Chapter 7 How Cells Harvest Energy
7.2: Glycolysis – Splitting Glucose
(Page 128; www.sparknotes.com/biology/cellrespiration/glycolysis/section1.html)
STAGE 1 – Priming (Endergonic Stage)
STEP 1:
1. Phosphorylation of glucose (REACTANT)
2. 1 molecule of ATP consumed (ACTIVATION ENERGY) Reason Step1 is
Endergonic!
3. Creates G6P (called this because 6th carbon of glucose acquires the phosphate group)
(PRODUCT)
4. Requires Mg (magnesium) to shield negative charges from phosphate group on the ATP
molecule. (COFACTOR)
5. Enzyme – hexokinase (CATALYST)
6. ADP + H (removed from hydroxyl group so phosphate group can be attached)
(PRODUCT)
STEP 2:
1. Isomerization - Convert G6P to fructose-6-phosphate (F6P) (REACTANT)
2. Enzyme – phosphoglucose isomerase (PI)
3. Isomerization reaction: reaction in which compound is transformed into any of its isomeric
forms. (Fructose is the isomer of glucose. Isomer – same formula; different structure)
4.
REACTANT
PRODUCT
G6P
F6P
STEP 3: Phosphorylation
1. F6P converted to fructose-1,6-biphosphate (FBP) (REACTANT)
2. 1 molecule of ATP (ACTIVATION ENERGY)
3. 2nd phosphate added to F6P to make FBP (PRODUCT)
4. Enzyme – phosphofructokinase (PFK) (CATALYST)
5. Requires Mg (see # 4 in Step 1) (COFACTOR)
6.
NOTE:1. STEP 1 is the phosphorylation that traps glucose in the cells.
2. Phosphorylation provides energy necessary to make the molecule of glucose
reactive– pushes over the energy threshold.
STAGE 2 – Cleavage
STEP 4:
1. Cleavage of FBP (REACTANT) into two 3-carbon molecules
a. G3P (glyceraldehyde-3-phosphate) (PRODUCTS)
b. Dihydroxyacetone phosphate (DHAP) (PRODUCTS)
2. Enzyme – aldolase (CATALYST)
3. DHAP molecule reorganized into GAP in reaction catalyzed by (TIM) triphosphate
isomerase. (ISOMERIZATION REACTION)
4. NOTE: glucose still not fully converted to pyruvate
STAGE 3 – Oxidation and ATP Formation
STEP 5: Exergonic Reaction
1. G3P oxidized = loses 2 e- + 1 proton (these are transferred to NAD+)
2. NAD (nicotinamide adenine dinucleotide) = NADH
3. Enzyme – GAPDH (glyceraldehydes – 3 – phosphate dehydrogenase)
4. G3P phosphorylated by addition of free Pi to form 1,3-bisphosphoglycerate (BPG)
(1 high-energy phosphate bond formed)
STEP 6:
1. BPG molecules attach to enzyme phosphoglycerate kinase
2. High-energy phosphate removed by (2) ADP
3. 2 ATP produced
4. Remaining form of sugar – 3-phosphoglycerate (3PG)
STEP 7:
1. 3PG attaches to enzyme – phosphoglyceromutase
2. Phosphate is removed
3. 2-phosphoglycerate (2PG) produced
STEP 8:
1.
2.
3.
a.
2PG attaches to enzyme – enolase
2 waters removed (H2O)
Yields (2) PEP molecules with high energy phosphate bonds
PEP = phosphoenolpyruvate
STEP 9:
1. 2 PEP (REACTANT) attach to pyruvate kinase (ENZYME)
2. ADP removes high-energy Phosphate from PEP molecules
3. Pyruvate + (2) ATP yielded (PRODUCT)
Net yield of glycolysis of 1 glucoses molecule =
2 molecules of pyruvate
2 ATP
2 NADH
2 H2O
End products move on to continue in cellular respiration (2 pyruvate), other metabolic reactions
(2ATP, 2 NADH), or removed as waste (2 H20)
7.3: Oxidation of Pyruvate to Produce Acetyl-CoA
1. End product of glycolysis, pyruvate, becomes the reactant for this metabolic chemical
reaction.
