Biological membranes and transport:Cell membrane Biochemistry: U.Satyanarayana

T h" plasma membrane is an envelope
I surrounding the cell \Refer Fig.l.l). lt
separates and protects the cell from the external
hostile environment. Besides being a protective
barrier, plasma membrane provides a connecting
system between the cell and its environment.
The subcellular organelles such as nucleus,
mitochondria, lysosomes are also surrounded by
membranes.
Chemical cormpcsitron
The membranes are composed of lipids,
proteins and carbohydrates. The actual compo-
sition differs from tissue to tissue. Among the
lipids, amphipathic lipids (containing hydro-
phobic and hydrophilic groups) namely phos-
pholipids, glycolipids and cholesterol, are found
in the animal membranes.
Manv animal cell membranes have thick
coating of complex polysaccharides referred to
as glycocalyx. The oligosaccharides of
glycocalyx interact with collagen of intercellular
matrix in the tissues.
$trsscture, of r*terit&'ra$:s :'
A lipid bilayer model originally proposed for
membrane structure in 1935 by Davson and
Danielle has been modified.
Fluid mosaic model, proposed by Singer and
Nicolson, is a more recent and acceptable model
for membrane structure. The biological
membranes usually have a thickness of 5-8 nm. A
membrane is essentially composed of a lipid
bilayer. The hydrophobic (nonpolar) regions of the
lipids face each other at the core of the bilayer
while the hydrophilic (polar) regions face outward.
Clobular proteins are irregularly embedded in the
lipid bilayer (Fi9.33.1). Membrane proteins are
categorized into two Broups.
1. Extrinsic (peripheral) membrane proteins
are loosely held to the surface of the membrane
and they can be easily separated e.g. cytochrome
c of mitochondria.
2. Intrinsic (integral) membrane proteins are
tightly bound to the lipid bilayer and they can
be separated only by the use of detergents ororganic solvents e.g. hormone
receptors, cytochrome P45g.
The membrane is asymmetric
due to the irregular distribution of
proteins. The lipid and protein
subunits of the membrane give an
appearance of mosaic or a ceramic
tile. Unlike a fixed ceramic tile, the
mernbrane freely changes, hence
the structure of the membrane is
considered as fluid mosaic. Fig.33,l : The fluid mosaic model of membrane structure.
Tvamsport &crqlss Fsnembrames
The biological membranes are relatively
impermeable. The membrane, therefore, forms a
barrier for the free passage of compounds across
it. At least three distinct mechanisms have been
identified lor the transoort of solutes
(metabolites) through the membrane (Fi9.33.A.
I . Passive diffusion
2. Facilitated diffusion
3. Active transoort;
1. Fassive diffusion : This is a simple process
which depends on the concentration gradient of
a oarticular substance across the membrane.
Fassage of water and gases through membrane
occurs by passive diffusion. This process does
not require energy.
2. Facilitated diffusion : This is somewhat
comparable with diffusion since the solute
moves along the concentration gradient (from
higher to lower concentration) and no energy is
needed. But the most important distinguishing
feature is that facilitated diffusion occurs through
the mediation of carrier or transport proteins.
Specific carrier proteins for the transport of
glucose, galactose, leucine, phenylalanine etc.
have been isolated and characterized.
Mechanism of facilitated diffusion : A ping-
pong model is put forth to explain the occurrence
of facilitated diffusion (Fig.33.3). According to
this mechanism, a transport (carrier) protein exists
in two conformations. In the pong conformation,
it is exposed to the side with high soluteconcentration. This allows the binding of solute
to specific sites on the carrier protein. The protein
then undergoes a conformational change (ping
state) to expose to the side with low solute
concentration where the solute molecule is
released. Hormones regulate facilitated diffusion.