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H8 campbell biology summary cell biology ldm125 animal management grade 1
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Celbiologie samenvattinghoofdstuk 8 Cell Membranes
8.1
Cellular membranes are fluid mosaics of lipids and proteins
Lipids and proteins are the staple ingredients of membranes, although carbohydrates are also
important. The most abundant lipids in most membranes are phospholipids. The ability of
phospholipids to fomr membranes is inherent in their molecular structure. A phospholipid is an
amphipathic molecule, meaning it has both a hydrophilic region and a hydrophobic region. A
phospholipid bilayer can exist as a stable boundary between two aqueous compartments because
the molecular arrangement shelters the hydophobic tails of the phospholipids from water while
exposing the hydrophilic heads to water. Like membrane lipids, most membrane proteins are
amphipathic. Such proteins can reside in the phospholipid bilayer with their hydrophilic regions
protruding. This molecular orientation maximizes contact of hydrophilic regions of proteins with
water in the cytosol and extacellular fluid, while providing their hydrophobic parts with a
nonaqueous enviroment. In the fluid mosaic model (blz. 199 figuur 8.3) the membrane is a mosaic of
protein molecules bobbing in a fluid bilayer of phospholipids. The proteins are not radomly
distributed in the membrane. Groups of proteins are often associated in long-lasting, specialized
patches, where they carry out common functions. The lipids themselves appear to form defined
regions as well.
The Fluidity of Membranes
A membrane is held together primarily by hydrophobic interactions. Most of the lipids and some of
the proteins can shift through laterally. Very rarely a lipid may flip-flop across the membrane,
switching from one phospholipid layer to the other.
The lateral movement of phospholipids within the membrane is rapid. Adjacent phospholipids switch
positions about 10^7 times per second, wich means that a phospholipid can travel about 2 µm in 1
second. Proteins are much larger than lipids and move more slowly, but some membrane proteins do
drift. Some membrane proteins move in a highly directed manner, perhaps driven along cytoskeletal
fibers in the cell by motor proteins connected to the membrane proteins’cytoplasmic regions.
The temperature at which a membrane solidifies depends on the types of lipids it is made of. The
membrane remains fluid to a lower teperature i fit is rich in phospholipids with unsaturated
hydrocarbon tails. Because of kinks in the tails where double bonds are locatedm unsaturated
hydrocarbon tails cannot pack together as closely as saturated hydrocarbon tails, making the
membrane more fluid.
The steriod cholestrol, which is wedged between phospholipid molecules in the plasma membranes
of animal cells, has different effects on membrane fluidity at different temperatures. At relatively
high temperatures (37 °C, the body temperature of humans) cholestrol makes the membrane less
fluid by restraining phospholipid movement. However, because cholestrol also hinders the close
packing of phopholipids, it lowers the temperature required fort he membrane to solidify.
, Membranes must bef luid to work properly; the fluidity of a membrane affects bot hits permeability
and the ability of membrane proteins to move to where their function is needed. Usually,
membranes are about as fluid as salad oil. When a membrane solidifies, its permeability changes,
and enzymatic proteins in the membrane may become inactive if their activity requires movement
within the membrane. However, membranes that re too fluid cannot support protein function either.
Evolution of differences in membrane lipid composition
Variations in the cell membrane lipid compositions of many species appear tob e evolutionar
adaptations that maintain the appropriate membrane fluidity under specific environmental
conditions. The ability to change the lipid composition of cell membranes in response to changing
temperatures has evolved in organisms that live where temperatures vary. In many plants that
tolerate extreme cold, such as winter wheat, the percentage of unsaturaed phospholipids increases
in autumn, an adjustment that keeps the membranes from solidifying during winter. Certain bacteria
and archaea can also change the proportion of unsaturated phospholipids in their cell membranes,
depending on the temperature at which they are growing. Overall, natural selections has apparently
favored organisms whose mic of membrane lipids ensures an appropriate level of membrane fluidity
for their enviroment.
Membrane proteins and their functions
Somewhat like a tile mosaic, a membrane is a collage of different proteins, often clustered together
in groups, embedded in the fluid matrix of the lipid bilayer. Phospholipids form the main fabric of the
membrane, but proteins determine most of the membrane’s functions. Different types of cells
contain different sets of membrane proteins, and the various membranes within a cell each have a
unique collection of proteins.
Notice in figure 8.3 that there are two major populations of membrane proteins: inegral proteins and
peripheral proteins. Integral proteins penetrate the hydrophobic interior of the lipid bilayer. The
majority of are transmembrane proteins, which span the membrane; other integral proteins extend
only partway into the hydrophobic interor. The hydrophobic regions of an integral protein consist of
one or more streches of nonpolar amino acids (figure 5.14), usually coiled into α helices (figure 8.6).
The hydrophilic parts of the molecule are exposed tot he aqueous solutions on eiter side of the
membrane. Some proteins also have one or more hydrophilic channels that allow passage through
the membrane of hydrophilic substances.
Peripheral proteins are not embedded in the lipid bilayer at all; they are appendages loosely bound
tot he surface of the membrane, often to exposed parts of integral proteins.
On the cytoplasmic side of the membrane, some membrane proteins are held in place by attachment
to the cytoskeleton. And on the extracelllar side, certain membrane proteins are attached to fibers of
the extracellular matrix. These attachments combine to give animal cells a stronger framework than
the plasma membrane alone could provide.
A single cell may have cell surface membrane proteins that carry out several different functions, such
as transport through the cell membrane, enzymatic activity, or attachting a cel lto either a
neighboring cell or the extracellular matrix. Furthermore, a single membrane protein may itself carry
out multiple functions. Thus, the membrane is not only a structural mosaic, but also a functional
mosaic. Figure 8.7 illustrates sic major functions performed by proteins of the plamsa membrane.
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