2. Reaction occurs in the presence of oxygen, which acts at the terminal electron acceptor
3. Location:
a. Takes place inside the mitochondria in eukaryotic cells
b. Takes place in the cytoplasm or plasma membrane in prokaryotic cells
4. Two Steps used to harvest energy in pyruvate:
STEP 1:
1. Transport protein brings pyruvate to the inner mitochondrial membrane
2. Enzyme removes one carbon from Pyruvate. (decarboxylation)
3. The carbon is released as CO2.
4. Pyruvate is now a 2-carbon compound called an acetyl group.
STEP 2:
5. Acetyl group is attached to coenzyme A, which is now called Acetyl-CoA.
6. A pair of electrons and one associated proton transferred to the electron carrier NAD+
to form NADH.
7. NET PRODUCT OF Oxydation of Pyruvate
a. 2 CO2
b. 2 NADH
c. 2 Acetyl Co-A
(Diagram 7.9, page 130, is a good visual reference to use as a review.)
NOTE: Tracking the carbons: Begin oxidation of pyruvate with 3 carbons on each pyruvate.
One carbon is removed from each pyruvate molecule, leaving 2 2-carbon molecules (acetyl) to
continue in the reaction. The two carbons that are removed are released as 2 CO 2 molecules.
7.4: The Krebs Cycle (AKA: Citric Acid Cycle)
1. 2 Acetyl Co-A molecules are the reactants for the Krebs Cycle.
2. Location: Within the mitochondrial matrix
3. Acetyl group is oxidized in series of 9 reactions called the Krebs Cycle
a. Reaction 1 – Condensation: 2-carbon acetyl-CoA combine with 4-carbon
oxaloacetate to form 6-carbon citrate molecule; Coenzyme A (CoA) is released to
be used again to make acetyl-CoA
b. Reaction 2 and 3 – Isomerization: Citrate is transformed to isocitrate(6-carbon
molecule)
c. Reaction 4 – First Oxidation: 1st energy yielding step; Isocitrate oxidized yielding pair
of electrons and one proton which are used to reduce NAD+ to NADH; oxidized
isocitrate undergoes decarboxylation to produce CO2; remaining intermediate is a
5-carbon alpha-ketoglutarate molecule
d. Reaction 5 – Second Oxidation: Alpha-ketoglutarate decarboxylized to release
another CO2; remaining 4-carbon intermediate, succinate, joins with coenzyme-A to
form succinyl-CoA, a 4-carbon molecule; two electrons extracted during process
used to reduce NAD+ to NADH.
e. Reaction 6 – Substrate level phosphorylation: linkage between 4-carbon succinyl
group with CoA is a high-energy bond; the bond is cleaved and energy released
driving phosporylation of ADP to produce ATP; 4-carbon succinate remains in Krebs
Cycle.
f. Reaction 7 – Third Oxidation: 4-carbon succinate oxidized, transferring electrons to
FAD+(electron carrier) in the inner mitochondrial membrane, thereby reducing FAD+
to FADH2; remaining intermediate is 4-carbon fumarate. (NOTE: FADH2 is so
tightly associated with its enzyme in the inner mitochondrial membrane, it can
only contribute electrons to the electron transport chain.)
g. Reaction 8 and 9 – Regeneration of Oxaloacetate: Addition of water to fumarate
creates malate, 4-carbon molecule; malate is then oxidized, yielding a 4-carbon
molecule of oxaloacetate and two electrons that reduce a molecule of NAD+ to
NADH. Oxaloacetate free to combine with another 2-carbon acetyl group from acetylCoA to begin the cycle again.
4.NET PRODUCTS OF KREBS CYCLE:
a. Per one pyruvate
3 NADH
1 FADH2
1 ATP
2 CO2
b. Per two pyruvate
6 NADH
2 FADH2
2 ATP
4 CO2
(Diagram 7.11, page 133, is a good visual reference to use as a review and will help you visual
how the carbons are removed from each intermediate carbon molecule as you move through
the Krebs Cycle. PLEASE REVIEW)
REMEMBER: During Aerobic respiration, glucose is entirely consumed to harvest its energy.
7.5 The Electron Transport Chain and Chemiosmosis (Refer to Fig. 7.12 )
A. Electron Transport Chain (ETC)
1. NADH and FADH2 molecules carry the pair of electrons each picked up when NAD and FAD
were reduced, to the inner mitochondrial membrane where the pair of electrons are transferred
to a series of membrane-associated proteins collectively called the electron transport chain.
2.Donated electrons flow across the electron transport chain (ETC) from complex to complex.
