Question

The melting temperature for phospholipid membranes is strongly impacted by fatty acid composition and cholesterol contain. You have 3 different membranes. Membrane 1 is formed by phosphoglycerides with 2 chains of behenate

Name	Number of carbons	Number of double bonds
myristate	14	0
stearate	18	0
linoleate	18	2

membrane 2 is formed by phosphoglycerides with 2 chains of stearate and membrane 3 is formed by phosphoglycerided with 2 chains of myristate. The structural properties of fatty acids are summarized in Table below. Which membrane (1, 2 or 3) has the lowest melting temperature and which membrane has the highest melting temperature. Explain your reasoning.

Answer 1

Longer the chain of carbon atom more will be melting point because larger the chain more the non polarity which results in more vanderwaal forces of attraction and hydrophobic interaction that result in increase in melting point of fatty acids.

Saturated fatty acid have higher melting point than unsaturated fatty acids (double bond) because double bond make unsaturated chain more rigid and forces a kink or bent in the hydrocarbon chain. Fatty acid with one or more kink cannot pack together tightly as compare to saturated fatty acids where free rotation around each single c-c bond gives hydrocarbon chain greater flexibility and that results in tight packaging in saturated fatty acids this leads to higher melting point in saturated fatty acid as compare to unsaturated fatty acid.

So here melting point of stearate fatty acid is maximum which is a saturated fatty acid with 18 carbon long chain

While melting point of linoleate is least because it contain two double bond which decreases its melting point as compare to two saturated fatty acid .

So melting point increasing order of fatty acid is

Linoleate< myristate< stearate

Melting point (MP) of membranes is according to their fatty acid composition hence increasing order of MP in membranes is:

Membrane 1 with two chains of myristate fatty acid< membrane 3 with two chains of myristate FA < membrane 2 with two chain of stearate FA

So Membrane 1 has least melting point while membrane 2has highest melting point .