From an organ system level to a cellular level, the
structure of all biological systems determines its function. An example of this is in human teeth, as each
teeth is designed in a way to match its role when tearing and grinding food. Another
example is finger like projections found in epithelial cells to increase
surface area so that more reabsorption can take place eg in the small intestine.
There are thousands of ways the human body is structured in a specific way that
relates to its function, and it would be impossible to mention them all in this
essay so I will be focusing on the structure of plasma membranes and how this
relates to the function of the cell.
The plasma membrane surrounds the cell, and has many
essential functions such as maintaining ion gradients, controlling what enters
and exits the cell, as well as supporting and maintaining cell shape. Membranes
control proteins such as channels, transporters, integrin’s, receptors and
enzymes. These are important for maintaining correct function of the cell. Transport
proteins are coded for by 30% of the genome and are extremely important to
maintain gradients. It is also a very energy consuming process and some
mammalian cells use up to 60% of their cells energy on transmembrane transport.
This shows the importance the membrane has on the cells function, and without
it the cell would cease to exist.
The membrane is made up of a phospholipid bilayer of lipids
with their hydrophilic heads facing outwards and their hydrophobic tails facing
inwards. This bilayer is also associated with cholesterol, proteins and
The main function of the plasma membrane is to separate the extracellular fluid
on the outside of the cell from the intracellular fluid on the inside of the
cell. In other words, it protects the cell from its surroundings so it cannot
be infected or harmed by anything outside the cell. In order to regulate the movement
of what goes into and out of the cell the membranes embedded proteins move specific
solutes into and out of the cell when instructed to do so by the cell.
Receptors are also positioned on the membrane to receive and
relay signals from neighbouring cells and are called transmembrane receptors.
Their job is to conduct signal transduction, converting signals received into
intracellular signals that can be converted by the cell. The 3 main types of
membrane receptors include; Ligand-gated ion channel receptors, enzyme coupled
receptors and G-protein-coupled receptors. Ligand-gated ion channel receptors
work when a ligand binds and the channel opens allowing the ion through. Enzyme
coupled receptors are also activated when the ligand binds causing the enzyme
to get to work. G protein coupled receptors are the largest and once the ligand
binds, GTP is activated which in turn activates an effector protein. Each of
these proteins are of a specific shape for their specific ligand to activate
them. A slight change in this structure and the ligand can no longer bind
making the receptor inactive and the signal can no longer be received.
The membrane plays a vital role in ATP synthesis and is
produced by a process called chesmiotic coupling. This is made possible by
protein complexes that are embedded into the phospholipid bilayer, with the
help of activated carrier molecules such as NAD and FAD. These molecules are
called carriers as they carry high energy electrons through electron carriers.
This occurs by travelling through covalent bonds and jumping across a 2mm gap
via electron tunnelling. It is this structure of the enzyme complexes allowing for
the crucial flow of electrons, resulting in our bodies production of energy in
the form of ATP.
Electron carriers make up the respiratory chain and are made up of 3 enzyme
complexes. The largest of the 3 is the NADH dehydrogenase complex, and consists
of 40 polypeptide chains. NADH releases the electrons it earlier accepted from
a redox reaction in a citric acid cycle. These electrons are then passed on to
ubiquinone. Ubiquinone accepts hydrogen atoms for every electron it accepts and
when these electrons are passed on to the 2nd enzyme complex (cytochrome
b-c1 complex) a proton is released into the inner mitochondrial membrane. Cytochrome
b-c1 passes the electron to the 3rd enzyme complex which is
cytochrome C. Cytochrome C acts as a motile carrier, passing electrons from one
complex to another. While this occurs, protons are pumped into the inner
mitochondrial membrane by each of the complexes to create a proton gradient.
The last complex to receive the electrons is cytochrome oxidase and accepts
electrons one at a time from cytochrome C.
