**Immediate Source of Cellular Energy: ATP**

**ATP: Adenosine Triphosphate**
ATP, or adenosine triphosphate, is a molecule that serves as the primary source of energy for cellular activities. It is often referred to as the "energy currency" of the cell because it stores and releases energy as needed. ATP consists of three components: an adenine molecule, a ribose sugar, and three phosphate groups.

**Formation of ATP**
ATP is formed through cellular respiration, a process that occurs in the mitochondria of cells. During cellular respiration, glucose, a simple sugar obtained from food, is broken down in the presence of oxygen to produce ATP. The process involves several steps, including glycolysis, the Krebs cycle, and oxidative phosphorylation.

**Glycolysis**
Glycolysis is the initial step of cellular respiration that occurs in the cytoplasm of the cell. During glycolysis, a molecule of glucose is converted into two molecules of pyruvic acid or pyruvate. This process generates a small amount of ATP through substrate-level phosphorylation.

**Krebs Cycle**
Following glycolysis, the pyruvate molecules enter the mitochondria and undergo further breakdown in the Krebs cycle. The Krebs cycle, also known as the citric acid cycle, generates ATP through oxidative phosphorylation. It involves a series of chemical reactions that release carbon dioxide and high-energy electrons used in the next step.

**Oxidative Phosphorylation**
The high-energy electrons produced in the Krebs cycle are transferred to the electron transport chain, located on the inner membrane of the mitochondria. As the electrons move through the electron transport chain, energy is released and used to pump hydrogen ions (protons) across the membrane, creating a gradient. This gradient drives the synthesis of ATP through a process called chemiosmosis, resulting in the production of a large amount of ATP.

**Role of ATP in Cellular Processes**
ATP plays a crucial role in numerous cellular processes, including:
1. Metabolic reactions: ATP provides the energy required for anabolic reactions (building complex molecules) and catabolic reactions (breaking down complex molecules).
2. Active transport: ATP powers the movement of substances across cell membranes against their concentration gradients.
3. Muscle contraction: ATP provides the energy needed for muscle fibers to contract.
4. Nerve impulse transmission: ATP is involved in the transmission of electrical signals in nerve cells.
5. DNA replication: ATP is required for DNA replication, which is essential for cell division and growth.

**Conclusion**
In summary, ATP serves as the immediate source of cellular energy. It is synthesized through cellular respiration and is involved in various energy-requiring processes within the cell. The other options, such as pyruvic acid, NADH, and glucose, are intermediates or molecules involved in energy production but are not the immediate source of energy like ATP.