Metabolic Pathways (8) Flashcards
△S < 0
The entropy has decreased (the disorder of the system has decreased).
- △G > 0
△G > 0
The potential energy has increased.
Energy in Covalent Bonds
Potential energy is stored in covalent bonds. The energy is stored in the electrons.
Covalent Bond
The sharing of two electrons between two atoms.
Entropy of Electrons in a Covalent Bond
The electrons have a certain degree of entropy but because the orbitals overlap in a covalent bond, there is less entropy.
Chemical energy of strong bonds
Strong bonds have a low chemical energy.
Chemical energy of weak bonds
Weak bonds have a high chemical energy.
Redox Reactions (Oxidation-Reduction Reactions)
Electrons are transferred from one atom to another. They are chemical reactions in which one of the reactants becomes oxidized and the other reactant becomes reduced. They are two reactions that always occur together.
In the cell, glucose is metabolized to…
Carbon dioxide, water, and energy.
C6H12O6 + 6O2 –> 6CO2 + 6H2O + energy
The energy yielding steps in the metabolism of glucose are…
Redox reactions. During the metabolism of glucose, glucose donates electrons. Glucose is oxidized while oxygen accepts electrons and is reduced.
The transfer of electrons…
This transfer of electrons is a transfer of energy. Transferring electrons from one molecule to another is a means by which energy can be transferred from one molecule to another.
Oxidized
Reactant that loses an electron in a redox reaction. This loss of electrons can be outright to form an ion or the electrons may be shared with a substance that has a greater affinity for the electrons, such as oxygen.
Reduced
Reactant that gains an electron in a redox reaction.
Oxidizing Agent
The reactant that accepts an electron or a hydrogen atom.
Reducing Agent
The reactant that donates an electron or a hydrogen atom.
Glucose Oxidation
During the metabolism of glucose, glucose donates electrons, glucose is oxidized. The electrons of glucose are first passed to an electron carrier.
Electron carriers of the cell
The cell has two electron carriers; NAD+ or FAD which accept the electrons from glucose and transfer them to the mitochondria, where ATP is synthesized.
Nicotinamide Adenine Dinucleotide (NAD)
A coenzyme that is an essential electron carrier in cellular redox reactions. It can be oxidized or reduced.
Oxidized form of NAD
NAD+
- Accepts two electrons + 1 hydrogen ion (H+) from two hydrogen atoms
Reduced form of NAD
NADH + H+
Reduction of NAD+
The reduction of NAD+ Requires an input of energy (endergonic).
NAD+ + 2H + energy -> NADH + H+
Oxidation of NADH + H+
The oxidation of NADH + H+ is exergonic.
NADH + H+ –> NAD+ + energy
Flavin Adenine Dinucleotide (FAD)
Electron transporter in cellular redox reactions.
FAD + 2H FADH2
Movement of Chemical Reactions
In principal, reactions can run in both directions.
Reactants Products
The direction each reaction goes depends on the concentration of products versus substrates. If there are more substrates, the reaction will move forward. If there are more products, the reaction will move in reverse.
Equilibrium in a Reaction
When the rate of the forward reaction is equal to the rate of the reverse reaction, the reaction is in equilibrium. This does not mean that there are equal concentrations of reactants and products.
Key Enzymes
Regulate the activity of each metabolic pathway. The production of D can be regulated by limiting the availability of B.
ABCD
E1 E2 E3
Metabolic Pathways
Metabolic pathways are a series of chemical reactions where the product of the first reaction serves as the substrate for the second reaction. The addition of A will push the reaction to the right, producing B. B is the substrate for the next reaction, so the reaction will continue toward the production of C. Metabolic pathways are similar in all organisms.
ABCD
E1 E2 E3
Enzymes of Metabolic Pathways
Each reaction is catalyzed by a separate enzyme.
ABCD
E1 E2 E3
Metabolic Pathways in the Aerobic Respiration of Glucose
- Glycolysis
- The Kreb’s Cycle
- Oxidative Phophorylation
Metabolic Pathways in Eukaryotes
Many of the metabolic pathways are compartmentalized in organelles.
Glucose
The sugar C6H12O6 is the most common form of energy molecule. The energy in glucose is stored in the chemical bonds.
Released from Glucose when heated or metabolized
Glucose releases CO2, H2O, and heat (energy).
Chemical Bonds of Glucose
The chemical bonds of glucose are broken to release energy (exergonic).
