Introduction:
Respiration is a vital process that occurs in living cells to generate energy. It involves the breakdown of glucose to produce ATP, the energy currency of cells. To initiate respiration, a living cell requires certain substances, which are discussed below.

Glucose:
The primary requirement for the start of respiration in a living cell is glucose. Glucose is a simple sugar that serves as the main source of energy for cellular activities. It is obtained from the food we consume and is transported into the cells. Glucose acts as the fuel for respiration and is broken down during the process to release energy.

Oxygen (O²):
Oxygen is another essential requirement for the start of respiration. It is obtained through the process of breathing and is transported to the cells via the bloodstream. Oxygen plays a crucial role in the final step of respiration called oxidative phosphorylation. It acts as the final electron acceptor in the electron transport chain, which leads to the production of ATP.

ATP:
ATP (adenosine triphosphate) is the energy molecule that powers various cellular processes. While glucose is the initial source of energy, it is converted into ATP during respiration. ATP is generated through a series of chemical reactions, including glycolysis, the Krebs cycle, and oxidative phosphorylation. It provides energy for cellular activities like muscle contraction, active transport, and synthesis of molecules.

Explanation:
When a living cell initiates respiration, glucose is first broken down in a process called glycolysis. This occurs in the cytoplasm and converts glucose into pyruvate, releasing a small amount of ATP. In the absence of oxygen, anaerobic respiration occurs, where pyruvate is converted into lactic acid.

However, in the presence of oxygen, aerobic respiration takes place. The pyruvate produced during glycolysis enters the mitochondria, where it undergoes further breakdown in the Krebs cycle. This cycle generates high-energy molecules like NADH and FADH₂.

The NADH and FADH₂ molecules then participate in oxidative phosphorylation, the final step of respiration. This process occurs in the inner mitochondrial membrane and requires oxygen as the final electron acceptor. As electrons flow through the electron transport chain, ATP is synthesized through chemiosmosis. This ATP is then utilized by the cell for various energy-requiring processes.

Therefore, for the start of respiration, a living cell requires glucose, oxygen, and ATP. Glucose acts as the initial source of energy, while oxygen is necessary for the efficient production of ATP through aerobic respiration. ATP, in turn, provides the energy required by the cell for its metabolic activities.