A glucose molecule originating from an apple has reached a muscle cell in your thigh. Identify the process and describe how it converts the glucose molecule into a usable form of energy, namely ATP.

Once a glucose molecule originating from an apple reaches a muscle cell, it undergoes a biological process called cellular respiration to convert into a usable form of energy, ATP (adenosine triphosphate). The process of converting glucose into ATP in a muscle cell consists of several stages:

1. Glycolysis: This is the first step and occurs in the cytoplasm of the cell. The glucose molecule, which is a six-carbon sugar, is broken down into two three-carbon molecules called pyruvate. This process produces a net gain of 2 ATP molecules and 2 NADH molecules (an electron carrier).

2. Pyruvate Oxidation: If oxygen is present, the pyruvate molecules are transported into the mitochondria. Here, each pyruvate molecule is converted into a molecule called acetyl-CoA. This process releases carbon dioxide and generates more NADH.

3. Citric Acid Cycle (Krebs Cycle): The acetyl-CoA enters the citric acid cycle. Throughout this cycle, acetyl-CoA is broken down and more carbon dioxide is released. This cycle produces 2 more ATP molecules per glucose molecule, along with additional NADH and another electron carrier called FADH2.

4. Electron Transport Chain (ETC) and Oxidative Phosphorylation: The NADH and FADH2 molecules produced in the previous steps carry electrons to the inner mitochondrial membrane, where the electron transport chain is located. As electrons pass along the chain through a series of redox reactions, protons are pumped from the mitochondrial matrix into the intermembrane space, creating a proton gradient. This gradient stores potential energy.

5. Chemiosmosis: Finally, the protons flow back into the mitochondrial matrix through an enzyme called ATP synthase. This flow drives the synthesis of ATP from ADP and inorganic phosphate. This final stage of the electron transport chain can produce around 34 ATP molecules per glucose molecule, depending on the efficiency and conditions in the cell.

Overall, from one glucose molecule, the process of cellular respiration can yield up to about 38 ATP molecules, although in many cases, it is more like 30-32 due to inefficiencies and the cost of transporting pyruvate into mitochondria and NADH produced in glycolysis into the mitochondrial matrix.