Question

How does the process of cellular respiration work?

Answer 1

The process by which organisms combine oxygen with food molecules, diverting chemical energy present them into life-sustaining activities and discarding the waste products, water and carbon dioxide is known as Cellular respiration. Organisms that do not depend on the oxygen degrade foodstuffs in a process called fermentation.

Complete answer:
The process of extracting energy from glucose in the food we eat, in the form of ATP is known as Cellular respiration. Cellular respiration occurs in a three step:
The reactants of cellular respiration are glucose and oxygen. When an organism breathes oxygen enters the body. Glucose enters the body when the organism eats. Water and carbon dioxide are the products of cellular respiration. Carbon dioxide is transported from mitochondria out of the cell, to red blood cells, and back to lungs to be exhaled. ATP is generated in this process. By breaking down one molecule of glucose, a total of 36 or 38 molecules of ATP can be generated. This occurs only in the presence of oxygen.
The overall chemical reaction for the cellular respiration is six molecules of oxygen (${{O}_{2}}$) and one molecule of glucose (${{C}_{6}}{{H}_{12}}{{O}_{6}}$) yields six molecules of water (${{H}_{2}}O$) and six molecules of carbon dioxide ($C{{O}_{2}}$). The equation can be represented as follows:
${{C}_{6}}{{H}_{12}}{{O}_{6}}~+\text{ }6{{O}_{2}}~\to \text{ }6C{{O}_{2}}~+\text{ }6{{H}_{2}}O$
Cellular respiration is a series of chemical reactions: glycolysis, the Krebs cycle, and the electron transport chain.

-Glycolysis
First stage of cellular respiration is glycolysis. Glycolysis is the lysis or splitting of glucose. In glycolysis, 6-carbon glucose is converted into two 3-carbon pyruvate molecules. This process occurs in the cell cytoplasm, and it occurs in the absence or presence of oxygen. During glycolysis a small amount of the NADH is made as are four ATP. During this process, two ATP are used, leaving a net gain of two ATP from the glycolysis. The NADH temporarily holds the energy, which will be used in stage three.

-The Krebs Cycle
Under aerobic conditions, in the presence of oxygen, pyruvate enters mitochondria to proceed into the Krebs cycle. The second stage of cellular respiration is the transfer of energy in pyruvate, which is the energy present initially in glucose, into two energy carriers, NADH and $FAD{{H}_{2}}$. A small amount of ATP is made during this process. This process occurs as a continuous cycle. The Krebs cycle uses the 2-carbon molecule (acetyl-CoA) derived from the pyruvate and produces the carbon dioxide.

-The Electron Transport Chain
Third stage of cellular respiration is the use of NADH and $FAD{{H}_{2}}$to generate ATP. This process occurs in two parts. First, the NADH and $FAD{{H}_{2}}$enter the electron transport chain, where by active transport, their energy is used to pump the protons (${{H}^{+}}$) into the intermembrane space of the mitochondria. This establishes a proton gradient across the inner membrane. By facilitating diffusion, these protons then flow down the concentration gradient, moving back into the matrix. During this process, by adding inorganic phosphate to ADP, ATP is made. Most of the ATP produced during the cellular respiration is made during this stage.

Note: For each glucose, that starts cellular respiration in presence of the oxygen (aerobic conditions), 36 to 38 ATP are generated. Without the presence of oxygen, under anaerobic conditions, only two ATP are produced.