Each of these enzyme complexes converts this energy from the flow of electrons
to pump protons across the membrane which is essential in the production of
ATP. This causes an electrochemical proton gradient, causing protons to want to
move downhill their concentration gradient down the enzyme ATP synthase and
synthesising ADP and an inorganic phosphate as this occurs. ATP synthase has
the ability to generate 100 molecules of ATP a second.
ATP synthase is formed from multiple subunits, and has a channel allowing
protons pass through. It is this movement of protons that causes the ring to
spin, converting the energy of protons into mechanical energy. It is this
structure that allows for the synthesis of ATP.
The membrane of a mitochondria is therefore crucial for the function it plays
in our body allowing for the main production of our bodies energy. It is this
structure of the enzyme complexes that allows for the production of an
electrochemical proton gradient, driving the synthesis of ATP.
The phospholipid bilayer of the membrane means that polar
molecules cannot pass through the hydrophobic inner layer of the bilayer. In order
for these important polar molecules to pass through membranes, transport
proteins are embedded into the membrane that allows them to pass through. The
molecules that are transported include water soluble molecules, ions, and large
The transport proteins that are embedded into the membrane can be sorted into 2
groups; channels and transporters. Channels work by forming aqueous pores that
open to allow solutes through, across the membrane, such as inorganic ions. Transporters
bind the specific solute, and the protein undergoes conformational changes
allowing the solute to be released on the other side.
Both of these require the solutes to move down a concentration gradient, but
channel proteins generally do this faster than transporter proteins.
Transporters can either work by passive diffusion or facilitated diffusion.
These work when specific solutes bind to the transporter proteins binding
sites, causing a conformational change in the protein and releasing the solute
on the other side of the membrane.
Each protein has a specific binding site structure that allows the solute to not
only fit in a certain way but also to cause a conformational change in a
certain way when the solute binds, allowing the solute to be released on the
other side of the membrane.
However not all solutes are needed to move down their concentration gradient eg
the uptake of glucose in the ileum. Moving solutes against their concentration gradient
can be done by either coupled transport or active transport.
Active transport moves solutes against their concentration gradient with the
use of ATP. On the other hand coupled transport moves one solute against their
concentration gradient by using the downhill concentration gradient of another
The use of couple transport is shown in the uptake of glucose in the ileum, to
bring glucose down from the epithelial cells lining the lumen into the
microvilli so it can be transported into the blood and around the body.
Without this strict regulation of what can enter and leave the cell, there
would be no maintenance of the specific concentrations of ions, glucose and other
substances as well as no removal of toxic by-products, resulting in apoptosis.
Ion regulation is extremely important in certain cell activities, such as the
sodium potassium pump that’s used to create sodium and potassium gradients in generating
nerve impulses. This is done by making the inside of the cell more negatively charged,
to create an electrochemical gradient.
These electrochemical gradients are important in regulating kidneys and
homeostasis, blood pressure, controlling cardiac contractions and much more.
Therefore a change in the mere structure of these transport proteins means they
are no longer specific to these certain ions and therefore the ion gradients
can no longer be regulated. This would cause many problems all over the body
and shows how important the structure of each of these transport proteins are,
and how without loss of function would occur in many organs.
A common example of how a change in the structure of a
membrane protein can hugely affect its function is in cystic fibrosis. Cystic
fibrosis’s main mutation accounting for 90% of cases is caused by a single
amino acid deletion. This one amino acid change in the structure reduces the individual’s
quality of life and their life expectancy.
This mutation stops the correct folding and processing of the cystic fibrosis
transmembrane conductance regulator (CFTR), so that chlorine gradients cannot
be maintained, and water is not properly regulated in tissues. It is this inability
to maintain water regulation in tissues that causes a thick sticky mucus build-up
and the other symptoms of cystic fibrosis. The absence of a single amino acid
leads to the incorrect function of a protein channel affecting the whole
function of the body.
In conclusion structure affects function so much so that a
slight change in these structures can cause a complete change in its function.