Metabolism refers to all chemical process carried out by living organisms. Two pathways involved in metabolism are anabolic pathways and catabolic pathways. Anabolic pathway or anabolism consists of reductive processes that require energy to synthesize a new substance, while catabolic pathway or catabolism includes oxidative processes that remove electrons from substrates to generate energy.

酵母玻璃纸ls perform anabolism for growth, reproduction, and repair of the cellular structure. Energy for yeast cell processes, including movement, transport, and synthesis of complex molecules is provided by catabolism.

酵母玻璃纸ls belong to the chemoautotroph group, i.e., yeasts obtain chemical energy from breaking down organic compounds. Chemoautotroph metabolism includes glycolysis, fermentation, and aerobic respiration. Glycolysis and fermentation do not require oxygen, and only a small amount of energy in the form of ATP is generated. Aerobic respiration needs oxygen as an electron acceptor and generates a relatively large amount of energy. Yeast cells can both ferment and respire. When conditions are anoxic and terminal acceptors are absent, yeast cells perform fermentation after glycolysis. When oxygen is available, yeasts can perform respiration after glycolysis. Fermentation generates lesser energy than respiration; therefore, the latter is preferred by yeasts. However, fermentation does not occur only when oxygen is absent.S. cerevisiaegrown with excess sugar, will also produce ethanol aerobically. This is called the "Crabtree-effect". It can be explained as carbon overflow. When the sugar uptake is higher than what can be used for growth, some carbon is diverted from respiration to ethanol production.

Substrate-level phosphorylation

In glycolysis, ATP is generated by substrate-level phosphorylation. During this process, ATP is directly synthesized from an energy-rich intermediate within substrate catabolism steps. If oxygen is present, pyruvate generated from glycolysis can be used in the Krebs cycle, and perform oxidative phosphorylation. If oxygen is absent, pyruvate is converted to ethanol, and no further energy is produced.

Oxidative phosphorylation

Oxidative phosphorylation occurs during aerobic respiration. In this process, ATP is produced at the expense of proton motive force. Oxidation is held using O₂as a terminal electron acceptor.

Table 1. Aerobic respiration and Fermentation

Culture MediaGlycolysis

Theory overview