What is the first step in the process of respiration?
A.
Electron transport chain.
B.
Fermentation.
C.
Krebs cycle.
D.
Glycolysis.
The Correct Answer and Explanation is:
The correct answer is D. Glycolysis.
Explanation:
Respiration is the process by which cells generate energy from glucose. It involves a series of metabolic pathways that convert glucose into adenosine triphosphate (ATP), the cell’s primary energy currency. The process of respiration can be divided into several stages, and the first of these stages is glycolysis.
Glycolysis:
Glycolysis occurs in the cytoplasm of the cell and is the initial step in both aerobic and anaerobic respiration. During glycolysis, one molecule of glucose (a six-carbon sugar) is broken down into two molecules of pyruvate (a three-carbon compound). This process occurs in a series of ten enzyme-catalyzed reactions and does not require oxygen, making it an anaerobic process.
The key features of glycolysis include:
- Energy Investment Phase: In the initial steps of glycolysis, the cell uses two molecules of ATP to phosphorylate glucose, forming fructose-1,6-bisphosphate. This investment is necessary to destabilize the glucose molecule, making it easier to split into two three-carbon molecules.
- Cleavage Phase: The six-carbon fructose-1,6-bisphosphate is then split into two three-carbon molecules of glyceraldehyde-3-phosphate (G3P).
- Energy Payoff Phase: Each G3P molecule undergoes a series of reactions that produce ATP and NADH (nicotinamide adenine dinucleotide, reduced form). In total, four ATP molecules are generated through substrate-level phosphorylation, and two NADH molecules are produced. However, since two ATP molecules were consumed in the energy investment phase, the net gain of ATP in glycolysis is two.
Importance of Glycolysis:
- ATP Production: Glycolysis provides a quick source of energy, especially in anaerobic conditions when oxygen is not available.
- NADH Generation: NADH produced in glycolysis can be used in the electron transport chain during aerobic respiration to generate more ATP.
- Pyruvate Production: The end product, pyruvate, is a critical intermediate that can enter the mitochondria for further oxidation in the Krebs cycle (also known as the citric acid cycle) during aerobic respiration or can undergo fermentation under anaerobic conditions.
Subsequent Steps:
After glycolysis, the fate of pyruvate depends on the presence of oxygen. If oxygen is available, pyruvate enters the mitochondria and is further processed in the Krebs cycle and the electron transport chain, leading to the production of a large amount of ATP. If oxygen is not available, pyruvate undergoes fermentation to regenerate NAD+ so that glycolysis can continue.
In summary, glycolysis is the first and essential step in the process of respiration, setting the stage for subsequent metabolic pathways that generate energy for the cell.