Energy Harvesting and Metabolism

Chapter 09: Pathways That Harvest Chemical Energy

Focus on how sunlight energy is converted into useful forms for organisms.
Topic includes discussion of photosynthesis:
- Photosynthesis in plants, algae, and some bacteria.
- Basic equation:
- ext{CO}2 + ext{H}2 ext{O}
  ightarrow ext{Sugars} + ext{O}_2
Energy Harvesting:
- Sugars and other organic molecules created by photosynthesis.
- Respiration in most living organisms, including plants and bacteria, where the reduction of CO2 is utilized.

Food is the energy source, broken down in a series of reactions to release energy.
CATABOLISM:
- Catabolic pathways involve the breakdown of molecules, losing energy as heat and forming smaller molecules for biosynthesis.
- Exergonic reactions: Energetically favorable reactions that release energy (e.g., oxidation of food molecules) and result in the production of ATP.
ANABOLISM:
- Anabolic pathways build larger molecules from smaller components and require energy (endergonic reactions).
- Resulting in newly synthesized cellular components necessary for growth and repair.

ATP (Adenosine Triphosphate):
- Serves as high-energy phosphate carrier, akin to batteries that store and release energy.
- Energy content: Approximately 12 kcal/mol free energy.
- Energy release when Pi breaks off leading to ADP and another phosphate:
- ext{ADP} + P + ext{Free Energy}
  ightarrow ext{ATP}
- The released energy can be utilized for various cellular functions.
NADH (Nicotinamide Adenine Dinucleotide):
- Electrons carrier molecule, storing high-energy electrons, mainly in mitochondria for ATP production.
- Energy content: Approximately 50 kcal/mol free energy.

Glucose and its Polymers:
- Most common forms of energy storage in cells, primarily glycogen.
- Catabolism of Glucose: Initial step involves glycolysis, a series of reactions resulting in energy release while forming pyruvate.
- Glycolysis leads to:
- Cellular respiration in aerobic conditions.
- Fermentation in anaerobic conditions.
- Glucose oxidation is exergonic and facilitates ATP synthesis.
- The extraction of energy is dependent on the specific pathways (aerobic vs anaerobic) followed by the cell.

Differences in oxidation states of molecules:
- Less oxygenated = More reduced: Higher potential energy (e.g., Glucose).
- More oxygenated = Less reduced: Lower potential energy (e.g., CO2).
Combustion of glucose equation:
- ext{C}6 ext{H}{12} ext{O}6 + 6 ext{O}2
  ightarrow 6 ext{CO}2 + 6 ext{H}2 ext{O} + ext{Free Energy}
- Change in Gibbs free energy (ΔG) for glucose oxidation:
- ext{ΔG} = -686 ext{ kcal/mol}
This indicates substantial energy release during glucose catabolism which drives ATP formation:
- ext{ADP} + P + ext{Free Energy}
  ightarrow ext{ATP}

With O2 (Aerobic Conditions):
- Glycolysis followed by:
- Pyruvate oxidation
- The Citric Acid Cycle (Krebs Cycle)
- Electron Transport Chain (ETC)
Without O2 (Anaerobic Conditions):
- Pyruvate is converted to either lactate or alcohol via fermentation, allowing NAD+ regeneration.

Eukaryotes:
- Glycolysis occurs in cytoplasm;
- Citric Acid Cycle and ETC occur within the mitochondrion.
Prokaryotes:
- All pathways occur in the cytoplasm or on the plasma membrane.

Oxidation:
- Refers to the loss of electrons from a substance, often associated with oxygen intake.
Reduction:
- Gain of electrons by a substance, occurring simultaneously with oxidation.
Importance of oxidizing and reducing agents in redox reactions, specifically in glucose metabolism.
OIL RIG:
- Oxidation Is Loss, Reduction Is Gain (of electrons).

NAD+: Critical electron carrier in oxidation-reduction reactions.
NADH Formation:
- NAD+ reduces by accepting 2 electrons and 1 proton during glucose oxidation.
Reaction:
- ext{NAD}^+ + 2 ext{H}
  ightarrow ext{NADH} + ext{H}^+
- The reduced form (NADH) then donates electrons to the ETC.
Oxidation of NADH:
- Coupled to the reduction of oxygen, resulting in energy release.

Pyruvate oxidation:
- Essential step wherein pyruvate is converted to acetyl CoA.
- 80% of free energy from glucose is released during this process followed by the Krebs cycle.
- Outputs:
- CO2 released, and electron carriers NADH and FADH2 generated.
Krebs Cycle:
- Operates in the mitochondrial matrix, linking the oxidation of acetyl CoA to CO2 emission and the production of GTP/ATP.

Electrons from NADH and FADH2 are transferred through complexes in the inner mitochondrial membrane.
Oxygen acts as the ultimate electron acceptor leading to water formation, and discrepancies in energy sit at various substrate levels.
The processes of ATP generation via chemiosmosis are highly reliant on creating a proton-motive force across the mitochondrial membrane.

Chemiosmosis:
- The process by which ATP is produced via the proton gradient set up by the electron transport chain. This occurs when protons are pumped out of the mitochondrial matrix.
ATP synthase uses this gradient to drive ATP formation from ADP and inorganic phosphate.
Typical yield per glucose from cellular respiration is approximately 38 ATP, with a higher yield through aerobic pathways versus fermentation (which nets only 2 ATP).

Fermentation allows for continued ATP production under anaerobic conditions, using pyruvate reduction and regenerating NAD+
While fermentation suffices for ATP production, aerobic respiration is significantly more efficient in terms of energy yield.

Energy production is regulated based on the cellular needs for ATP.
Key control points in glycolysis and the Krebs cycle, primarily involving enzymes like phosphofructokinase (inhibited by high ATP levels) and isocitrate dehydrogenase, respectively.

The energy metabolism pathways are vital in integrating metabolic processes functioning in anabolism.
Glucose serves as a precursor for various biosynthetic pathways facilitating different cellular functions.
Key intermediates include acetyl CoA and alpha-ketoglutarate, further utilized for synthesizing fatty acids and nucleotides, respectively.