What is synthesized by the endoplasmic reticulum?

The endoplasmic reticulum (ER) plays a crucial role in the synthesis of proteins and lipids, making it essential for cellular function and maintaining homeostasis. The rough endoplasmic reticulum, which has ribosomes on its surface, is primarily responsible for the synthesis of proteins. These proteins are either secreted from the cell, incorporated into the cell's plasma membrane, or sent to an organelle. The smooth endoplasmic reticulum, on the other hand, is involved in the synthesis of lipids, including phospholipids and cholesterol, which are critical components of cell membranes and precursors for various hormones.

Considering the functions of the endoplasmic reticulum, it's clear why this option is accurate. The ER's ability to synthesize both proteins and lipids is fundamental to the production of biomolecules that are vital for life processes, contributing to the complex tapestry of cellular architecture and signaling pathways.