Final answer:
The Golgi apparatus is the organelle that modifies the carbohydrate tree of glycosylated proteins. This crucial cellular structure refines and alters protein-bound carbohydrates, ensuring proper post-translational modifications before proteins reach their final destinations within the cell.
Step-by-step explanation:
The Golgi apparatus is an essential cellular organelle responsible for organizing, modifying, packaging, and tagging proteins and lipids that have been synthesized in the endoplasmic reticulum (ER). Proteins that are passed on to the Golgi from the ER are often glycosylated, and it is within the Golgi that they undergo further modifications such as the addition of short chains of sugar molecules to produce glycolipids and glycoproteins. These modified molecules then play important roles in cell-cell recognition and signaling once they are incorporated into the plasma membrane.