The difference in structure and function between the rough and smooth endoplasmic reticulum is what’s being tested. The rough ER is studded with ribosomes, making it the site where proteins destined for secretion or for membranes are synthesized and folded, then processed as they enter the ER lumen or become part of the ER membrane. The smooth ER lacks ribosomes and thus does not synthesize proteins; instead it focuses on lipid production for membranes and on detoxification processes, such as metabolizing drugs and toxins—especially in liver cells. So, saying that the rough ER has ribosomes and makes secreted and membrane proteins, while the smooth ER lacks ribosomes and makes lipids and handles detoxification, captures the key contrast. The other statements misstate ribosome presence or the primary roles of each ER type, or note functions (like DNA replication) that aren’t tied to ER activity.

Ribosomal RNA is the most abundant RNA in cells because it forms the core of ribosomes, the molecular machines that translate mRNA into proteins. Ribosomes are present in huge numbers to meet the demand for protein synthesis, and each ribosome contains multiple rRNA molecules that make up its structural and catalytic centers. Because of that immense demand, rRNA accounts for the majority of total cellular RNA across many cell types and conditions. mRNA, the templates that carry gene-specific instructions for protein production, is present in far fewer copies at any given time and fluctuates with gene expression. Transfer RNA is also essential for bringing amino acids to the ribosome and is relatively abundant, but it does not reach the same overall quantity as rRNA. Small nuclear RNA participates in RNA processing and is typically present in smaller amounts.

DNA’s backbone is the sugar-phosphate chain. Covalent phosphodiester bonds connect the deoxyribose sugar of one nucleotide to the phosphate group of the next nucleotide, creating a continuous sugar–phosphate backbone along each strand. This backbone runs the length of the molecule, while the nitrogenous bases extend inward and pair with the opposite strand via hydrogen bonds. So the covalent link that forms the backbone is between the sugar of one nucleotide and the phosphate of the next.

Stomata are openings on the leaf surface that primarily regulate gas exchange and water loss. Their main job is to control the movement of carbon dioxide into the leaf for photosynthesis and the release of oxygen, while also allowing water vapor to exit in a process called transpiration. Guard cells surrounding each stoma change shape to open or close the pore in response to light, CO2 levels, humidity, and signals like drought stress, which helps balance photosynthetic needs with water conservation. When stomata are open, CO2 diffuses into the leaf for fixation, and O2 exits as a byproduct; transpiration helps pull water up from the roots through the xylem, aiding nutrient transport and temperature regulation. Absorbing sunlight happens in chloroplasts inside leaf cells, not through stomata; water transport from roots is done by the xylem; and chlorophyll is produced in chloroplasts, not in the stomata.