Surrounding every cell is some sort of covering that keeps what's inside the cell inside and prevents harmful particles in the external environment from diffusing into the cell. There are two types of organelles which serve as a covering for the cell: the cell membrane and the cell wall. All cells have a cell membrane, and certain cells also have a cell wall. We'll discuss each of these separately, because they perform very different functions.
The cell membrane's main purpose is to regulate the movement of materials into and out of the cell. By doing so, the internal environment of the cell can be different than the external environment, since only certain materials can pass through. Scientists say that the cell membrane
is selectively permeable, which means that only certain substances can
permeate (go through) the membrane. The next section discusses how the cell membrane accomplishes this very important function, but for now, let's discuss what the cell membrane is made of.
Cell membranes consist mainly of phospholipids. Phospholipids have two parts: a polar phosphate "head" and two nonpolar fatty acid "tails." These molecules are arranged in what is called a bilayer (a double layer) so that the polar heads face the external and the internal environments of the cell and the fatty acids form the inside of the membrane. The phospholipids
are free to move around, often switching places with their neighbors.
This allows for the membrane to stretch and change shape.
Also contained within the membrane are large protein molecules. Some of them jut out on either side of the membrane, while others are only on one side.
Unlike the cell membrane which is found in all cells, not all cells have a cell wall, another structure which surrounds the cell membrane. In particular, prokaryotes usually have a cell wall of some sort, and a type of eukaryotes called algae may also have a cell well. The cell walls in each of these two types of organisms is different.
For prokaryotes, the cell wall usually contains large polymers called peptidoglycans. These molecules provide for the strength of the bacterial cell wall. Some prokaryotic cell walls have two layers: an inner layer made of peptidoglycans and an outer layer composed of lipoproteins and lipopolysaccharides.
You will learn later that in eukaryotes, the cell wall has three main parts: the primary cell wall, the middle lamella, and the secondary cell wall. The primary cell wall is located closest to the inside of the cell. It is made mostly of cellulose which allows the wall to stretch as the cell grows. The middle lamella is composed of polysaccharides called pectins. Outside of the middle lamella is the secondary cell wall, which contains both cellulose and a strong material called lignin. Lignin strengthens the wall and gives the cell a somewhat rectangular shape.
STRUCTURAL PROPERTIES OF CELL MEMBRANES
Formation of cell membranes is based upon the properties
of lipids. All are bi-layers of phospholipids with associated proteins. As
previously mentioned, phospholipids are amphipathic meaning that
they possess both a hydrophobic and hydrophilic end.
Polar head groups are in contact with water while fatty acid tails
aggregate in the interior of the membrane. The four major phospholipids found in cell membranes:
phosphatidyl choline
phosphatidyl ethanolamine
phosphatidyl serine
sphingomyelin - a non-glycerol phospholipid
Various glycolipids are also found in the outer leaflet of the
cell membrane. Cholesterol is another important constituent of the animal cell
membrane. Lipid composition differs in the different types of cells and
in different types of organisms. The average eukaryotic plasma membrane
- ~50% of mass is lipid and 50% protein.
The lipid bilayer behaves as a fluid.
FLUID MOSAIC MODEL
The fluid mosaic model was first proposed by Singer and Nicolson . Lipids and proteins can readily move laterally and can also undergo
rotation. The degree of membrane fluidity is determined by temperature
and lipid composition. Lipids with shorter fatty acid chains are less rigid and remain
fluid at lower temperatures. This is because interactions between shorter
chains is weaker than for longer chains. Lipids containing unsaturated fatty acids increase membrane fluidity
The = bonds introduce kinks, preventing tight packing of the fatty acids.
Cholesterol with
its hydrocarbon ring structure plays a distinct role in determining membrane
fluidity.
Polar hydroxyl group positions close to the phosphate head group. Rigid
rings interact with regions of fatty acid chain adjacent to phospholipid
head groups. This interaction decreases the mobility of the outer portions
of the fatty acid chains, making this region of the membrane more rigid,
even at higher temperatures.
On the other hand...
Insertion of cholesterol interferes with interactions between fatty
acids, thereby maintaining fluidity at lower temperatures. Cholesterol is not present in bacteria or plant cells. Plant cell membranes do contain sterols which function in a manner
similar to cholesterol. In the fluid mosaic model of the membrane ----there are membrane
proteins inserted into the lipid bilayer. The lipids provide the basic
structure, but proteins carry out the specific functions of the different
types of membranes.