Basic Steps in Creating ATP
- Glycolysis (Glucose 6-Carbon)
- Pyruvate Oxidation (Pyruvate 3-Carbon)
- Kreb’s Cycle
- Acetyl CoA (2-Carbon + CO2)
- NADH + 2CO2 - Respiratory Chain
- ATP
ATP Produced from Glucose Molecule
Complete aerobic metabolism of one glucose molecule will produce 36 ATP.
Glycolysis
Begins glucose metabolism in all cells. A 6-Carbon glucose molecule is converted to two 3-Carbon pyruvate molecules. This produces 2 ATP molecules.
Location of Glycolysis
Occurs in the cytoplasm.
Two Stages of Glycolysis
- Energy-investing reactions that use ATP
2. Energy-harvesting reactions that produce ATP
Energy-Investing Reactions of Glycolysis
- Glucose Molecule
- ATP —> ADP + Pi
- Glucose 6-phosphate (glucose molecule with phosphate group attached)
- Fructose 6-phosphate
- ATP —-> ADP + Pi
- Fructose 1,6 biphosphate (glucose molecule with two phosphate groups attached)
- Aldolase splits the 6-carbon molecule into two 3-carbon molecules that become glyceraldehyde 3-phosphate (G3P) if more ATP will be made or dihydroxyacetone phosphate (DAP) if no more ATP is needed
- NAD+ —> NADH
- ADP —-> ATP
- ADP —> ATP
- G3P becomes a pyruvate molecule. This occurs twice (steps 8-10) since there are two molecules.
Aldolase
An enzyme that splits the molecule into two 3-C molecules that become glyceraldehyde 3-phosphate (G3P).
G3P and DAP
Isomerase of one another. G3P continues the reaction while DAP stops the reaction. If G3P is made, two of the molecules are made.
Energy-Harvesting Reaction of Glycolysis
Occurs after the energy-investing reactions.
- An oxidation reaction occurs, releasing free energy that is used to make two molecules of NADH + H+, one for each of the G3P molecules.
- The final product of glycolsis is two 3-C molecules of pyruvate. The next step is pyruvate oxidation.
Substrate-Level Phosphorylation
The generation of ATP when the glucose molecules become pyruvate molecules. In addition, for each glucose molecule 2 NAD+ are reduced to NADH and 4 ATP molecules are generated from ADP. In substrate level phosphorylation, a phosphate group (P) is transferred from one molecule to ADP to form ATP.
Phosphofructokinase (PFK)
The rate limiting enzyme. A step that occurs between Fructose-6-Phosphate (F6P) and Fructose 1,6-biphosphate (FBP). It is the main control point in glycolysis and is inhibited by ATP and activated by ADP and AMP.
Metabolic Homeostasis
The levels of the products and substrates of energy metabolism are constant. Cells regulate the enzymes of catabolism and anabolism to maintain balance. Metabolic pathways work together to maintain cellular homeostasis.
Positive and Negative Feedback
Control whether a molecule of glucose is used in anabolic or catabolic pathways.
If there is no ATP
Glucose is used for energy
If there is plenty of ATP
Glucose is used to synthesize molecules, usually fat.
Allosteric Control
The amount and balance of products a cell has is regulated by allosteric control of enzyme activities. Control points use both positive and negative feedback mechanisms.
The final product of glycolsis
The final product of glycolysis is two 3-C molecules of pyruvate.
Pyruvate Oxidation
Occurs in the mitochondria and starts with two 3-C molecules of pyruvate.
14. Pyruvate (3C) is oxidized to acetyl CoA (2C) and a CO2 molecule. One NAD is reduced to NADH + H+ during this reaction for each pyruvate.
Acetyl CoA
The only molecule that can enter at the start of the Kreb’s Cycle.
Pyruvate Dehydrogenase
Pyruvate oxidation is a multistep reaction catalyzed by an enzyme complex (pyruvate dehydrogenase) attached to the inner mitochondrial membrane.
Molecules Generated by the Kreb’s Cycle
For each turn of the Kreb’s cycle (for each acetyl-CoA), the following molecules are generated:
3 NADH + H+
1 ATP
1 FADH2
2 CO2
For one glucose molecule, two acetyl CoA molecules are generated.
Isocitrate Dehydrogenase
The rate limiting enzyme during the Kreb’s cycle. Used between the 6-C molecule and the 5-C molecule. It converts isocitrate to a-ketoglutarate.
Isocitrate Dehydrogenase is inhibited by…
NADH + H+ and ATP
Isocitrate Dehydrogenase is activated by…
NAD+ and ADP
Steps in the Kreb’s Cycle
1.
