Question

Bio Ch 3 What makes a cell start to burn its stores of lipids or proteins when glucose runs out? Then when the supplies of glucose return, what stops the cell from burning all of its lipids or proteins, literally eating itself up? And finally, what triggers a cell to build molecules only when they are needed?

Answer 1

In the absence of glucose, the body switches to burning lipids and, if necessary, proteins to produce energy, sparing essential proteins as much as possible. When food is reintroduced, glucose replenishes glycogen reserves first, then excess is converted to fat. Cellular metabolism is carefully regulated to balance ATP production and utilization, conserving body resources.

Step-by-step explanation:

When the body experiences a lack of glucose, it enters starvation mode where the metabolism of lipids and proteins is altered to provide sufficient energy. Initially, glucose levels support brain function but as they diminish, the body starts to burn fat reserves by converting fatty acids into ketones through ketogenesis. As glucose levels drop, glycolysis shuts off in cells that can use alternative fuels; for example, muscles will use fatty acids instead of glucose to create ATP (adhosphine triphosphate).

When the body re-enters an absorptive state (with the reintroduction of food), glucose is used first to replenish glycogen reserves. Once these reserves are refilled, the anabolic metabolism of fats and proteins resumes, thus preventing the body from depleting its own crucial reserves. Glucose utilization prioritizes brain function until fat reserves are depleted, after which protein catabolism begins, sparing muscle mass as much as possible.

Finally, the regulation of cellular respiration ensures that cells generate ATP as needed, directing intermediates to pathways for synthesizing necessary compounds, rather than producing excess ATP. This metabolic control is fundamental for maintaining energy balance and conserving resources within the body.