ATP, the "Currency" of Cell Energy

Section 2: ATP, the "Currency" of Cell Energy

Fireflies glow by utilizing cellular energy. Similarly, within cells, active transport of substances, synthesis of molecules, and muscle fiber contraction all require energy. Where does this energy come from? Organic compounds like sugars and fats stored in cells contain chemical energy, but the direct provider of energy for cellular activities is another type of organic molecule: ATP.

ATP: A High-Energy Phosphate Compound

ATP, short for adenosine triphosphate, drives cellular activities directly. The structure of an ATP molecule can be simplified as A-PPP, where A represents adenosine and P represents phosphate groups, with ~ indicating a special chemical bond. Due to repulsion between the negatively charged phosphate groups, this bond is unstable, and the terminal phosphate group has a tendency to leave ATP and bind with other molecules, possessing high transfer potential energy. When ATP undergoes hydrolysis catalyzed by enzymes, the released terminal phosphate group can bind to other molecules, thereby causing changes. Thus, the hydrolysis of ATP is a process that releases energy, with 1 mole of ATP hydrolysis releasing up to 30.54 kJ of energy, making ATP a high-energy phosphate compound.

Interconversion of ATP and ADP

After hydrolysis, ATP converts into ADP (adenosine diphosphate), a more stable compound than ATP. If the released phosphate group is not transferred to other molecules, it becomes free inorganic phosphate (Pi). With the action of specific enzymes, ADP can accept energy and bind with Pi to regenerate ATP.

For normal cellular activities, this mutual conversion between ATP and ADP occurs continuously in dynamic equilibrium. It's estimated that during intense physical activity, approximately 0.5 kg of ATP per minute converts to ADP, releasing energy for muscle movement. The generated ADP can then, under certain conditions, convert back into ATP. The energy supply mechanism involving the interconversion of ATP and ADP is universal across all cells in the biological world, demonstrating biological unity.

Utilization of ATP

Most energy-consuming cellular activities rely directly on ATP, such as brain functions, electric discharge in electric rays, and active transport of substances. By referring to Figure 5-6, can you provide other examples of ATP utilization?

How is the energy released by ATP hydrolysis utilized in the above-mentioned life activities? Figure 5-6 illustrates how ATP provides energy for active transport.

The phosphate groups released by ATP hydrolysis phosphorylate molecules like proteins, a common occurrence in cells. Once phosphorylated, these molecules undergo structural changes and altered activity, enabling them to participate in various chemical reactions.

Chemical reactions within cells can be classified into energy-absorbing (endothermic) and energy-releasing (exothermic) reactions. The former includes processes like protein synthesis, which require energy absorbed from ATP hydrolysis; the latter includes processes like glucose oxidation, which release energy. Many energy-absorbing reactions are linked to ATP hydrolysis, which provides the necessary energy, while energy released by exothermic reactions is stored in ATP, used to directly supply energy for energy-absorbing reactions. Thus, energy circulates between ATP molecules in energy-absorbing and energy-releasing reactions. Therefore, ATP can metaphorically be likened to the circulating "currency" of energy within cells.

It is precisely because cells possess ATP as this energy "currency" that they can timely and continuously meet the energy demands of various life activities.

The glowing cells at the rear of firefly bodies contain luciferin and luciferase. Luciferin, activated by energy provided by ATP, reacts chemically with oxygen under the catalysis of luciferase to form oxidized luciferin and emit light. Scientists have applied this principle by introducing the luciferase gene into plants and irrigating them with luciferin solution, resulting in genetically modified plants that glow in darkness, thus cultivating a luminous "fluorescent tree."