3.1.3 - lipids

give the 2 groups of lipid

• triglycerides

• phospholipids

order alcohols, proteins, carbs and lipids from most energy to least energy

MOST

1. lipids

2. alcohol

3. protein

4. carbs

LEAST

what do all lipids contain

carbon, hydrogen, and oxygen (C, H, O)

• are lipids soluble or insoluble in water

• are lipids soluble or insoluble in organic solvents (e.g. alcohol or acetane)

• insoluble

• soluble

summarise role of lipids (WIPS)

• waterproofing

• insulation

• protection

• source of energy

what do lipids release (apart from energy) when oxidised that is useful?

water

• why are lipids useful for waterproofing

• examples of this in mammals and plants

• they are insoluble in water

• plants having waxy, lipid cuticles —→ these conserve water

• mammals producing an oily secretion from the sebaceous glands in their skin

• how do lipids help to retain body heat

• (extra) when do fats act as electrical insulators

• fats are slow conductors of heat

• in the myelin sheath around nerve cells

why is it good that fats help with protection

they are often stored around delicate organs, e.g. the kidney

what are triglycerides

fats and oils

how are triglycerides formed

by the condensation of 3 molecules of fatty acid (tri) and 1 molecule of glycerol (glyceride).

• what bond forms between each fatty acid and glycerol?

• this bond is formed in a…(type of reaction?)

• an ester bond

• condensation reaction

therefore, what does hydrolysis of a triglyceride produce?

glycerol and 3 fatty acids

useful diagram of the formation of a triglyceride

EXACT wording for describing the condensation reaction in this case (learn this!)

there is a reaction between the hydroxyl group (—OH) of the glycerol and the carboxyl groups (—COOH) of the fatty acids.

HOW are the properties of triglycerides different, even though the glycerol is the same?

• different triglycerides can have different fatty acids. we’ll go into this now

• general formula for a fatty acid (from spec)

• hence, all fatty acids have a c__________ group (—COOH) with a h____________ c_____ attached.

• RCOOH

• hence, all fatty acids have a carboxyl group (—COOH) with a hydrocarbon chain attached

• (the R represents the hydrocarbon chain, think of it as a variable - it can change)

• when is the fatty acid described as saturated

• why is this

• if the chain has no C==C double bonds (all single C—C bonds)

• bc all the C atoms are bonded to the maximum no. of H atoms possible (no bonds can be opened up, and therefore no atoms can be added)

• when is the fatty acid described as unsaturated

• what is a mono-unsaturated fatty acid

• what is the fatty acid called when it has more than 1 double C==C bond

• if a double C==C bond is present

• when it has 1 double C==C bond

• poly-unsaturated

diagram of diff. fatty acids

(we’ve already covered this, this is just the wording used in the spec) the R-group of a fatty acid may be…

unsaturated or saturated

apart from a fatty acid being saturated or unsaturated, what else may differ between them?

how many C atoms they have (and therefore, how long the hydrocarbon chain is)

• why are unsaturated lipids liquid at room temperature (make ref. to structure)

• the double bonds in the hydrocarbon chain cause kinks in the molecule’s structure (which cause it to bend)

• so the fatty acid tails cannot pack together closely

• which weakens the IMF between the unsaturated fatty acids

• and weak IMF require less energy to overcome, so the lipids have lower MPs

why are triglycerides a good source of energy, in terms of structure

high ratio of energy-storing C—H bonds to C atoms

why are triglycerides good storage molecules, in terms of their structure

• have a low mass to energy ratio

• so more energy can be stored in a small vol. (useful in animals, as reduces mass they have to carry as they move around)

• why are triglycerides insoluble

• hence what 2 things does their storage not affect

• they are large + non-polar

• osmosis in cells, or the water potential of cells

role of phospholipids

• contribute to the flexibility of membranes

• contribute to transfer of lipid-soluble substances across membranes

what is different between a triglyceride and a phospholipid in terms of structure?

one of the fatty acids of a triglyceride = substituted by a phosphate molecule, and a phospholipid has no glycerol

what is different between a triglyceride and a phospholipid in terms of relationship with water?

• fatty acids are hydrophobic (repel water)

• phospholipids are hydrophilic (attract water)

what 2 main parts is a phospholipid made up of

• a hydrophilic phosphate head (interacts w/ water, but not fats)

• 2 hydrophobic tails (orients themselves away from water, but mixes readily w/ fat)

diagram of a structure of a phospholipid

• what are the ends of a molecule called 

• what do you call molecules that have 2 poles that behave differently in this way?

• poles

• polar molecules

hence, when phospholipids are placed in water, how do they position themselves

• so that the hydrophilic heads are as close to the water as possible 

• and the hydrophobic tails are as far away from the water as possible

why do phospholipids, when in aquaeous environments, form a bilayer within cell-surface membranes

• phospholipids = polar molecules (hydrophilic phosphate head and hydrophobic fatty acid tails)

• so a hydrophobic barrier is formed between the inside and outside of the cell

question: why is the barrier hydrophobic if the hydrophilic heads are facing outward

• what does the phospholipid structure allow them to form

• how

• glycolipids (these are important in cell recognition)

• by combining w/ carbohydrates within the cell-surface membrane

what is the test for lipids called

emulsion test

how to carry out emulsion test for a sample

• crush / grind food and put in test tube

• add ethanol

• then add water (water MUST be added after ethanol, won’t get the mark otherwise)

• shake tube thoroughly. if cloudy white emulsion forms, lipids are present

can make a control by repeating the procedures and using water instead of the sample. final solution should remain clear.