Which of the following correctly describes lipids?

Lipids are a diverse group of molecules that play critical roles in biological systems, and option C accurately describes two of their primary functions. Firstly, lipids serve as a major energy storage form within organisms. They are more energy-dense than carbohydrates, providing a concentrated source of energy that can be utilized by the body when needed.

Secondly, lipids are key components of cell membranes. Phospholipids, a type of lipid, form a bilayer that serves as a barrier to protect the interior of the cell, while also allowing for the selective passage of substances in and out of the cell. This structural role is essential for maintaining cellular integrity and function.

The other options do not describe lipids correctly. The first option mischaracterizes lipids as polymers made of amino acids, which is more accurately a description of proteins. The second option incorrectly claims that lipids are mainly soluble in water; in reality, lipids are hydrophobic and generally insoluble in water, which is why they can form membranes that separate different environments within a cell. The last option pertains to nucleic acids, such as DNA and RNA, which are responsible for genetic information storage, rather than lipids. Hence, the correct answer highlights the essential roles of