Explanation

Irritability in biology means the ability of cells and organisms to detect and respond to changes in their environment. Stimuli are environmental changes like light, temperature, chemicals, or touch. Cells can sense these changes and react appropriately to survive.

For example, nerve cells respond to stimuli by sending electrical signals. Plant cells respond to light by growing toward it. White blood cells respond to bacteria by attacking them. This responsiveness helps organisms adapt to their surroundings.

Generating energy, replicating DNA, and making proteins are all important cell functions. However, these processes are not specifically about responding to external changes. Only the ability to detect and respond to stimuli defines cellular irritability.