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Complex chemical transformations in the cell do not occur in a single reaction, but in a number of small steps that are connected in a
pathway
Each reaction is catalyzed by a specific enzyme
Metabolic pathways are similar in all organisms
Many metabolic pathways are compartmentalized, with certain steps occuring inside an organelle
Metabolic pathways in organisms are regulated by the activites of a few enzymes
I. Obtaining Energy and Electrons from Glucose (C6H12O6)
A. Cells trap free energy while metabolizing glucose
1. C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (heat and light)
2. Three metabolic processes utilize glucose for energy
a. Glycolysis
b. Cellular respiration
c. Fermentation
B. Redox reactions transfer electrons and produce energy
1. When an atom gains one or more electrons, = reduction
2. The loss of one or more electrons= oxidation
3. Some think of it in terms of H atoms instead
4. Oxidation and reduction always occur together
C. The coenzyme NAD (nictotinamide adenine dinucleotide) is a key electron carrier in redox reactions
1. NAD comes in two chemically distinct forms
a. NAD+
(oxidized)
b. NADH + H+ (reduced)
c. Which one is oxidized and which one is reduced?
2. NAD acts as an energy intermediary
II. An Overview: Releasing Energy from Glucose
A. Glycolysis begins both major energy pathways
B. If oxygen is present (cellular respiration)
1. Glycolysis
2. Cellular respiraton
a. pyruvate oxidation
b. citric acid cycle
c. electron transport chain
3. Eukaryotes- glycolysis- outside mitochondria, rest associated with matrix or plasma membrane of mitochondria.
4. Prokaryotes- steps occur either in the cytosol or inner surface of the plasma membrane
C. If oxygen is not present
1. Glycolysis
2. Fermentation  lactic acid and carbon dioxide
III. Glycolysis: From Glucose to Pyruvate (cytoplasm)
A. The energy-investing reactions of glycolysis require ATP
1. 2 molecules of ATP are used to add phosphate groups to the modified glucose molecule
2. The six carbon molecule is broken into 2, 3 carbon molecule products (each with one phosphate group) (G3P or glyceraldehyde 3phosphate)
B. The energy-harvesting reactions of glycolysis yield ATP and NADH + H+
1. Free phosphate group is attached to 3 carbon sugar molecule, two molecules of NADH + H + are formed
2. Two phosphates (1 from each molecule) is transferred to ADP to yield ATP.
3. Further modification of the 3 carbon sugars finally release two more phosphate groups to form 2 more molecules of ATP.
4. Net yield= 2 ATP's and 2 Pyruvates and 2 NADH
IV. Pyruvate Oxidation
A. Pyruvate moves into the inner mitochondrial matrix (enzyme attached to inner membrane)
1. Pyruvate looses 2 hydrogen atoms (oxidized) and a carboxyl group that yields a two carbon acetyl group. Carbon dioxide is released
2. The acetyl group is linked to a coenzyme called coenzyme A (CoA) to form acetyl CoA.
3. Part of the energy from the oxidation is saved by the reduction of NAD+ to NADH + H+ ( in this reaction, how many electrons
is NAD+ accepting?)
4. Glycolysis and Pyruvate oxidation overview
V. The Citric Acid Cycle
A. AcetylCoA enters the cycle and combines with oxaloacetate to form a six carbon citrate (citric acid)
B. Rearrangement of the molecule yield NADH and CO2
C. Loss of the CoA drives GDP to GTP which drives ADP to ATP
D. FAD is reduced to FADH2
E. More rearrangements produce NADH and oxaloacetate
F. Net Yield= 6 NADH, 2 ATP, and CO2
G. Biggest yield is the trapping of electrons to use in the respiratory chain
VI. The Respiratory Chain: Electrons, Proton Pumping, and ATP- inner membrane of mitochondria
A. The respiratory chain transports electrons and releases energy
1. NADH, FADH2 leave matrix and enter the respiratory chain
2. Protein complexes imbedded or associated with membrane act as channels and/or enzymes
3. NADH and FADH2 are oxidized which feeds the associated "freed" electrons into the membrane.
B. Active proton transport is followed by diffusion coupled to ATP synthesis
1. H+ (protons) are able to move through the channels and into the area between the inner and outer membrane
2. The intermembrane space and the matrix are NOT in equilibrium. The protons desperately want to move back into the matrix
3. Protons return to the matrix by passing through an ATP synthase. This energy drives the formation of ATP.
4. This all occurs in the presence of oxygen and is termed oxidative phosphorylation
5. Net yield 32 ATP, H2O
C. Two experiments demonstrate the Chemiosmotic Mechanism
1. Experiment 1
2. Experiment 2
VII. Fermentation: ATP from Glucose, without O2
A. Some fermenting cells produce lactic acid and others produce alcohol
B. Yield of ATP is low (only what glycolysis can produce using the oxidized NAD+
VIII. Contrasting Energy Yields
A. Total Yield from Cellular respiration with glucose as the starting molecule= 36ATP
B. Yields vary depending on starting molecule
IX. Metabolic Pathways
A. Catabolism and anabolism involve interconversions using carbon skeletons
1. Polysaccharides, lipids and proteins can all be catabolized for energy
2. Many intermediates of cellular respiration can be shuttled to form glycogen, fatty acids, proteins or even nucleotides
B. Catabolism and anabolism are integrated
1. Your body tries to be efficient and use these molecules in your best interest
2. For example, fats store energy very efficiently and are used as such unless not enough food is available.
3. Proteins are needed for so many processes that it is not efficient to use them in cellular respiration unless deficient in glucose
4. Once protein reserves are used up, critical proteins will be broken down
X. Regulating Energy Pathways
A. Allostery regulates metabolism
1. Positive and negative feedback determines path of reaction
2. Example, feedback regulation using products of the citric acid cycle to inhibit steps in glycolysis
B. Evolution has led to metabolic efficiency