Key Points

Cellular respiration is the mechanism of breaking down food materials, like glucose ($C_6H_{12}O_6$), within the cell to release energy and trap it in the form of ATP.

The compounds that are oxidized during respiration to release energy are called respiratory substrates. The most common substrate is glucose, but fats, proteins, and organic acids can also be used.

Plants do not have specialized respiratory organs. Gas exchange occurs through stomata in leaves and lenticels in stems, with most plant parts managing their own needs.

Glycolysis is the partial oxidation of glucose into two molecules of pyruvic acid. It occurs in the cytoplasm and results in a net gain of 2 ATP and 2 NADH molecules.

Under anaerobic conditions, pyruvic acid is incompletely oxidized to either ethanol and $\text{CO}_2$ (alcoholic fermentation) or lactic acid. It yields only a net of 2 ATP per glucose molecule.

Aerobic respiration, the complete oxidation of organic substances, takes place within the mitochondria. It requires the presence of oxygen.

In the mitochondrial matrix, pyruvic acid is converted to acetyl CoA. This reaction releases one molecule of $\text{CO}_2$ and produces one molecule of NADH per pyruvate.

Acetyl CoA enters the Krebs' cycle in the mitochondrial matrix. One turn of the cycle produces 3 NADH, 1 $\text{FADH}_2$, 1 ATP (via GTP), and releases 2 molecules of $\text{CO}_2$.

The ETS is a series of protein complexes located on the inner mitochondrial membrane. It facilitates the transfer of electrons from NADH and $\text{FADH}_2$ to oxygen.

This is the process where energy released from the electron transport in the ETS is used to synthesize ATP. Oxygen is the final electron acceptor, which gets reduced to water ($H_2O$).

The oxidation of one molecule of NADH in the ETS yields 3 molecules of ATP. The oxidation of one molecule of $\text{FADH}_2$ yields 2 molecules of ATP.

ATP synthase uses the energy from a proton gradient, created by the ETS across the inner mitochondrial membrane, to synthesize ATP from ADP and inorganic phosphate.

Theoretically, the complete aerobic respiration of one glucose molecule can yield a net gain of 38 ATP molecules. This is an estimate based on a set of ideal assumptions.

The respiratory pathway is considered amphibolic because it involves both catabolism (breakdown) and anabolism (synthesis). Intermediates like acetyl CoA can be withdrawn to synthesize molecules like fatty acids.

RQ is the ratio of the volume of $\text{CO}_2$ evolved to the volume of $\text{O}_2$ consumed in respiration. The formula is $\text{RQ} = \frac{\text{volume of } CO_2 \text{ evolved}}{\text{volume of } O_2 \text{ consumed}}$

The RQ value is 1.0 for carbohydrates, less than 1 for fats (around 0.7), and about 0.9 for proteins. The value depends on the type of respiratory substrate being oxidized.