Lipids include several categories of organic compounds, which are not water soluble.
The two main groups are:
- Triglycerides (fats and oils); also known as neutral fats
Both of these are based on glycerol , a 3-carbon compound with 3 -OH groups: a triple alcohol.
Each of these hydroxyl groups may react with other sustances.
A triglyceride is formed as a result of a condensation reaction between glycerol and three fatty acids.
The (3) bonds so formed are known as ester linkages
-C-O-C- , another form of oxygen bridge.
This condensation process cannot continue indefinitely as in carbohydrates, proteins, and nucleic acids, so it would be wrong to describe lipids as polymers.
Fatty acids - often written as RCOOH - have an alkyl 'hydrocarbon' section R - quite a long chain of carbon atoms surrounded by hydrogen atoms
- and a carboxylic acid group: -COOH.
These R groups may be described as saturated
In saturated fatty acids, all the carbon atoms are connected to other C atoms by single bonds as a chain of -CH2
- units, whereas unsaturated fatty acids have one or more double bonds: -CH=CH- (and consequently fewer hydrogens).
In triglycerides, the 3 long hydrocarbon chains and glycerol residue contain many carbon atoms surrounded by hydrogen atoms.
This means that the triglyceride molecule:
- contains a large amount of chemical energy locked into the connecting bonds, which can be released as it is respired in a parallel pathway to carbohydrate respiration
- has a hydrocarbon nature i.e. it is overall hydrophobic (water repelling) but is able to interact with other compounds of a similar nature.
This spacefilling model of a triglyceride molecule shows three tails formed from fatty acid residues extending to the right, attached to glycerol on the left. Carbon atoms are shown in grey, hydrogens in white and oxygens in red.
Phospholipids have a similar structure to triglycerides but one of the fatty acids is
substituted by a phosphate group, possibly attached to another group.
This group is able to ionise and interact with water, so it adds a hydrophilic section ("head") at the other end of the two remaining hydrophobic fatty acid residues ("tails").
This spacefilling model of a phospholipid molecule shows the phosphate "head" on the left (phosphorus in orange, surrounded by oxygens in red, with another triangular section on the far left). This is extending away from the two "tails".
This knowledge can be used to explain the biological functions of fats and oils and phospholipids
|(long term) Energy storage
||Chemical potential energy stored in C-C bonds and C-H bonds
Fat tends to be stored in less accessible parts of the body, and oil is accumulated in seeds
Waterproofing of skin, hair & feathers
- also leaf cuticle in plants
Large sections of triglyceride molecule are composed of carbon surrounded by hydrogens, resulting in hydrophobic properties of fats and oils - repelling water
| Storage of certain vitamins (A,D,E,K) and metabolic intermediates
||These organic compounds are soluble in hydrophobic sections of lipid molecules ('like dissolves like')
|Protective packing in the skin and around certain internal organs
||Triglycerides (especially those with saturated fatty acids) in adipose tissue pack fairly evenly but will not be easily compressed together
Fatty tissues are less dense than tissues containing more water (smaller molecules)
|Especially important in some species for thermal insulation (blubber of sea mammals etc)
||Heat does not easily pass through fat - less molecular movement than smaller molecules of liquids and gases
|Form cell membranes - keeping cell contents enclosed
also form many organelles within cells - forming compartments for different metabolic reactions
|Hydrophilic heads face out of cells into extracellular fluid or into cell interior (both aqueous environments)
Hydrophobic tails interact to form bilayers away from both - not allowing water-based compounds to pass easily
|Act as electrical insulation
in axons of nerve cells - several layers of myelin sheaths
||Hydrophilic heads sandwich hydrophobic tails only allowing water-based ions to pass via protein channels
The Emulsion test - for Fats and Oils
This test uses the differential solubility of lipids as described above.
In principle a solution of lipid in ethanol is produced, which is clear.
This is then converted into an emulsion: a suspension of small droplets of lipid in water. This is whitish and cloudy, like milk.
Take two test tubes.
Nearly fill one with water - tap water is adequate - no need for distilled/deionised water - and set it to one side - in a test tube rack.
The other tube is for the sample.
For oils, only a drop or two will be enough. Run the sample into the test tube.
For fats (butter, lard, 'spreads' or white material from the edge of meats), a small amount on the tip of a spatula will be enough. If a hot waterbath is available, the tube may be gently warmed to melt its contents.
More solid food products, such as meat itself, cheese, etc. may be cut up small enough to drop into the tube. Snack items such as crisps, biscuits etc may also be tried.
Next a small amount of ethanol (alcohol, industrial methylated spirit) is added to the tube containing the test substance. 1cm depth is more than enough. With liquids, you may be able to see a distinct line of separation between them and the ethanol.
Shake the mixture or heat it gently in a waterbath, to encourage the lipid content to dissolve in the ethanol.
The ethanolic layer should be fairly clear. If not, it could be filtered or diluted.
Now take the other tube containing (tap)water, and pour the ethanolic solution (prepared above) into the top.
A white (milk-like) emulsion indicates the presence of fats or oils.
This forms because lipids dissolve on alcohol, but not in water. So when the ethanol is effectively diluted by mixing with water, the lipid is 'thrown out of solution', rather like a precipitate in a chemical reaction (which is a solid which soon falls through the surrounding liquid) - but an emulsion is a mixture of small liquid droplets in another liquid and it may take a long time to separate out.
For comparison it may be useful to repeat the process omitting the sample, i.e. add just ethanol to water (from a separate, clean, tube).
