Lipids are important components of our nutrition, and they fulfill a variety of important roles in t — Analytical Chemistry Chemistry Question
Lipids
Lipids are important components of our nutrition, and they fulfill a variety of important roles in the body – although we do not always want to be reminded of their presence!
Lipids can be classified according to their hydrophobicity: apolar or neutral lipids with overall hydrophobic structures store energy in our fat cells, whereas polar lipids, which contain a polar “head group” and one or more apolar “tails”, are found in the membranes around each cell of our body.
In addition to the common phospholipids like lecithin, other polar lipids like cerebroside are present in membranes surrounding human cells.
[VISUAL]
Lecithin (phosphatidyl choline) Cerebroside
Name the building blocks of lecithin. Indicate the head and tail structures of both lipids in the structure above.
Model Answer
Glycerol, phosphate, choline, 2 fatty acids (in this case stearic and linoleic acid)
[VISUAL]
polar heads hydrophobic tails
If lipids are mixed with water, what aggregates can they form? Describe two characteristic superstructures which are commonly found in biological systems, including our food. How are the lipid head groups oriented towards the water? Which factor determines the superstructure formed by a lipid?
Model Answer
Micelles and vesicles (lipid monolayers, lipid bilayers).
Micelles: spheroidal with (hydrophilic) head groups facing outwards; diameter depends on tail lengths, no water inside.
Vesicles: spheroidal lipid bilayers with head groups facing inwards and outwards, filled with water.
Micelles will form if the polar head group of the lipid has a much larger cross section than the hydrophobic part. Naturally occurring phospholipids carry two bulky fatty acids which do not fit into a micelle, therefore a vesicle (a lipid bilayer) forms.
Together with other lipids cerebrosides are found on the surface of human cells. In contrast to the head group of cholesterol which points to the inside as well as to the outside, the head group of cerebrosides is found exclusively pointing to the outer surface of human cells.
Why does this arrangement not dissipate into the entropically favoured arrangement with the head groups of the cerebrosides pointing to the inside and outside?
Model Answer
The polar head groups from the outer layer of the membrane would have to cross the hydrophobic part of the membrane bilayer surrounding the cell to reach the more stable symmetric arrangement. This so–called “flip flop” mechanism has a high activation energy preventing the rearrangement of the cerebroside molecules into a symmetric distribution.
The differential scanning calorimetry plot below refers to a mixture of 60% disteaoryl phosphatidyl choline and 40% water.
[VISUAL]
A: heat uptake (rel. units) B: temperature (K)
Explain the two peaks in the diagram. How can a living cell control the position of the second peak to adapt the properties of its membrane to the demands of life?
Model Answer
The peak at about 273 K indicates the phase transition from ice to water. The peak at about 335 K results from a phase transition of the phosphatidyl choline in the vesicles: At low temperatures, in the so–called liquid–crystalline phase, the C–C bonds of the hydrocarbon chains of the saturated fatty acids are in the single–trans conformation leading to a rigid, highly ordered array of many straight chains. Above the transition temperature, this order is disturbed by kinks in the hydrocarbon chains due to different conformations in some of the C–C bonds leading to a more disordered, fluid phase of the vesicles.
The fluidity of biological membranes is well controlled. Cells can reduce the transition temperature by introducing lipids with shorter fatty acids or with unsaturated fatty acids (the naturally occurring cis–conformation leads to a kink and disrupts the order). The incorporation of cholesterol, which prevents the packing of hydrocarbon chains of the other lipids is another way to control the phase of the membrane.
In blood, lipids are transported in the form of lipoproteins, which consist of polar and apolar lipids, as well as proteins with hydrophilic and hydrophobic surfaces.
In western countries lipoprotein levels are elevated in the blood of many people due to a high fat diet. Especially high amounts of cholesterol and cholesterol–esters in some lipoproteins lead to modifications of blood vessels and lipid deposition (atherosclerosis). This can finally result in a blockage of the blood flow in the arteries supplying the heart with oxygen: a heart attack occurs, one of the most common causes of death.
How could lipids and proteins form lipoproteins, stable superstructures which can be easily transported in blood? How would
a) cholesterol
b) esters of cholesterol with fatty acids be incorporated into lipoproteins?
Model Answer
Lipoproteins are supramolecular structures of lipids and proteins forming micelles with the polar surfaces of the proteins and the head groups of the polar lipids (phospholipids, free cholesterol) facing outwards. The apolar lipids (triacylglycerols, cholesteryl esters) together with the hydrophobic surfaces of the proteins and the hydrophobic part of the polar lipids are hidden in the interior.
a) The OH–group of cholesterol represents the polar head group and faces outwards, the apolar steroid ring system faces inwards.
b) Cholesteryl esters are hydrophobic lipids buried in the interior of the lipoproteins.
[VISUAL]
Structure of a lipoprotein (adapted from Lehninger, Biochemistry)