Structure and Function of Epithelia II
Cytoskeleton provides structural support of a cell, it is like the skeleton of the cell
contained within the cytoplasm. It maintains the cell shape and protects the cell and
enabling cellular motions such as developing structures such as flagella and cilia. It
also plays a role in intracellular transport and cellular division. There are mainly
made up of 3 filaments, microfilaments (actin), intermediate filaments and
This shows a actin filament at the brush
border of an epithelial cell, actin filament are
also known as microfilament. It is made up
of actin proteins joined in a long chain and it
is the thinnest filament. It is made up of G-
actin monomer linking into a chain called F-
Actin polymer, which then pair off and
interwind along eachother making it
They are found in a adherens junction and
Although microtubules and actin filaments are
found in all eucaryotes, intermediate filaments
are only found in some soft tissued animals.
They provide lots of strength and they are found
in desomosome junctions. So 2 monomers
associate with eachother creating a twisted
dimer helical structure, then 2 dimers line up next to eachother forming a staggered
tetramer and arranged in opposite orentiations. Then the tetramers will link end to
end building up one strand of intermediate filament. Then 8 filaments come
together twisting like rope giving it stronge strength. Microtubules are thickest, and they are formed
by tubuline proteins joining into long tube.
Structures like cilia and tail of sperm are made
up of microtubules. They also provide tracks
for transportation in the cytoplasma such as
transporting vesicles around. They also help
separate the chromosomes to opposite end during cell division thus cell motility.
Cilium have this 9 pairs with 2 in the middle called a axenome structure. There are
dynein arms that help slide microtubules along eachother causing the beating of
cilia and ATP is used.
Dynein move towards the -ve end while kinesin move towards +ve end. Membrane permeability
a permeable structure allows passage of substances through it, an impermeable
structure does not. Plasma membrane have a selective permeability depending on
it's properties and type. The impermeable properties allows cell to set up a
concentration gradient across the 2 sides of the membrane.
Membrane are permeable to non-polar substances that are
hydrophobic such as steroid hormones, and small uncharged
molecules such as oxygen and carbondioxide.
However they are impermeable to ions and molecules that is
large in size even though it is uncharged, such as ions and
glucose, thus they cannot cross the membrane.
They are slightly permeable to small unchared polar molecules
such as water and urea via diffusion. During movement of
membrane of fluid mosiac model, this allows gaps to be
formed in the plasa membrane so water and urea can pass
through those gaps.
Note that although phosphate and proteins are constantly moving, but they are only
moving in their own side, thus flip flop does not occur. Unsaturated tails will also
provide loose packing of membrane phosphate lipids thus providing gaps.
So when partition is removed, you will get a large
movement of atom from A to B and little from B to A,
thus giving a net movement of A to B so over a period
of time A and B would share the same conecentration
of ions. But ions are still constantly moving between B
and A, just no net movement occurs.
Atoms and molecules are continually moving
(brownian motion) and over time they will move to a
region of lower concentration gradient.
The Brownian motion is the random movement motion of particles suspended in
fluid (liquid or gas). So over a period of time, the atoms or molecules will spread
out equally. Protein mediated transport
solutes that cannot diffuse across the plasma membrane may
still be able to enter a cell if the plasma membrane contains an
internal membrane protein that allows a solute to pass through
into the cell. They will allow a specific molecule to pass
through, it has side hydrophobic amino acids on side in contact
with the fatty acid tails and hydrophilic on inside pore and
ends of the channel.
Substances such as Na, K and gluose all have transporters
embedded in the plasma membrane that span the bilayer of the
Substance moved by transporter is much more faster than diffusion.
Facilitated (passive) transport is a mediated movement of substance down it's
electrochemical gradient with no energy required, thus both electric and
concentration gradient. This includes pore-mediated, channel-mediated and carrier
mediated transport. They cannot move substances against their concentration
Pore-mediated transport such as a aquaporines (AQP1) which are water
cahnnels that made up of four protein subunits coming together to form a
pore that only water can fit through.