lipids

Triglyceride

Formed by the condensation of one

molecule of glycerol and three

molecules of fatty acids

forming 3 ester bonds

Fatty acid

structure

carboxyl group and a long

hydrocarbon chain

can be saturated or unsaturated

Ester bond

–COO– chemical bond

formed between glycerol and fatty

acids

Describe how an

ester bond form?

A condensation reaction between

glycerol and a fatty acid (RCOOH)

What small molecule is released when an ester bond forms?

Water

What is the R

group of a fatty

acid?

the hydrocarbon chain.

it can be saturated or unsaturated

How is the fatty acid generally represented in the specification?

As RCOOH

What functional group do fatty acids contain?

The carboxyl (–COOH) group.

What does the R in the fatty acid formula RCOOH represent?

The hydrocarbon side chain, which can vary between fatty acids

Saturated

fatty acid

a long hydrocarbon chain with a

carboxyl group at one end

only single bonds between carbon

atoms

Unsaturated

fatty acid

a long hydrocarbon chain with a

carboxyl group at one end

at least one double bond between

carbon atoms

What specific bond is present in the hydrocarbon chain of an unsaturated fatty acid?

A C=C double bond.

What is the difference

between saturated and

unsaturated fatty

acid?

A saturated fatty acid has no

double bonds between carbon

atoms

whereas unsaturated fatty acids

have at least one double bond

between carbon atoms

What effect do double bonds in fatty acid chains have on the shape of the chain?

They introduce kinks, so the chain is not straight

How do double bonds in fatty acids affect melting point?

They lower the melting point, making the lipid more likely to be liquid (oils).

Why are triglycerides used as energy storage molecules?

They contain a high proportion of energy-rich C–H bonds and can be oxidised to release lots of energy.

How does the structure

of a triglyceride relate

to its function?

high ratio of C–H bonds to carbon

atoms → lots of energy released

when broken so more ATP can be generated during respiration

High hydrogen to oxygen ratio →

release water when oxidised

(metabolic water source)

Large, non-polar → insoluble in

water, doesn’t affect water potential

Low mass to energy ratio → energy-

rich storage molecule making them efficient energy stores

Why is it important that triglycerides are insoluble in water?

They do not dissolve in cytoplasm or blood plasma.

How are triglycerides arranged in cells due to their insolubility?

They form insoluble droplets or globules within cells.

Phospholipid

Formed by the condensation of one

molecule of glycerol and two

molecules of fatty acid

held by two ester bonds

a phosphate group is attached to

the glycerol

How does the structure of

a phospholipid relate to

its function?

phospholipids have two charged

regions, so they are polar

In water, they are positioned so

that the heads are exposed to

water and the tails are not.

This forms a phospholipid bilayer

which makes up the plasma

membrane around cells.

Polar molecule

A molecule that has an uneven

distribution of charge

Explain the

arrangement of

phospholipids in a cell-

surface membrane

In water, phospholipids form a

bilayer

Hydrophobic (fatty acid) tails point

away as they are repelled from

water

Hydrophilic (phosphate) heads

point to and are attracted to water

Hydrophobic

The tendency to repel and not mix

with water

e.g. tail region on a phospholipid

Hydrophilic

The ability to mix, interact or

attract water

e.g. head region on a phospholipid

Describe the

structure of a

phospholipid

bilayer

a phospholipid has:

A hydrophilic (polar) head that is

attracted to water

Two hydrophobic (non-polar)

tails that repel water

In a bilayer, phospholipids form two

layers with heads on the outside and

tails on the inside.

Plasma

membrane

phospholipid bilayer

cell surface membranes and

organelle membranes

Why is the phospholipid bilayer a useful barrier in cells?

It forms a partially permeable barrier between cell contents and the external environment.

How does the presence of a charged phosphate group affect interactions with water and ions?

The charged head can interact with water and charged particles, helping stabilise the bilayer at the water interface

How do the structural differences between triglycerides and phospholipids explain their different roles?

Triglycerides, with three hydrophobic tails, are suited to energy storage, while phospholipids, with hydrophilic heads and hydrophobic tails, are suited to forming membranes

How does the

structure of a

triglyceride and

phospholipid differ?

Triglyceride: one glycerol + three fatty

acids

Phospholipid: one glycerol + two fatty

acids + one phosphate group

Triglycerides are non-polar and

hydrophobic

Phospholipids have a polar

(hydrophilic) head and non-polar

(hydrophobic) tails

Test for lipids

Add ethanol and shake

to dissolve lipids

then add water

milky white emulsion forms

What is the first step in the emulsion test for lipids?

Mix the sample with ethanol.

Why is ethanol added in the emulsion test?

Ethanol dissolves lipids, allowing them to be carried into solution

What would the result look like if no lipid is present?

The solution remains clear and colourless.

Why does a cloudy white emulsion form if lipid is present?

Lipid droplets come out of solution when the ethanol–lipid mixture is added to water and scatter light, producing the cloudy appearance

Why must the sample and ethanol be shaken thoroughly in the emulsion test?

To ensure any lipid present dissolves fully in the ethanol.

Why is water added after dissolving the sample in ethanol?

It forces the dissolved lipids to form an emulsion if present

How could you increase the reliability of the emulsion test results?

Use a control with no lipid, replicate samples, and standardise volumes and times

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