Biology Reference
In-Depth Information
cholesterol, howdoes themitochondrial inner membrane accomplish the same basic functions
as the cholesterol-rich plasma membrane? Cholesterol plays an important role in preventing
leakiness tomany solutes across the plasma membrane, yet the cholesterol-free mitochondrial
inner membrane must be very impermeable to the smallest solute, a proton, to function.
For many years it was believed that cholesterol was nearly absent from plants. However,
its presence in higher plants is now universally accepted. Still plants produce a staggering
number of plant sterols collectively referred to as phytosterols. More than 250 different
phytosterols have been discovered to date. With such a large number of sterols commonly
found in plants, it is obvious that large quantities must enter the human diet
[34]
. Yet photo-
sterols are systematically excluded from the human body, primarily at the level of the intes-
tinal epithelium.
Interestingly,
it
is well documented that plant sterols, particularly
b
-sitosterol, do compete with cholesterol uptake, thus providing a potentially beneficial
role in reducing human cholesterol levels.
E. MEMBRANE LIPID DISTRIBUTION
Table 5.1
lists an 'average' mammalian cell polar lipid composition (these numbers are
representative of many examples, for example see
[35]
). Note the values are reported as
a range and not a precise number. Unlike protein and nucleic acid compositions that are
defined, lipid compositions can vary considerably with different organisms, different tissues
of the same organism, different organelles within a cell, different membrane domains within
a single membrane and even different acyl chains of phospholipids in the same cell. Lipid
composition can vary with diet, environmental conditions and age of the organism. However
there are some general conclusions that are usually observed for mammalian cells. The major
structural lipids are usually PC
PE. There are no cationic lipids. The anionic lipids, PA, PS,
PI, PG and CL, are all found at lower levels than the structural lipids PC and PE. At physio-
logical pH, PC is a true zwitterion and so has no net charge, while PE is almost a full zwit-
terion that has a slight net negative charge. Cholesterol can vary from trace amounts (inner
mitochondrial membrane) to being the major polar lipid (over 50 mol% in the plasma
membrane) depending on the particular membrane. CL is found almost exclusively in the
mitochondrial inner membrane. Free, unesterified fatty acids and lysophospholipids
(
Figure 5.24
) are found in very low amounts where they adversely affect membrane structure.
High enough amounts of free fatty acids or lysophospholipids will kill a cell through the
'detergent effect'. Fatty acids are distributed unevenly between the two phospholipid acyl
chains. The
>
-1 chains are predominantly, but not exclusively, saturated (often 16:0 or
18:0), while the
sn
-2 chains are mostly unsaturated (18, 20 or 22 carbons long). The large
number of unsaturated acyl chains means that, on average, at physiological temperature,
most, if not all, of the membrane is in the melted, fluid state.
sn
F. PLANT LIPIDS
The most characteristic feature of plants is photosynthesis and photosynthetic organisms
have evolved unique membrane lipids
[36]
to support this process. Photosynthetic organisms
