The concentration gradient of protons feeds into ATP synthase structure, which creates ATP. Water does not feed into ATP synthase. ADP does not accept the hydrogen to create ATP, rather it is the substrate onto which a phosphate group is added to create ATP. NADH gives up a hydrogen in the electron transport chain to create the concentration gradient. It does not pass through ATP synthase.

The mitochondrion is composed of an outer membrane, matrix, inner membrane and inner membrane space. The components of the electron transport chain, namely the cytochrome proteins and ATP synthase, are arranged on the inner membrane to allow for the movement of protons between the inner membrane space and matrix in order to create ATP.

The final electron acceptor of the electron transport chain in cellular respiration is oxygen. Additionally carbon dioxide is produced as a waste product during the citric acid cycle phase of cellular respiration.

A reducing agent works as an electron donor, so it is ultimately giving away electrons to another molecule and ultimately making that other molecule more negative in charge, or reduced in charge. Note that a reducing agent is itself oxidized. Also recall the mnemonic OIL RIG (Oxidation Is Loss of electrons, Reduction Is Gain of electrons).

Cellular respiration begins with the process of breathing in oxygen and consuming glucose. Through a series of reactions they eventually produce energy in the form of  and heat, as well as byproducts such as , which is exhaled, and water molecules.

Glucose is the primary molecule that heterotrophs use to make energy at a cellular level. Heterotrophs use glucose for a starting material in both fermentation and cellular respiration. However fermentation is performed without oxygen (anaerobic), while cellular respiration requires oxygen as a final electron receptor at the end of the electron transport chain.

Cellular respiration divided into three steps: glycolysis, citric acid cycle (also known as the Krebs or TCA cycle), and the electron transport chain (also known as oxidative phosphorylation). While glycolysis takes place in the cytosol, the other two steps occur in the mitochondria.

The major processes that occur in mitochondria are the citric acid cycle and the electron transport chain. The citric acid cycle forms electron carries  that are used in the electron transport chain to reduce oxygen to water and produce ATP.

ATP Synthase work by converting the energy in the protons electrochemical gradient into production of ATP. This gradient is oriented across in the inner mitochondrial membrane so that protons is at a higher concentration in the intermembrane space than the matrix, and thus will flow from the intermembrane space to the matrix. This gradient is produced by the electron transport chain pumping protons from the matrix across the inner membrane to the intermembrane space.

During the reduction of oxygen to water, reactive oxygen species such as superoxide or hydrogen peroxide can be produced. These molecules are highly reactive and such can react with proteins or DNA to cause cellular damage or mutations.