Answer & explanation:
Given that these characteristics are not described in the question (only the alternatives), I will summarize lipids and explain why steroids are classified as lipids.
Lipids are generally classified into four types: Triglycerides, cerids, phospholipids and steroids.
- Triglycerides are fats (animal origin) and oils (vegetable origin). Triglycerides are composed of glycerol (alcohol) associated with three fatty acid molecules;
- Cerides are composed of an alcohol other than glycerol. They have solid consistency and their main function is waterproofing, avoiding water loss on surfaces subject to dehydration. It can be of animal origin, such as beeswax, constituting the hive, and ear wax, protecting the ear canal.
- Phospholipids are the lipids associated with phosphoric acid. They are amphipathic molecules, in other words, they have one region (head) which is hydrophilic, and another hydrophobic region (tail).
- Steroids are another class of lipid molecules, identified by their four interconnected ring structure. Although not structurally similar to other lipids, steroids are included in this category because they are also hydrophobic and insoluble in water. Cholesterol is the most important steroid, being part of the structure of cell membranes, giving it greater rigidity, and acting as a precursor in the synthesis of biologically active steroids, such as steroid hormones and bile acids and salts. However, its excess in the blood is usually associated with heart disease such as myocardial infarction.
