Source: Biology, University of Michigan
Tags: lipids, fats, triglycerides, fatty acids, saturated fat, unsaturated fat, trans fat, phospholipids, steroids, cholesterol, nucleic acids, DNA, RNA, nucleotides, phosphodiester linkage, base pairing, adenine, thymine, guanine, cytosine, uracil
Difficulty: Introductory | Prerequisites: Carbohydrates and Proteins sections of Chapter 5, basic understanding of covalent bonds and functional groups
Lipids and nucleic acids round out the four macromolecule families. Lipids are the odd ones out: they are not true polymers, they are hydrophobic, and they serve roles ranging from long-term energy storage to forming every cell membrane in your body. Nucleic acids (DNA and RNA) carry and execute the genetic instructions that tell a cell which proteins to build, connecting this chapter directly to genetics and gene expression. By this point in the chapter you should be comfortable with dehydration reactions and the monomer-to-polymer logic covered in carbohydrates and proteins.
Lipids are hydrophobic molecules made mostly of C, H and O. The most common are triglycerides (energy storage), phospholipids (cell membranes) and steroids (cholesterol, hormones). Nucleic acids are polymers of nucleotides joined by phosphodiester bonds. DNA stores genetic information as a double helix; RNA reads and carries that information to build proteins. DNA uses the bases A, T, G, C; RNA swaps thymine for uracil (U).
Lipid
A broad category of hydrophobic (water-fearing) organic molecules composed mainly of C, H and O. Lipids are not true polymers, but they are assembled by dehydration reactions.
Triglyceride (fat)
A lipid consisting of one glycerol molecule bonded to three fatty acid chains via ester linkages. Think of it as the body's main long-term energy storage molecule.
Fatty acid
A long hydrocarbon chain with a carboxyl group (–COOH) at one end. The building block of fats and phospholipids.
Saturated fat
A fat in which all three fatty acid chains carry the maximum number of hydrogen atoms (no double bonds between carbons). Solid at room temperature. In simple terms, the carbon chain is "saturated" with hydrogens.
Unsaturated fat
A fat with one or more double bonds in at least one fatty acid chain, resulting in fewer hydrogen atoms and kinks in the chain. Liquid at room temperature (oils).
Trans fat
An unsaturated fat that has been artificially hydrogenated, converting some double bonds to a "trans" configuration. Associated with increased health risks.
Hydrogenation
The industrial process of adding hydrogen to unsaturated fats, converting them toward saturation. This is how liquid vegetable oils become solid (margarine) and how trans fats are produced.
Ester linkage
The covalent bond between a hydroxyl group (on glycerol) and a carboxyl group (on a fatty acid), formed by a dehydration reaction. The lipid equivalent of a glycosidic or peptide bond.
Phospholipid
A lipid with two fatty acid tails and a phosphate-group head. The head is hydrophilic (polar); the tails are hydrophobic. Phospholipids form the bilayer of every cell membrane.
Steroid
A lipid whose carbon skeleton is arranged in four fused rings, structurally distinct from fats. Cholesterol is the base steroid from which the body makes sex hormones and other signalling molecules.
Cholesterol
A steroid found in animal cell membranes that helps maintain membrane fluidity. Also the precursor for steroid hormones (oestrogen, testosterone).
Nucleic acid
A polymer of nucleotides that stores and transmits genetic information. The two types are DNA and RNA.
Nucleotide
The monomer of nucleic acids. Each nucleotide has three parts: a phosphate group, a pentose sugar (deoxyribose in DNA, ribose in RNA) and a nitrogenous base.
Phosphodiester linkage
The covalent bond that links nucleotides in a nucleic acid strand, formed when a phosphate group connects the sugars of two adjacent nucleotides.
Base pairing (complementary base pairing)
The rule that adenine (A) pairs with thymine (T) in DNA (or uracil (U) in RNA), and guanine (G) pairs with cytosine (C), held together by hydrogen bonds. In simple terms, A always goes with T (or U), and G always goes with C.
DNA (deoxyribonucleic acid)
A double-stranded nucleic acid that carries the genetic instructions for building proteins. Its sugar is deoxyribose, and it uses the bases A, T, G, C.
