Final answer:
Carbohydrates can exist within cells for an extended period because their breakdown is controlled by enzymes and metabolic pathways, ensuring that energy is released only when necessary. These pathways also enable the synthesis of carbohydrates, allowing cells to store energy efficiently.
Step-by-step explanation:
The breakdown of carbohydrate to CO2 and H2O is indeed a spontaneous and favorable process, releasing free energy in the form of ATP through cellular respiration. However, carbohydrates can exist within cells for a long time due to the regulated nature of metabolic pathways. Carbohydrate molecules are stable forms of energy storage, and their breakdown is tightly controlled by enzymes within the cell. These enzymes only catalyze the breakdown of carbohydrates when energy is needed, allowing cells to regulate the use of carbohydrates efficiently and maintain an energy balance.
Carbohydrates serve not only as fuel but also as part of cellular structures, such as glycoproteins and glycolipids in cell membranes. The stable carbon backbone of carbohydrate molecules comes from carbon dioxide, which is fixed into organic molecules through the Calvin Cycle in plants and some microorganisms.
In the absence of enzymes and regulatory mechanisms, carbohydrates would quickly be consumed, releasing energy but leaving the cell without a reserve. It is this intricate regulation and the ability of cells to synthesize carbohydrates through photosynthesis using ATP and NADPH from the light-dependent reactions, and incorporate carbon from CO2, that allow for the long-term existence of carbohydrates within cells.