Lecture: Lipids – Structure, Classification, and Functions

A comprehensive set of Q&A flashcards covering definitions, classifications, properties, reactions, and biological roles of lipids, suitable for exam review.

What is the general definition of a lipid?

An organic compound found in living organisms that is insoluble in water but soluble in non-polar organic solvents.

What is saponification?

A basic (alkaline) hydrolysis reaction where ester bonds in a lipid are broken, producing glycerol and fatty-acid salts (soaps).

List four major biological functions of lipids.

Energy storage, insulation/protection of organs, prevention of heat loss, and supply of carbon atoms for biosynthesis.

Which lipid class is the most abundant energy-storage lipid?

Triacylglycerols (TAG).

Membrane lipids are chiefly composed of which three lipid types?

Phospholipids, sphingolipids, and cholesterol.

Give two examples of emulsification lipids.

Bile acids such as cholic acid and deoxycholic acid.

Name the two broad groups of lipids based on their ability to undergo saponification.

Saponifiable lipids and non-saponifiable lipids.

Identify any two saponifiable lipids.

Triacylglycerols and phospholipids (others: sphingoglycolipids, biological waxes).

Identify any two non-saponifiable lipids.

Cholesterol and eicosanoids (others: steroid hormones, bile acids).

What structural feature do most naturally occurring fatty acids share?

They contain an even number of unbranched carbon atoms.

Define a saturated fatty acid (SFA).

A fatty acid whose carbon chain contains only carbon-carbon single bonds.

Define a monounsaturated fatty acid (MUFA).

A fatty acid whose carbon chain contains exactly one carbon-carbon double bond.

Define a polyunsaturated fatty acid (PUFA).

A fatty acid whose carbon chain contains two or more carbon-carbon double bonds (up to six for biologically important PUFA).

How is an omega-3 fatty acid structurally defined?

Its terminal (endmost) double bond is three carbon atoms away from the methyl end of the chain.

How is an omega-6 fatty acid structurally defined?

Its terminal double bond is six carbon atoms away from the methyl end of the chain.

How does carbon-chain length affect fatty-acid water solubility?

Solubility decreases as chain length increases; short chains are slightly soluble, long chains are insoluble.

How do chain length and saturation influence fatty-acid melting point?

Longer chains and fewer double bonds raise the melting point; more double bonds lower it.

Why are long-chain unsaturated fatty acids liquid at room temperature?

Cis double bonds create bends that prevent tight packing, weakening intermolecular attractions and lowering melting point.

What is triacylglycerol chemically?

A triester of glycerol and three fatty acids; commonly called triglyceride.

Where in the body are triacylglycerols primarily stored?

Adipocytes in skin, abdominal cavity, and mammary glands.

Differentiate fats and oils with respect to room-temperature state and source.

Fats are mostly solid/semi-solid at 25 °C and usually animal-derived; oils are liquid at 25 °C and typically plant or fish derived.

Which dietary fats are considered ‘good’ for cardiovascular health?

Monounsaturated fats (e.g., olive, canola, avocado oils).

Why are saturated fats regarded as ‘bad’ fats?

High intake is linked to increased heart-disease risk and higher LDL cholesterol.

Give one benefit and one risk of polyunsaturated fats.

Benefit: reduce heart disease risk; Risk: may increase risk of certain cancers.

Name the two essential fatty acids for humans.

Linoleic acid and linolenic acid.

State one symptom of essential-fatty-acid deficiency.

Red, irritated skin (other symptoms: susceptibility to infection/dehydration, liver abnormalities, growth impairment in infants).

Linoleic acid is the precursor to which important 20-carbon fatty acid?

Arachidonic acid.

List two omega-3 derivatives formed from linoleic/linolenic acids that are vital for brain and retina.

EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid).

Which laboratory reaction converts unsaturated fats to saturated fats by adding hydrogen?

Hydrogenation.

What undesirable by-product can be produced during partial hydrogenation of oils?

Trans fatty acids.

How do trans fatty acids affect blood lipids?

They raise LDL (bad) cholesterol but do not raise HDL (good) cholesterol.

Name two pancreatic enzymes that hydrolyze dietary triacylglycerols.

Lipase and trypsin (lipase is primary; trypsin assists).

What chemical reaction of triacylglycerols causes rancid odor in stored fats?

Oxidation of double bonds by atmospheric oxygen producing aldehydes and carboxylic acids.

Which vitamins are commonly added to commercial fats as antioxidants?

Vitamin C and Vitamin E.

What is lecithin chemically, and where is it used?

Phosphatidylcholine (a glycerophospholipid); used as an emulsifier in products such as mayonnaise, ice cream, and custards.

What are cephalins and where are they abundant?

Phosphatidylethanolamines and phosphatidylserines found in heart, liver, and especially brain; important in blood clotting.

Which sphingophospholipid is a key component of the myelin sheath?

Sphingomyelin.

Differentiate cerebrosides and gangliosides by complexity and location.

Cerebrosides are simpler sphingoglycolipids found in brain, myelin, and nerves; gangliosides are more complex and concentrated in gray matter of the brain.

Where is cholesterol synthesized in the body, and roughly how much daily?

Mainly in liver and intestines; about 800–1000 mg per day.

What lipoprotein transports cholesterol from liver to tissues?

Low-density lipoprotein (LDL), known as ‘bad’ cholesterol.

What lipoprotein transports excess cholesterol back to the liver?

High-density lipoprotein (HDL), known as ‘good’ cholesterol.

Give two dietary items that can help raise HDL cholesterol.

Oatmeal and fatty fish such as salmon (others: nuts, citrus fruits, buckwheat, etc.).

What is the physiological role of bile acids?

They emulsify dietary lipids in the small intestine to aid digestion and absorption.

List the three main classes of steroid sex hormones and one function of each.

Estrogens – female secondary characteristics; Androgens – male secondary characteristics and muscle growth; Progestins – prepare uterus for implantation and maintain pregnancy.

Name two major adrenocortical hormone groups and a key role of each.

Mineralocorticoids – regulate Na⁺/K⁺ balance (e.g., aldosterone); Glucocorticoids – control glucose metabolism and counteract inflammation (e.g., cortisol).

What type of messenger lipids are prostaglandins, thromboxanes, and leukotrienes?

Eicosanoids.

How does aspirin reduce pain and fever?

By inactivating enzymes necessary for prostaglandin synthesis, thereby reducing inflammation and fever.

Which eicosanoid promotes platelet aggregation and clot formation?

Thromboxane.

Which eicosanoids rise during allergic or hypersensitivity reactions?

Leukotrienes.

Give two common applications of carnuba wax.

Automobile wax and floor/boat wax (others: shoe polish).

Why is lanolin valuable in skin creams?

It enhances water retention and softens skin, acting as an effective moisturizer.

State the five broad functional categories of lipids mentioned in the lecture.

Energy-storage lipids, membrane lipids, emulsification lipids, messenger lipids, and protective-coating (biological wax) lipids.