Yeast is a single-celled organism, multiplying rapidly in the presence of sugar. Its products differ in aerobic and anaerobic conditions. Yeast and human cells share similar aerobic processes.
This makes yeast a 'model' for functions in human cells. In aerobic respiration, yeast use oxygen (O) to generate energy. Their fundamental metabolic pathways are shared by the cells of all eukaryotic organisms.
A eukaryote cell has a true nucleus. Bacteria and archaea lack this development, though they have many traits of their own.
Aerobic respiration is the process by which cells convert glucose into energy (ATP) in the presence of oxygen. Occurring in the mitochondria, or cell organelles, it starts with oxygen and glucose.
From this the cell creates carbon dioxide (CO2), water (H2O), and ATP energy. The chemical equation is
C6H12O6 + 6O2 → 6CO2 + 6H2O
(glucose + oxygen -> carbon dioxide + water)
Yeast uses the energy for growth, reproduction and other metabolic functions.
The aerobic respiration process in yeast can be divided into main stages including glycolysis, the Krebs cycle and the electron transport chain.
Glycolysis
Glycolysis occurs in the cytoplasm of yeast cells as the initial step in the aerobic respiration process. It doesn't specifically need oxygen and happens also in anaerobic environments.
Glucose is broken down into two molecules of pyruvate. A small amount of ATP and NADH (an electron carrier) are produced.
Pyruvate Decarboxylation: Preparing for the Krebs Cycle
Each pyruvate molecule from glycolysis is transported into the mitochondria. Here it undergoes decarboxylation, releasing a molecule of carbon dioxide and forming acetyl-CoA.
This process also generates NADH. Acetyl-CoA is the link between glycolysis and the Krebs stage.
The Krebs Cycle (Citric Acid Cycle)
In the mitochondrial matrix, the acetyl-CoA combines with oxaloacetate to form citric acid, starting a cycle of reactions. These regenerate oxaloacetate, allowing the cycle to continue.
More NADH and FADH2 (another electron carrier) are produced, along with ATP and carbon dioxide as byproducts.
Energy Production: The Krebs cycle turns twice for each glucose molecule, leading to the production of three NADH, one FADH2, and one ATP per cycle.
Electron Transport Chain (ETC)
The final stage of aerobic respiration in yeast occurs in the inner mitochondrial membrane, where the electron transport chain is located.
Electron Transfer: NADH and FADH2 produced in previous stages donate their electrons to a series of protein complexes in the electron transport chain. As electrons move through these complexes, protons (H+) are pumped into the intermembrane space, creating an electrochemical gradient.
ATP Synthesis: Protons flow back into the mitochondrial matrix through ATP synthase, driving the conversion of ADP (adenosine diphosphate) and inorganic phosphate into ATP, a process known as oxidative phosphorylation.
Water Formation: At the end of the electron transport chain, electrons are transferred to molecular oxygen, the final electron acceptor, forming water as a byproduct. The yeast needs a certain amount of H2O to live.
While yeast enjoy anaerobic conditions, powering fermentation, aerobic respiration offers advantages.
Higher ATP Yield: Aerobic respiration generates significantly more ATP per glucose molecule compared to fermentation (around 36-38 ATP vs. 2 ATP). This provides cells with more energy for growth, reproduction and other activities.
Efficient Energy Use: Yeast can completely oxidize glucose to carbon dioxide and water, extracting the maximum amount of energy possible.
Factors Affecting Aerobic Respiration in Yeast
Oxygen Availability: Oxygen is essential for the final electron acceptor in the ETC; its absence will cause yeast to switch to fermentation.
Glucose Concentration: High glucose concentrations can suppress aerobic respiration, even in the presence of oxygen, a phenomenon known as the "Crabtree effect."
Temperature: Optimal temperatures are required for the proper functioning of enzymes involved in respiration.
Nutrient Availability: The presence of essential nutrients like nitrogen and phosphorus is crucial for supporting the growth and metabolism required for respiration.
Fermentation
While yeast is famous for anaerobic respiration (fermentation), the presence of oxygen when it's added to the wort allows it to produce more energy, strengthen and grow for the upcoming task.
The fermentation processes in humans produce lactic acid.
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