For a start, what is a cell membrane? Cell membrane is
the barrier and exchange between the cell and its environment.
For most of us, when we mention cell membrane, we only
remember specifically the plasma membrane.
We must not forget that internal
membranes are also present inside the cell. Examples of internal membranes
are: Golgi apparatus and endoplasmic reticulum.
The most important feature of the cell membrane is the
lipid bilayer structure, formed mainly by phospholipids. The figure below shows
the fluid mosaic model of a cell membrane structure, with all the membrane
contents, which will be talked about later on.
Membrane contents
|
Membrane
contents
|
||
|
Carbohydrates
|
Lipids
|
Proteins
|
|
- Glycoproteins
- Glycolipids
|
4 types:
-
Phospholipids
- Steroids
- Neutral
fats
- Glycolipid
|
- Intergral
o Single
span
o Multiple
span
- Peripheral
o Exofacial
o Endofacial
Function wise:
ü Anchoring
ü Receptor-transducer
function
|
The table above shows a basic summary of the chemical
contents that form up the cell membrane. But in detail…
Carbohydrates
There are various carbohydrate chains which will be
attached to different structures on the cell membrane. If the carbohydrate
chain is attached to a protein, is it a Glycoprotein.
If the carbohydrate chain is attached to a lipid, it
is a Glycolipid.
Lipids
The
most important lipid is the Phospholipid. Structure is as shown below:
We can see that there is a polar head, which confers
its hydrophilic (dissolves in water) characteristic, and there are 2 fatty acid
chains attached to it, which is hydrophobic. The hydrophilic polar head and
hydrophobic lipid tails gives phospholipids its AMPHIPATHIC nature.
Lipids can be saturated (C-C) or unsaturated (C=C), as
shown in the picture above. This is an important feature, as the degree of
unsaturation affects the quality of the cell membrane.
For example, decrease in unsaturation means more C-C. The
lipid chains are more ‘straight’, which increases rigidity with decreased
packing of chain. This effectively decreases the mobility of the phospholipid,
and on a whole, makes the cell membrane more rigid. Cell membranes which are
more rigid, are less capable in withstanding physical pressures.
Phospholipids spontaneously form bilayers in aqueous
solutions, and this process stabilises the membrane structure. This is because,
the polar heads will form polar attractions with each other, while there is
tail-tail interaction between the hydrophobic tails.
Steroids refers to cholesterol and its esters. They
are also amphipathic, but they have relatively large C-H portion.
Neutral fats mainly refers to glycerides, and
Glycolipids (as mentioned before), are lipids with carbohydrate chains attached
to them.
Proteins
Structure wise, membrane proteins are classified as
integral (transmembrane) and peripheral. The integral membrane passes through the
cell membrane, and they can be single span or multiple span. Single span passes
through the membrane once, while multiple membrane passes through the membrane
multiple times.
Whereas for peripheral proteins, it stays attached to
the membrane surface and does not pass through the cell membrane. Peripheral
proteins can be either endofacial (on the inner membrane) or exofacial (on the
outer membrane).
Function wise, different structures of membrane
proteins have different kinds of structures. Multiple spanning transmembrane
proteins usually form a “pore” in the cell membrane to allow transport of
certain ions or molecules in and out of the cell. Single spanning proteins may
act as anchoring for other proteins, or they have Receptor-transducer function.
Exofacial peripheral proteins usually act as antigens
for recognition by immune cells. The endofacial peripheral proteins can be
enzymes that act as one of the downstream messengers when a receptor on the
cell membrane gets activated.
Amino acid
residues in proteins
Amino acids are the basic building blocks of proteins.
The various amino acids have a different charge, depending on what their side
chain is. There are 3 main types of amino acid residues in proteins:
I.
Neutral: no charge
II.
Cationic: Basic (positively charged)
III.
Anionic: Acidic (negatively charged)
A neutral amino acid can be Zwitterionic. Zwitterionic means the molecules have both separate
groups of positively charged and negatively charged side chains.
Examples of basic amino acids would be Arginine and
Lysine, and examples of acidic anionic side chains would be glutamate and
aspartate.
Protein
electrical charge distribution
Electrical charge distribution along a protein varies,
and it is dependent on the primary structure of protein.
Regions of high charge density (presence of a lot of
+/- residue) are relatively, hydrophilic. This is usually found on the cell
membrane surface, where the polar heads are.
Regions of low charge density will be relatively
hydrophobic, which is inside the cell membrane, where the lipid tails are.
Using that principle, we can use a hydropathy plot to find out the
structure of a certain transmembrane protein. Referring to the example below…
Hydropathy plots are done by plotting the mean
hydrophobicity against the amino acid sequence of the protein. The higher the
hydrophobic point, the more it suggests that the protein is inside the core of
the cell membrane. Thus, we can predict the number of times a protein passes
through a cell membrane, giving us an idea of the protein structure.
So that is the main bulk on the structure and feature
of how the cell membrane act as a barrier. In the next post, I will talk about
how the cell membrane acts as an exchange for ions and molecules in greater
details.
Preview:
The 3 different kinds of mechanism by which solutes
are transported through the membrane: diffusion, osmosis and transport.
