2.2.82 · Biological Molecules

Phospholipids — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 7 parts Lesson 14 of 25 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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2.2.8 Phospholipids — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the structure of phospholipids and explain their role in cell membranes.Components — a glycerol molecule bonded to two fatty acid tails and a phosphate head.Difference from triglycerides — …

Amphipathic
Arrangement
Building block
Compartments
Protein orientation
Polysaccharide

Part 1 of 5Structure of a phospholipid

Structure of a phospholipid

Components

Components a glycerol molecule bonded to two fatty acid tails and a phosphate head.

Difference from triglycerides

Difference from triglycerides one fatty acid is replaced by a phosphate ion (PO₄^3-).

Head

Head the polar phosphate is hydrophilic and soluble in water.

Tails

Tails the non-polar fatty acid chains are hydrophobic and insoluble in water.

Part 2 of 5Amphipathic behaviour in water

Amphipathic

Amphipathic phospholipids have both hydrophobic and hydrophilic parts.

Arrangement

Arrangement in water they form monolayers or bilayers.
At an air–water surface the heads sit in the water and the tails stick out.
In a bilayer the heads face the water and the tails face inwards.

Part 3 of 5Roles in membranes

Roles in membranes

Building block

Building block phospholipids are the main component of cell membranes.

Barrier

Barrier the hydrophobic core of the bilayer blocks water-soluble molecules.

Compartments

Compartments hydrophilic heads hydrogen bond with water, allowing cells to form organelles with specific roles.

Fluidity

Fluidity mainly unsaturated tails make a membrane more fluid, and mainly saturated tails make it less fluid.

Protein orientation

Protein orientation weak hydrophobic interactions hold proteins in the membrane while allowing them to move.

Part 4 of 5Phospholipids compared with triglycerides

Feature	Phospholipid	Triglyceride
Fatty acid tails	2	3
Phosphate group	Yes	No
Ester bonds	2	3
Water molecules released on formation	2	3
Polarity	Polar head, non-polar tails	Non-polar
Function	Cell membrane component	Energy storage

Part 5 of 5More from the specification

More from the specification

Condensation Reactions and Glycosidic Bonds

Monosaccharides like α-glucose or β-glucose link together via covalent glycosidic bonds formed during condensation reactions, eliminating a water molecule to build complex carbohydrate polymers.

Polysaccharides for Energy Storage

Starch and Glycogen — Plants store chemical energy as starch,
- (composed of amylose and amylopectin polymers of α-glucose)
while animal cells store glucose as highly branched glycogen polysaccharides in the liver and muscle tissue.

Cellulose Structure and Plant Cell Walls

Cellulose is a structural polysaccharide composed of unbranched chains of β-glucose monomers joined by β-1,4-glycosidic bonds,
where adjacent chains form hydrogen bonds to assemble into high-tensile microfibrils.

Triglycerides vs. Phospholipids

While polysaccharides are polymers of sugars, lipids are formed from glycerol bonded to fatty acids via ester bonds;
- phospholipids substitute one fatty acid chain with a hydrophilic phosphate group,
forming amphipathic molecules essential for cell membranes.

Condensation Reactions and Glycosidic Bonds

Monosaccharides like α-glucose or β-glucose link together via covalent glycosidic bonds formed during condensation reactions, eliminating a water molecule to build complex carbohydrate polymers.

Polysaccharides for Energy Storage

Starch and Glycogen — Plants store chemical energy as starch,
- (composed of amylose and amylopectin polymers of α-glucose)
while animal cells store glucose as highly branched glycogen polysaccharides in the liver and muscle tissue.

Cellulose Structure and Plant Cell Walls

Cellulose is a structural polysaccharide composed of unbranched chains of β-glucose monomers joined by β-1,4-glycosidic bonds,
where adjacent chains form hydrogen bonds to assemble into high-tensile microfibrils.

Triglycerides vs. Phospholipids

While polysaccharides are polymers of sugars, lipids are formed from glycerol bonded to fatty acids via ester bonds;
- phospholipids substitute one fatty acid chain with a hydrophilic phosphate group,
forming amphipathic molecules essential for cell membranes.

Condensation

A reaction that joins molecules and removes water.

Sucrose

The sugar mainly transported in phloem.

Covalent bond

A shared electron pair linking monomers.

Glycosidic bond

The covalent link between two sugar units made by condensation.

Non-reducing sugar

A sugar that cannot reduce Benedict's reagent until hydrolysed.

Condensation reaction

A reaction that joins two molecules by a covalent bond and releases water.

Glucose

A simple sugar; the unit that starch, glycogen and cellulose are built from.

Cellulose

A structural polysaccharide of β-glucose.

Starch

An insoluble polysaccharide of glucose used by plants for energy storage.

Glycogen

The polysaccharide in which glucose is stored in animals.

Polysaccharide

Many sugars joined, such as starch, glycogen, cellulose or chitin.

Polymer

A large molecule made of many repeating monomers.

Monosaccharide

A single sugar unit such as glucose, fructose or galactose.

Carbohydrate

A molecule of carbon, hydrogen and oxygen, including sugars, starch, glycogen and cellulose.

Key terms

Term	Meaning
Phospholipid	A lipid with glycerol, two fatty acids and a phosphate group.
Amphipathic	Having both hydrophilic and hydrophobic regions.
Hydrophilic	Attracted to and soluble in water.
Hydrophobic	Repelled by water and insoluble.
Bilayer	A double layer of phospholipids with tails facing inwards.
Membrane fluidity	How freely the molecules in a membrane move.

Avoid these mistakes

Giving a phospholipid three fatty acids — it has two, plus a phosphate.

Saying phospholipid tails are hydrophilic — tails are hydrophobic and heads are hydrophilic.

Saying saturated tails increase fluidity — unsaturated tails make membranes more fluid.

Saying heads face inwards in a bilayer — heads face the water outside and tails face inwards.

Last updated: 15 October 2026