The respiratory pathways of glycolysis, the tricarboxylic acid (TCA) cycle and the mitochondrial electron transport chain are ubiquitous throughout nature. They are essential for both energy provision in heterotrophic cells and a wide range of other physiological functions. Although the series of enzymes and proteins that participate in these pathways have long been known, their regulation and control are much less well understood. Further complexity arises due to the extensive interaction among these pathways in particular, and also between cytosolic and mitochondrial metabolism in general. These interactions include those between mitochondrial function in the photosynthetic and photorespiratory processes, amino-acid biosynthesis and the regulation of cellular redox. Recently, a wide range of molecular and biochemical strategies have been adopted to elucidate the functional significance of these interactions.

Glycolysis, TCA cycle, and ETS cycle are interconnected pathways involved in cellular respiration. The correct sequential order of these pathways is option A: Glycolysis, TCA cycle, and ETS cycle.

1. Glycolysis:
Glycolysis is the first step in cellular respiration and takes place in the cytoplasm of the cell. It is an anaerobic process that occurs in both aerobic and anaerobic organisms. During glycolysis, a molecule of glucose is broken down into two molecules of pyruvate. This process generates a small amount of ATP and NADH.

2. TCA cycle (also known as the Krebs cycle or citric acid cycle):
The TCA cycle takes place in the mitochondria of the cell. It is an aerobic process that requires oxygen. The pyruvate molecules generated during glycolysis enter the mitochondria and undergo further oxidation. Each pyruvate molecule is converted into acetyl-CoA, which enters the TCA cycle. In the TCA cycle, acetyl-CoA is oxidized, releasing CO2 and generating more NADH and FADH2. These molecules carry high-energy electrons to the next stage of cellular respiration.

3. ETS cycle (also known as oxidative phosphorylation or electron transport chain):
The ETS cycle occurs in the inner mitochondrial membrane. It is an aerobic process that requires oxygen. The high-energy electrons carried by NADH and FADH2 from glycolysis and the TCA cycle are transferred through a series of protein complexes in the inner mitochondrial membrane. This transfer of electrons generates a proton gradient across the membrane. The energy from the electron transfer is used to pump protons from the mitochondrial matrix to the intermembrane space. The protons then flow back into the matrix through ATP synthase, generating ATP through a process called chemiosmosis.

Therefore, the correct sequential order of these pathways is Glycolysis, TCA cycle, and ETS cycle. This order ensures that glucose is first broken down into pyruvate during glycolysis, which then enters the mitochondria to be further oxidized in the TCA cycle. The high-energy electrons generated in the TCA cycle are then used in the ETS cycle to generate ATP through oxidative phosphorylation.