Cellular respiration is the process by which cells break down organic molecules, typically glucose, to produce energy in the form of adenosine triphosphate (ATP). It occurs in three main stages: glycolysis, the citric acid cycle (Krebs cycle), and the electron transport chain.

Respiratory Substrate:

The respiratory substrate is the molecule that is oxidized during cellular respiration to produce ATP. While glucose is the most common respiratory substrate, other molecules such as fatty acids and amino acids can also serve as respiratory substrates depending on the cellular energy needs and availability of these molecules.

Respiratory Quotient (RQ):

The respiratory quotient is a ratio that indicates the type of respiratory substrate being used during cellular respiration. It is calculated as the ratio of the volume of carbon dioxide (CO2) produced to the volume of oxygen (O2) consumed during respiration. The RQ value varies depending on the type of substrate being oxidized. For example:

• RQ = 1: When glucose is the substrate, as in complete carbohydrate metabolism.

• RQ < 1: When lipids (fats) are the substrate, indicating incomplete oxidation.

• RQ > 1: In rare cases, such as in the metabolism of certain organic acids.

Types of Cellular Respiration: There are two primary types of cellular respiration:

a. Aerobic Respiration: This is the most common and efficient type of cellular respiration. It occurs in the presence of oxygen and involves glycolysis, the citric acid cycle, and the electron transport chain. Aerobic respiration yields a large amount of ATP and is the primary mode of energy production in eukaryotic cells.

b. Anaerobic Respiration: Anaerobic respiration occurs in the absence of oxygen or when oxygen is in limited supply. It includes processes like lactic acid fermentation and alcoholic fermentation. While less efficient than aerobic respiration in terms of ATP production, anaerobic respiration allows cells to produce ATP when oxygen is scarce.

Glycolysis:

Glycolysis is the first step in cellular respiration and occurs in the cytoplasm of the cell. It involves the breakdown of one molecule of glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon compound). During glycolysis, ATP and NADH (a coenzyme) are produced, and some ATP is consumed. The net gain of ATP in glycolysis is two molecules of ATP per molecule of glucose. Glycolysis can occur under both aerobic and anaerobic conditions. If oxygen is available, pyruvate is further oxidized in the citric acid cycle and electron transport chain as part of aerobic respiration. If oxygen is lacking, pyruvate may undergo fermentation to generate ATP.