The following question is in the public domain. It concerns a description of the unique characteristics of chemical substances and macromolecules used by living systems, as that subject relates to the correlation between stucture and function of biologically important molecules and their relationship to cell processes. It was directly lifted from the web site of public release questions posted by a state department of education on the date of posting (link).
Which statement describes the major role of lipids within a cell?
A. They cause DNA to replicate.
B. They move RNA in the cytoplasm.
C. They catalyze chemical reactions in the cell cytoplasm.
D. They are the main structural components of membranes.
The main roles of lipids in cells are the following:
a. Store energy
b. Make membranes
But since storing energy wasn’t one of the choices, we should choose Option (D), “They are the main structural components of cell membranes.”
Carbohydrates, especially sugars and starches, also store energy for our cells, so putting “store energy” as one of the other answers would have made the question inaccurate.
For Option (C), “They catalyze chemical reactions in the cell cytoplasm,” that is one of the primary roles of proteins within a cell. Most catalysts in our cells are proteins.
The primary roles of proteins within a cell are:
a. Transport molecules
— Across membranes (sodium-potassium pump)
— In circulation (hemoglobin, cytochromes)
b. Catalyze chemical reactions
c. Regulation, such as of osmotic pressure or gene expression
d. Support, as in collagen fibers
e. Motion, as in muscles (actin, myosin)
f. Defense (immunoglobulins, cell surface antigens)
Finally, lipids play almost no direct role in the replaction or transport of RNA or the replication of DNA.