Which is the most important energy carrier in living cells?

ATP

The correct answer is option 3. ATP.

ATP (adenosine triphosphate) is the primary energy carrier molecule in living cells, often referred to as the "energy currency" of the cell. Its importance stems from its ability to store and transfer energy for various cellular processes. ATP is composed of three main components:

Adenine: A nitrogenous base.

Ribose: A five-carbon sugar molecule.

Three Phosphate Groups (Triphosphate): These are attached to the ribose molecule, forming the "tri-" part of ATP.

The bonds between the phosphate groups in ATP are high-energy bonds. These bonds are referred to as phosphoanhydride bonds and store a considerable amount of energy due to electrostatic repulsion between the negatively charged phosphate groups. The third phosphate group is attached to the rest of the molecule via a high-energy bond.

When ATP is hydrolyzed (broken down) by the enzyme ATPase, a molecule of water is added, resulting in the cleavage of one of the phosphate groups. This reaction releases energy, converting ATP into ADP (adenosine diphosphate) and inorganic phosphate (Pi). The released energy is then used to drive various energy-requiring cellular processes.

\(ATP → ADP + Pi + \text{ Energy}\)

The released energy from ATP hydrolysis is used to power cellular processes such as:

• Muscle contraction
• Active transport of ions and molecules across cell membranes
• Biosynthesis of macromolecules such as proteins, DNA, and RNA
• Cellular signaling and communication
• Mechanical work, such as movement of cilia and flagella

ADP and inorganic phosphate can be recombined to form ATP through cellular processes such as cellular respiration and photosynthesis, where energy from the breakdown of organic molecules (such as glucose) is used to drive the synthesis of ATP.

In summary, ATP is the most important energy carrier in living cells due to its ability to store and transfer energy in high-energy phosphate bonds. Its hydrolysis provides the energy needed for a wide range of cellular processes, making ATP essential for the survival and function of all living organisms.