lipids

what are lipids made of

hydrocarbons

are lipids soluble

only in organic solvents like alcohols

structure of triglycerides

1 glycerol with 3 fatty acids bonded by ester bonds formed in a condensation reaction

the R group is the hydrocarbon tail

the hydrocarbon tail is hydrophobic and non-polar, therefore insoluble in water

at the end is a carboxyl group

to form a triglyceride…

condensation reaction

the hydroxyl groups of each carbon on the glycerol reacts with the hydroxyl group of the fatty acids to form ester bonds

3 fatty acids bond to a glycerol

E structure

esterification

saturated lipids/fatty acids

do not have any double bonds

straight line of hydrocarbons bonded by single bonds

unsaturated lipids/fatty acids

there is at least 1 double bond between carbon atoms

causes a kink in the chain

melting point of a saturated lipid

higher melting point

no double bonds in saturated fatty acids, so no double bonds, no kinks in the chain

this causes the molecules to pack closely together

so the intermolecular (van der waals) forces are stronger

melting point of an unsaturated lipid

lower melting point

double bonds present in unsaturated fatty acids, kinks present in the chain

molecules do not pack as closely together

so weaker intermolecular (van der waals) forces

why can triglycerides be easily stored

insoluble because they are non-polar

this means that they do not affect water potential or osmosis

to form a phospholipid…

2 fatty acids + glycerol + phosphate group

structure of a phospholipid

the two fatty acids are long chain hydrocarbons, and are therefore hydrophobic (hydrocarbon = hydrophobic)

the phosphate group is polar and hydrophilic - attracts water

cell membranes

made of a phospholipid bi-layer

the hydrophobic tails face inwards - prevents water moving in

the hydrophilic heads face outwards

this makes the membrane selective - does not allow water-soluble molecules to pass through

membrane fluidity

the ability of any substances to move through the membrane

the presence of unsaturated fatty acids disrupts the tight packing due to kinks

thus increasing membrane fluidity since there are bends, so substances can move more easily through the membrane

cholesterol

another type of lipid attached to the hydrocarbon tail - hydrophobic

has a ring structure which has a polar hydroxyl group, so cholesterol is soluble in water

cholesterol function

maintains fluidity of cell membrane

it fits between phospholipid molecules and binds to the hydrophobic tails of the phospholipids - causing tighter packing of molecules

this makes the membrane less fluid - less space, more rigid

helps to maintain membrane fluidity at high and low temperatures; at high temperatures, the cholesterol restrains phospholipid movement, at low temperatures, cholesterol stops the membrane from becoming too rigid