There should be no emulsion - only a few swirls - "concentration lines" - as the ethanol mixes with (dissolves in) the water - not forming a distinct line, or an emulsion.
Structure of glycerol : propane-1,2,3-triol
A generalised fatty acid: RCOOH or HOOCR
The formation of a triglyceride (triacylglycerol , triacylglyceride) by the condensation reaction between 1 glycerol and 3 fatty acid molecules
Both glycerol and fatty acid molecules have -OH groups which interact with water, forming hydrogen bonds, and this explains their solubility in water. However a triglyceride has no -OH groups. This greatly reduces the chance of hydrogen bond formation which is responsible for the solubility of many biological compounds.
The triglyceride molecule is said to be non-polar
as there is no variation of electronic charge on its surface.
Saturated fatty acids have fairly straight R groups, whereas unsaturated fatty acids may have one or more bends.
Fats are likely to be solid at room temperature (25 °C), and oils liquid.
In general, fats are of animal origin and a high proportion of their fatty acids are saturated, and oils are of plant origin, with at least part of their fatty acids unsaturated. This is the result of different packing of the fatty acid residues in the triglyceride molecules, with fats consisting of parallel hydrocarbon tails quite closely packed, and oils with more open hydrocarbon tails, causing a more fluid structure.
The 3 fatty acids in a triglyceride (R1
) can be all the same (as seen above) or different (as seen below).
Click to show these using
You should be able to put a name to the fatty acid residues in the triglyceride above.
Use the mouse cursor to check your answers.
In digestion in animals, triglycerides need to be partly hydrolysed to monoglycerides or diglycerides (together with free fatty acids) before they can be absorbed into the cells lining the intestine. Here they re-form into triglycerides and are packaged into spherical structures called chylomicrons, mostly 330-350 nm in diameter, with an outer coating of phospholipids. These chylomicrons may be passed into the blood circulation and around the body.
Triglycerides are accumulated within cells of adipose tissue, around organs and in skin.
More about phospholipids
Substituting one of the fatty acid residues with a phosphate-containing group results in a phospholipid. This is a polar group
, as electrons are unevenly spread because one of the oxygens on the phosphate group is negatively charged.
Phospholipids can also be described as amphipathic or amphiphilic, as their molecules have both polar (hydrophilic) and non polar (hydrophobic) regions.
The hydrophilic head can interact with aqueous environments, and the hydrophobic tails remain aligned away from them.
Click to show this using
More than politically correct
The head (in the blue circle above) is in fact phosphatidyl choline
: often abbreviated to PC. It is in fact forming a phospholipid with palmitic acid and oleic acid - making it 1-Oleoyl-2-palmitoyl-phosphatidylcholine- and it is also known as lecithin
This compound is generally found in the outer (exoplasmic) layer of the bilayer making a cell membrane surrounding cells. It is thought to be transported between membranes within the cell by phosphatidylcholine transfer protein (PCTP
Another phospholipid on the the exoplasmic side of the plasma membrane is sphingomyelin
, also with a phosphocholine head group, attached to two 'tails' : sphingosine - an 18-carbon amino alcohol with an unsaturated hydrocarbon chain - and a single fatty acid tail.
A cellular postscript, and some great enzyme names
(PS) is normally found only on the other inner (cytosolic) side of the cell membrane.
An enzyme called flippase
is responsible for this. Floppases
transport phospholipids in the reverse direction.
At the end of a cell's life it undergoes the process of apoptosis, and PS may be transferred by the enzyme scramblase
to the outer surface, where it acts as a signal for macrophages to engulf the cell.
Other phospholipids on the the cytosolic side of the plasma membrane include phosphatidylethanolamine (PE) and phosphatidylinositol (PI).
More odd lipid compounds
Lipids can combine with sugars to form glycolipids
. Although these do not have a phosphate group they still align in membranes like phospholipids because the sugars have -OH groups which interact with the aqueous environment. In animal cells these may have a role in cell recognition, and act as antigens.
One special case is galactolipids
- lipid compounds with one or two galactose units instead of a phosphate group, and these form a large part of chloroplast membranes in plants - in fact the most common plant lipid on the planet!. It is said that monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG)) are indispensable for the efficiency of photosynthetic light reactions.
Mitochondrial membranes contain an unusual phospholipid cardiolipin
which consists of 2 diglyceride units joined via phosphate groups to a central glycerol, so it is effectively a phospholipid with 4 tails. This compound appears essential for the location of compounds involved in electron transfer reactions.
Spontaneous assembly of bilayers
Phospholipids usually assemble into a double layer with the hydrophobic tails together in the middle. The outer surfaces have hydrophilic heads projecting out into the aqueous liquid next to them. They thus form membranes on the outside of cells, and around organelles inside.
a phospholipid bilayer
In cell membranes there is also another amphipathic lipid, cholesterol
. This has an -OH (alcohol) group which gives a hydrophilic section at one end of the molecule. Leading away from this are four fused aromatic rings and a branched alkyl chain, all of which give it a hydrophobic nature. Cholesterol can thus embed and align itself in the phospholipid bilayer, modifying its texture and fluidity.
Cholesterol is converted into other compounds (steroids) - some of which function as hormones.
Oestradiol is an oestrogen (female sex hormone).
This also has four fused aromatic rings but instead of the branched alkyl chain there is another -OH group, which explains why it is a diol.
Testosterone is an androgen (male sex hormone).
It has a similar structure, but instead of one -OH it has =O, a ketone group.