Moof’s Medical Biochemistry Video Course: https://moof-university.thinkific.com/courses/medical-biochemistry-for-usmle-step-1-exam

Questions Answered in This Video:

- What are lipids? How are lipids defined?
- Are lipids fats?
- How can lipids be classified? What are the different types or classes of lipids?
- What are the different functions of lipids?
- How do lipids relate or different from the other classes of macromolecules?
- What are the monomers and polymers of lipids?

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Video Content Summary:

In this video, I begin the introduction to lipids, what they are, what kinds there are, and what their functions are.

A common misconception is that lipids are fats. Though fats are lipids, not all lipids are fats. Lipids are defined, essentially, as molecules that are mostly nonpolar or hydrophobic, and, thus, insoluble in water, at least for the most part. Many lipids, however, are amphipathic or amphiphilic because they have some hydrophobic (nonpolar) and some hydrophilic (polar) portions.

The functions of lipids vary widely. Some lipids can store energy and/or be used as fuel. Some make up membranes and are known as membrane lipids. Some can act as hormones, and hormones are signaling molecules. Others are key nutrients, as some vitamins are lipids, though that’s not discussed much in this video or the rest of the videos in this series.

Lipids are set apart from the other classes of macromolecules – carbohydrates, proteins, and nucleic acids – because they do not have monomers or polymers in the way that the other three do. Carbohydrate monomers are monosaccharides, and their polymers are polysaccharides. For proteins, the monomers are amino acids, and the polymers are polypeptides, which can fold and become functional proteins. Nucleic acids have nucleotide monomers, and polynucleotide polymers, which are simply called nucleic acids. With lipids, this set-up isn’t the case. There aren’t any monomeric or polymeric units. This is something that is seen when discussing the specific structures of other lipids in the other videos of this lipid series.

The lipids mentioned in this video are 1) free fatty acids 2) triacylglycerols or triglycerides 3) phospholipids 4) sphingolipid 5) glycolipids 6) steroids. Free fatty acids are the simplest lipid, and they are used for fuel, as they can be broken down for energy via beta oxidation, or they can be created via fatty acid synthesis. Triacylglycerols or triglycerides are two names for the same thing, and they are used for fuel storage — they are a key storage form of energy in cells. Phospholipids, sphingolipids, and glycolipids are all membrane lipids because they all show up are membrane components. Phospholipids have phosphate groups, sphingolipids, have a sphingosine backbone, and glycolipids have sugar moieties attached to them. Steroids, finally, are important in membranes and as hormones or signaling molecules.