Place of glycolysis in energy metabolism
Catabolism (dissimilation) is a set of enzymatic reactions involving the breakdown of organic substances, which are accompanied by the release of energy and the formation of end products of degradation: carbon dioxide and water. The released energy is stored in high-energy bonds of ATP (adenosine triphosphate) molecules through phosphorylation.
Glycolysis is always preceded by the preparatory stage. In multicellular organisms, it occurs in the gastrointestinal tract, and inside the cell, in lysosomes. At this stage, enzymes break down complex polymers into simple monomers:
- Proteins are degraded to amino acids;
- Fats into glycerol and fatty acids;
- Complex carbohydrates (starch) into monosaccharides (glucose);
- Nucleic acids into nucleotides.
The main feature of the first stage is that all the released energy is completely dissipated as heat. No ATP synthesis occurs here. Only after glucose is formed does the cell proceed to the second, anaerobic stage.
Mechanism of anaerobic breakdown
The second stage of energy metabolism is glycolysis itself. It takes place in the liquid medium of the cytoplasm without the participation of oxygen.
During the multi-step reaction, one glucose molecule is broken down into two molecules of pyruvic acid (pyruvate). Concurrently, cellular energy is generated. The overall equation for this stage is:
Glucose + 2 ADP + 2 inorganic phosphates + 2 NAD+ → 2 pyruvate + 2 ATP + 2 NADH+H+
The breakdown of one mole of glucose releases 200 kJ of energy. Of this, 120 kJ (60%) is irreversibly lost as heat, and the remaining 80 kJ (40%) is accumulated in the high-energy bonds of two ATP molecules. In addition, two molecules of reduced hydrogen carrier NADH+H+ are formed, which will play their role in subsequent stages.
Fermentation: pyruvate pathways in hypoxia
The further fate of pyruvic acid depends on whether oxygen is available to the cell. If conditions are anaerobic, fermentation processes are triggered.
- Lactic acid fermentation. This pathway is characteristic of certain bacteria, as well as animal muscle cells under conditions of acute oxygen shortage (hypoxia). Pyruvate interacts with hydrogen from carriers (NADH+H+) and is reduced to lactic acid (lactate).
- Alcoholic fermentation. Found in yeasts and plants. First, carbon dioxide is cleaved from pyruvate, leading to the formation of acetaldehyde. Then this acetaldehyde is reduced to the end product — ethanol.
Aerobic completion of catabolism
If oxygen is present, pyruvate does not undergo fermentation, but is transported from the cytoplasm to the mitochondria for complete oxidation. This final stage takes place in two compartments:
- In the mitochondrial matrix. Pyruvate is converted into Acetyl-CoA and enters the tricarboxylic acid cycle (Krebs cycle). Here, carbon dioxide is released, 2 more ATP molecules are synthesized, and new reduced carriers are formed.
- On the inner membrane (cristae). The respiratory electron transport chain functions. With the participation of oxygen and hydrogen carriers, metabolic water is synthesized and a colossal amount of energy is generated — 34 ATP molecules.
Thus, the complete breakdown cycle of one glucose molecule yields a total energy output of 38 ATP molecules (2 from glycolysis, 2 from the Krebs cycle, and 34 from the respiratory chain).
To easily remember the key products of glycolysis per one glucose molecule, use the phrase "Two Pirates Carry Two Batteries": Two Pirates = 2 Pyruvates, Carry = 2 NADH+H+, Two Batteries = 2 ATP.
Class and exam questions
Is ATP synthesized during the preparatory stage of metabolism?
No, during the breakdown of complex organic substances into monomers (for example, in lysosomes or the digestive tract), all energy is dissipated exclusively as heat.
Where exactly do the reactions of glycolysis take place?
Glycolysis occurs in the cell cytoplasm and is a strictly anaerobic process not associated with the membranes of any organelles.
In which human cells can fermentation occur?
In the human body, lactic acid fermentation can be triggered in muscle cells. This occurs during hypoxia when there is not enough oxygen for full aerobic oxidation.
What else to study on this topic
- Mechanism of action of digestive tract enzymes and lysosomes
- Tricarboxylic acid cycle (Krebs cycle) in the mitochondrial matrix
- Structure and function of the electron transport chain on the cristae
- Structure of the ATP molecule and features of high-energy bonds
- Biochemical differences between alcoholic and lactic acid fermentation
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