Here's why:

* Temperature:

* High temperatures can cause molecules to vibrate more rapidly, potentially breaking weak bonds like hydrogen bonds and van der Waals interactions. This can disrupt the shape and function of proteins (enzymes), DNA, and other biological molecules.

* Low temperatures slow down molecular motion, making it harder for molecules to interact properly and for reactions to occur. This can also affect enzyme activity.

* pH:

* Extreme pH values (very acidic or very basic) can disrupt the charges on amino acids in proteins. This can cause proteins to denature (lose their shape) and become inactive.

Examples:

* Enzymes: Enzymes are proteins that catalyze (speed up) chemical reactions. Their activity is highly dependent on their shape, which is maintained by weak bonds. Changes in temperature or pH can disrupt these bonds, causing the enzyme to lose its shape and become inactive.

* DNA: DNA is a double helix held together by hydrogen bonds. Extreme temperatures or pH changes can break these bonds, leading to damage and degradation of the DNA molecule.

In summary, changes in temperature and pH can disrupt the weak bonds that hold biological molecules together, altering their shape and function.