In cellular energy transformations, through which of the following conversions is energy directly released for cellular work?

Introduction / Context:
This question focuses on bioenergetics, specifically on how energy is stored and released in living cells. Cells use high energy molecules to capture energy from nutrients or light and then release that energy when needed to drive biological work, such as muscle contraction, active transport and biosynthesis. Understanding which conversions store energy and which release it is essential in interpreting processes like photosynthesis and cellular respiration.

Given Data / Assumptions:

• Options include photosynthesis (CO2 and H2O to glucose and O2), NADP+ to NADPH, ADP to ATP, ATP to ADP and glucose to glycogen.
• Energy release refers to a conversion that drives cellular work by making energy available.
• Energy storage refers to building more energy rich molecules from less energy rich ones.
• Standard roles of ATP, NADPH and glucose in metabolism are assumed.

Concept / Approach:
ATP (adenosine triphosphate) is the immediate energy currency of the cell. When ATP is broken down to ADP (adenosine diphosphate) and inorganic phosphate, energy is released that can be coupled to various cellular processes. Conversely, forming ATP from ADP and phosphate requires an input of energy, which is captured during cellular respiration or photosynthesis. Similarly, forming NADPH from NADP+ and forming glucose or glycogen from simpler molecules store energy rather than release it. Thus, among the given options, the conversion ATP to ADP is the one that directly releases energy for cell use.

Step-by-Step Solution:
Step 1: Examine the photosynthesis reaction: CO2 and H2O to glucose and O2. This process requires light energy and stores energy in glucose; it is not a net energy releasing step. Step 2: Consider NADP+ to NADPH. This conversion involves adding high energy electrons and a proton to NADP+, storing energy in NADPH, which is later used in biosynthetic reactions. Step 3: Evaluate ADP to ATP. This reaction requires an input of energy from respiration or photosynthesis to form the high energy phosphate bond, so it is an energy storing process. Step 4: Look at ATP to ADP. Hydrolysis of ATP to ADP and inorganic phosphate releases energy that can be coupled to endergonic cellular reactions. Step 5: Glucose to glycogen is a storage process in which energy is used to build polymeric glycogen from glucose units, again not a net energy releasing conversion. Step 6: Therefore, the conversion that directly releases energy for work is ATP to ADP.

Verification / Alternative check:
Textbooks frequently illustrate ATP hydrolysis as ATP + H2O → ADP + Pi + energy, highlighting that energy is released when the terminal phosphate bond is broken. This energy is then used to power processes such as active transport (for example, Na+/K+ pump), mechanical work (muscle contraction) and chemical synthesis (building macromolecules). In contrast, forming ATP from ADP is depicted with energy input from nutrient oxidation in mitochondria or light capture in chloroplasts. NADPH is always described as a reducing power carrier, storing energy in electrons and used later in the Calvin cycle and other biosynthetic pathways. These roles confirm that ATP to ADP is the key energy releasing step.

Why Other Options Are Wrong:
The conversion CO2 and H2O to glucose and O2 is the overall reaction for photosynthesis, which requires energy input from sunlight and stores that energy in glucose. NADP+ to NADPH stores energy in the reduced coenzyme form. ADP to ATP is an energy storing step in which the cell invests energy to build ATP. Glucose to glycogen is a storage reaction in animals and plants, creating an energy reserve rather than releasing usable energy. None of these processes are direct energy releasing steps for immediate cellular work in the way that ATP hydrolysis to ADP is.

Common Pitfalls:
Students sometimes confuse the idea of energy rich products with energy release. They may incorrectly choose ADP to ATP or NADP+ to NADPH because they know these molecules are high energy forms, but the question is specifically asking about a conversion that releases energy, not one that stores it. Another common mistake is to think that any reaction that produces ATP must release energy, without recognising that energy had to be supplied to form ATP in the first place. To avoid such confusion, always ask whether energy flows into the molecule (storage) or out of it (release) during the conversion.

Final Answer:
Energy is directly released for cellular work during the conversion of ATP to ADP.