In cellular respiration energy is made available to organisms by

Analyze the question: It asks how energy becomes available to cells during cellular respiration. The key molecule is ATP, the cell's "energy currency."

Explain the concept:

• During respiration, glucose is broken down and the energy released is used to build ATP from ADP and an inorganic phosphate (Pi).
• ADP + Pi + energy → ATP
• When the cell needs energy for muscle contraction, active transport, or protein synthesis, it breaks the terminal phosphate bond of ATP (hydrolysis).
• ATP → ADP + Pi + energy

Evaluate the options:

• A. Removal of a phosphate group from ADR: "ADR" is not a biological molecule. It is probably a typo for ADP, and even if it were ADP, the energy released is much smaller. ATP is the main energy source, so this is not the best answer.
• B. Breaking off a phosphate group from ATP: Correct. Hydrolysis of the terminal phosphate bond of ATP releases energy for cell activities.
• C. Adding a phosphate group to glucose: This is phosphorylation, an early step of glycolysis. It uses energy rather than making it available to the cell.
• D. Breaking off a hydrogen ion from NADPH: NADPH is mainly used in photosynthesis and biosynthesis, not as the direct energy supply in respiration (that role belongs to NADH and ATP).

Answer: The correct option is B.

Tip: Remember "ATP is spent, not ADP." Energy is released when ATP loses a phosphate and stored when a phosphate is added to ADP. When you see "energy made available" in an exam, look for the option about ATP breaking down.