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1.5Chemistry of Life

Lipids — AP BIOLOGY Revision Notes & Topical Questions

5 min read 8 parts Lesson 5 of 7 1 quiz

📚 AP BIOLOGY Hub / Topical Revision
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Lesson summary

1.5 Lipids — Notes — AP Biology notes: Describe the structure and function of lipids.Composition — lipids contain carbon, hydrogen and oxygen and are typically nonpolar, hydrophobic moleculesPolymers — lipids are not true polymersFatty acids — a hydrocarbon chain (R group) with a…

  • Triglyceride
  • Degree of unsaturation
  • Membrane fluidity
  • Insulation
  • Cholesterol
  • Energy per gram

Part 1 of 6General features of lipids

General features of lipids
Composition
  • Composition lipids contain carbon, hydrogen and oxygen and are typically nonpolar, hydrophobic molecules
Polymers
  • Polymers lipids are not true polymers
Fatty acids
  • Fatty acids a hydrocarbon chain (R group) with a methyl group at one end and a carboxyl group at the other, with shorthand formula RCOOH
Variation
  • Variation fatty acids vary in the length and the saturation of the hydrocarbon chain
Triglyceride
  • Triglyceride glycerol plus three fatty acids, joined by three ester bonds through dehydration synthesis that releases 3 H₂O

Part 2 of 6Saturated and unsaturated fatty acids

FeatureSaturatedUnsaturated
C–C bondsOnly single bondsAt least one C=C double bond
Chain shapeStraightBent (kinked) at each double bond
PackingPack together tightlyCannot pack tightly
State at room temperatureSolid, e.g. butterLiquid, e.g. olive oil
Degree of unsaturation
  • Degree of unsaturation the more double bonds in a fatty acid tail, the more unsaturated the lipid and the more liquid it is at room temperature
Saturated
  • Saturated a saturated lipid is saturated with hydrogen atoms and cannot hold more
Membrane fluidity
  • Membrane fluidity cis double bonds introduce kinks that increase membrane fluidity

Part 3 of 6Functions of lipids

Functions of lipids
Energy
  • Energy lipids are energy-dense and provide energy storage, about twice the energy per gram of carbohydrates
Insulation
  • Insulation subcutaneous fat provides thermal insulation, as in whale blubber, and cushions organs
Steroids
  • Steroids a class of lipids with four fused rings that act as hormones supporting growth and development, energy metabolism and homeostasis
Cholesterol
  • Cholesterol it provides structural stability to animal cell membranes
Waxes
  • Waxes hydrophobic lipids that reduce water loss, as in the plant cuticle
Energy per gram
  • Energy per gram fats store about 38 kJ g⁻¹ compared with about 17 kJ g⁻¹ for carbohydrates
Storage
  • Storage triglycerides are insoluble, so stored fat does not change the water potential of cells
Metabolic water
  • Metabolic water respiring fat yields more metabolic water than carbohydrate, as in a camel's hump
Other roles
  • Buoyancy and waterproofing, as in the waxy leaf cuticle and preening oils.
  • Electrical insulation of axons by myelin.
  • Pigments such as carotenoids.
  • Glycolipids act as cell-surface recognition molecules.

Part 4 of 6Phospholipids

Structure
  • Structure glycerol, two fatty acid tails (hydrophobic) and a phosphate head (hydrophilic), so a phospholipid is amphipathic
Bilayer
  • Bilayer in water, heads face the aqueous environment and tails face inward, forming the lipid bilayers of plasma and cell membranes
Micelles
  • Micelles phospholipids can also form micelles in water

Part 5 of 6Emulsion test for lipids

Method
  • Method dissolve the sample in ethanol, then pour the solution into water
Result
  • Result a milky-white emulsion shows lipid, because the nonpolar lipid comes out of solution, whereas a tube with no lipid stays clear

Part 6 of 6More from the specification

More from the specification
Nucleotide
  • The unit of DNA made of a phosphate, a deoxyribose sugar and a base.
Covalent bond
  • A shared electron pair linking monomers.
Primary structure
  • The sequence of amino acids in a polypeptide.
Glycosidic bond
  • The covalent link between two sugar units made by condensation.
Double helix
  • Two DNA strands wound around each other.
Protein
  • A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.
Monomer
  • A small molecule that links with others of its kind to build a polymer.
Glucose
  • A simple sugar; the unit that starch, glycogen and cellulose are built from.
Polypeptide
  • A chain of three or more amino acids.
Starch
  • An insoluble polysaccharide of glucose used by plants for energy storage.
Hydrolysis
  • A reaction that splits a molecule by adding water.

Key terms

TermMeaning
TriglycerideGlycerol joined to three fatty acids by ester bonds.
Saturated fatty acidNo C=C double bonds; straight and solid at room temperature.
Unsaturated fatty acidOne or more C=C bonds; kinked and liquid.
PhospholipidHydrophilic phosphate head and two hydrophobic tails.
SteroidLipid with four fused rings, such as cholesterol and hormones.
AmphipathicHaving both hydrophilic and hydrophobic parts.

Avoid these mistakes

1
Calling lipids true polymers.
2
Saying unsaturated fats have straight chains — double bonds kink the chain.
3
Confusing the hydrophilic head and hydrophobic tails of a phospholipid.
4
Thinking trans fats are healthy — they are linked to heart disease.
5
Forgetting that a triglyceride forms three ester bonds and releases three water molecules.

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