Organelles and Their Roles

Objective: Connect genetic information to protein construction.

The nucleus houses most of a eukaryotic cell’s DNA and supports transcription of many genes into RNA. Nuclear pores regulate exchange with the cytoplasm. Ribosomes read messenger RNA and assemble amino acids into proteins; they may be free in cytosol or associated with rough endoplasmic reticulum. Ribosomes themselves are not membrane-bound and occur in both prokaryotic and eukaryotic cells. A gene’s product depends on expression and processing, so DNA is an instruction source rather than a tiny finished protein stored in the nucleus.

Objective: Trace a secreted protein through the endomembrane system.

Ribosomes on rough endoplasmic reticulum make many proteins destined for secretion, membranes, or certain organelles. The ER assists folding and initial processing. Transport vesicles move cargo to the Golgi apparatus, where it can be modified, sorted, and sent onward. Smooth ER has roles including lipid synthesis and other functions that vary by cell type. This is a general route, not a rule that every protein passes through the Golgi; proteins made on free ribosomes often follow different destinations.

Objective: Compare cellular respiration and photosynthetic compartments.

Mitochondria carry out major steps of aerobic cellular respiration in many eukaryotic cells, helping convert energy from fuel molecules into ATP. Chloroplasts in photosynthetic eukaryotes capture light energy and support synthesis of sugars from carbon dioxide and water. Plants generally have both mitochondria and chloroplasts in photosynthetic tissues; chloroplasts do not replace respiration. Both organelles have internal membranes and their own DNA, features relevant to endosymbiotic explanations of their evolutionary origins. ATP and sugars serve different roles, and neither organelle “creates energy.”