RNA (ribonucleic acid)
A usually single-stranded nucleic acid involved in translating genetic instructions into proteins. Its sugar is ribose (one extra –OH group compared to deoxyribose), and it uses U in place of T.
Composed primarily of C, H and O (same elements as carbohydrates, but in different proportions)
Hydrophobic (non-polar): do not dissolve in water
Lipids are NOT true polymers; they lack a repeating monomer unit, though they are still assembled via dehydration reactions
Three main categories: fats (triglycerides), phospholipids and steroids
Structure: one glycerol molecule + three fatty acid chains, joined by ester linkages
Function: long-term energy storage, cushioning of organs, thermal insulation
Fats store roughly twice as much energy per gram as carbohydrates
Saturated fats
No double bonds in the fatty acid chains; chains are straight and pack tightly
Solid at room temperature (butter, animal fat)
A fat is only classified as saturated if all three fatty acid chains are fully saturated
Unsaturated fats
One or more double bonds create kinks in the chain, preventing tight packing
Liquid at room temperature (olive oil, fish oil)
Monounsaturated = one double bond; polyunsaturated = two or more
Hydrogenation and trans fats
Hydrogenation adds hydrogen atoms to unsaturated fats, removing double bonds and making the fat more solid
This process can produce trans fats, in which the remaining double bonds have a "trans" configuration
Trans fats are associated with increased cardiovascular risk
Structure: glycerol + two fatty acid tails + one phosphate-group head
The phosphate head is hydrophilic; the fatty acid tails are hydrophobic
This amphipathic nature is what allows phospholipids to form the lipid bilayer of cell membranes: heads face the water, tails face each other
Structure: four fused carbon rings (distinct from fats)
Cholesterol is the base steroid in animal cells
Embedded in cell membranes, where it helps regulate fluidity
Precursor to sex hormones (oestrogen, testosterone) and other signalling steroids
Anabolic steroids are synthetic variants of testosterone used to promote muscle growth
Polymers of nucleotides, joined by phosphodiester linkages
Contain C, H, O, N and P (phosphorus distinguishes nucleic acids from carbohydrates and most proteins)
Two types: DNA and RNA
Function: store genetic information (DNA) and translate it into proteins (RNA)
Each nucleotide consists of three components:
A phosphate group
A pentose (five-carbon) sugar: deoxyribose in DNA, ribose in RNA
A nitrogenous base: adenine (A), guanine (G), cytosine (C) and either thymine (T) in DNA or uracil (U) in RNA
Nucleotides are linked by phosphodiester bonds: the phosphate of one nucleotide connects to the sugar of the next.
Double-stranded helix
Sugar: deoxyribose (one fewer –OH group than ribose)
Bases: A, T, G, C
Base-pairing rules: A pairs with T (two hydrogen bonds), G pairs with C (three hydrogen bonds)
Located in the nucleus of the cell
Carries the complete set of genetic instructions for building proteins
Usually single-stranded
Sugar: ribose (has an extra –OH group on the 2' carbon compared to deoxyribose)
Bases: A, U, G, C (uracil replaces thymine)
Found in both the nucleus and the cytoplasm
Functions in translating DNA's instructions into proteins (mRNA, tRNA, rRNA)
The phospholipid bilayer is the structural basis of every cell membrane, which is why understanding phospholipids matters far beyond this chapter. Trans fats were largely banned from processed foods in many countries after research linked them to heart disease, a direct application of the saturated/unsaturated distinction covered here. DNA's base-pairing rules are the foundation of forensic DNA fingerprinting, paternity testing and the entire field of genetic engineering, including CRISPR gene editing.
Students often call lipids "polymers of fatty acids." Lipids are not true polymers. They are assembled by dehydration reactions, but a triglyceride is three fatty acids on a glycerol backbone, not a repeating chain of identical subunits.
"Saturated" and "unsaturated" refer to hydrogen saturation of the carbon chain, not to how much fat you have eaten. A fat molecule is saturated when every carbon-carbon bond is single and the chain carries the maximum possible hydrogens.
Students frequently forget that DNA and RNA differ in both sugar and base. DNA has deoxyribose + thymine; RNA has ribose + uracil. Both differences matter.
