein + an iron group (most common)
Events of the electron transport chain:
1. NAD+ and FAD collected energy in the form of hydrogens (electrons) from organic
molecules during Glycolysis, Decarboxylation, and the Citric Acid Cycle becoming the
reduced forms NADH and FADH2
2. NADH and FADH2 are oxidized and pass hydrogens (electrons and protons) to the electron
transport chain consisting of flavoproteins, cytochromes, and coenzyme Q. As electrons are
passed along the chain, protons are pushed out through the membrane. This sets up a
concentration gradient with protons (positive charge) on the outside and electrons (negative
charge) on the inside.
3. At the end of the chain the electrons are accepted by oxygen creating an anion (O-) inside,
which has a strong affinity for the cations (H+) outside.
4. Chemiosmosis generates ATP: H+ from the outside moves toward O- on the inside through
special membrane channels that are coupled to ATP synthase, and the high-energy diffusion
of H+ drives the reaction ADP + P → ATP. The energy from 1 NADH from glycolysis can
generate 2 ATP, the energy from 1 NADH from decarboxylation and the Citric Acid Cycle
can generate 3 ATP, and that from 1 FADH2 can generate 2 ATP for a total of 32 ATP.
5. H+ combines with O- inside the mitochondria creating water (H2O).
Summary of Electron Transport:
2 NADH from Glycolysis + 2 NADH from Decarboxylation + 6 NADH from Citric Acid Cycle
+ 2 FADH2 from Citric Acid Cycle + 6 O2 + 32 ADP + 32 P →
12 H2O + 32 ATP + 10 NAD+ + 2 FAD
Final Summary For Aerobic Respiration:
C6H12O6 + 6 O2 + 36 ADP + 36 P → 6 CO2 + 6 H2O + 36 ATP
(In most Eukaryotic cells, 36 ATP are produced from 1 glucose molecule: 2 from
Glycolysis and 2 from the Citric Acid Cycle by substrate level phosphorylation, and 32 from
Electron Transport by oxidative phosphorylation. Glycolysis occurs in the cytoplasm, the Citric
Acid Cycle occurs in the matrix of the mitochondria, and Electron Transport occurs on the
cristae of the mitochondria. Some energy is lost as electrons are carried from the cytoplasm
across the mitochondrial membrane and thus the NADH from Glycolysis can generate only four
total ATP instead of the 3 each observed for NADH created inside the mitochondria.)
Amy Warenda Czura, Ph.D.
4
SCCC BIO130 Chapter 25 Cellular Respiration Handout
Cellular Respiration Review:
1. Glycolysis:
- anaerobic, occurs in cytoplasm
- 1 glucose oxidized into 2 pyruvic acids
- 2 ATP produced by substrate level phosphorylation
- 2 NADH produced by reduction of NAD via oxidation of glucose
- If no O2 available, pyruvic acid reduced to lactic acid (fermentation)
- Erythrocytes (RBCs) → glycolysis only (no mitochondria!)
- Skeletal muscle → fermentation when no O2
- Neurons and cardiac muscle cannot ferment, need O2
2. Decarboxylation + Krebs / Citric Acid Cycle:
-occur in matrix of mitochondria
-Decarboxylation:
- 2 pyruvic acid decarboxylated and oxidized into 2 acetyl Co A +
2 CO2 with 2 NADH
-Citric Acid Cycle:
- 2 acetyl combined with 2 oxaloacetic acids creating 2 citric acids
- citric acid decarboxylated and oxidized → 4 CO2, 6 NADH, 2 FADH2
- 2 ATP generated by substrate level phosphorylation
3. Electron Transport:
- aerobic, occurs on cristae of mitochondria
- NADH and FADH2 reduced during glycolysis and citric acid cycle are oxidized
- electrons are passed to cytochromes, finally accepted by oxygen
- 32 ATP generated by chemiosmosis / oxidative phosphorylation
- 12 H2O produced as waste from oxidation of oxygen
* With oxygen, 1 glucose will produce 36 ATP in most human tissue cells.
* Without oxygen, 1 glucose will produce 2 ATP in human tissue cells that are capable of
fermentation (not neurons or cardiac muscle).
Amy Warenda Czura, Ph.D.
5
SCCC BIO130 Chapter 25 Cellular Respiration Handout
Lipolysis (Lipid Catabolism)
The process of lipid catabolism hydrolyzes triglycerides, the storage form of fat, into glycerol
and three fatty acids. The glycerol is converted into pyruvic acid in the cytoplasm and
catabolized through the Citric Acid Cycle in the mitochondria. The fatty acids are catabolized by
Beta-oxidation in the mitochondria to be entered into the Citric Acid Cycle as two-carbon
fragments. For each two-carbon fragment of fatty acid produced by Beta-oxidation, the cell can
generate 17 molecules of ATP. This is 1.5 times the energy production (when compared carbon
to carbon) as with glucose. Although lipolysis generates more energy, it requires more oxygen
and occurs much more slowly than equal carbohydrate metabolism.
Protein and Amino Acid Catabolism
For proteins to be used for energy production, they must first be broken down into individual
amino acids. Then for the amino acids to be used in the Citric Acid Cycle they must first have
the amino group (NH2) removed in a process called deamination, which requires vitamin B6. The
amino group is removed in conjunction with a hydrogen creating ammonia (NH3) which is very
toxic. The liver must then convert the ammonia to urea, which is relatively harmless, for
excretion by the kidney. The remaining amino acid carbon chains are then used at various stages
in the Citric Acid Cycle to generate ATP. The amount of ATP produced varies with the type of
amino acid, depending on which step into the Citric Acid Cycle it was entered.
Catabolism of amino acids is not a practical source of quick energy and is typically only used in
starvation situations. Proteins are harder to break apart than carbohydrates or lipids, their
catabolism generates toxic waste products, and they are the structural and functional parts of
every cell, and thus tend to only be used when no other energy source is available. Instead,
amino acids are usually simply recycled by hydrolysis of peptide bonds in one protein, to be
reassembled by dehydration synthesis into the next.
Nucleic Acid Catabolism
DNA is never catabolized for energy. RNA can be broken down into simple sugars and
nitrogenous bases. The sugars are metabolized in glycolysis but only the pyrimidine bases,
uracil and cytosine, can be processed into the Citric Acid Cycle. The purines, adenine and
guanine, are deaminated and excreted as uric acid making RNA metabolism very inefficient.
Typically nucleotides are simply recycled into new nucleic acid molecules and are not used for
energy production.
Amy Warenda Czura, Ph.D.
6
SCCC BIO130 Chapter 25 Cellular Respiration Handout