Question

Where and how is water formed during aerobic respiration?

Answer 1

Hint: All cells, eukaryotes and prokaryotes, respire. Cellular respiration involves the breakdown of food molecules to generate energy-rich ATP (adenosine triphosphate) molecules. Cellular respiration can be classified into two types, aerobic respiration, and anaerobic respiration. Oxygen is required for aerobic respiration. Anaerobic respiration does not require oxygen. While eukaryotes and prokaryotes can undergo both types of cellular respiration, most eukaryotes respire aerobically.

Complete answer:
Aerobic respiration involves three major events- glycolysis, Krebs cycle, and electron transport chain. In Prokaryotes and eukaryotes, glycolysis occurs in the cytoplasm of the cell. The glucose is broken down to pyruvate. Oxygen is not utilized during glycolysis. ATP and NADH molecules are also produced during glycolysis.

Pyruvate is converted to Acetyl-CoA before the Krebs cycle begins (decarboxylation of pyruvate). This step releases NADH molecules. In eukaryotes, the Krebs cycle takes place in the mitochondrial matrix. In prokaryotes, the Krebs cycle takes place in the cytoplasm as they do not have cell organelle mitochondria. In eukaryotes, during this cycle oxygen is utilized to produce ATP, NADH, and FADH.Carbon dioxide is produced in the end as the byproduct.

Coming to the electron transport chain, this occurs in the inner membrane of the mitochondria. Oxygen is required for this step as well. The FADHand NADH produced during glycolysis and Krebs cycle will release electrons and protons. The electrons pass through the electron transport chain (proteins that accept the electrons). The protons remain in the matrix while electrons enter the electron transport chain in the inner membrane. The proteins of the transport chain are divided into four complexes. As the electron travels from one complex to the next it releases energy. This energy is utilized to pump a pair of protons from the matrix to the intermembrane space of the mitochondria. When the electron reaches the last complex (cytochrome complex) no energy is released. This electron is accepted by oxygen in the end.

The oxygen receives electrons from the electron transport chain and it receives a pair of protons from the matrix. Together they will form a water molecule.
In prokaryotes, water is synthesized in the cytoplasm.

Note: After the water is produced, ATPase goes on to catalyze the production of ATP in the mitochondria (in eukaryotes). The protons (that were pumped out as the electron passed through the first three complexes) have a tendency to move back to the matrix because the concentration of protons there is less. While moving into the matrix they release energy that helps ATPase catalyze the reaction between ADP and inorganic phosphate to make ATP. This is called oxidative phosphorylation.