Cholesterol is often treated as purely harmful. In biology, cholesterol is essential: it sits in animal cell membranes to regulate fluidity and is the precursor for steroid hormones.
⚠️ Know why lipids are not true polymers, even though they use dehydration reactions.
⚠️ Be able to distinguish saturated, unsaturated and trans fats by bond type and physical state at room temperature.
⚠️ Understand phospholipid structure and explain why the bilayer forms spontaneously in water (hydrophilic heads out, hydrophobic tails in).
⚠️ Memorise the base-pairing rules: A–T (DNA) or A–U (RNA), and G–C. Expect a question requiring you to write the complementary strand of a given DNA or RNA sequence.
⚠️ Know the three components of a nucleotide and be able to state what differs between DNA and RNA nucleotides (sugar and one base).
True or false: Lipids are polymers of fatty acids. (False. Lipids are not true polymers.)
Fill in the blank: A triglyceride consists of one ______ molecule bonded to three ______ chains. (glycerol, fatty acid)
True or false: Unsaturated fats are solid at room temperature. (False. Unsaturated fats are liquid at room temperature.)
Fill in the blank: In DNA, adenine pairs with ______ and guanine pairs with ______. (thymine, cytosine)
True or false: RNA uses deoxyribose as its sugar. (False. RNA uses ribose; DNA uses deoxyribose.)
Q: Why are lipids classified as hydrophobic, and what consequence does this have for cell biology?
A: Lipids are composed mostly of non-polar C–H bonds, so they do not dissolve in water. This hydrophobicity is what allows phospholipids to spontaneously form a bilayer in aqueous environments, creating the barrier that defines every cell.
Q: Describe the structural difference between a saturated and an unsaturated fatty acid. How does this affect the fat's physical state?
A: A saturated fatty acid has only single bonds between carbons and carries the maximum number of hydrogens; its chain is straight. An unsaturated fatty acid has one or more double bonds, which create kinks. Straight chains pack tightly (solid at room temperature); kinked chains cannot pack as closely (liquid at room temperature).
Q: What is hydrogenation, and why does it produce trans fats?
A: Hydrogenation is the addition of hydrogen to unsaturated fats, converting double bonds to single bonds. During the process, some remaining double bonds shift to a trans configuration rather than the natural cis form. These trans fats are associated with increased cardiovascular risk.
Q: List the three components of a nucleotide and state how DNA and RNA nucleotides differ.
A: A nucleotide has a phosphate group, a pentose sugar and a nitrogenous base. In DNA the sugar is deoxyribose and the bases are A, T, G, C. In RNA the sugar is ribose (an extra –OH on the 2' carbon) and uracil (U) replaces thymine (T).
Q: Given the DNA strand 5'-ATCGGA-3', write the complementary strand.
A: 3'-TAGCCT-5'. Apply the base-pairing rules: A pairs with T, T pairs with A, C pairs with G, G pairs with C.
Q: Explain why phospholipids arrange themselves into a bilayer in water.
A: Phospholipids are amphipathic: the phosphate head is hydrophilic and the two fatty acid tails are hydrophobic. In water, the heads face outward toward the aqueous environment while the tails face inward, shielded from water. This arrangement is energetically favourable and forms spontaneously.
Phospholipids connect directly to Chapter 7 (cell membranes and transport), where the bilayer's selective permeability is central. DNA and RNA connect to genetics and gene expression (Chapters 10 onwards): DNA replication, transcription (DNA to RNA) and translation (RNA to protein) all depend on the base-pairing rules and nucleotide structure covered here. Steroids and cholesterol will reappear in discussions of cell signalling and the endocrine system.
lipids, fats, triglycerides, glycerol, fatty acids, saturated fat, unsaturated fat, monounsaturated, polyunsaturated, trans fat, hydrogenation, ester linkage, phospholipid, lipid bilayer, cell membrane, amphipathic, hydrophobic, hydrophilic, steroids, cholesterol, anabolic steroids, testosterone, nucleic acids, DNA, RNA, nucleotide, phosphodiester linkage, deoxyribose, ribose, nitrogenous base, adenine, thymine, guanine, cytosine, uracil, base pairing, complementary strand, double helix, single-stranded, macromolecules, BIO 101, Chapter 5