Lecture Notes on Cellular Respiration and Fermentation

Cells generally have enough ATP to sustain activity for 30 seconds to a few minutes.
ATP is considered unstable; hence, it is continuously produced by cells.
Glucose sources:
- Plants synthesize glucose during photosynthesis.
- Other organisms acquire glucose through food.
Organisms also store glucose typically as glycogen (in animals) or starch (in plants).

The process of glucose oxidation can be described by the following chemical equation:
C{6}H{12}O{6} + 6 O{2}
ightarrow 6 CO{2} + 6 H{2}O + energy (686 ext{ kcal/mol})
During oxidation:
- Oxygen is reduced to form water.
- Glucose undergoes a long series of controlled redox reactions.
- The energy released is utilized for ATP synthesis, which encompasses cellular respiration.

ATP's high energy level is due to the closely spaced negative charges of its phosphate groups.
Hydrolysis of ATP is exergonic.
Energy released during ATP hydrolysis is transferred to proteins through a process known as phosphorylation, typically causing a change in the protein’s shape.

Cellular respiration refers to the set of reactions that utilize electrons from high-energy molecules to produce ATP.
The four major steps are:
1. Glycolysis: Glucose is broken down into pyruvate.
2. Pyruvate Processing: Pyruvate is oxidized to form acetyl CoA.
3. Citric Acid Cycle (Krebs Cycle or TCA cycle): Acetyl CoA is oxidized to CO2.
4. Electron Transport Chain (ETC) & Chemiosmosis: Compounds reduced in steps 1-3 are oxidized, leading to ATP production.

Involve breakdown of molecules to produce ATP by using stored chemical energy.
Initial substrates are typically carbohydrates, followed by fats and proteins as energy sources if carbohydrates are scarce.

Involve synthesis of larger molecules from smaller components and often utilize energy in the form of ATP.

Fats are broken down into:
- Glycerol: enters glycolysis.
- Fatty acids: are converted to acetyl CoA, entering the citric acid cycle.
Proteins are decomposed into amino acids:
- Amino groups are excreted as waste.
- The remaining carbon compounds convert to pyruvate or other intermediates to be utilized in glycolysis or the citric acid cycle.
Metabolism encompasses a multitude of chemical reactions organized into pathways which can be regulated to maintain homeostasis.

Glycolysis is a series of 10 chemical reactions occurring in the cytosol:
- Glucose is converted into two 3-carbon molecules of pyruvate, with the released potential energy used to phosphorylate ADP to form ATP.

Energy Investment Phase:
- Two ATP molecules are consumed.
Energy Payoff Phase:
- Glucose is split to form two pyruvate molecules.
- Two NAD+ are reduced to NADH.
- Four ATP molecules are produced by substrate-level phosphorylation, resulting in a net gain of 2 ATP.

Glycolytic pathway regulation occurs primarily at the third step, which involves the conversion of Fructose-6-phosphate to Fructose-1,6-bisphosphate. Once produced, Fructose-1,6-bisphosphate commits to the glycolytic pathway.
Phosphofructokinase, the enzyme catalyzing this step:
- Exhibits allosteric inhibition by ATP. When ATP levels are high, it binds to a regulatory site and inhibits the enzyme, modulating the pathway's activity.

Pyruvate produced in glycolysis is transported into the mitochondria:
- Mitochondria are characterized by inner and outer membranes, forming the inner membrane space and matrix.
- Pyruvate processing occurs in an enzyme complex known as pyruvate dehydrogenase.
The process details:
- One carbon from pyruvate is oxidized to CO2, to produce NADH.
- The remaining 2-carbon unit binds to coenzyme A to form acetyl CoA.
Summary of pyruvate processing:
- Inputs: Pyruvate, NAD+, and CoA; Outputs: CO2, NADH, and Acetyl CoA.

Each acetyl CoA is oxidized to produce two CO2 molecules.
The cycle occurs in:
- Mitochondrial matrix in eukaryotes or cytosol in prokaryotes.
- Starts with the combination of acetyl CoA with oxaloacetate to form citrate and regains oxaloacetate at the cycle's completion.

The potential energy release includes:
1. Reducing three NAD+ to NADH.
2. Reducing one FAD to FADH2.
3. Producing ATP from ADP.
The cycle turns twice for each glucose molecule, due to two pyruvate produced by glycolysis.

The ETC oxidizes NADH and FADH2, facilitating the generation of a proton gradient.
Composed of four protein complexes (I-IV) that transfer electrons, ultimately passing them to oxygen to form water, utilizing cytochrome c for electron transfer.
Chemiosmosis: the process through which ATP synthase utilizes the proton motive force to synthesize ATP.
The ATP yield is estimated at 29 ATP per glucose molecule.

Fermentation is a metabolic pathway that regenerates NAD+ from NADH, allowing glycolysis to continue when oxygen is not available.
Different fermentation pathways include:
- Lactic Acid Fermentation: Muscle cells convert pyruvate to lactate and regenerate NAD+ during oxygen deprivation.
- Alcohol Fermentation: Yeast converts pyruvate to acetaldehyde before producing ethanol and NAD+ when oxygen is unavailable.
Fermentation produces only 2 ATP per glucose, much less efficient than ~29 ATP from respiration.

Aerobic Respiration: Utilizes oxygen as the final electron acceptor, providing a greater yield of ATP.
Anaerobic Respiration: Uses alternative electron acceptors, pertinent in low-oxygen environments.
Facultative Anaerobes can switch between using oxygen and fermentation depending on availability, thus ensuring cellular energy production under varying environmental conditions.