Structure and Function of Epithelia I
So cellular membranes acts as a barrier for a cell and
some organelles so there are many different varieties
of membranes. Membrane of mitochondria have
electron transport chain and thus it is useful for ATP
The cellular membrane is flexible and asymmetric
with hydrophilic head facing the outside and lipid
tails on the inside, thus like a mirror image of lipid
bilaye. The membrane have both lipids and proteins
but protein population will vary between membranes.
Lipid components of plasma membrane
The plasma membrane is a lipid bilayer of mainly phospholipids
(75%). There are different types of phosphlipids, mainly
phosphatidylcholine (PC) is present in large amount, but other types
are phosphatidylserine, phosphatidylethanolamine and
phosphatidylinositol. Phosphatidylinositol present in very small
amounts because it can be released by cell as a signalling molecule.
Phospholipids have a head and 2 fatty acid tails and it is a
amphipathic structure which just means 2 different characteristic in
one molecule. The choline head group is hydrophilic (polar) and the
fatty acid chain which is hydrophobic and is non-polar.
Phospholipids in a aquous environment can spontaneously form a vesicle or lipid
Although phospholipids are a very stable structure, it can also be broken down such
as soap which breaks down the phospholipids so cell can be penetrated.
Cholesterol is another hydrophobic structure and is present as 20% in a plasma
membrane. Glycolipids are lipids with a sugar chain on top or a sugar associated
group, they only make up about 5% of the lipids in a plasma membrane
composition. Fluid mosaic model
Phospholipids are continously moving, and the fluidity of the membrane will
depend on the type of membrane so some can be quite stiff while other are very
fluid like. Lipid molecules can move sideways in their own half of the bilayer,
exchanging places with their neighbours.
This fluidity can occur because of the chemical structure of lipids, on the fatty acid
tails, if there is a lot of double carbon bonds (unsaturated fatty acids) this will un-
straighten the tail and giving it a kink in the fatty acids so the phospholipids cannot
pack as tightly together thus they can move around more freely and faster. But with
saturated fatty acids and cholesterol present, they have straight structure so they can
pack together and tightly giving very little room for movement thus very stiff and
lipids move slower.
Because lipids and protein can move, this means that membrane can break apart
and re-seal such as exocytosis. Proteins are embedded in the lipid bilayer, but can
move within the bilayers. Lipid molecules can move sideways in their own half of
the bilayer and exchange places with their neighbour. They are flexible for cell
growth, secretion and self sealing.
Integral (intrinsic) membrane proteins are transmembrane proteins that span
through out the lipid bilayer, many are glycoproteins. Integral membrane proteins
help to form glycocalyx which is the glycoprotein on extracellular material.
Peripheral proteins are proteins which only associate with one leflet of the lipid
bilayer. These proteins have a hydrophobic domain which allowing it to insert and
anchore itself on a side of the lipid layer into the fatty acid tails within. There are
many other ways in which proteins can associate with the membrane.
So different populations of protein are expressed will depend on the cell type,
proteins are associated with membrane because that's normally where they function.
Membrane need to be flexible to allow sealing and growing.
Example insuline is a polar hormone so it targets liver and skeletal muscle and is a
polar hormone, this means that insuline receptors must be embedded within the
plasma membrane of those cells but not necessaryly in other cells. Ion channels and carrier proteins are integral proteins which provide ways for
substance to move across the membrane barrier layer of lipid bilayer which are
Receptor is when a ligand just binds onto it which then will sent a signal or second
messenger cascade into the cell. Enzyme also works similar in which substrate
binds to the outside of the enzyme and catalytic activity occurs and products is
Linker or adhersion proteins are not necessarily integral proteins but they link to a
integral membrane protein to help form a stable complex of proteins.
Because epithelia cells are specilised
for transport of ions and molecules, so
the population of proteins must be
different between the apical and the
So this shows a sodium glucose co-
transporter (SGLT) which maybe
found in the proximal tubule of
kidney or small intestine, this will
allow Na and glucose to come into the
cell. On the basolateral side, there are
Na-K ATPase which will pump Na
back out of the cell and K into the cell with the use of ATP, glucose will be
transported in the glucose transporter into the capillary. So this is a transcellular
pathway which means transport of substance from external to internal environment
via the epithelia cell. Depending on the epithelia, some molecules may enter the internal environment
between the epithelia cells called paracellular pathway, in this pathway the
molecule must penetrate via the tight junction. In some epithelia nothing goes via
the paracellular pathway while some a lot of things goes through.
So here shows 4 epithelial cells packed together with
4 different type