3. Hydrogen are pumped out by each complex and enter the intermembrane space.
4. Spent electrons are picked up by the terminal electron acceptor (TEA), which is oxygen for
aerobic respiration.
5. Oxygen also picks up hydrogen and water is produced.
B. Chemiosmosis
1. Hydrogen ions (H+) are allowed to come back in through ATP synthase, which uses the
energy from the proton gradient produced by the H+ to synthesize ATP from ADP and Pi.
2. O2 plus the 10 NADH and 2 FADH2 from Glycolysis, Oxidation of pyruvate and Krebs yields
34 ATP and 4 H2O.
3. This is oxidative phosphorylation - a metabolic pathway that generates ATP from ADP
through phosphorylation that derives the energy from the oxidation of nutrients; an enzymatic
process that occurs in both prokaryotes and eukaryotes. In eukaryotes, the process occurs as
part of cellular respiration within the mitochondrion. In prokaryotes, it occurs in the cell
membrane itself. This process is a more efficient method to produce ATP (in terms of net ATP
yield) than fermentation.
7.6 Energy Yield of Aerobic Respiration
A. Total Net Yield from Aerobic Respiration
1. Glycolysis – 2 ATP
2. Krebs – 2 ATP
3. ETC/Chemiosmosis – 34 ATP
4. 38 is the total theoretical yield of ATP at the end of cellular respiration of one glucose
molecule.
5. Actual yield is 30 ATP in eukaryotes
6. Energy yield can fluctuate.
7. NADH from glycolysis can’t enter the mitochondrion without the use of energy from an
ATP molecule;It must pass its electrons to transport proteins. This costs ATP.
8. NADH passes electrons at a point on the chain that produces 3 ATP.
9. FADH2 makes only 2 ATP.
7.7 Regulation of Aerobic Respiration
1. Cells control the rate of cellular respiration through a system of feedback inhibition.
2. When ATP levels are high, the metabolic pathway of cellular respiration is shut down and
ATP production stops.
3. As ATP is used by the cell, it triggers the production of more when ADP activates enzymes in
the pathways of carbohydrate catabolism to stimulate the production of more ATP.
4. See Fig. 7.17
7.8 Oxidation without Oxygen
1. Anaerobic cellular respiration:
A. Uses the same metabolic pathway (glycolysis, pyruvate
oxidation, Krebs Cycle, ETC, and Chemiosmosis).
B. It does not use oxygen as its terminal electron acceptor. TEA
C. Sulfur, nitrate, carbon dioxide or even inorganic metals serve as TEAs.
2. Fermentation
A. Starts with glycolysis/
B. Glycolysis yields 2 ATP, 2 pyruvate, 2 NADH
C. Uses organic substance formed from glucose as the TEA
D. Regenerates NAD+
3. Two types of Fermentation
A. Alcoholic fermentation:
1. Pyruvate is converted to acetaldehyde
2. Acetaldehyde accepts electrons from NADH and becomes ethanol (alcohol)
3. CO2 is produced
4. Prominent in baking and brewing industries
a. Baking – CO2 makes bread light and airy, ethanol evaporates away
b. Brewing – yeast feed on sugar in grapes and produce ethanol
B. Lactate fermentation:
1. Pyruvate accepts electrons from NADH
2. This regenerates NAD+
3. Electron transfer converts pyruvate to lactate
4. Used to produce yogurt, cheese, sauerkraut, etc.
5. Muscles use when there is an O2 deficit
6. Lactate can be converted back to pyruvate when O2 is available again
7. Occurs in animals, some fungi and some bacteria
EXAMPLE: Our muscles have slow-twitch muscle fibers for prolonged activity using aerobic
respiration. These cells are darkened due to the large numbers of myoglobin (bind and store
oxygen). Our muscles also have fast-twitch muscle fibers for short bursts of immediate intense
use. These cells are paler and perform anaerobic fermentation when needed. Muscle
composition is the difference between great sprinters and great marathon runners.
7.9 Catabolism of Proteins and Fats
Energy can be acquired from proteins and lipids.
1. Nucleic Acids: nitrogenous bases are converted to nitrogeneous waste; carbons enter
glycolysis or Krebs Cycle.
2. Proteins: amino group (nitrogen group) is removed as waste and carbon backbone goes into
glycolysis and/or Krebs Cycle.
3. Fats: glycerol goes into glycolysis and carbon backbone goes into Krebs.